ios: add chat fork actions

ios/Packages/Sybil/Sources/Sybil/SybilAPIClient.swift

@@ -74,6 +74,16 @@ actor SybilAPIClient: SybilAPIClienting {
         return response.chat
     }
 
+    func forkChat(chatID: String, messageID: String? = nil) async throws -> ChatSummary {
+        let response = try await request(
+            "/v1/chats/\(chatID)/fork",
+            method: "POST",
+            body: AnyEncodable(ChatForkBody(messageId: messageID)),
+            responseType: ChatCreateResponse.self
+        )
+        return response.chat
+    }
+
     func updateChatTitle(chatID: String, title: String) async throws -> ChatSummary {
         let response = try await request(
             "/v1/chats/\(chatID)",
@@ -704,6 +714,10 @@ private struct ChatTitleUpdateBody: Encodable {
     var title: String
 }
 
+private struct ChatForkBody: Encodable {
+    var messageId: String?
+}
+
 private struct StarUpdateBody: Encodable {
     var starred: Bool
 }

ios/Packages/Sybil/Sources/Sybil/SybilAPIClienting.swift

@@ -11,6 +11,7 @@ protocol SybilAPIClienting: Sendable {
         messages: [CompletionRequestMessage]?
     ) async throws -> ChatSummary
     func getChat(chatID: String) async throws -> ChatDetail
+    func forkChat(chatID: String, messageID: String?) async throws -> ChatSummary
     func updateChatTitle(chatID: String, title: String) async throws -> ChatSummary
     func updateChatStar(chatID: String, starred: Bool) async throws -> ChatSummary
     func deleteChat(chatID: String) async throws

ios/Packages/Sybil/Sources/Sybil/SybilChatTranscriptView.swift

@@ -1,5 +1,6 @@
 import MarkdownUI
 import SwiftUI
+import UIKit
 
 struct SybilChatTranscriptView: View {
     var messages: [Message]
@@ -9,6 +10,8 @@ struct SybilChatTranscriptView: View {
     var topContentInset: CGFloat = 0
     var bottomContentInset: CGFloat = 0
     var bottomPinRequestID: Int = 0
+    var isForking: Bool = false
+    var onForkResponse: ((String) -> Void)? = nil
 
     @State private var hasTrackedToolCallMessages = false
     @State private var knownToolCallMessageIDs: Set<String> = []
@@ -45,7 +48,11 @@ struct SybilChatTranscriptView: View {
                             MessageBubble(
                                 message: message,
                                 isSending: isSending,
-                                streamingAssistantPresentation: streamingAssistantPresentation
+                                streamingAssistantPresentation: streamingAssistantPresentation,
+                                isForking: isForking,
+                                onFork: {
+                                    onForkResponse?(message.id)
+                                }
                             )
                                 .frame(maxWidth: .infinity)
                         case let .toolGroup(id, messages):
@@ -146,6 +153,8 @@ private struct MessageBubble: View {
     var message: Message
     var isSending: Bool
     var streamingAssistantPresentation: StreamingAssistantPresentation?
+    var isForking: Bool
+    var onFork: (() -> Void)?
 
     private var toolCallMetadata: ToolCallMetadata? {
         message.toolCallMetadata
@@ -164,7 +173,37 @@ private struct MessageBubble: View {
         return streamingAssistantPresentation
     }
 
+    private var canFork: Bool {
+        message.role == .assistant &&
+            !message.id.hasPrefix("temp-") &&
+            onFork != nil
+    }
+
     var body: some View {
+        Group {
+            if canFork {
+                bubbleContent
+                    .contextMenu {
+                        Button {
+                            UIPasteboard.general.string = message.content
+                        } label: {
+                            Label("Copy", systemImage: "doc.on.doc")
+                        }
+
+                        Button {
+                            onFork?()
+                        } label: {
+                            Label("Fork", systemImage: "arrow.triangle.branch")
+                        }
+                        .disabled(isForking)
+                    }
+            } else {
+                bubbleContent
+            }
+        }
+    }
+
+    private var bubbleContent: some View {
         HStack(alignment: .top, spacing: 0) {
             leadingSpacer
 

ios/Packages/Sybil/Sources/Sybil/SybilPhoneShellView.swift

@@ -130,7 +130,10 @@ struct SybilPhoneShellView: View {
             route: route,
             onRequestBack: { _ in showSidebarOverlay() },
             onRequestNewChat: sidebarWorkspaceNewChatAction,
-            onShowSidebar: showSidebarOverlay
+            onShowSidebar: showSidebarOverlay,
+            onOpenForkedChat: { chatID in
+                showRoute(.chat(chatID))
+            }
         )
         .background(SybilTheme.background)
         .blur(radius: SidebarOverlaySwipeMetrics.workspaceBlurRadius(for: sidebarOverlayProgress))
@@ -511,6 +514,9 @@ private struct SybilPhoneSidebarRoot: View {
                 },
                 onSelect: { item in
                     onSelect(item.selection)
+                },
+                onOpenForkedChat: { chatID in
+                    onSelect(.chat(chatID))
                 }
             )
         }
@@ -585,6 +591,7 @@ private struct SybilPhoneDestinationView: View {
     let onRequestBack: (_ animateNavigation: Bool) -> Void
     let onRequestNewChat: (() -> Void)?
     let onShowSidebar: () -> Void
+    let onOpenForkedChat: (String) -> Void
 
     var body: some View {
         SybilWorkspaceView(
@@ -593,7 +600,8 @@ private struct SybilPhoneDestinationView: View {
             navigationLeadingControl: .showSidebar,
             onShowSidebar: onShowSidebar,
             onRequestBack: onRequestBack,
-            onRequestNewChat: onRequestNewChat
+            onRequestNewChat: onRequestNewChat,
+            onOpenForkedChat: onOpenForkedChat
         )
         .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
         .task(id: route) {

ios/Packages/Sybil/Sources/Sybil/SybilSidebarView.swift

@@ -111,6 +111,7 @@ struct SybilSidebarItemList: View {
     @Bindable var viewModel: SybilViewModel
     var isSelected: (SidebarItem) -> Bool
     var onSelect: (SidebarItem) -> Void
+    var onOpenForkedChat: ((String) -> Void)? = nil
     @State private var renameTarget: SidebarItem?
     @State private var renameTitle = ""
 
