Add conversation branching to insight chat (rewind preserves forks)

Store chat history as a tree in training_messages instead of a flat
array. Rewind now sets active_leaf_id to the target node rather than
truncating, so discarded paths survive as alternate branches. Fork
indicators ("X/Y") mark divergence points and let the client load or
switch to alternate paths.

- llm_client: StoredChatNode/ChatHistoryStore with tree traversal
  helpers (path_to_leaf, children_of, fork_at_node, leaves*, etc.) and
  from_flat_array for backward-compatible reads of the old flat format.
- insight_chat: load_history/chat_turn/rewind_history operate on the
  tree; new switch_branch and get_branches. render_tree_path walks every
  node (including non-rendered tool-dispatch nodes) when computing fork
  info, so a fork whose diverging child is a tool call still surfaces on
  the following rendered message.
- handlers: GET /insights/chat/branches, POST /insights/chat/switch-branch,
  and a branch_id param on the history endpoint.

Backward-compatible: flat arrays are converted to a tree lazily on read.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ai/handlers.rs

@@ -1310,6 +1310,11 @@ pub struct ChatHistoryQuery {
     pub path: String,
     #[serde(default)]
     pub library: Option<String>,
+    /// When set, load the branch anchored at this leaf ID instead of the
+    /// active branch. Allows the client to preview alternate conversation
+    /// paths without making them active.
+    #[serde(default)]
+    pub branch_id: Option<u64>,
 }
 
 #[derive(Debug, Serialize)]
@@ -1318,6 +1323,18 @@ pub struct ChatHistoryHttpResponse {
     pub turn_count: usize,
     pub model_version: String,
     pub backend: String,
+    pub active_leaf_id: u64,
+    /// The branch leaf ID being viewed. Equals `active_leaf_id` when viewing
+    /// the active branch, or the `branch_id` query param for alternates.
+    pub viewing_branch_id: u64,
+    #[serde(skip_serializing_if = "Vec::is_empty")]
+    pub fork_info: Vec<Option<ChatForkInfo>>,
+}
+
+#[derive(Debug, Clone, Serialize)]
+pub struct ChatForkInfo {
+    pub position: usize,
+    pub total: usize,
 }
 
 #[derive(Debug, Serialize)]
@@ -1414,8 +1431,22 @@ pub async fn chat_history_handler(
         .flatten()
         .unwrap_or_else(|| app_state.primary_library());
 
-    match app_state.insight_chat.load_history(library.id, &query.path) {
-        Ok(view) => HttpResponse::Ok().json(ChatHistoryHttpResponse {
+    match app_state
+        .insight_chat
+        .load_history(library.id, &query.path, query.branch_id)
+    {
+        Ok(view) => {
+            let fork_info: Vec<Option<ChatForkInfo>> = view
+                .fork_info
+                .into_iter()
+                .map(|f| {
+                    f.map(|fi| ChatForkInfo {
+                        position: fi.position,
+                        total: fi.total,
+                    })
+                })
+                .collect();
+            HttpResponse::Ok().json(ChatHistoryHttpResponse {
                 messages: view
                     .messages
                     .into_iter()
@@ -1438,7 +1469,11 @@ pub async fn chat_history_handler(
                 turn_count: view.turn_count,
                 model_version: view.model_version,
                 backend: view.backend,
-        }),
+                active_leaf_id: view.active_leaf_id,
+                viewing_branch_id: view.viewing_branch_id,
+                fork_info,
+            })
+        }
         Err(e) => {
             let msg = format!("{}", e);
             if msg.contains("no insight found") {
@@ -1452,6 +1487,94 @@ pub async fn chat_history_handler(
     }
 }
 
+/// GET /insights/chat/branches — return the list of branch leaf IDs for a
+/// photo's conversation tree. Each branch is a leaf node; the active branch
+/// is the one matching `active_leaf_id`. Use with `branch_id` on the
+/// history endpoint to load an alternate conversation path.
+#[get("/insights/chat/branches")]
+pub async fn chat_branches_handler(
+    _claims: Claims,
+    query: web::Query<ChatHistoryQuery>,
+    app_state: web::Data<AppState>,
+) -> impl Responder {
+    let library = libraries::resolve_library_param_state(&app_state, query.library.as_deref())
+        .ok()
+        .flatten()
+        .unwrap_or_else(|| app_state.primary_library());
+
+    let (branches, active_leaf_id) =
+        match app_state.insight_chat.get_branches(library.id, &query.path) {
+            Ok(result) => result,
+            Err(e) => {
+                let msg = format!("{}", e);
+                if msg.contains("no insight found") {
+                    return HttpResponse::NotFound().json(serde_json::json!({ "error": msg }));
+                } else if msg.contains("no chat history") {
+                    return HttpResponse::Conflict().json(serde_json::json!({ "error": msg }));
+                } else {
+                    return HttpResponse::InternalServerError()
+                        .json(serde_json::json!({ "error": msg }));
+                }
+            }
+        };
+
+    HttpResponse::Ok().json(serde_json::json!({
+        "branches": branches,
+        "active_leaf_id": active_leaf_id,
+    }))
+}
+
+/// POST /insights/chat/switch-branch — switch the active branch to a
+/// different conversation path. The new branch becomes the active
+/// conversation; the previous active branch becomes a regular fork.
+#[post("/insights/chat/switch-branch")]
+pub async fn chat_switch_branch_handler(
+    _claims: Claims,
+    request: web::Json<ChatSwitchBranchRequest>,
+    app_state: web::Data<AppState>,
+) -> impl Responder {
+    let library =
+        match libraries::resolve_library_param_state(&app_state, request.library.as_deref()) {
+            Ok(Some(lib)) => lib,
+            Ok(None) => app_state.primary_library(),
+            Err(e) => {
+                return HttpResponse::BadRequest().json(serde_json::json!({
+                    "error": format!("invalid library: {}", e)
+                }));
+            }
+        };
+
+    match app_state
+        .insight_chat
+        .switch_branch(library.id, &request.file_path, request.branch_id)
+        .await
+    {
+        Ok(()) => HttpResponse::Ok().json(serde_json::json!({ "success": true })),
+        Err(e) => {
+            let msg = format!("{}", e);
+            log::error!("Switch branch failed: {}", msg);
+            if msg.contains("no insight found") {
+                HttpResponse::NotFound().json(serde_json::json!({ "error": msg }))
+            } else if msg.contains("no chat history") {
+                HttpResponse::Conflict().json(serde_json::json!({ "error": msg }))
+            } else if msg.contains("not found") || msg.contains("not a leaf") {
+                HttpResponse::BadRequest().json(serde_json::json!({ "error": msg }))
+            } else {
+                HttpResponse::InternalServerError().json(serde_json::json!({ "error": msg }))
+            }
+        }
+    }
+}
+
+#[derive(Debug, Deserialize)]
+pub struct ChatSwitchBranchRequest {
+    pub file_path: String,
+    #[serde(default)]
+    pub library: Option<String>,
+    /// The leaf node ID to switch to. Must be a valid leaf in the tree.
+    pub branch_id: u64,
+}
+
 /// POST /insights/chat/stream — streaming variant of /insights/chat.
 /// Returns `text/event-stream` with one event per chat stream event.
 #[post("/insights/chat/stream")]

