Add chat thread forking

docs/api/rest.md

@@ -89,6 +89,8 @@ Behavior notes:
       "type": "chat",
       "id": "chat-id",
       "title": "optional title",
+      "parentChatId": null,
+      "titleGenerationPending": false,
       "createdAt": "2026-02-14T00:00:00.000Z",
       "updatedAt": "2026-02-14T00:00:00.000Z",
       "starred": true,
@@ -117,7 +119,8 @@ Behavior notes:
 Behavior notes:
 - This endpoint is intended for combined conversation/search lists such as sidebars.
 - The legacy `GET /v1/chats` and `GET /v1/searches` endpoints remain available for clients that need separate collections.
-- The response currently combines up to 100 chats and up to 100 searches.
+- The response currently combines the 100 most recently updated chats, any additional root chats needed to group those forks, and up to 100 searches.
+- Root chats have `parentChatId: null`. Every fork points directly to its single root chat, including a fork created from another fork, so clients can group rows without traversing a fork chain.
 - `starred`/`starredAt` are backed by membership in a reserved `Project` with id `starred`; future project folders can reuse the same project item model.
 
 ## Chats
@@ -148,15 +151,32 @@ Behavior notes:
 
 Behavior notes:
 - `provider` and `model` must be supplied together when present.
+- Newly created non-fork chats have `parentChatId: null` and `titleGenerationPending: false`.
 - When `provider`/`model` are supplied, the new chat initializes `initiatedProvider`/`initiatedModel` and `lastUsedProvider`/`lastUsedModel`.
 - `additionalSystemPrompt` is trimmed and stored on the chat; blank values are stored as `null`.
 - `enabledTools` stores the enabled Sybil-managed tool names for future chat completions. Unknown tool names are ignored; omitted values default to all currently available tools.
 - Optional `messages` are inserted as the initial transcript. Attachment metadata uses the same schema and limits as chat completion messages.
 
+### `POST /v1/chats/:chatId/fork`
+- Body: `{ "messageId"?: string }`
+- Response: `{ "chat": ChatSummary }`
+- Source chat not found: `404 { "message": "chat not found" }`
+- Supplied message missing from the source chat: `404 { "message": "message not found in chat" }`
+- Supplied message is not an assistant response: `400 { "message": "fork message must be an assistant response" }`
+
+Behavior notes:
+- With no `messageId`, the server copies the complete source transcript. With an assistant response `messageId`, it copies the transcript through that response, inclusive.
+- The source chat is unchanged. Copied messages receive new ids while retaining their role, content, name, creation time, order, and metadata except for the source request's transport-only `clientRequestId`. Attachments and tool-call metadata are retained. LLM call logs, star/project memberships, and other child threads are not copied.
+- The fork inherits the source chat's user, initiated/last-used provider and model, additional system prompt, and enabled tool settings.
+- A fork of a root or another fork always sets `parentChatId` to the single root chat id. Root chats keep `parentChatId: null`.
+- A whole-chat fork starts with `Fork of <source title>` (or `Fork of Untitled chat`). A message fork starts with `Fork of '<message snippet>'`.
+- The placeholder title is returned with `titleGenerationPending: true`. After the first new prompt, call `POST /v1/chats/title/suggest`; the placeholder is eligible for replacement exactly once.
+
 ### `PATCH /v1/chats/:chatId`
 - Body: any subset of `{ "title": string, "additionalSystemPrompt": string|null, "enabledTools": string[] }`
 - Response: `{ "chat": ChatSummary }`
 - Blank titles are rejected. The server trims surrounding whitespace before storing the title.
+- Setting a title clears `titleGenerationPending`, preventing an in-flight or later automatic suggestion from replacing the manual title.
 - `additionalSystemPrompt: null` clears the stored prompt. Blank string values are also stored as `null`.
 - `enabledTools: []` disables Sybil-managed tools for this chat. Omitted settings are left unchanged.
 - Updating chat fields changes the returned chat's `updatedAt`.
@@ -183,14 +203,19 @@ Behavior notes:
 - Response: `{ "chat": ChatSummary }`
 
 Behavior notes:
-- If the chat already has a non-empty title, server returns the existing chat unchanged.
+- If the chat already has a non-empty title and `titleGenerationPending` is false, server returns the existing chat unchanged.
+- A fork placeholder with `titleGenerationPending: true` is eligible for the same title-generation flow as an untitled original chat. A successful or fallback suggestion clears the flag.
 - If a title is set while suggestion generation is in flight, server returns the current chat instead of overwriting that title.
-- When no title exists at write time, server uses OpenAI `gpt-4.1-mini` to generate a one-line title (up to ~4 words), updates the chat title, and returns the updated chat.
+- For an eligible untitled chat or pending fork placeholder, server uses OpenAI `gpt-4.1-mini` to generate a one-line title (up to ~4 words), updates the chat title, and returns the updated chat.
 - If the title provider is unavailable or rejects the request, server still persists a deterministic title derived from the first line of `content` instead of leaving the chat untitled.
 
 ### `DELETE /v1/chats/:chatId`
 - Response: `{ "deleted": true }`
 - Not found: `404 { "message": "chat not found" }`
+- Active chat or fork: `409 { "message": "chat or fork has an active stream" }`
+- Concurrent family deletion: `409 { "message": "chat family deletion already in progress" }`
+- Deleting a root chat also deletes its grouped forks. Deleting a fork leaves the root and sibling forks unchanged.
+- A root cannot be deleted while it or any grouped fork has an active completion stream. A fork cannot be deleted while its own completion stream is active.
 
 ### `GET /v1/chats/:chatId`
 - Response: `{ "chat": ChatDetail }`
@@ -429,6 +454,8 @@ Behavior notes:
 {
   "id": "...",
   "title": null,
+  "parentChatId": null,
+  "titleGenerationPending": false,
   "createdAt": "...",
   "updatedAt": "...",
   "starred": false,
@@ -481,6 +508,8 @@ Behavior notes:
 {
   "id": "...",
   "title": null,
+  "parentChatId": null,
+  "titleGenerationPending": false,
   "createdAt": "...",
   "updatedAt": "...",
   "starred": false,

docs/api/streaming-chat.md

@@ -73,8 +73,10 @@ Notes:
 
 Persisted chat streams with a `chatId` are backend-owned active runs:
 - Once started, the backend keeps the stream running even if the HTTP client disconnects or refreshes.
+- The backend reserves the active run before persisting submitted messages, preventing a root or fork deletion from interleaving with stream setup.
 - While running, `GET /v1/active-runs` includes the `chatId`.
 - Starting a second persisted stream for the same active `chatId` returns `409`, unless its `clientRequestId` matches the active submission, in which case the existing stream is replayed.
+- Starting a persisted stream while that chat family is being deleted returns `409 { "message": "chat family deletion already in progress" }`.
 - Clients can reattach with `POST /v1/chats/:chatId/stream/attach`.
 
 ## Attach Endpoint

ios/Packages/Sybil/Sources/Sybil/SybilModels.swift

@@ -154,6 +154,8 @@ public struct ChatAttachment: Codable, Hashable, Identifiable, Sendable {
 public struct ChatSummary: Codable, Identifiable, Hashable, Sendable {
     public var id: String
     public var title: String?
+    public var parentChatId: String? = nil
+    public var titleGenerationPending = false
     public var createdAt: Date
     public var updatedAt: Date
     public var starred = false
@@ -164,6 +166,39 @@ public struct ChatSummary: Codable, Identifiable, Hashable, Sendable {
     public var lastUsedModel: String?
 }
 
+extension ChatSummary {
+    private enum CodingKeys: String, CodingKey {
+        case id
+        case title
+        case parentChatId
+        case titleGenerationPending
+        case createdAt
+        case updatedAt
+        case starred
+        case starredAt
+        case initiatedProvider
+        case initiatedModel
+        case lastUsedProvider
+        case lastUsedModel
+    }
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        id = try container.decode(String.self, forKey: .id)
+        title = try container.decodeIfPresent(String.self, forKey: .title)
+        parentChatId = try container.decodeIfPresent(String.self, forKey: .parentChatId)
+        titleGenerationPending = try container.decodeIfPresent(Bool.self, forKey: .titleGenerationPending) ?? false
+        createdAt = try container.decode(Date.self, forKey: .createdAt)
+        updatedAt = try container.decode(Date.self, forKey: .updatedAt)
+        starred = try container.decodeIfPresent(Bool.self, forKey: .starred) ?? false
+        starredAt = try container.decodeIfPresent(Date.self, forKey: .starredAt)
+        initiatedProvider = try container.decodeIfPresent(Provider.self, forKey: .initiatedProvider)
+        initiatedModel = try container.decodeIfPresent(String.self, forKey: .initiatedModel)
+        lastUsedProvider = try container.decodeIfPresent(Provider.self, forKey: .lastUsedProvider)
+        lastUsedModel = try container.decodeIfPresent(String.self, forKey: .lastUsedModel)
+    }
+}
+
 public struct SearchSummary: Codable, Identifiable, Hashable, Sendable {
     public var id: String
     public var title: String?
@@ -184,6 +219,8 @@ public struct WorkspaceItem: Codable, Identifiable, Hashable, Sendable {
     public var id: String
     public var title: String?
     public var query: String?
+    public var parentChatId: String? = nil
+    public var titleGenerationPending = false
     public var createdAt: Date
     public var updatedAt: Date
     public var starred = false
@@ -198,6 +235,8 @@ public struct WorkspaceItem: Codable, Identifiable, Hashable, Sendable {
         self.id = chat.id
         self.title = chat.title
         self.query = nil
+        self.parentChatId = chat.parentChatId
+        self.titleGenerationPending = chat.titleGenerationPending
         self.createdAt = chat.createdAt
         self.updatedAt = chat.updatedAt
         self.starred = chat.starred
@@ -213,6 +252,8 @@ public struct WorkspaceItem: Codable, Identifiable, Hashable, Sendable {
         self.id = search.id
         self.title = search.title
         self.query = search.query
+        self.parentChatId = nil
+        self.titleGenerationPending = false
         self.createdAt = search.createdAt
         self.updatedAt = search.updatedAt
         self.starred = search.starred
@@ -228,6 +269,8 @@ public struct WorkspaceItem: Codable, Identifiable, Hashable, Sendable {
         return ChatSummary(
             id: id,
             title: title,
+            parentChatId: parentChatId,
+            titleGenerationPending: titleGenerationPending,
             createdAt: createdAt,
             updatedAt: updatedAt,
             starred: starred,
@@ -253,6 +296,43 @@ public struct WorkspaceItem: Codable, Identifiable, Hashable, Sendable {
     }
 }
 