@@ -169,6 +170,13 @@ struct SybilSidebarItemList: View {
                                 }
 
                                 if item.kind == .chat {
+                                    Button {
+                                        forkChat(item)
+                                    } label: {
+                                        Label("Fork", systemImage: "arrow.triangle.branch")
+                                    }
+                                    .disabled(viewModel.isForkingChat || item.isRunning)
+
                                     Button {
                                         renameTarget = item
                                         renameTitle = item.title
@@ -215,6 +223,28 @@ struct SybilSidebarItemList: View {
             .disabled(renameTitle.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty)
         }
     }
+
+    private func forkChat(_ item: SidebarItem) {
+        guard case let .chat(chatID) = item.selection else {
+            return
+        }
+
+        let selectionAtStart = viewModel.selectedItem
+        let draftKindAtStart = viewModel.draftKind
+        Task {
+            guard let forkedChat = await viewModel.forkChat(chatID: chatID),
+                  viewModel.selectedItem == selectionAtStart,
+                  viewModel.draftKind == draftKindAtStart else {
+                return
+            }
+
+            if let onOpenForkedChat {
+                onOpenForkedChat(forkedChat.id)
+            } else {
+                viewModel.select(.chat(forkedChat.id))
+            }
+        }
+    }
 }
 
 struct SybilSidebarRow: View {

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

@@ -170,6 +170,7 @@ final class SybilViewModel {
     var isLoadingCollections = false
     var isLoadingSelection = false
     var isCreatingSearchChat = false
+    var isForkingChat = false
     var chatBottomPinRequestID = 0
     var errorMessage: String?
 
@@ -958,6 +959,36 @@ final class SybilViewModel {
         }
     }
 
+    @discardableResult
+    func forkChat(chatID: String, messageID: String? = nil) async -> ChatSummary? {
+        guard isAuthenticated, !isForkingChat else {
+            return nil
+        }
+
+        guard messageID != nil || !isChatRowRunning(chatID) else {
+            errorMessage = "Wait for the response to finish before forking this chat."
+            return nil
+        }
+
+        let forkDescription = messageID.map { "chat \(chatID) from response \($0)" } ?? "chat \(chatID)"
+        SybilLog.info(SybilLog.ui, "Forking \(forkDescription)")
+        errorMessage = nil
+        isForkingChat = true
+        defer {
+            isForkingChat = false
+        }
+
+        do {
+            let forkedChat = try await client().forkChat(chatID: chatID, messageID: messageID)
+            applyChatSummary(forkedChat, moveToFront: true)
+            return forkedChat
+        } catch {
+            errorMessage = normalizeAPIError(error)
+            SybilLog.error(SybilLog.ui, "Fork failed", error: error)
+            return nil
+        }
+    }
+
     func renameChat(chatID: String, title: String) async {
         guard isAuthenticated else {
             return

ios/Packages/Sybil/Sources/Sybil/SybilWorkspaceView.swift

@@ -19,6 +19,7 @@ struct SybilWorkspaceView: View {
     var onShowSidebar: (() -> Void)? = nil
     var onRequestBack: ((_ animateNavigation: Bool) -> Void)? = nil
     var onRequestNewChat: (() -> Void)? = nil
+    var onOpenForkedChat: ((String) -> Void)? = nil
     @FocusState private var composerFocused: Bool
     @Environment(\.dismiss) private var dismiss
     @State private var isShowingAttachmentOptions = false
@@ -196,7 +197,9 @@ struct SybilWorkspaceView: View {
                         streamingAssistantPresentation: viewModel.displayedStreamingAssistantPresentation,
                         topContentInset: showsCustomWorkspaceNavigation ? customWorkspaceNavigationContentInset : 0,
                         bottomContentInset: viewModel.showsComposer ? composerOverlayContentInset : 0,
-                        bottomPinRequestID: viewModel.chatBottomPinRequestID
+                        bottomPinRequestID: viewModel.chatBottomPinRequestID,
+                        isForking: viewModel.isForkingChat,
+                        onForkResponse: forkResponse
                     )
                     .id(transcriptScrollContextID)
                 }
@@ -230,6 +233,28 @@ struct SybilWorkspaceView: View {
         }
     }
 
+    private func forkResponse(messageID: String) {
+        guard viewModel.draftKind == nil,
+              case let .chat(chatID) = viewModel.selectedItem else {
+            return
+        }
+
+        let selectionAtStart = viewModel.selectedItem
+        Task {
+            guard let forkedChat = await viewModel.forkChat(chatID: chatID, messageID: messageID),
+                  viewModel.selectedItem == selectionAtStart,
+                  viewModel.draftKind == nil else {
+                return
+            }
+
+            if let onOpenForkedChat {
+                onOpenForkedChat(forkedChat.id)
+            } else {
+                viewModel.select(.chat(forkedChat.id))
+            }
+        }
+    }
+
     private var customWorkspaceNavigationBar: some View {
         HStack(spacing: 14) {
             workspaceNavigationLeadingControl