src/ai/insight_chat.rs

@@ -8,7 +8,9 @@ use tokio::sync::Mutex as TokioMutex;
 
 use crate::ai::backend::{BackendKind, ResolvedBackend, SamplingOverrides};
 use crate::ai::insight_generator::InsightGenerator;
-use crate::ai::llm_client::{ChatMessage, LlmStreamEvent, Tool};
+use crate::ai::llm_client::{
+    BranchLeafInfo, ChatHistoryStore, ChatMessage, LlmStreamEvent, StoredChatNode, Tool,
+};
 use crate::ai::turn_registry::TurnEntry;
 use crate::ai::turn_registry::TurnRegistry;
 use crate::database::InsightDao;
@@ -137,19 +139,20 @@ impl InsightChatService {
         &self.insight_dao
     }
 
-    /// Load the rendered transcript for chat-UI display. Filters internal
-    /// scaffolding (system message, tool turns, tool-dispatch-only assistant
-    /// messages) and drops base64 images from user turns to keep payloads
-    /// small. The first remaining user message is flagged `is_initial`.
+    /// Load the rendered transcript for chat-UI display. Deserializes the
+    /// `training_messages` tree, traverses to `active_leaf_id`, and renders
+    /// the path from root to that leaf. Backward-compatible: if the stored
+    /// value is a flat array (old format), converts to a tree automatically.
     ///
     /// `library_id` scopes the lookup to one library — without it, a
     /// regenerate on lib1 can be shadowed on the next refresh by an
     /// untouched `is_current=true` row in lib2 for the same rel_path.
-    /// Falls back to the cross-library `get_insight` only when the
-    /// scoped lookup misses, preserving the cross-library "show this
-    /// photo's primary insight" merge for the case where the active
-    /// library has no insight but another library does.
-    pub fn load_history(&self, library_id: i32, file_path: &str) -> Result<HistoryView> {
+    pub fn load_history(
+        &self,
+        library_id: i32,
+        file_path: &str,
+        branch_id: Option<u64>,
+    ) -> Result<HistoryView> {
         let normalized = normalize_path(file_path);
         let cx = opentelemetry::Context::new();
         let mut dao = self.insight_dao.lock().expect("Unable to lock InsightDao");
@@ -168,98 +171,32 @@ impl InsightChatService {
             .training_messages
             .as_ref()
             .ok_or_else(|| anyhow!("insight has no chat history (pre-agentic insight)"))?;
-        let messages: Vec<ChatMessage> = serde_json::from_str(raw)
-            .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?;
 
-        let mut rendered = Vec::new();
-        let mut user_turns_seen = 0usize;
-        let mut assistant_turns_seen = 0usize;
+        // Backward-compatible deserialization: flat array (old) vs tree (new).
+        let store: ChatHistoryStore = if let Ok(arr) = serde_json::from_str::<Vec<ChatMessage>>(raw)
+        {
+            ChatHistoryStore::from_flat_array(arr)
+        } else {
+            serde_json::from_str(raw)
+                .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?
+        };
 
-        // Accumulate tool invocations seen since the last user turn. An
-        // invocation is: one assistant tool_call message (which may hold
-        // multiple calls) + the N following tool-role messages (one per call,
-        // in order). They attach to the next assistant-with-content, which
-        // is the "final" reply for the current turn.
-        //
-        // Wire shape from the model:
-        //   assistant { tool_calls: [A, B], content: "" }
-        //   tool      { content: "result of A" }
-        //   tool      { content: "result of B" }
-        //   assistant { content: "here's the answer" }  ← rendered as final
-        let mut pending_tools: Vec<ToolInvocation> = Vec::new();
-        // Queue of (name, arguments) awaiting a tool_result to pair with.
-        let mut pending_calls: std::collections::VecDeque<(String, serde_json::Value)> =
-            std::collections::VecDeque::new();
+        // Use branch_id if provided, otherwise use active_leaf_id.
+        let target_leaf = branch_id.unwrap_or(store.active_leaf_id);
+        let path = store
+            .path_to_leaf(target_leaf)
+            .ok_or_else(|| anyhow!("branch_id {} not found in tree", target_leaf))?;
 