+extension WorkspaceItem {
+    private enum CodingKeys: String, CodingKey {
+        case type
+        case id
+        case title
+        case query
+        case parentChatId
+        case titleGenerationPending
+        case createdAt
+        case updatedAt
+        case starred
+        case starredAt
+        case initiatedProvider
+        case initiatedModel
+        case lastUsedProvider
+        case lastUsedModel
+    }
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        type = try container.decode(WorkspaceItemType.self, forKey: .type)
+        id = try container.decode(String.self, forKey: .id)
+        title = try container.decodeIfPresent(String.self, forKey: .title)
+        query = try container.decodeIfPresent(String.self, forKey: .query)
+        parentChatId = try container.decodeIfPresent(String.self, forKey: .parentChatId)
+        titleGenerationPending = try container.decodeIfPresent(Bool.self, forKey: .titleGenerationPending) ?? false
+        createdAt = try container.decode(Date.self, forKey: .createdAt)
+        updatedAt = try container.decode(Date.self, forKey: .updatedAt)
+        starred = try container.decodeIfPresent(Bool.self, forKey: .starred) ?? false
+        starredAt = try container.decodeIfPresent(Date.self, forKey: .starredAt)
+        initiatedProvider = try container.decodeIfPresent(Provider.self, forKey: .initiatedProvider)
+        initiatedModel = try container.decodeIfPresent(String.self, forKey: .initiatedModel)
+        lastUsedProvider = try container.decodeIfPresent(Provider.self, forKey: .lastUsedProvider)
+        lastUsedModel = try container.decodeIfPresent(String.self, forKey: .lastUsedModel)
+    }
+}
+
 public struct Message: Codable, Identifiable, Hashable, Sendable {
     public var id: String
     public var createdAt: Date
@@ -391,6 +471,8 @@ public enum JSONValue: Codable, Hashable, Sendable {
 public struct ChatDetail: Codable, Identifiable, Hashable, Sendable {
     public var id: String
     public var title: String?
+    public var parentChatId: String? = nil
+    public var titleGenerationPending = false
     public var createdAt: Date
     public var updatedAt: Date
     public var starred = false
@@ -402,6 +484,41 @@ public struct ChatDetail: Codable, Identifiable, Hashable, Sendable {
     public var messages: [Message]
 }
 
+extension ChatDetail {
+    private enum CodingKeys: String, CodingKey {
+        case id
+        case title
+        case parentChatId
+        case titleGenerationPending
+        case createdAt
+        case updatedAt
+        case starred
+        case starredAt
+        case initiatedProvider
+        case initiatedModel
+        case lastUsedProvider
+        case lastUsedModel
+        case messages
+    }
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        id = try container.decode(String.self, forKey: .id)
+        title = try container.decodeIfPresent(String.self, forKey: .title)
+        parentChatId = try container.decodeIfPresent(String.self, forKey: .parentChatId)
+        titleGenerationPending = try container.decodeIfPresent(Bool.self, forKey: .titleGenerationPending) ?? false
+        createdAt = try container.decode(Date.self, forKey: .createdAt)
+        updatedAt = try container.decode(Date.self, forKey: .updatedAt)
+        starred = try container.decodeIfPresent(Bool.self, forKey: .starred) ?? false
+        starredAt = try container.decodeIfPresent(Date.self, forKey: .starredAt)
+        initiatedProvider = try container.decodeIfPresent(Provider.self, forKey: .initiatedProvider)
+        initiatedModel = try container.decodeIfPresent(String.self, forKey: .initiatedModel)
+        lastUsedProvider = try container.decodeIfPresent(Provider.self, forKey: .lastUsedProvider)
+        lastUsedModel = try container.decodeIfPresent(String.self, forKey: .lastUsedModel)
+        messages = try container.decode([Message].self, forKey: .messages)
+    }
+}
+
 public struct SearchResultItem: Codable, Identifiable, Hashable, Sendable {
     public var id: String
     public var createdAt: Date

ios/Packages/Sybil/Sources/Sybil/SybilViewModel.swift

@@ -406,14 +406,17 @@ final class SybilViewModel {
                 } else {
                     initiatedLabel = nil
                 }
+                let starOwner = item.parentChatId.flatMap { parentChatID in
+                    workspaceItems.first(where: { $0.type == .chat && $0.id == parentChatID })
+                } ?? item
 
                 return SidebarItem(
                     selection: .chat(item.id),
                     kind: .chat,
                     title: chatTitle(title: item.title, messages: nil),
                     updatedAt: item.updatedAt,
-                    starred: item.starred,
-                    starredAt: item.starredAt,
+                    starred: starOwner.starred,
+                    starredAt: starOwner.starredAt,
                     initiatedLabel: initiatedLabel,
                     isRunning: isChatRowRunning(item.id)
                 )
@@ -687,6 +690,8 @@ final class SybilViewModel {
             selectedChat = ChatDetail(
                 id: chat.id,
                 title: chat.title,
+                parentChatId: chat.parentChatId,
+                titleGenerationPending: chat.titleGenerationPending,
                 createdAt: chat.createdAt,
                 updatedAt: chat.updatedAt,
                 starred: chat.starred,
@@ -896,7 +901,8 @@ final class SybilViewModel {
                 let client = try client()
                 switch selection {
                 case let .chat(chatID):
-                    let updated = try await client.updateChatStar(chatID: chatID, starred: starred)
+                    let rootChatID = chatFamilyRootID(for: chatID)
+                    let updated = try await client.updateChatStar(chatID: rootChatID, starred: starred)
                     applyChatSummary(updated, moveToFront: false)
                 case let .search(searchID):
                     let updated = try await client.updateSearchStar(searchID: searchID, starred: starred)
@@ -1454,6 +1460,8 @@ final class SybilViewModel {
 
         if selectedChat?.id == chat.id {
             selectedChat?.title = chat.title
+            selectedChat?.parentChatId = chat.parentChatId
+            selectedChat?.titleGenerationPending = chat.titleGenerationPending
             selectedChat?.updatedAt = chat.updatedAt
             selectedChat?.starred = chat.starred
             selectedChat?.starredAt = chat.starredAt
@@ -1505,6 +1513,19 @@ final class SybilViewModel {
         workspaceItems.insert(item, at: 0)
     }
 
+    private func chatFamilyRootID(for chatID: String) -> String {
+        if let selectedChat, selectedChat.id == chatID, let parentChatID = selectedChat.parentChatId {
+            return parentChatID
+        }
+        if let parentChatID = chats.first(where: { $0.id == chatID })?.parentChatId {
+            return parentChatID
+        }
+        if let parentChatID = workspaceItems.first(where: { $0.type == .chat && $0.id == chatID })?.parentChatId {
+            return parentChatID
+        }
+        return chatID
+    }
+
     private func attachToVisibleActiveRunIfNeeded() {
         guard draftKind == nil else {
             return
@@ -1854,6 +1875,8 @@ final class SybilViewModel {
                 selectedChat = ChatDetail(
                     id: created.id,
                     title: created.title,
+                    parentChatId: created.parentChatId,
+                    titleGenerationPending: created.titleGenerationPending,
                     createdAt: created.createdAt,
                     updatedAt: created.updatedAt,
                     starred: created.starred,
@@ -1912,7 +1935,7 @@ final class SybilViewModel {
         let streamLifecycleGeneration = appLifecycleGeneration
         let streamStartedWhileInactive = !isAppActive
 
-        if isUntitledChat(chatID: chatID, detail: currentSelectedChat) {
+        if shouldRequestChatTitle(baseChat) {
             Task { [weak self] in
                 guard let self else { return }
                 do {
@@ -2623,20 +2646,10 @@ final class SybilViewModel {
         )
     }
 
-    private func isUntitledChat(chatID: String, detail: ChatDetail?) -> Bool {
-        if let detail, detail.id == chatID {
-            if let title = detail.title?.trimmingCharacters(in: .whitespacesAndNewlines), !title.isEmpty {
-                return false
-            }
+    private func shouldRequestChatTitle(_ chat: ChatDetail) -> Bool {
+        if chat.titleGenerationPending {
             return true
         }
-
-        if let summary = chats.first(where: { $0.id == chatID }) {
-            if let title = summary.title?.trimmingCharacters(in: .whitespacesAndNewlines), !title.isEmpty {
-                return false
-            }
-        }
-
-        return true
+        return chat.title?.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty ?? true
     }
 }

ios/Packages/Sybil/Tests/SybilTests/SybilTests.swift

@@ -10,7 +10,10 @@ private struct MockClientCallSnapshot: Sendable {
     var createChat = 0
     var getChat = 0
     var updateChatTitle = 0
+    var suggestChatTitle = 0
     var updateChatStar = 0
+    var lastUpdateChatStarID: String?
+    var lastUpdateChatStarred: Bool?
     var updateSearchStar = 0
     var getSearch = 0
     var getActiveRuns = 0
@@ -36,6 +39,7 @@ private actor MockSybilClient: SybilAPIClienting {
     private let searchDetails: [String: SearchDetail]
     private let createChatResponse: ChatSummary?
     private let updateChatTitleResponses: [String: ChatSummary]
+    private let suggestChatTitleResponses: [String: ChatSummary]
     private let updateChatStarResponses: [String: ChatSummary]
     private let updateSearchStarResponses: [String: SearchSummary]
     private let activeRunsResponse: ActiveRunsResponse
@@ -64,6 +68,7 @@ private actor MockSybilClient: SybilAPIClienting {
         searchDetails: [String: SearchDetail] = [:],
         createChatResponse: ChatSummary? = nil,
         updateChatTitleResponses: [String: ChatSummary] = [:],
+        suggestChatTitleResponses: [String: ChatSummary] = [:],
         updateChatStarResponses: [String: ChatSummary] = [:],
         updateSearchStarResponses: [String: SearchSummary] = [:],
         activeRunsResponse: ActiveRunsResponse = ActiveRunsResponse(),
@@ -76,6 +81,7 @@ private actor MockSybilClient: SybilAPIClienting {
         self.searchDetails = searchDetails
         self.createChatResponse = createChatResponse
         self.updateChatTitleResponses = updateChatTitleResponses
+        self.suggestChatTitleResponses = suggestChatTitleResponses
         self.updateChatStarResponses = updateChatStarResponses
         self.updateSearchStarResponses = updateSearchStarResponses
         self.activeRunsResponse = activeRunsResponse
@@ -204,6 +210,8 @@ private actor MockSybilClient: SybilAPIClienting {
 
     func updateChatStar(chatID: String, starred: Bool) async throws -> ChatSummary {
         snapshot.updateChatStar += 1
+        snapshot.lastUpdateChatStarID = chatID
+        snapshot.lastUpdateChatStarred = starred
         guard let summary = updateChatStarResponses[chatID] else {
             throw UnexpectedClientCall()
         }
@@ -215,7 +223,11 @@ private actor MockSybilClient: SybilAPIClienting {
     }
 
     func suggestChatTitle(chatID: String, content: String) async throws -> ChatSummary {
-        throw UnexpectedClientCall()
+        snapshot.suggestChatTitle += 1
+        guard let summary = suggestChatTitleResponses[chatID] else {
+            throw UnexpectedClientCall()
+        }
+        return summary
     }
 
     func listSearches() async throws -> [SearchSummary] {
@@ -420,6 +432,46 @@ private func makeToolCallMessage(id: String, date: Date, summary: String = "Ran
     )
 }
 