-        for msg in &messages {
-            match msg.role.as_str() {
-                "system" => continue,
-                "tool" => {
-                    if let Some((name, arguments)) = pending_calls.pop_front() {
-                        let (result, result_truncated) = truncate_tool_result(&msg.content);
-                        pending_tools.push(ToolInvocation {
-                            name,
-                            arguments,
-                            result,
-                            result_truncated,
-                        });
-                    }
-                    // If there's no pending call, the tool message is an
-                    // orphan (shouldn't happen in practice) — skip silently.
-                }
-                "assistant" => {
-                    let has_tool_calls = msg
-                        .tool_calls
-                        .as_ref()
-                        .map(|c| !c.is_empty())
-                        .unwrap_or(false);
-                    if has_tool_calls && msg.content.trim().is_empty() {
-                        // Tool-dispatch turn: enqueue calls, wait for tool
-                        // results on subsequent messages.
-                        if let Some(ref tcs) = msg.tool_calls {
-                            for tc in tcs {
-                                pending_calls.push_back((
-                                    tc.function.name.clone(),
-                                    tc.function.arguments.clone(),
-                                ));
-                            }
-                        }
-                        continue;
-                    }
-                    // Final assistant reply for this turn — drain accumulated
-                    // tools into it.
-                    assistant_turns_seen += 1;
-                    let tools = std::mem::take(&mut pending_tools);
-                    pending_calls.clear(); // any leftover unpaired calls are dropped
-                    rendered.push(RenderedMessage {
-                        role: "assistant".to_string(),
-                        content: msg.content.clone(),
-                        is_initial: false,
-                        tools,
-                    });
-                }
-                "user" => {
-                    let is_initial = user_turns_seen == 0;
-                    user_turns_seen += 1;
-                    // New user turn resets any in-flight tool state.
-                    pending_tools.clear();
-                    pending_calls.clear();
-                    rendered.push(RenderedMessage {
-                        role: "user".to_string(),
-                        content: msg.content.clone(),
-                        is_initial,
-                        tools: Vec::new(),
-                    });
-                }
-                _ => continue,
-            }
-        }
+        let (rendered, turn_count, _node_ids, fork_info) = render_tree_path(&store, &path);
 
         Ok(HistoryView {
             messages: rendered,
-            turn_count: assistant_turns_seen,
+            turn_count,
             model_version: insight.model_version,
             backend: insight.backend,
+            active_leaf_id: store.active_leaf_id,
+            viewing_branch_id: target_leaf,
+            fork_info,
         })
     }
 
@@ -315,8 +252,21 @@ impl InsightChatService {
                 anyhow!("insight has no chat history; regenerate this insight in agentic mode")
             })?
             .clone();
-        let mut messages: Vec<ChatMessage> = serde_json::from_str(&raw_history)
-            .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?;
+
+        // Backward-compatible: flat array (old) vs tree (new).
+        let mut store: ChatHistoryStore =
+            if let Ok(arr) = serde_json::from_str::<Vec<ChatMessage>>(&raw_history) {
+                ChatHistoryStore::from_flat_array(arr)
+            } else {
+                serde_json::from_str(&raw_history)
+                    .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?
+            };
+
+        // Build messages from the path to the active leaf.
+        let path = store
+            .path_to_leaf(store.active_leaf_id)
+            .ok_or_else(|| anyhow!("active_leaf_id {} not found in tree", store.active_leaf_id))?;
+        let mut messages: Vec<ChatMessage> = path.iter().map(|n| n.message.clone()).collect();
 
         // 3. Resolve effective backend. Reject the unsupported switch.
         let stored_backend = insight.backend.clone();
@@ -509,8 +459,22 @@ impl InsightChatService {
         // 9. Persist. Append mode rewrites the JSON blob in place; amend
         //    mode regenerates the title and inserts a new insight row,
         //    relying on store_insight to flip prior rows' is_current=false.
-        let json = serde_json::to_string(&messages)
-            .map_err(|e| anyhow!("failed to serialize chat history: {}", e))?;
+        // Append new messages (this turn's user + assistant exchanges) as
+        // tree nodes chained from the previous active_leaf_id.
+        let path_len = path.len();
+        let new_messages = messages[path_len..].to_vec();
+        let mut parent_id = Some(store.active_leaf_id);
+        for msg in &new_messages {
+            let new_id = store.append_node(parent_id, msg.clone());
+            parent_id = Some(new_id);
+        }
+        // Update active_leaf_id to the last node appended this turn.
+        if let Some(last_id) = parent_id {
+            store.active_leaf_id = last_id;
+        }
+
+        let json = serde_json::to_string(&store)
+            .map_err(|e| anyhow!("failed to serialize chat tree: {}", e))?;
 
         let mut amended_insight_id: Option<i32> = None;
         if req.amend {
@@ -597,11 +561,14 @@ impl InsightChatService {
         })
     }
 