+@Test func chatForkMetadataDecodesBackwardCompatiblyAndSurvivesWorkspaceConversions() throws {
+    let decoder = JSONDecoder()
+    let legacySummary = try decoder.decode(
+        ChatSummary.self,
+        from: Data(#"{"id":"legacy-chat","title":"Legacy","createdAt":0,"updatedAt":1}"#.utf8)
+    )
+    let legacyWorkspaceItem = try decoder.decode(
+        WorkspaceItem.self,
+        from: Data(#"{"type":"chat","id":"legacy-chat","title":"Legacy","createdAt":0,"updatedAt":1}"#.utf8)
+    )
+    let legacyDetail = try decoder.decode(
+        ChatDetail.self,
+        from: Data(#"{"id":"legacy-chat","title":"Legacy","createdAt":0,"updatedAt":1,"messages":[]}"#.utf8)
+    )
+    let forkDetail = try decoder.decode(
+        ChatDetail.self,
+        from: Data(#"{"id":"fork-chat","title":"Fork of Legacy","parentChatId":"root-chat","titleGenerationPending":true,"createdAt":0,"updatedAt":1,"messages":[]}"#.utf8)
+    )
+
+    #expect(legacySummary.parentChatId == nil)
+    #expect(!legacySummary.titleGenerationPending)
+    #expect(legacyWorkspaceItem.parentChatId == nil)
+    #expect(!legacyWorkspaceItem.titleGenerationPending)
+    #expect(legacyDetail.parentChatId == nil)
+    #expect(!legacyDetail.titleGenerationPending)
+    #expect(forkDetail.parentChatId == "root-chat")
+    #expect(forkDetail.titleGenerationPending)
+
+    var fork = legacySummary
+    fork.parentChatId = "root-chat"
+    fork.titleGenerationPending = true
+
+    let workspaceItem = WorkspaceItem(chat: fork)
+    let restoredSummary = try #require(workspaceItem.chatSummary)
+    #expect(workspaceItem.parentChatId == "root-chat")
+    #expect(workspaceItem.titleGenerationPending)
+    #expect(restoredSummary.parentChatId == "root-chat")
+    #expect(restoredSummary.titleGenerationPending)
+}
+
 @Test func transcriptRenderItemsGroupAdjacentToolCalls() async throws {
     let date = Date(timeIntervalSince1970: 1_700_000_000)
     let user = Message(id: "user-1", createdAt: date, role: .user, content: "Search this", name: nil)
@@ -567,18 +619,16 @@ private func makeToolCallMessage(id: String, date: Date, summary: String = "Ran
 @MainActor
 @Test func renameChatUpdatesSidebarAndSelectedTranscriptTitle() async throws {
     let date = Date(timeIntervalSince1970: 1_700_000_150)
-    let original = makeChatSummary(id: "chat-rename", date: date)
-    let renamed = ChatSummary(
-        id: "chat-rename",
-        title: "Renamed chat",
-        createdAt: date,
-        updatedAt: date.addingTimeInterval(60),
-        initiatedProvider: .openai,
-        initiatedModel: "gpt-4.1-mini",
-        lastUsedProvider: .openai,
-        lastUsedModel: "gpt-4.1-mini"
-    )
-    let detail = makeChatDetail(id: "chat-rename", date: date, body: "existing transcript")
+    var original = makeChatSummary(id: "chat-rename", date: date)
+    original.parentChatId = "root-chat"
+    original.titleGenerationPending = true
+    var renamed = original
+    renamed.title = "Renamed chat"
+    renamed.titleGenerationPending = false
+    renamed.updatedAt = date.addingTimeInterval(60)
+    var detail = makeChatDetail(id: "chat-rename", date: date, body: "existing transcript")
+    detail.parentChatId = original.parentChatId
+    detail.titleGenerationPending = original.titleGenerationPending
     let client = MockSybilClient(
         chatsResponse: [original],
         updateChatTitleResponses: ["chat-rename": renamed]
@@ -596,7 +646,13 @@ private func makeToolCallMessage(id: String, date: Date, summary: String = "Ran
     let snapshot = await client.currentSnapshot()
     #expect(snapshot.updateChatTitle == 1)
     #expect(viewModel.sidebarItems.first?.title == "Renamed chat")
+    #expect(viewModel.chats.first?.parentChatId == "root-chat")
+    #expect(viewModel.chats.first?.titleGenerationPending == false)
+    #expect(viewModel.workspaceItems.first?.parentChatId == "root-chat")
+    #expect(viewModel.workspaceItems.first?.titleGenerationPending == false)
     #expect(viewModel.selectedChat?.title == "Renamed chat")
+    #expect(viewModel.selectedChat?.parentChatId == "root-chat")
+    #expect(viewModel.selectedChat?.titleGenerationPending == false)
     #expect(viewModel.errorMessage == nil)
 }
 
@@ -635,6 +691,54 @@ private func makeToolCallMessage(id: String, date: Date, summary: String = "Ran
     #expect(viewModel.sidebarItems.first(where: { $0.selection == .search("search-star") })?.starred == true)
 }
 
+@MainActor
+@Test func unstarringForkTargetsRootWithoutReplacingSelectedChild() async throws {
+    let date = Date(timeIntervalSince1970: 1_700_000_180)
+    var root = makeChatSummary(id: "chat-root", date: date)
+    root.starred = true
+    root.starredAt = date.addingTimeInterval(5)
+
+    var child = makeChatSummary(id: "chat-child", date: date.addingTimeInterval(1))
+    child.parentChatId = root.id
+    child.titleGenerationPending = true
+
+    var childDetail = makeChatDetail(id: child.id, date: date, body: "forked transcript")
+    childDetail.title = child.title
+    childDetail.parentChatId = root.id
+    childDetail.titleGenerationPending = true
+
+    var unstarredRoot = root
+    unstarredRoot.starred = false
+    unstarredRoot.starredAt = nil
+
+    let client = MockSybilClient(
+        chatsResponse: [root, child],
+        updateChatStarResponses: [root.id: unstarredRoot]
+    )
+    let viewModel = SybilViewModel(settings: testSettings(named: #function)) { _ in client }
+    viewModel.isAuthenticated = true
+    viewModel.isCheckingSession = false
+    viewModel.chats = [root, child]
+    viewModel.workspaceItems = [WorkspaceItem(chat: root), WorkspaceItem(chat: child)]
+    viewModel.selectedItem = .chat(child.id)
+    viewModel.selectedChat = childDetail
+
+    #expect(viewModel.sidebarItems.first(where: { $0.selection == .chat(child.id) })?.starred == true)
+
+    await viewModel.setItemStarred(.chat(child.id), starred: false)
+
+    let snapshot = await client.currentSnapshot()
+    #expect(snapshot.updateChatStar == 1)
+    #expect(snapshot.lastUpdateChatStarID == root.id)
+    #expect(snapshot.lastUpdateChatStarred == false)
+    #expect(viewModel.chats.first(where: { $0.id == root.id })?.starred == false)
+    #expect(viewModel.chats.first(where: { $0.id == child.id }) == child)
+    #expect(viewModel.sidebarItems.first(where: { $0.selection == .chat(child.id) })?.starred == false)
+    #expect(viewModel.selectedItem == .chat(child.id))
+    #expect(viewModel.selectedChat == childDetail)
+    #expect(viewModel.errorMessage == nil)
+}
+
 @MainActor
 @Test func foregroundSearchRefreshReloadsSelectedSearch() async throws {
     let date = Date(timeIntervalSince1970: 1_700_000_200)
@@ -781,6 +885,72 @@ private func makeToolCallMessage(id: String, date: Date, summary: String = "Ran
     #expect(viewModel.chatBottomPinRequestID == initialPinRequestID + 1)
 }
 
+@MainActor
+@Test func firstPromptInPendingForkRequestsAndAppliesGeneratedTitle() async throws {
+    let date = Date(timeIntervalSince1970: 1_700_000_247)
+    var fork = makeChatSummary(id: "chat-fork", date: date)
+    fork.title = "Fork of Original chat"
+    fork.parentChatId = "root-chat"
+    fork.titleGenerationPending = true
+
+    var forkDetail = makeChatDetail(id: fork.id, date: date, body: "forked transcript")
+    forkDetail.title = fork.title
+    forkDetail.parentChatId = fork.parentChatId
+    forkDetail.titleGenerationPending = true
+
+    var titledFork = fork
+    titledFork.title = "Investigating the follow-up"
+    titledFork.titleGenerationPending = false
+    titledFork.updatedAt = date.addingTimeInterval(1)
+
+    var titledForkDetail = forkDetail
+    titledForkDetail.title = titledFork.title
+    titledForkDetail.titleGenerationPending = false
+    titledForkDetail.updatedAt = titledFork.updatedAt
+
+    let client = MockSybilClient(
+        chatsResponse: [titledFork],
+        chatDetails: [fork.id: titledForkDetail],
+        suggestChatTitleResponses: [fork.id: titledFork]
+    )
+    await client.setCompletionStreamEvents(
+        [.done(CompletionStreamDone(text: "Follow-up answer"))],
+        delayNanoseconds: 100_000_000
+    )
+
+    let viewModel = SybilViewModel(settings: testSettings(named: #function)) { _ in client }
+    viewModel.isAuthenticated = true
+    viewModel.isCheckingSession = false
+    viewModel.chats = [fork]
+    viewModel.workspaceItems = [WorkspaceItem(chat: fork)]
+    viewModel.selectedItem = .chat(fork.id)
+    viewModel.selectedChat = forkDetail
+    viewModel.composer = "Investigate this follow-up"
+
+    let sendTask = Task {
+        await viewModel.sendComposer()
+    }
+
+    for _ in 0..<20 {
+        let snapshot = await client.currentSnapshot()
+        if snapshot.suggestChatTitle == 1,
+           viewModel.selectedChat?.title == titledFork.title,
+           viewModel.selectedChat?.titleGenerationPending == false {
+            break
+        }
+        try await Task.sleep(nanoseconds: 5_000_000)
+    }
+
+    let titleSnapshot = await client.currentSnapshot()
+    #expect(titleSnapshot.suggestChatTitle == 1)
+    #expect(viewModel.chats.first?.title == titledFork.title)
+    #expect(viewModel.workspaceItems.first?.title == titledFork.title)
+    #expect(viewModel.selectedChat?.title == titledFork.title)
+    #expect(viewModel.selectedChat?.titleGenerationPending == false)
+
+    await sendTask.value
+}
+
 @MainActor
 @Test func quickQuestionRunsNonPersistentCompletionStream() async throws {
     let client = MockSybilClient()

server/prisma/migrations/20260816000000_add_chat_forks/migration.sql

@@ -0,0 +1,5 @@
+-- Add durable root grouping and one-time fork title generation state.
+ALTER TABLE "Chat" ADD COLUMN "parentChatId" TEXT REFERENCES "Chat"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+ALTER TABLE "Chat" ADD COLUMN "titleGenerationPending" BOOLEAN NOT NULL DEFAULT false;
+
+CREATE INDEX "Chat_parentChatId_idx" ON "Chat"("parentChatId");

server/prisma/schema.prisma

@@ -51,7 +51,8 @@ model Chat {
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
 
-  title String?
+  title                  String?
+  titleGenerationPending Boolean @default(false)
 
   initiatedProvider Provider?
   initiatedModel    String?
@@ -64,11 +65,17 @@ model Chat {
   user   User?   @relation(fields: [userId], references: [id])
   userId String?
 