-    /// Truncate the stored conversation so the rendered message at
-    /// `discard_from_rendered_index` (and everything after it — including
-    /// the tool-call scaffolding that produced a discarded assistant reply)
-    /// is removed. The initial user turn cannot be discarded; attempting to
-    /// do so returns an error.
+    /// Rewind the conversation to the rendered message at
+    /// `discard_from_rendered_index` by setting `active_leaf_id` to the
+    /// node just before the discarded message. Unlike the legacy flat-array
+    /// approach, this preserves all fork branches — the discarded path
+    /// remains accessible as an alternate branch.
+    ///
+    /// The initial user turn cannot be discarded; attempting to do so
+    /// returns an error.
     ///
     /// Holds the per-file chat mutex so it serialises with `chat_turn`.
     pub async fn rewind_history(
@@ -636,15 +603,30 @@ impl InsightChatService {
             .training_messages
             .as_ref()
             .ok_or_else(|| anyhow!("insight has no chat history"))?;
-        let messages: Vec<ChatMessage> = serde_json::from_str(raw_history)
-            .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?;
 
-        let cut_at = find_raw_cut(&messages, discard_from_rendered_index)
+        let mut store: ChatHistoryStore =
+            if let Ok(arr) = serde_json::from_str::<Vec<ChatMessage>>(raw_history) {
+                ChatHistoryStore::from_flat_array(arr)
+            } else {
+                serde_json::from_str(raw_history)
+                    .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?
+            };
+
+        let path = store
+            .path_to_leaf(store.active_leaf_id)
+            .ok_or_else(|| anyhow!("active_leaf_id not found in tree"))?;
+
+        let (_rendered, _turn_count, node_ids, _fork_info) = render_tree_path(&store, &path);
+
+        // The last kept rendered message is at index `discard_from_rendered_index - 1`.
+        let last_kept_idx = discard_from_rendered_index - 1;
+        let new_active_leaf_id = *node_ids
+            .get(last_kept_idx)
             .ok_or_else(|| anyhow!("discard_from_rendered_index out of range"))?;
 
-        let truncated = &messages[..cut_at];
-        let json = serde_json::to_string(truncated)
-            .map_err(|e| anyhow!("failed to serialize truncated history: {}", e))?;
+        store.active_leaf_id = new_active_leaf_id;
+        let json = serde_json::to_string(&store)
+            .map_err(|e| anyhow!("failed to serialize tree: {}", e))?;
 
         let dao = self.insight_dao.clone();
         let rows = retry_with_backoff("update_training_messages", 3, || {
@@ -652,7 +634,7 @@ impl InsightChatService {
             let mut d = dao.lock().expect("Unable to lock InsightDao");
             d.update_training_messages(&cx, library_id, &normalized, &json)
         })
-        .map_err(|e| anyhow!("failed to persist truncated history: {:?}", e))?;
+        .map_err(|e| anyhow!("failed to persist rewound history: {:?}", e))?;
         if rows == 0 {
             log::warn!(
                 "update_training_messages (rewind) updated 0 rows for {} (lib {}), \
@@ -664,6 +646,108 @@ impl InsightChatService {
         Ok(())
     }
 