+  // Forks always point directly to the single root chat, never to another fork.
+  parentChat   Chat?  @relation("ChatForks", fields: [parentChatId], references: [id], onDelete: Cascade)
+  parentChatId String?
+  childChats   Chat[] @relation("ChatForks")
+
   messages Message[]
   calls    LlmCall[]
   projectItems ProjectItem[]
 
   @@index([userId])
+  @@index([parentChatId])
 }
 
 model Message {

server/src/routes.ts

@@ -1,3 +1,4 @@
+import { randomUUID } from "node:crypto";
 import { performance } from "node:perf_hooks";
 import { z } from "zod";
 import type { FastifyInstance, FastifyReply, FastifyRequest } from "fastify";
@@ -18,6 +19,7 @@ import type { ChatAttachment } from "./llm/types.js";
 
 const ProviderSchema = z.enum(["openai", "anthropic", "xai", "gemini", "hermes-agent"]);
 const MAX_ADDITIONAL_SYSTEM_PROMPT_CHARS = 12_000;
+const MAX_FORK_MESSAGE_SNIPPET_CHARS = 48;
 const EnabledToolsSchema = z.array(z.string().trim().min(1).max(80)).max(20).transform((value) => normalizeEnabledChatTools(value));
 
 type IncomingChatMessage = {
@@ -94,7 +96,7 @@ async function storeNonAssistantMessages(chatId: string, messages: IncomingChatM
 
   const existing = await prisma.message.findMany({
     where: { chatId },
-    orderBy: { createdAt: "asc" },
+    orderBy: [{ createdAt: "asc" }, { id: "asc" }],
     select: { role: true, content: true, name: true, metadata: true },
   });
   const existingNonAssistant = existing.filter((m) => m.role !== "assistant" && !isToolCallLogMessage(m));
@@ -317,6 +319,21 @@ function normalizeSuggestedTitle(raw: string, fallback: string) {
   return words.slice(0, 4).join(" ").slice(0, 64).trim() || fallback;
 }
 
+export function buildForkTitle(originalTitle: string | null, messageContent?: string) {
+  if (messageContent !== undefined) {
+    const snippet = truncateContextPart(messageContent.replace(/\s+/g, " "), MAX_FORK_MESSAGE_SNIPPET_CHARS) ?? "message";
+    return `Fork of '${snippet}'`;
+  }
+  return `Fork of ${originalTitle?.trim() || "Untitled chat"}`;
+}
+
+export function copyForkMessageMetadata(metadata: unknown) {
+  if (metadata === null || metadata === undefined) return undefined;
+  if (typeof metadata !== "object" || Array.isArray(metadata)) return metadata;
+  const { clientRequestId: _clientRequestId, ...copied } = metadata as Record<string, unknown>;
+  return Object.keys(copied).length ? copied : undefined;
+}
+
 async function generateChatTitle(content: string) {
   const systemPrompt =
     "You create short chat titles. Return exactly one line, maximum 4 words, no quotes, no trailing punctuation.";
@@ -415,6 +432,7 @@ type SearchRunRequest = z.infer<typeof SearchRunBody>;
 
 const activeChatStreams = new Map<string, ActiveSseStream>();
 const activeChatStreamRequestIds = new Map<string, string>();
+const chatDeletionRoots = new Set<string>();
 const activeSearchStreams = new Map<string, ActiveSseStream>();
 const STARRED_PROJECT_ID = "starred";
 
@@ -427,6 +445,8 @@ const starredProjectItemsSelect = {
 const chatSummarySelect = {
   id: true,
   title: true,
+  titleGenerationPending: true,
+  parentChatId: true,
   createdAt: true,
   updatedAt: true,
   initiatedProvider: true,
@@ -507,6 +527,29 @@ async function getSearchSummary(searchId: string) {
   return search ? serializeSearchLike(search) : null;
 }
 
+async function listRecentChatsWithRoots() {
+  const chats = await prisma.chat.findMany({
+    orderBy: { updatedAt: "desc" },
+    take: 100,
+    select: chatSummarySelect,
+  });
+  const includedIds = new Set(chats.map((chat) => chat.id));
+  const missingRootIds = [
+    ...new Set(
+      chats
+        .map((chat) => chat.parentChatId)
+        .filter((id): id is string => id !== null && !includedIds.has(id))
+    ),
+  ];
+  if (!missingRootIds.length) return chats;
+
+  const roots = await prisma.chat.findMany({
+    where: { id: { in: missingRootIds } },
+    select: chatSummarySelect,
+  });
+  return [...chats, ...roots].sort(compareUpdatedAtDesc);
+}
+
 async function setChatStarred(chatId: string, starred: boolean) {
   const exists = await prisma.chat.findUnique({ where: { id: chatId }, select: { id: true } });
   if (!exists) return null;
@@ -545,11 +588,7 @@ async function setSearchStarred(searchId: string, starred: boolean) {
 
 async function listWorkspaceItems() {
   const [chats, searches] = await Promise.all([
-    prisma.chat.findMany({
-      orderBy: { updatedAt: "desc" },
-      take: 100,
-      select: chatSummarySelect,
-    }),
+    listRecentChatsWithRoots(),
     prisma.search.findMany({
       orderBy: { updatedAt: "desc" },
       take: 100,
@@ -605,7 +644,7 @@ function mapChatStreamEvent(ev: StreamEvent): SseStreamEvent {
   return { event: ev.type, data: ev };
 }
 
-function registerActiveChatStream(chatId: string, clientRequestId?: string) {
+export function registerActiveChatStream(chatId: string, clientRequestId?: string) {
   const stream = new ActiveSseStream();
   activeChatStreams.set(chatId, stream);
   if (clientRequestId) {
@@ -616,7 +655,7 @@ function registerActiveChatStream(chatId: string, clientRequestId?: string) {
   return stream;
 }
 
-function clearActiveChatStream(chatId: string, stream: ActiveSseStream) {
+export function clearActiveChatStream(chatId: string, stream: ActiveSseStream) {
   if (activeChatStreams.get(chatId) !== stream) return;
   activeChatStreams.delete(chatId);
   activeChatStreamRequestIds.delete(chatId);
@@ -647,12 +686,6 @@ function executeActiveChatStream(chatId: string, body: z.infer<typeof Completion
   })();
 }
 
-function startActiveChatStream(chatId: string, body: z.infer<typeof CompletionStreamBody>) {
-  const stream = registerActiveChatStream(chatId, body.clientRequestId);
-  executeActiveChatStream(chatId, body, stream);
-  return stream;
-}
-
 function getMetadataClientRequestId(metadata: unknown) {
   if (!metadata || typeof metadata !== "object" || Array.isArray(metadata)) return null;
   const clientRequestId = (metadata as Record<string, unknown>).clientRequestId;
@@ -888,11 +921,7 @@ export async function registerRoutes(app: FastifyInstance) {
 
   app.get("/v1/chats", async (req) => {
     requireAdmin(req);
-    const chats = await prisma.chat.findMany({
-      orderBy: { updatedAt: "desc" },
-      take: 100,
-      select: chatSummarySelect,
-    });
+    const chats = await listRecentChatsWithRoots();
     return { chats: chats.map((chat) => serializeChatLike(chat)) };
   });
 
@@ -951,6 +980,80 @@ export async function registerRoutes(app: FastifyInstance) {
     return { chat: serializeChatLike(chat) };
   });
 
+  app.post("/v1/chats/:chatId/fork", async (req) => {
+    requireAdmin(req);
+    const Params = z.object({ chatId: z.string() });
+    const Body = z.object({ messageId: z.string().trim().min(1).optional() });
+    const { chatId } = Params.parse(req.params);
+    const parsed = Body.safeParse(req.body ?? {});
+    if (!parsed.success) return app.httpErrors.badRequest(parsed.error.message);
+
+    const result = await prisma.$transaction(async (tx) => {
+      const source = await tx.chat.findUnique({
+        where: { id: chatId },
+        select: {
+          id: true,
+          title: true,
+          parentChatId: true,
+          initiatedProvider: true,
+          initiatedModel: true,
+          lastUsedProvider: true,
+          lastUsedModel: true,
+          additionalSystemPrompt: true,
+          enabledTools: true,
+          userId: true,
+          messages: { orderBy: [{ createdAt: "asc" }, { id: "asc" }] },
+        },
+      });
+      if (!source) return { status: "chat-not-found" as const };
+
+      let messages = source.messages;
+      let selectedMessage: (typeof source.messages)[number] | undefined;
+      if (parsed.data.messageId) {
+        const selectedIndex = messages.findIndex((message) => message.id === parsed.data.messageId);
+        if (selectedIndex < 0) return { status: "message-not-found" as const };
+        selectedMessage = messages[selectedIndex];
+        if (selectedMessage.role !== "assistant") return { status: "message-not-assistant" as const };
+        messages = messages.slice(0, selectedIndex + 1);
+      }
+
+      const forkMessageIdPrefix = `fork-${randomUUID()}`;
+      const chat = await tx.chat.create({
+        data: {
+          title: buildForkTitle(source.title, selectedMessage?.content),
+          titleGenerationPending: true,
+          parentChatId: source.parentChatId ?? source.id,
+          initiatedProvider: source.initiatedProvider,
+          initiatedModel: source.initiatedModel,
+          lastUsedProvider: source.lastUsedProvider,
+          lastUsedModel: source.lastUsedModel,
+          additionalSystemPrompt: source.additionalSystemPrompt,
+          enabledTools: (source.enabledTools ?? undefined) as any,
+          userId: source.userId,
+          messages: messages.length
+            ? {
+                create: messages.map((message, index) => ({
+                  id: `${forkMessageIdPrefix}-${String(index).padStart(8, "0")}`,
+                  createdAt: message.createdAt,
+                  role: message.role,
+                  content: message.content,
+                  name: message.name,
+                  metadata: copyForkMessageMetadata(message.metadata) as any,
+                })),
+              }
+            : undefined,
+        },
+        select: chatSummarySelect,
+      });
+      return { status: "created" as const, chat };
+    });
+
+    if (result.status === "chat-not-found") return app.httpErrors.notFound("chat not found");
+    if (result.status === "message-not-found") return app.httpErrors.notFound("message not found in chat");
+    if (result.status === "message-not-assistant") return app.httpErrors.badRequest("fork message must be an assistant response");
+    return { chat: serializeChatLike(result.chat) };
+  });
+
   app.patch("/v1/chats/:chatId", async (req) => {
     requireAdmin(req);
     const Params = z.object({ chatId: z.string() });
@@ -963,7 +1066,10 @@ export async function registerRoutes(app: FastifyInstance) {
     const body = Body.parse(req.body ?? {});
 
     const data: Record<string, unknown> = {};
-    if (body.title !== undefined) data.title = body.title;
+    if (body.title !== undefined) {
+      data.title = body.title;
+      data.titleGenerationPending = false;
+    }
     if (body.additionalSystemPrompt !== undefined) data.additionalSystemPrompt = normalizeAdditionalSystemPrompt(body.additionalSystemPrompt);
     if (body.enabledTools !== undefined) data.enabledTools = body.enabledTools;
 