+    /// Switch the active branch to the given leaf ID. The new branch becomes
+    /// the active conversation path; the previous active branch becomes a
+    /// regular fork. If the leaf ID doesn't exist, returns an error.
+    pub async fn switch_branch(
+        &self,
+        library_id: i32,
+        file_path: &str,
+        leaf_id: u64,
+    ) -> Result<()> {
+        let normalized = normalize_path(file_path);
+
+        let lock_key = (library_id, normalized.clone());
+        let entry_lock = {
+            let mut locks = self.chat_locks.lock().await;
+            locks
+                .entry(lock_key.clone())
+                .or_insert_with(|| Arc::new(TokioMutex::new(())))
+                .clone()
+        };
+        let _guard = entry_lock.lock().await;
+
+        let insight = {
+            let cx = opentelemetry::Context::new();
+            let mut dao = self.insight_dao.lock().expect("Unable to lock InsightDao");
+            dao.get_current_insight_for_library(&cx, library_id, &normalized)
+                .map_err(|e| anyhow!("failed to load insight: {:?}", e))?
+                .ok_or_else(|| anyhow!("no insight found for path"))?
+        };
+        let raw_history = insight
+            .training_messages
+            .as_ref()
+            .ok_or_else(|| anyhow!("insight has no chat history"))?;
+
+        let mut store: ChatHistoryStore =
+            if let Ok(arr) = serde_json::from_str::<Vec<ChatMessage>>(raw_history) {
+                ChatHistoryStore::from_flat_array(arr)
+            } else {
+                serde_json::from_str(raw_history)
+                    .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?
+            };
+
+        // Validate the leaf_id exists and is a leaf.
+        if !store.nodes.iter().any(|n| n.id == leaf_id) {
+            bail!("branch_id {} not found in tree", leaf_id);
+        }
+        if !store.children_of(leaf_id).is_empty() {
+            bail!("branch_id {} is not a leaf node", leaf_id);
+        }
+
+        store.active_leaf_id = leaf_id;
+        let json = serde_json::to_string(&store)
+            .map_err(|e| anyhow!("failed to serialize tree: {}", e))?;
+
+        let dao = self.insight_dao.clone();
+        let rows = retry_with_backoff("update_training_messages", 3, || {
+            let cx = opentelemetry::Context::new();
+            let mut d = dao.lock().expect("Unable to lock InsightDao");
+            d.update_training_messages(&cx, library_id, &normalized, &json)
+        })
+        .map_err(|e| anyhow!("failed to persist branch switch: {:?}", e))?;
+        if rows == 0 {
+            log::warn!(
+                "update_training_messages (switch_branch) updated 0 rows for {} (lib {}), \
+                 concurrent regenerate likely flipped is_current",
+                normalized,
+                library_id
+            );
+        }
+        Ok(())
+    }
+
+    /// Return branch leaf metadata (id, snippet, message_count) and the
+    /// active leaf ID for a photo's conversation tree.
+    pub fn get_branches(
+        &self,
+        library_id: i32,
+        file_path: &str,
+    ) -> Result<(Vec<BranchLeafInfo>, u64)> {
+        let normalized = normalize_path(file_path);
+        let cx = opentelemetry::Context::new();
+        let mut dao = self.insight_dao.lock().expect("Unable to lock InsightDao");
+        let insight = dao
+            .get_current_insight_for_library(&cx, library_id, &normalized)
+            .map_err(|e| anyhow!("failed to load insight: {:?}", e))?
+            .ok_or_else(|| anyhow!("no insight found for path"))?;
+
+        let raw = insight
+            .training_messages
+            .as_ref()
+            .ok_or_else(|| anyhow!("insight has no chat history"))?;
+
+        let store: ChatHistoryStore = if let Ok(arr) = serde_json::from_str::<Vec<ChatMessage>>(raw)
+        {
+            ChatHistoryStore::from_flat_array(arr)
+        } else {
+            serde_json::from_str(raw)
+                .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?
+        };
+
+        Ok((store.leaves_with_info(), store.active_leaf_id))
+    }
+
     /// Streaming variant of `chat_turn`. Emits user-facing events as the
     /// conversation progresses: iteration starts, tool dispatch + result,
     /// text deltas from the final assistant reply, and a terminal `Done`
@@ -828,8 +912,21 @@ impl InsightChatService {
         let raw_history = insight.training_messages.as_ref().ok_or_else(|| {
             anyhow!("insight has no chat history; regenerate this insight in agentic mode")
         })?;
-        let mut messages: Vec<ChatMessage> = serde_json::from_str(raw_history)
-            .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?;
+
+        // Backward-compatible: flat array (old) vs tree (new).
+        let mut store: ChatHistoryStore =
+            if let Ok(arr) = serde_json::from_str::<Vec<ChatMessage>>(raw_history) {
+                ChatHistoryStore::from_flat_array(arr)
+            } else {
+                serde_json::from_str(raw_history)
+                    .map_err(|e| anyhow!("failed to deserialize chat history: {}", e))?
+            };
+
+        let path = store
+            .path_to_leaf(store.active_leaf_id)
+            .ok_or_else(|| anyhow!("active_leaf_id {} not found in tree", store.active_leaf_id))?;
+        let path_len = path.len();
+        let mut messages: Vec<ChatMessage> = path.iter().map(|n| n.message.clone()).collect();
 
         let stored_backend = insight.backend.clone();
         let effective_backend = req
@@ -931,8 +1028,19 @@ impl InsightChatService {
             restore_system_prompt_override(&mut messages, override_stash);
         }
 
-        let json = serde_json::to_string(&messages)
-            .map_err(|e| anyhow!("failed to serialize chat history: {}", e))?;
+        // Append new messages as tree nodes.
+        let new_messages = messages[path_len..].to_vec();
+        let mut parent_id = Some(store.active_leaf_id);
+        for msg in &new_messages {
+            let new_id = store.append_node(parent_id, msg.clone());
+            parent_id = Some(new_id);
+        }
+        if let Some(last_id) = parent_id {
+            store.active_leaf_id = last_id;
+        }
+
+        let json = serde_json::to_string(&store)
+            .map_err(|e| anyhow!("failed to serialize chat tree: {}", e))?;
 
         let mut amended_insight_id: Option<i32> = None;
         if req.amend {
@@ -2164,6 +2272,7 @@ pub enum ChatStreamEvent {
 /// Is this raw message visible in the rendered transcript? Must match
 /// `load_history`'s filter exactly — `find_raw_cut` depends on it to map
 /// rendered indices back to raw positions.
+#[allow(dead_code)]
 fn is_rendered(m: &ChatMessage) -> bool {
     match m.role.as_str() {
         "user" => true,
@@ -2189,6 +2298,7 @@ fn is_rendered(m: &ChatMessage) -> bool {
 /// regenerating after a failed turn — its optimistic user bubble lives at
 /// the index just past the server's persisted history. Strictly past the end
 /// (`discard > rendered_count`) returns `None`.
+#[allow(dead_code)]
 pub(crate) fn find_raw_cut(
     messages: &[ChatMessage],
     discard_from_rendered_index: usize,
@@ -2349,6 +2459,112 @@ pub(crate) fn restore_system_prompt_override(
     }
 }
 