@@ -1004,7 +1110,7 @@ export async function registerRoutes(app: FastifyInstance) {
       select: chatSummarySelect,
     });
     if (!existing) return app.httpErrors.notFound("chat not found");
-    if (existing.title?.trim()) return { chat: serializeChatLike(existing) };
+    if (existing.title?.trim() && !existing.titleGenerationPending) return { chat: serializeChatLike(existing) };
 
     const fallback = body.content.split(/\r?\n/)[0]?.trim().slice(0, 48) || "New chat";
     let suggestedRaw = "";
@@ -1022,8 +1128,12 @@ export async function registerRoutes(app: FastifyInstance) {
     const title = normalizeSuggestedTitle(suggestedRaw, fallback);
 
     await prisma.chat.updateMany({
-      where: { id: body.chatId, title: existing.title },
-      data: { title },
+      where: {
+        id: body.chatId,
+        title: existing.title,
+        titleGenerationPending: existing.titleGenerationPending,
+      },
+      data: { title, titleGenerationPending: false },
     });
 
     const chat = await getChatSummary(body.chatId);
@@ -1039,14 +1149,46 @@ export async function registerRoutes(app: FastifyInstance) {
 
     req.log.info({ chatId }, "delete chat requested");
 
-    const result = await prisma.chat.deleteMany({ where: { id: chatId } });
-    if (result.count === 0) {
+    const target = await prisma.chat.findUnique({
+      where: { id: chatId },
+      select: { parentChatId: true },
+    });
+    if (!target) {
       req.log.warn({ chatId }, "delete chat target not found");
       return app.httpErrors.notFound("chat not found");
     }
 
-    req.log.info({ chatId }, "chat deleted");
-    return { deleted: true };
+    const familyRootId = target.parentChatId ?? chatId;
+    if (chatDeletionRoots.has(familyRootId)) {
+      return app.httpErrors.conflict("chat family deletion already in progress");
+    }
+
+    chatDeletionRoots.add(familyRootId);
+    try {
+      const familyIds = target.parentChatId
+        ? [chatId]
+        : (
+            await prisma.chat.findMany({
+              where: { OR: [{ id: chatId }, { parentChatId: chatId }] },
+              select: { id: true },
+            })
+          ).map((chat) => chat.id);
+      if (familyIds.some((id) => activeChatStreams.has(id))) {
+        req.log.warn({ chatId }, "delete chat rejected while chat family is active");
+        return app.httpErrors.conflict("chat or fork has an active stream");
+      }
+
+      const result = await prisma.chat.deleteMany({ where: { id: chatId } });
+      if (result.count === 0) {
+        req.log.warn({ chatId }, "delete chat target no longer exists");
+        return app.httpErrors.notFound("chat not found");
+      }
+
+      req.log.info({ chatId }, "chat deleted");
+      return { deleted: true };
+    } finally {
+      chatDeletionRoots.delete(familyRootId);
+    }
   });
 
   app.get("/v1/searches", async (req) => {
@@ -1336,7 +1478,7 @@ export async function registerRoutes(app: FastifyInstance) {
     const chat = await prisma.chat.findUnique({
       where: { id: chatId },
       include: {
-        messages: { orderBy: { createdAt: "asc" } },
+        messages: { orderBy: [{ createdAt: "asc" }, { id: "asc" }] },
         calls: { orderBy: { createdAt: "desc" } },
         projectItems: starredProjectItemsSelect,
       },
@@ -1430,10 +1572,15 @@ export async function registerRoutes(app: FastifyInstance) {
     if (!parsed.success) return app.httpErrors.badRequest(parsed.error.message);
     const body = withRequestUserLocation(parsed.data, req);
 
-    // ensure chat exists if provided
+    // Ensure the chat exists and identify its family before reserving a stream.
+    let chatFamilyRootId: string | null = null;
     if (body.chatId) {
-      const exists = await prisma.chat.findUnique({ where: { id: body.chatId }, select: { id: true } });
+      const exists = await prisma.chat.findUnique({
+        where: { id: body.chatId },
+        select: { id: true, parentChatId: true },
+      });
       if (!exists) return app.httpErrors.notFound("chat not found");
+      chatFamilyRootId = exists.parentChatId ?? exists.id;
     }
 
     if (body.persist !== false && body.chatId) {
@@ -1445,32 +1592,31 @@ export async function registerRoutes(app: FastifyInstance) {
         return app.httpErrors.conflict("chat completion already running");
       }
 
-      if (body.clientRequestId) {
-        const reservedStream = registerActiveChatStream(body.chatId, body.clientRequestId);
-        try {
+      if (chatFamilyRootId && chatDeletionRoots.has(chatFamilyRootId)) {
+        return app.httpErrors.conflict("chat family deletion already in progress");
+      }
+
+      const reservedStream = registerActiveChatStream(body.chatId, body.clientRequestId);
+      try {
+        if (body.clientRequestId) {
           const completedSubmission = await findCompletedChatSubmission(body.chatId, body.clientRequestId);
           if (completedSubmission) {
             completeChatSubmissionStream(reservedStream, body.chatId, body, completedSubmission.content);
             clearActiveChatStream(body.chatId, reservedStream);
             return streamActiveRun(req, reply, reservedStream);
           }
-
-          // Store only new non-assistant messages to avoid duplicate history entries.
-          await storeNonAssistantMessages(body.chatId, body.messages, body.clientRequestId);
-          const configuredBody = await applyStoredChatSettings(body);
-          executeActiveChatStream(body.chatId, configuredBody, reservedStream);
-          return streamActiveRun(req, reply, reservedStream);
-        } catch (err) {
-          reservedStream.complete({ event: "error", data: { message: getErrorMessage(err) } });
-          clearActiveChatStream(body.chatId, reservedStream);
-          throw err;
         }
-      }
 
-      // Legacy requests without an idempotency key retain the original behavior.
-      await storeNonAssistantMessages(body.chatId, body.messages);
-      const stream = startActiveChatStream(body.chatId, await applyStoredChatSettings(body));
-      return streamActiveRun(req, reply, stream);
+        // Reserve the stream before persistence so deletion cannot interleave with setup.
+        await storeNonAssistantMessages(body.chatId, body.messages, body.clientRequestId);
+        const configuredBody = await applyStoredChatSettings(body);
+        executeActiveChatStream(body.chatId, configuredBody, reservedStream);
+        return streamActiveRun(req, reply, reservedStream);
+      } catch (err) {
+        reservedStream.complete({ event: "error", data: { message: getErrorMessage(err) } });
+        clearActiveChatStream(body.chatId, reservedStream);
+        throw err;
+      }
     }
 
     reply.raw.writeHead(200, buildSseHeaders(typeof req.headers.origin === "string" ? req.headers.origin : undefined));

server/tests/chat-forks.test.ts

@@ -0,0 +1,252 @@
+import assert from "node:assert/strict";
+import { execFileSync } from "node:child_process";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { dirname, join, resolve } from "node:path";
+import test from "node:test";
+import { fileURLToPath } from "node:url";
+import Fastify from "fastify";
+import sensible from "@fastify/sensible";
+
+const serverRoot = resolve(dirname(fileURLToPath(import.meta.url)), "..");
+const databaseDir = mkdtempSync(join(tmpdir(), "sybil-chat-forks-"));
+process.env.DATABASE_URL = `file:${join(databaseDir, "test.db")}`;
+process.env.OPENAI_API_KEY = "";
+delete process.env.ADMIN_TOKEN;
+
+execFileSync(process.execPath, [join(serverRoot, "node_modules/prisma/build/index.js"), "migrate", "deploy"], {
+  cwd: serverRoot,
+  env: { ...process.env, PRISMA_HIDE_UPDATE_MESSAGE: "1" },
+  stdio: "pipe",
+});
+
+const [{ clearActiveChatStream, registerActiveChatStream, registerRoutes }, { prisma }] = await Promise.all([
+  import("../src/routes.js"),
+  import("../src/db.js"),
+]);
+const app = Fastify({ logger: false });
+await app.register(sensible);
+await registerRoutes(app);
+await app.ready();
+
+test.after(async () => {
+  await app.close();
+  await prisma.$disconnect();
+  rmSync(databaseDir, { recursive: true, force: true });
+});
+
+test("forks copy bounded history and keep every child grouped under the root", async () => {
+  const timestamp = new Date("2026-08-16T12:00:00.000Z");
+  const source = await prisma.chat.create({
+    data: {
+      title: "Planning session",
+      initiatedProvider: "openai",
+      initiatedModel: "gpt-4.1-mini",
+      lastUsedProvider: "anthropic",
+      lastUsedModel: "claude-sonnet-4-20250514",
+      additionalSystemPrompt: "Keep answers concise.",
+      enabledTools: ["web_search"],
+      messages: {
+        create: [
+          {
+            id: "source-message-0001",
+            createdAt: timestamp,
+            role: "user",
+            content: "Plan a trip",
+            metadata: {
+              clientRequestId: "source-request",
+              attachments: [{ kind: "text", filename: "notes.txt" }],
+            },
+          },
+          {
+            id: "source-message-0002",
+            createdAt: timestamp,
+            role: "assistant",
+            content: "First assistant response",
+            metadata: { clientRequestId: "source-request", citations: ["https://example.com"] },
+          },
+          {
+            id: "source-message-0003",
+            createdAt: timestamp,
+            role: "user",
+            content: "Only visible in a whole-chat fork",
+          },
+        ],
+      },
+      calls: {
+        create: {
+          provider: "openai",
+          model: "gpt-4.1-mini",
+          request: { input: "Plan a trip" },
+        },
+      },
+    },
+    include: { messages: { orderBy: [{ createdAt: "asc" }, { id: "asc" }] } },
+  });
+
+  const forkResponse = await app.inject({
+    method: "POST",
+    url: `/v1/chats/${source.id}/fork`,
+    payload: { messageId: source.messages[1].id },
+  });
+  assert.equal(forkResponse.statusCode, 200, forkResponse.body);
+  const forkSummary = forkResponse.json().chat;
+  assert.equal(forkSummary.parentChatId, source.id);
+  assert.equal(forkSummary.title, "Fork of 'First assistant response'");
+  assert.equal(forkSummary.titleGenerationPending, true);
+  assert.equal(forkSummary.starred, false);
+
+  const detailResponse = await app.inject({ method: "GET", url: `/v1/chats/${forkSummary.id}` });
+  assert.equal(detailResponse.statusCode, 200, detailResponse.body);
+  const fork = detailResponse.json().chat;
+  assert.deepEqual(
+    fork.messages.map((message: any) => [message.role, message.content]),
+    [
+      ["user", "Plan a trip"],
+      ["assistant", "First assistant response"],
+    ]
+  );
+  assert.notEqual(fork.messages[0].id, source.messages[0].id);
+  assert.equal(fork.messages[0].createdAt, timestamp.toISOString());
+  assert.deepEqual(fork.messages[0].metadata, {
+    attachments: [{ kind: "text", filename: "notes.txt" }],
+  });
+  assert.deepEqual(fork.messages[1].metadata, { citations: ["https://example.com"] });
+  assert.equal(fork.initiatedProvider, "openai");
+  assert.equal(fork.initiatedModel, "gpt-4.1-mini");
+  assert.equal(fork.lastUsedProvider, "anthropic");
+  assert.equal(fork.lastUsedModel, "claude-sonnet-4-20250514");
+  assert.equal(fork.additionalSystemPrompt, "Keep answers concise.");
+  assert.deepEqual(fork.enabledTools, ["web_search"]);
+  assert.deepEqual(fork.calls, []);
+
+  const childForkResponse = await app.inject({
+    method: "POST",
+    url: `/v1/chats/${forkSummary.id}/fork`,
+    payload: {},
+  });
+  assert.equal(childForkResponse.statusCode, 200, childForkResponse.body);
+  const childFork = childForkResponse.json().chat;
+  assert.equal(childFork.parentChatId, source.id);
+  assert.equal(childFork.title, "Fork of Fork of 'First assistant response'");
+  assert.equal(childFork.titleGenerationPending, true);
+
+  const wholeForkResponse = await app.inject({
+    method: "POST",
+    url: `/v1/chats/${source.id}/fork`,
+    payload: {},
+  });
+  assert.equal(wholeForkResponse.statusCode, 200, wholeForkResponse.body);
+  const wholeFork = wholeForkResponse.json().chat;
+  assert.equal(wholeFork.parentChatId, source.id);
+  assert.equal(wholeFork.title, "Fork of Planning session");
+  const wholeForkDetail = await app.inject({ method: "GET", url: `/v1/chats/${wholeFork.id}` });
+  assert.equal(wholeForkDetail.statusCode, 200, wholeForkDetail.body);
+  assert.deepEqual(
+    wholeForkDetail.json().chat.messages.map((message: any) => message.content),
+    ["Plan a trip", "First assistant response", "Only visible in a whole-chat fork"]
+  );
+  assert.equal(wholeForkDetail.json().chat.messages[2].metadata, null);
+
+  const suggestedTitleResponse = await app.inject({
+    method: "POST",
+    url: "/v1/chats/title/suggest",
+    payload: { chatId: wholeFork.id, content: "Compare rail and air options" },
+  });
+  assert.equal(suggestedTitleResponse.statusCode, 200, suggestedTitleResponse.body);
+  assert.equal(suggestedTitleResponse.json().chat.title, "Compare rail and air");
+  assert.equal(suggestedTitleResponse.json().chat.titleGenerationPending, false);
+
+  const repeatedTitleResponse = await app.inject({
+    method: "POST",
+    url: "/v1/chats/title/suggest",
+    payload: { chatId: wholeFork.id, content: "This must not overwrite the generated title" },
+  });
+  assert.equal(repeatedTitleResponse.statusCode, 200, repeatedTitleResponse.body);
+  assert.equal(repeatedTitleResponse.json().chat.title, "Compare rail and air");
+
+  const sourceAfterForks = await prisma.chat.findUniqueOrThrow({
+    where: { id: source.id },
+    include: { messages: true, calls: true },
+  });
+  assert.equal(sourceAfterForks.messages.length, 3);
+  assert.equal(sourceAfterForks.calls.length, 1);
+
+  const manualTitleResponse = await app.inject({
+    method: "PATCH",
+    url: `/v1/chats/${forkSummary.id}`,
+    payload: { title: "Manual branch title" },
+  });
+  assert.equal(manualTitleResponse.statusCode, 200, manualTitleResponse.body);
+  assert.equal(manualTitleResponse.json().chat.titleGenerationPending, false);
+
+  const activeForkStream = registerActiveChatStream(wholeFork.id);
+  try {
+    const deleteActiveRootResponse = await app.inject({ method: "DELETE", url: `/v1/chats/${source.id}` });
+    assert.equal(deleteActiveRootResponse.statusCode, 409, deleteActiveRootResponse.body);
+    assert.equal(deleteActiveRootResponse.json().message, "chat or fork has an active stream");
+
+    const deleteActiveForkResponse = await app.inject({ method: "DELETE", url: `/v1/chats/${wholeFork.id}` });
+    assert.equal(deleteActiveForkResponse.statusCode, 409, deleteActiveForkResponse.body);
+  } finally {
+    clearActiveChatStream(wholeFork.id, activeForkStream);
+  }
+
+  const deleteChildResponse = await app.inject({ method: "DELETE", url: `/v1/chats/${childFork.id}` });
+  assert.equal(deleteChildResponse.statusCode, 200, deleteChildResponse.body);
+  assert.equal(await prisma.chat.count({ where: { id: { in: [source.id, forkSummary.id, wholeFork.id] } } }), 3);
+
+  const unrelated = await prisma.chat.create({
+    data: {
+      messages: { create: { role: "assistant", content: "Unrelated response" } },
+    },
+    include: { messages: true },
+  });
+  const countBeforeInvalidForks = await prisma.chat.count();
+
+  const wrongChatResponse = await app.inject({
+    method: "POST",
+    url: `/v1/chats/${source.id}/fork`,
+    payload: { messageId: unrelated.messages[0].id },
+  });
+  assert.equal(wrongChatResponse.statusCode, 404, wrongChatResponse.body);
+  assert.equal(wrongChatResponse.json().message, "message not found in chat");
+
+  const nonAssistantResponse = await app.inject({
+    method: "POST",
+    url: `/v1/chats/${source.id}/fork`,
+    payload: { messageId: source.messages[0].id },
+  });
+  assert.equal(nonAssistantResponse.statusCode, 400, nonAssistantResponse.body);
+  assert.equal(nonAssistantResponse.json().message, "fork message must be an assistant response");
+  assert.equal(await prisma.chat.count(), countBeforeInvalidForks);
+
+  const concurrentTarget = await prisma.chat.create({ data: { title: "Concurrent delete" } });
+  const concurrentDeleteResponses = await Promise.all([
+    app.inject({ method: "DELETE", url: `/v1/chats/${concurrentTarget.id}` }),
+    app.inject({ method: "DELETE", url: `/v1/chats/${concurrentTarget.id}` }),
+  ]);
+  assert.equal(concurrentDeleteResponses.filter((response) => response.statusCode === 200).length, 1);
+  assert.equal(concurrentDeleteResponses.some((response) => response.statusCode >= 500), false);
+
+  const concurrentRoot = await prisma.chat.create({ data: { title: "Concurrent family delete" } });
+  const concurrentChildResponse = await app.inject({
+    method: "POST",
+    url: `/v1/chats/${concurrentRoot.id}/fork`,
+    payload: {},
+  });
+  assert.equal(concurrentChildResponse.statusCode, 200, concurrentChildResponse.body);
+  const concurrentChild = concurrentChildResponse.json().chat;
+  const concurrentFamilyDeleteResponses = await Promise.all([
+    app.inject({ method: "DELETE", url: `/v1/chats/${concurrentRoot.id}` }),
+    app.inject({ method: "DELETE", url: `/v1/chats/${concurrentChild.id}` }),
+  ]);
+  assert.equal(concurrentFamilyDeleteResponses.some((response) => response.statusCode >= 500), false);
+  assert.equal(concurrentFamilyDeleteResponses.some((response) => response.statusCode === 200), true);
+  await prisma.chat.deleteMany({ where: { id: concurrentRoot.id } });
+
+  const deleteRootResponse = await app.inject({ method: "DELETE", url: `/v1/chats/${source.id}` });
+  assert.equal(deleteRootResponse.statusCode, 200, deleteRootResponse.body);
+  assert.equal(await prisma.chat.count({ where: { id: { in: [source.id, forkSummary.id, childFork.id, wholeFork.id] } } }), 0);
+  assert.equal(await prisma.chat.count({ where: { id: unrelated.id } }), 1);
+});

tui/src/index.ts

@@ -127,21 +127,27 @@ function upsertWorkspaceItem(items: WorkspaceItem[], item: WorkspaceItem) {
 }
 
 function buildSidebarItems(items: WorkspaceItem[]): SidebarItem[] {
+  const chatsById = new Map<string, ChatSummary>();
+  for (const item of items) {
+    if (item.type === "chat") chatsById.set(item.id, item);
+  }
+
   return items.map((item) => {
     if (item.type === "chat") {
       const chat = item;
+      const starOwner = chatsById.get(chat.parentChatId ?? chat.id) ?? chat;
       return {
-      kind: "chat" as const,
-      id: chat.id,
-      title: getChatTitle(chat),
-      updatedAt: chat.updatedAt,
-      createdAt: chat.createdAt,
-      starred: chat.starred,
-      starredAt: chat.starredAt,
-      initiatedProvider: chat.initiatedProvider,
-      initiatedModel: chat.initiatedModel,
-      lastUsedProvider: chat.lastUsedProvider,
-      lastUsedModel: chat.lastUsedModel,
+        kind: "chat" as const,
+        id: chat.id,
+        title: getChatTitle(chat),
+        updatedAt: chat.updatedAt,
+        createdAt: chat.createdAt,
+        starred: starOwner.starred,
+        starredAt: starOwner.starredAt,
+        initiatedProvider: chat.initiatedProvider,
+        initiatedModel: chat.initiatedModel,
+        lastUsedProvider: chat.lastUsedProvider,
+        lastUsedModel: chat.lastUsedModel,
       };
     }
 
@@ -979,11 +985,11 @@ async function main() {
     focusComposer();
   }
 
-  async function maybeSuggestTitle(chatId: string, content: string) {
-    const chatSummary = chats.find((chat) => chat.id === chatId);
-    const hasExistingTitle = Boolean(selectedChat?.id === chatId ? selectedChat.title?.trim() : chatSummary?.title?.trim());
-    if (hasExistingTitle || pendingTitleGeneration.has(chatId)) return;
+  async function maybeSuggestTitle(chat: ChatDetail, content: string) {
+    const needsGeneratedTitle = chat.titleGenerationPending || !chat.title?.trim();
+    if (!needsGeneratedTitle || pendingTitleGeneration.has(chat.id)) return;
 