+/// Fork indicator for a rendered message: position and total branches at
+/// the nearest divergence point upstream in the conversation tree.
+#[derive(Debug, Clone, serde::Serialize)]
+pub struct ForkInfo {
+    pub position: usize,
+    pub total: usize,
+}
+
+/// Render a path of tree nodes into the UI-friendly `RenderedMessage` format,
+/// matching the same filtering logic as the legacy flat-array `load_history`.
+/// Returns `(rendered_messages, turn_count, node_ids, fork_info)` where
+/// `node_ids[i]` is the tree node ID that produced `rendered_messages[i]` and
+/// `fork_info[i]` carries the nearest divergence point at or upstream of it.
+///
+/// Fork detection walks *every* node in the path — including the non-rendered
+/// tool-dispatch/tool/system nodes — so a fork whose diverging child is a
+/// tool-dispatch assistant (empty content + tool_calls) is still attributed to
+/// the next rendered message. Detecting forks only on rendered nodes would miss
+/// regenerations where the model replied with a tool call.
+fn render_tree_path(
+    store: &ChatHistoryStore,
+    path: &[&StoredChatNode],
+) -> (Vec<RenderedMessage>, usize, Vec<u64>, Vec<Option<ForkInfo>>) {
+    let mut rendered = Vec::new();
+    let mut user_turns_seen = 0usize;
+    let mut assistant_turns_seen = 0usize;
+    let mut pending_tools: Vec<ToolInvocation> = Vec::new();
+    let mut pending_calls: std::collections::VecDeque<(String, serde_json::Value)> =
+        std::collections::VecDeque::new();
+    let mut node_ids: Vec<u64> = Vec::new();
+    let mut fork_info: Vec<Option<ForkInfo>> = Vec::new();
+    // Nearest divergence seen so far, carried forward across nodes (rendered or
+    // not) so every message at/after a fork shows the indicator.
+    let mut last_fork: Option<ForkInfo> = None;
+
+    for node in path {
+        // Update the carried fork BEFORE rendering: a non-rendered fork child
+        // (e.g. a tool-dispatch assistant) must colour the rendered message
+        // that follows it.
+        if let Some((position, total)) = store.fork_at_node(node.id) {
+            last_fork = Some(ForkInfo { position, total });
+        }
+        let msg = &node.message;
+        match msg.role.as_str() {
+            "system" => continue,
+            "tool" => {
+                if let Some((name, arguments)) = pending_calls.pop_front() {
+                    let (result, result_truncated) = truncate_tool_result(&msg.content);
+                    pending_tools.push(ToolInvocation {
+                        name,
+                        arguments,
+                        result,
+                        result_truncated,
+                    });
+                }
+            }
+            "assistant" => {
+                let has_tool_calls = msg
+                    .tool_calls
+                    .as_ref()
+                    .map(|c| !c.is_empty())
+                    .unwrap_or(false);
+                if has_tool_calls && msg.content.trim().is_empty() {
+                    if let Some(ref tcs) = msg.tool_calls {
+                        for tc in tcs {
+                            pending_calls.push_back((
+                                tc.function.name.clone(),
+                                tc.function.arguments.clone(),
+                            ));
+                        }
+                    }
+                    continue;
+                }
+                assistant_turns_seen += 1;
+                let tools = std::mem::take(&mut pending_tools);
+                pending_calls.clear();
+                rendered.push(RenderedMessage {
+                    role: "assistant".to_string(),
+                    content: msg.content.clone(),
+                    is_initial: false,
+                    tools,
+                });
+                node_ids.push(node.id);
+                fork_info.push(last_fork.clone());
+            }
+            "user" => {
+                let is_initial = user_turns_seen == 0;
+                user_turns_seen += 1;
+                pending_tools.clear();
+                pending_calls.clear();
+                rendered.push(RenderedMessage {
+                    role: "user".to_string(),
+                    content: msg.content.clone(),
+                    is_initial,
+                    tools: Vec::new(),
+                });
+                node_ids.push(node.id);
+                fork_info.push(last_fork.clone());
+            }
+            _ => continue,
+        }
+    }
+
+    (rendered, assistant_turns_seen, node_ids, fork_info)
+}
+
 /// View returned to clients for chat-UI rendering.
 #[derive(Debug)]
 pub struct HistoryView {
@@ -2356,6 +2572,15 @@ pub struct HistoryView {
     pub turn_count: usize,
     pub model_version: String,
     pub backend: String,
+    /// ID of the leaf node the current view is anchored to.
+    pub active_leaf_id: u64,
+    /// ID of the branch leaf being viewed. Equals `active_leaf_id` when
+    /// viewing the active branch, or the `branch_id` parameter when
+    /// viewing an alternate branch.
+    pub viewing_branch_id: u64,
+    /// Fork info for each rendered message. `None` means no divergence at
+    /// or before that point in the tree.
+    pub fork_info: Vec<Option<ForkInfo>>,
 }
 
 #[derive(Debug)]
@@ -2581,6 +2806,61 @@ mod tests {
         assert!(!dropped);
     }
 
+    #[test]
+    fn render_tree_path_detects_fork_at_tool_dispatch_child() {
+        // Regression: a regeneration where the model replies with a tool call
+        // forks at the user turn, but the diverging children are non-rendered
+        // tool-dispatch assistant nodes. The indicator must still surface on
+        // the rendered final reply that follows — earlier code only inspected
+        // rendered nodes and silently dropped the fork, orphaning the branch.
+        let mut store = ChatHistoryStore::empty();
+        let u = store.append_node(None, ChatMessage::user("q1"));
+        // Original branch: tool-dispatch → tool result → final reply.
+        let d1 = store.append_node(Some(u), assistant_with_tool_call("lookup"));
+        let t1 = store.append_node(Some(d1), ChatMessage::tool_result("data1"));
+        let _a1 = store.append_node(Some(t1), assistant_text("answer 1"));
+        // Regenerated branch: also a tool reply, forking at the user turn.
+        let d2 = store.append_node(Some(u), assistant_with_tool_call("lookup"));
+        let t2 = store.append_node(Some(d2), ChatMessage::tool_result("data2"));
+        let a2 = store.append_node(Some(t2), assistant_text("answer 2"));
+        store.active_leaf_id = a2;
+
+        let path = store.path_to_leaf(a2).expect("path to active leaf");
+        let (rendered, _turns, node_ids, fork_info) = render_tree_path(&store, &path);
+
+        assert_eq!(rendered.len(), 2, "user turn + final reply are rendered");
+        assert_eq!(rendered[1].content, "answer 2");
+        assert_eq!(node_ids, vec![u, a2]);
+        assert!(
+            fork_info[0].is_none(),
+            "user turn sits before the divergence"
+        );
+        let f = fork_info[1]
+            .as_ref()
+            .expect("fork indicator carried onto the rendered reply");
+        assert_eq!((f.position, f.total), (2, 2));
+    }
+
+    #[test]
+    fn render_tree_path_detects_fork_at_rendered_child() {
+        // Non-tool case: two direct assistant replies to the same user turn.
+        // The diverging child is itself rendered, so the indicator lands on it.
+        let mut store = ChatHistoryStore::empty();
+        let u = store.append_node(None, ChatMessage::user("q1"));
+        let _a1 = store.append_node(Some(u), assistant_text("answer 1"));
+        let a2 = store.append_node(Some(u), assistant_text("answer 2"));
+        store.active_leaf_id = a2;
+
+        let path = store.path_to_leaf(a2).expect("path to active leaf");
+        let (rendered, _turns, _node_ids, fork_info) = render_tree_path(&store, &path);
+
+        assert_eq!(rendered.len(), 2);
+        let f = fork_info[1]
+            .as_ref()
+            .expect("fork indicator on the divergent rendered reply");
+        assert_eq!((f.position, f.total), (2, 2));
+    }
+
     #[test]
     fn rewind_strips_assistant_and_tool_scaffolding() {
         // Rendered: [user1, asst1, user2, asst2] → cut at rendered index 3

src/ai/llm_client.rs

@@ -171,6 +171,178 @@ pub struct ModelCapabilities {
     pub has_tool_calling: bool,
 }
 