+    const chatId = chat.id;
     pendingTitleGeneration.add(chatId);
     try {
       const updated = await api.suggestChatTitle({ chatId, content });
@@ -993,6 +999,8 @@ async function main() {
         selectedChat = {
           ...selectedChat,
           title: updated.title,
+          parentChatId: updated.parentChatId,
+          titleGenerationPending: updated.titleGenerationPending,
           updatedAt: updated.updatedAt,
           starred: updated.starred,
           starredAt: updated.starredAt,
@@ -1049,6 +1057,8 @@ async function main() {
       selectedChat = {
         id: chat.id,
         title: chat.title,
+        parentChatId: chat.parentChatId,
+        titleGenerationPending: chat.titleGenerationPending,
         createdAt: chat.createdAt,
         updatedAt: chat.updatedAt,
         starred: chat.starred,
@@ -1068,13 +1078,13 @@ async function main() {
       throw new Error("Unable to initialize chat");
     }
 
-    void maybeSuggestTitle(chatId, content);
-
     let baseChat = selectedChat;
     if (!baseChat || baseChat.id !== chatId) {
       baseChat = await api.getChat(chatId);
     }
 
+    void maybeSuggestTitle(baseChat, content);
+
     const requestMessages: CompletionRequestMessage[] = [
       ...baseChat.messages
         .filter((message) => !isToolCallLogMessage(message))
@@ -1392,6 +1402,8 @@ async function main() {
       selectedChat = {
         ...selectedChat,
         title: updated.title,
+        parentChatId: updated.parentChatId,
+        titleGenerationPending: updated.titleGenerationPending,
         updatedAt: updated.updatedAt,
         initiatedProvider: updated.initiatedProvider,
         initiatedModel: updated.initiatedModel,
@@ -1405,12 +1417,16 @@ async function main() {
   async function handleToggleStarSelection() {
     if (!selectedItem) return;
 
-    const currentItem = getSidebarItems().find((item) => item.kind === selectedItem?.kind && item.id === selectedItem?.id);
-    const nextStarred = !currentItem?.starred;
     setError(null);
 
     if (selectedItem.kind === "chat") {
-      const updated = await api.updateChatStar(selectedItem.id, nextStarred);
+      const selectedSummary = chats.find((chat) => chat.id === selectedItem?.id);
+      const selectedParentChatId = selectedChat?.id === selectedItem.id
+        ? selectedChat.parentChatId
+        : selectedSummary?.parentChatId;
+      const rootChatId = selectedParentChatId ?? selectedItem.id;
+      const rootSummary = chats.find((chat) => chat.id === rootChatId);
+      const updated = await api.updateChatStar(rootChatId, !rootSummary?.starred);
       chats = chats.map((chat) => (chat.id === updated.id ? updated : chat));
       if (!chats.some((chat) => chat.id === updated.id)) chats = [updated, ...chats];
       workspaceItems = workspaceItems.map((item) => (item.type === "chat" && item.id === updated.id ? chatWorkspaceItem(updated) : item));
@@ -1421,6 +1437,8 @@ async function main() {
         selectedChat = {
           ...selectedChat,
           title: updated.title,
+          parentChatId: updated.parentChatId,
+          titleGenerationPending: updated.titleGenerationPending,
           updatedAt: updated.updatedAt,
           starred: updated.starred,
           starredAt: updated.starredAt,
@@ -1431,7 +1449,8 @@ async function main() {
         };
       }
     } else {
-      const updated = await api.updateSearchStar(selectedItem.id, nextStarred);
+      const currentItem = getSidebarItems().find((item) => item.kind === "search" && item.id === selectedItem?.id);
+      const updated = await api.updateSearchStar(selectedItem.id, !currentItem?.starred);
       searches = searches.map((search) => (search.id === updated.id ? updated : search));
       if (!searches.some((search) => search.id === updated.id)) searches = [updated, ...searches];
       workspaceItems = workspaceItems.map((item) => (item.type === "search" && item.id === updated.id ? searchWorkspaceItem(updated) : item));

tui/src/types.ts

@@ -13,6 +13,8 @@ export type ModelCatalogResponse = {
 export type ChatSummary = {
   id: string;
   title: string | null;
+  parentChatId: string | null;
+  titleGenerationPending: boolean;
   createdAt: string;
   updatedAt: string;
   starred: boolean;
@@ -68,6 +70,8 @@ export type ToolCallEvent = {
 export type ChatDetail = {
   id: string;
   title: string | null;
+  parentChatId: string | null;
+  titleGenerationPending: boolean;
   createdAt: string;
   updatedAt: string;
   starred: boolean;

web/src/components/chat/chat-composer.tsx

@@ -0,0 +1,140 @@
+import { useLayoutEffect, useRef, useState } from "preact/hooks";
+import { Paperclip, Search, SendHorizontal } from "lucide-preact";
+import { Button } from "@/components/ui/button";
+import { Textarea } from "@/components/ui/textarea";
+import { cn } from "@/lib/utils";
+
+type MutableValueRef = {
+  current: string;
+};
+
+type Props = {
+  draftRef: MutableValueRef;
+  draftRevision: number;
+  error: string | null;
+  isSearchMode: boolean;
+  isSending: boolean;
+  pendingAttachmentCount: number;
+  attachmentButtonDisabled: boolean;
+  onOpenAttachmentPicker: () => void;
+  onPaste: (event: ClipboardEvent) => void;
+  onSend: (draft: string) => void | Promise<void>;
+};
+
+const MIRROR_SENTINEL = "\u200b";
+const HAS_NON_WHITESPACE = /\S/;
+
+export function ChatComposer({
+  draftRef,
+  draftRevision,
+  error,
+  isSearchMode,
+  isSending,
+  pendingAttachmentCount,
+  attachmentButtonDisabled,
+  onOpenAttachmentPicker,
+  onPaste,
+  onSend,
+}: Props) {
+  // The draft stays in the DOM/ref so typing never schedules a render of the workspace transcript.
+  const textareaContainerRef = useRef<HTMLDivElement>(null);
+  const mirrorRef = useRef<HTMLDivElement>(null);
+  const [hasDraft, setHasDraft] = useState(() => HAS_NON_WHITESPACE.test(draftRef.current));
+  const hasDraftRef = useRef(hasDraft);
+
+  const getTextarea = () => textareaContainerRef.current?.querySelector("textarea") ?? null;
+
+  const updateMirror = (value: string) => {
+    // The overlapping mirror lets normal layout size the textarea without synchronous scrollHeight reads.
+    if (mirrorRef.current) mirrorRef.current.textContent = `${value}${MIRROR_SENTINEL}`;
+  };
+
+  const updateHasDraft = (value: string) => {
+    const nextHasDraft = HAS_NON_WHITESPACE.test(value);
+    if (nextHasDraft !== hasDraftRef.current) {
+      hasDraftRef.current = nextHasDraft;
+      setHasDraft(nextHasDraft);
+    }
+  };
+
+  useLayoutEffect(() => {
+    const value = draftRef.current;
+    const textarea = getTextarea();
+    if (textarea && textarea.value !== value) {
+      textarea.value = value;
+    }
+    updateMirror(value);
+    updateHasDraft(value);
+  }, [draftRevision]);
+
+  const submit = () => {
+    const textarea = getTextarea();
+    const draft = textarea?.value ?? draftRef.current;
+    const canSend = HAS_NON_WHITESPACE.test(draft) || (!isSearchMode && pendingAttachmentCount > 0);
+    if (isSending || !canSend) return;
+
+    draftRef.current = "";
+    if (textarea) textarea.value = "";
+    updateMirror("");
+    updateHasDraft("");
+    void onSend(draft);
+  };
+
+  return (
+    <>
+      <div ref={textareaContainerRef} className="grid max-h-40 min-h-0 overflow-hidden">
+        <div
+          ref={mirrorRef}
+          className="pointer-events-none invisible col-start-1 row-start-1 max-h-40 min-h-0 overflow-x-hidden overflow-y-auto whitespace-pre-wrap break-words px-3 py-3 text-base"
+          aria-hidden="true"
+        />
+        <Textarea
+          id="composer-input"
+          rows={1}
+          onInput={(event) => {
+            const value = event.currentTarget.value;
+            draftRef.current = value;
+            updateMirror(value);
+            updateHasDraft(value);
+          }}
+          onPaste={(event) => {
+            onPaste(event);
+          }}
+          onKeyDown={(event) => {
+            if (event.key === "Enter" && !event.shiftKey && !event.isComposing) {
+              event.preventDefault();
+              submit();
+            }
+          }}
+          placeholder={isSearchMode ? "Search the web" : "Enter prompt..."}
+          className="col-start-1 row-start-1 h-full max-h-40 min-h-0 resize-none overflow-y-auto border-0 bg-transparent px-3 py-3 text-base text-violet-50 shadow-none placeholder:text-violet-200/45 focus-visible:ring-0"
+          disabled={isSending}
+        />
+      </div>
+      <div className={cn("flex items-center gap-3 px-2 pb-1", error ? "justify-between" : "justify-end")}>
+        {error ? <p className="min-w-0 truncate text-xs text-rose-300">{error}</p> : null}
+        {!isSearchMode ? (
+          <Button
+            className="h-10 w-10 rounded-lg"
+            onClick={onOpenAttachmentPicker}
+            size="icon"
+            variant="secondary"
+            disabled={attachmentButtonDisabled}
+            aria-label="Attach files"
+          >
+            <Paperclip className="h-4 w-4" />
+          </Button>
+        ) : null}
+        <Button
+          className="h-10 w-10 rounded-lg"
+          onClick={submit}
+          size="icon"
+          disabled={isSending || (!hasDraft && (isSearchMode || pendingAttachmentCount === 0))}
+          aria-label={isSearchMode ? "Search" : "Send message"}
+        >
+          {isSearchMode ? <Search className="h-4 w-4" /> : <SendHorizontal className="h-4 w-4" />}
+        </Button>
+      </div>
+    </>
+  );
+}

web/src/components/chat/chat-messages-panel.tsx

@@ -10,6 +10,7 @@ type Props = {
   messages: Message[];
   isLoading: boolean;
   isSending: boolean;
+  onMessageContextMenu?: (event: MouseEvent, messageId: string) => void;
 };
 
 type ToolLogMetadata = {
@@ -395,7 +396,7 @@ function ToolCallStack({
   );
 }
 
-export function ChatMessagesPanel({ messages, isLoading, isSending }: Props) {
+export function ChatMessagesPanel({ messages, isLoading, isSending, onMessageContextMenu }: Props) {
   const hasPendingAssistant = messages.some((message) => message.id.startsWith("temp-assistant-") && message.content.trim().length === 0);
   const renderItems = useMemo(() => buildMessageRenderItems(messages), [messages]);
   const toolCallMessageIDs = useMemo(() => getToolCallMessageIDs(messages), [messages]);
@@ -467,6 +468,11 @@ export function ChatMessagesPanel({ messages, isLoading, isSending }: Props) {
                     ? "rounded-xl border border-violet-300/24 bg-[linear-gradient(135deg,hsl(258_86%_48%_/_0.86),hsl(278_72%_29%_/_0.86))] px-4 py-3 text-sm leading-6 text-fuchsia-50 shadow-sm"
                     : "text-base leading-7 text-violet-50"
                 )}
+                onContextMenu={
+                  message.role === "assistant" && !message.id.startsWith("temp-") && onMessageContextMenu
+                    ? (event) => onMessageContextMenu(event, message.id)
+                    : undefined
+                }
               >
                 {attachments.length ? <ChatAttachmentList attachments={attachments} tone={isUser ? "user" : "assistant"} /> : null}
                 {isPendingAssistant ? (

web/src/index.css

@@ -131,6 +131,45 @@ textarea {
   }
 }
 
+@media (horizontal-viewport-segments: 2) {
+  .app-safe-frame {
+    padding:
+      max(0.5rem, var(--safe-area-top))
+      max(0.5rem, var(--safe-area-right))
+      max(0.5rem, var(--safe-area-bottom))
+      max(0.5rem, var(--safe-area-left));
+  }
+
+  .workspace-shell {
+    display: grid;
+    grid-template-columns:
+      calc(env(viewport-segment-width 0 0) - max(0.5rem, var(--safe-area-left)))
+      calc(env(viewport-segment-width 1 0) - max(0.5rem, var(--safe-area-right)));
+    column-gap: calc(env(viewport-segment-left 1 0) - env(viewport-segment-right 0 0));
+  }
+
+  .workspace-sidebar {
+    position: static;
+    width: 100%;
+    max-width: none;
+    transform: none;
+    border-width: 1px;
+    border-radius: 1rem;
+  }
+
+  .workspace-content {
+    min-width: 0;
+    border-width: 1px;
+    border-radius: 1rem;
+    touch-action: auto;
+  }
+
+  .workspace-sidebar-backdrop,
+  .workspace-sidebar-trigger {
+    display: none;
+  }
+}
+
 .glass-panel {
   background:
     linear-gradient(180deg, hsl(243 42% 12% / 0.88), hsl(236 48% 5% / 0.92)),

web/src/lib/api.ts

@@ -1,6 +1,8 @@
 export type ChatSummary = {
   id: string;
   title: string | null;
+  parentChatId: string | null;
+  titleGenerationPending: boolean;
   createdAt: string;
   updatedAt: string;
   starred: boolean;
@@ -58,6 +60,8 @@ export type ToolCallEvent = {
 export type ChatDetail = {
   id: string;
   title: string | null;
+  parentChatId: string | null;
+  titleGenerationPending: boolean;
   createdAt: string;
   updatedAt: string;
   starred: boolean;
@@ -291,6 +295,14 @@ export async function getChat(chatId: string) {
   return data.chat;
 }
 
+export async function forkChat(chatId: string, messageId?: string) {
+  const data = await api<{ chat: ChatSummary }>(`/v1/chats/${chatId}/fork`, {
+    method: "POST",
+    body: JSON.stringify(messageId ? { messageId } : {}),
+  });
+  return data.chat;
+}
+
 export async function updateChatTitle(chatId: string, title: string) {
   const data = await api<{ chat: ChatSummary }>(`/v1/chats/${chatId}`, {
     method: "PATCH",

web/src/lib/chat-forking.ts

@@ -0,0 +1,82 @@
+export type ForkGroupingItem = {
+  kind: "chat" | "search";
+  id: string;
+  parentChatId: string | null;
+};
+
+type ChatTitleState = {
+  title: string | null;
+  titleGenerationPending: boolean;
+};
+
+type ForkPresentationItem = ForkGroupingItem & {
+  updatedAt: string;
+  starred: boolean;
+  starredAt: string | null;
+};
+
+export function groupForkedChatItems<T extends ForkGroupingItem>(items: T[]): T[][] {
+  const groups = new Map<string, { firstIndex: number; items: Array<{ item: T; index: number }> }>();
+
+  items.forEach((item, index) => {
+    const groupKey = item.kind === "chat" ? `chat:${item.parentChatId ?? item.id}` : `search:${item.id}`;
+    const group = groups.get(groupKey);
+    if (group) {
+      group.items.push({ item, index });
+      return;
+    }
+    groups.set(groupKey, { firstIndex: index, items: [{ item, index }] });
+  });
+
+  return [...groups.values()]
+    .sort((a, b) => a.firstIndex - b.firstIndex)
+    .map((group) =>
+      group.items
+        .sort((a, b) => {
+          const aIsRoot = a.item.kind === "chat" && a.item.parentChatId === null;
+          const bIsRoot = b.item.kind === "chat" && b.item.parentChatId === null;
+          if (aIsRoot !== bIsRoot) return aIsRoot ? -1 : 1;
+          return a.index - b.index;
+        })
+        .map(({ item }) => item)
+    );
+}
+
+export function filterSidebarItemsWithForkGroups<T extends ForkGroupingItem>(items: T[], matches: (item: T) => boolean): T[] {
+  return groupForkedChatItems(items)
+    .flatMap((group) => {
+      const matchingItems = group.filter(matches);
+      if (!matchingItems.length) return [];
+      const root = group.find((item) => item.kind === "chat" && item.parentChatId === null);
+      if (!root || matchingItems.includes(root)) return group;
+      return group.filter((item) => item === root || matchingItems.includes(item));
+    });
+}
+
+export function getForkGroupPresentation<T extends ForkPresentationItem>(groupItems: T[], allItems: T[]) {
+  const firstItem = groupItems[0];
+  if (!firstItem) return null;
+
+  const rootId = firstItem.kind === "chat" ? firstItem.parentChatId ?? firstItem.id : null;
+  const familyItems = rootId
+    ? allItems.filter((item) => item.kind === "chat" && (item.id === rootId || item.parentChatId === rootId))
+    : groupItems;
+  const updatedAt = familyItems.reduce(
+    (newest, item) => (new Date(item.updatedAt).getTime() > new Date(newest).getTime() ? item.updatedAt : newest),
+    firstItem.updatedAt
+  );
+  const starOwner = rootId
+    ? familyItems.find((item) => item.kind === "chat" && item.parentChatId === null)
+    : firstItem;
+
+  return {
+    updatedAt,
+    starred: starOwner?.starred ?? false,
+    starredAt: starOwner?.starredAt ?? null,
+  };
+}
+
+export function shouldRequestChatTitle(chat: ChatTitleState | null | undefined, isRequestInFlight: boolean) {
+  if (!chat || isRequestInFlight) return false;
+  return chat.titleGenerationPending || !chat.title?.trim();
+}

web/tests/chat-forking.test.mjs

@@ -0,0 +1,59 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import {
+  filterSidebarItemsWithForkGroups,
+  getForkGroupPresentation,
+  groupForkedChatItems,
+  shouldRequestChatTitle,
+} from "../src/lib/chat-forking.ts";
+
+const root = { kind: "chat", id: "root", parentChatId: null };
+const firstFork = { kind: "chat", id: "first-fork", parentChatId: "root" };
+const nestedFork = { kind: "chat", id: "nested-fork", parentChatId: "root" };
+const search = { kind: "search", id: "search", parentChatId: null };
+
+test("fork groups are positioned by their newest member and always render root first", () => {
+  assert.deepEqual(groupForkedChatItems([nestedFork, search, root, firstFork]), [
+    [root, nestedFork, firstFork],
+    [search],
+  ]);
+});
+
+test("sidebar filtering keeps a matching fork with its root", () => {
+  assert.deepEqual(
+    filterSidebarItemsWithForkGroups([search, root, firstFork, nestedFork], (item) => item.id === "nested-fork"),
+    [root, nestedFork]
+  );
+});
+
+test("sidebar filtering keeps all children when their root matches", () => {
+  assert.deepEqual(
+    filterSidebarItemsWithForkGroups([search, root, firstFork, nestedFork], (item) => item.id === "root"),
+    [root, firstFork, nestedFork]
+  );
+});
+
+test("a filtered family keeps its full-family date and root-owned star state", () => {
+  const fullFamily = [
+    { ...root, updatedAt: "2026-08-10T00:00:00.000Z", starred: true, starredAt: "2026-08-15T00:00:00.000Z" },
+    { ...firstFork, updatedAt: "2026-08-11T00:00:00.000Z", starred: false, starredAt: null },
+    { ...nestedFork, updatedAt: "2026-08-16T00:00:00.000Z", starred: false, starredAt: null },
+  ];
+  const filteredFamily = [fullFamily[0], fullFamily[1]];
+
+  assert.deepEqual(getForkGroupPresentation(filteredFamily, fullFamily), {
+    updatedAt: "2026-08-16T00:00:00.000Z",
+    starred: true,
+    starredAt: "2026-08-15T00:00:00.000Z",
+  });
+});
+
+test("a fork placeholder title is replaced on its first submitted prompt", () => {
+  assert.equal(shouldRequestChatTitle({ title: "Fork of original", titleGenerationPending: true }, false), true);
+  assert.equal(shouldRequestChatTitle({ title: "Generated title", titleGenerationPending: false }, false), false);
+});
+
+test("title generation remains compatible with untitled chats and deduplicates in-flight requests", () => {
+  assert.equal(shouldRequestChatTitle({ title: null, titleGenerationPending: false }, false), true);
+  assert.equal(shouldRequestChatTitle({ title: "Fork of original", titleGenerationPending: true }, true), false);
+});

web/tsconfig.tsbuildinfo

@@ -1 +1 @@
-{"root":["./src/App.tsx","./src/main.tsx","./src/pwa.ts","./src/root-router.tsx","./src/vite-env.d.ts","./src/components/sybil-character.tsx","./src/components/auth/auth-screen.tsx","./src/components/chat/chat-attachment-list.tsx","./src/components/chat/chat-messages-panel.tsx","./src/components/markdown/markdown-content.tsx","./src/components/search/search-results-panel.tsx","./src/components/ui/button.tsx","./src/components/ui/input.tsx","./src/components/ui/scroll-area.tsx","./src/components/ui/separator.tsx","./src/components/ui/textarea.tsx","./src/hooks/use-session-auth.ts","./src/lib/api.ts","./src/lib/chat-model-selection.ts","./src/lib/sidebar-selection.ts","./src/lib/utils.ts","./src/pages/search-route-page.tsx"],"version":"5.9.3"}
+{"root":["./src/App.tsx","./src/main.tsx","./src/pwa.ts","./src/root-router.tsx","./src/vite-env.d.ts","./src/components/sybil-character.tsx","./src/components/auth/auth-screen.tsx","./src/components/chat/chat-attachment-list.tsx","./src/components/chat/chat-composer.tsx","./src/components/chat/chat-messages-panel.tsx","./src/components/markdown/markdown-content.tsx","./src/components/search/search-results-panel.tsx","./src/components/ui/button.tsx","./src/components/ui/input.tsx","./src/components/ui/scroll-area.tsx","./src/components/ui/separator.tsx","./src/components/ui/textarea.tsx","./src/hooks/use-session-auth.ts","./src/lib/api.ts","./src/lib/chat-forking.ts","./src/lib/chat-model-selection.ts","./src/lib/sidebar-selection.ts","./src/lib/utils.ts","./src/pages/search-route-page.tsx"],"version":"5.9.3"}