+/// A single node in the conversation tree. Wraps ChatMessage with structural
+/// metadata so the tree can be persisted as JSON in `training_messages`.
+#[derive(Serialize, Deserialize, Clone, Debug)]
+pub struct StoredChatNode {
+    pub id: u64,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub parent_id: Option<u64>,
+    pub message: ChatMessage,
+}
+
+/// Top-level container for tree-based chat history. Stored as JSON in the
+/// `training_messages` column. Backward-compatible reader detects whether
+/// the stored value is a flat array (old format) or this object (new format).
+#[derive(Serialize, Deserialize, Clone, Debug)]
+pub struct ChatHistoryStore {
+    pub nodes: Vec<StoredChatNode>,
+    pub active_leaf_id: u64,
+}
+
+impl ChatHistoryStore {
+    /// Create an empty store. Test-only: production stores are always built
+    /// from `from_flat_array` or deserialized from `training_messages`.
+    #[cfg(test)]
+    pub fn empty() -> Self {
+        Self {
+            nodes: Vec::new(),
+            active_leaf_id: 0,
+        }
+    }
+
+    /// Convert a flat `Vec<ChatMessage>` (old format) into a tree.
+    /// Each message becomes a node chained by parent_id.
+    pub fn from_flat_array(messages: Vec<ChatMessage>) -> Self {
+        let mut nodes = Vec::with_capacity(messages.len());
+        let mut prev_id: Option<u64> = None;
+        for (i, msg) in messages.into_iter().enumerate() {
+            let id = (i + 1) as u64;
+            nodes.push(StoredChatNode {
+                id,
+                parent_id: prev_id,
+                message: msg,
+            });
+            prev_id = Some(id);
+        }
+        let active_leaf_id = prev_id.unwrap_or(0);
+        Self {
+            nodes,
+            active_leaf_id,
+        }
+    }
+
+    /// Allocate the next node ID (max existing + 1, or 1 if empty).
+    pub fn next_id(&self) -> u64 {
+        self.nodes.iter().map(|n| n.id).max().unwrap_or(0) + 1
+    }
+
+    /// Append a new node as a child of the given parent. Returns the new node's id.
+    pub fn append_node(&mut self, parent_id: Option<u64>, message: ChatMessage) -> u64 {
+        let id = self.next_id();
+        self.nodes.push(StoredChatNode {
+            id,
+            parent_id,
+            message,
+        });
+        id
+    }
+
+    /// Traverse from root to the given leaf_id, returning the path in order.
+    /// Returns None if the leaf_id doesn't exist.
+    pub fn path_to_leaf(&self, leaf_id: u64) -> Option<Vec<&StoredChatNode>> {
+        // Build a map from id to node for fast lookup.
+        let node_map: std::collections::HashMap<u64, usize> = self
+            .nodes
+            .iter()
+            .enumerate()
+            .map(|(i, n)| (n.id, i))
+            .collect();
+        let &_leaf_idx = node_map.get(&leaf_id)?;
+
+        let mut path_indices = Vec::new();
+        let mut current_id = leaf_id;
+        while let Some(&idx) = node_map.get(&current_id) {
+            path_indices.push(idx);
+            match self.nodes[idx].parent_id {
+                Some(pid) => current_id = pid,
+                None => break,
+            }
+        }
+        path_indices.reverse();
+        Some(path_indices.into_iter().map(|i| &self.nodes[i]).collect())
+    }
+
+    /// All direct children of a given node, ordered by id (chronological).
+    pub fn children_of(&self, node_id: u64) -> Vec<&StoredChatNode> {
+        self.nodes
+            .iter()
+            .filter(|n| n.parent_id == Some(node_id))
+            .collect()
+    }
+
+    /// For a given node, return (position, total) where:
+    /// - total = number of children of the parent of this node (i.e., siblings + self)
+    /// - position = 1-based rank of this node among siblings, ordered by id
+    ///
+    /// Returns None if the node has no parent (root) or the parent has 0 or 1 child.
+    pub fn fork_at_node(&self, node_id: u64) -> Option<(usize, usize)> {
+        let node = self.nodes.iter().find(|n| n.id == node_id)?;
+        let parent_id = node.parent_id?;
+        let children = self.children_of(parent_id);
+        let total = children.len();
+        if total <= 1 {
+            return None;
+        }
+        let position = children.iter().position(|c| c.id == node_id)? + 1;
+        Some((position, total))
+    }
+
+    /// All leaf node IDs (nodes with no children).
+    pub fn leaves(&self) -> Vec<u64> {
+        let child_ids: std::collections::HashSet<u64> =
+            self.nodes.iter().filter_map(|n| n.parent_id).collect();
+        self.nodes
+            .iter()
+            .filter(|n| !child_ids.contains(&n.id))
+            .map(|n| n.id)
+            .collect()
+    }
+
+    /// The parent of the given node, if any. Test-only helper.
+    #[cfg(test)]
+    pub fn parent_of(&self, node_id: u64) -> Option<&StoredChatNode> {
+        let node = self.nodes.iter().find(|n| n.id == node_id)?;
+        let parent_id = node.parent_id?;
+        self.nodes.iter().find(|n| n.id == parent_id)
+    }
+
+    /// Return all leaf nodes with metadata: id, snippet (first 100 chars of
+    /// the first user-visible message after the divergence point), and
+    /// message_count (number of nodes in the path from root to leaf).
+    pub fn leaves_with_info(&self) -> Vec<BranchLeafInfo> {
+        let leaf_ids = self.leaves();
+        leaf_ids
+            .into_iter()
+            .map(|leaf_id| {
+                let path = self.path_to_leaf(leaf_id).unwrap_or_default();
+                let message_count = path.len();
+                // Snippet: first 100 chars of the first user/assistant message
+                // (skip system messages).
+                let snippet = path
+                    .iter()
+                    .find(|n| n.message.role != "system")
+                    .map(|n| n.message.content.chars().take(100).collect::<String>())
+                    .unwrap_or_default();
+                BranchLeafInfo {
+                    id: leaf_id,
+                    snippet,
+                    message_count,
+                }
+            })
+            .collect()
+    }
+}
+
+/// Metadata about a branch leaf, returned by the branches endpoint.
+#[derive(Serialize, Clone, Debug)]
+pub struct BranchLeafInfo {
+    pub id: u64,
+    #[serde(skip_serializing_if = "String::is_empty")]
+    pub snippet: String,
+    pub message_count: usize,
+}
+
 /// Strip a leading `<think>…</think>` reasoning block from model output.
 ///
 /// Thinking models sometimes emit chain-of-thought inside think tags before
@@ -221,4 +393,135 @@ mod tests {
         let raw = "<think>thinking forever";
         assert_eq!(strip_think_blocks(raw), raw);
     }
+
+    // ── ChatHistoryStore tree traversal tests ──────────────────────────
+
+    fn mk_msg(role: &str, content: &str) -> ChatMessage {
+        match role {
+            "system" => ChatMessage::system(content),
+            "user" => ChatMessage::user(content),
+            _ => ChatMessage {
+                role: role.to_string(),
+                content: content.to_string(),
+                tool_calls: None,
+                images: None,
+            },
+        }
+    }
+
+    #[test]
+    fn store_from_flat_array_chains_parent_ids() {
+        let msgs = vec![
+            mk_msg("system", "sys"),
+            mk_msg("user", "hello"),
+            mk_msg("assistant", "hi there"),
+        ];
+        let store = ChatHistoryStore::from_flat_array(msgs);
+        assert_eq!(store.nodes.len(), 3);
+        assert_eq!(store.nodes[0].id, 1);
+        assert_eq!(store.nodes[0].parent_id, None);
+        assert_eq!(store.nodes[1].parent_id, Some(1));
+        assert_eq!(store.nodes[2].parent_id, Some(2));
+        assert_eq!(store.active_leaf_id, 3);
+    }
+
+    #[test]
+    fn store_path_to_leaf_returns_ordered_path() {
+        let mut store = ChatHistoryStore::empty();
+        let n1 = store.append_node(None, mk_msg("user", "u1"));
+        let n2 = store.append_node(Some(n1), mk_msg("assistant", "a1"));
+        let n3 = store.append_node(Some(n2), mk_msg("user", "u2"));
+        let path = store.path_to_leaf(n3).unwrap();
+        assert_eq!(path.len(), 3);
+        assert_eq!(path[0].id, n1);
+        assert_eq!(path[1].id, n2);
+        assert_eq!(path[2].id, n3);
+    }
+
+    #[test]
+    fn store_path_to_leaf_returns_none_for_missing_id() {
+        let store = ChatHistoryStore::empty();
+        assert!(store.path_to_leaf(999).is_none());
+    }
+
+    #[test]
+    fn store_children_of_returns_direct_children() {
+        let mut store = ChatHistoryStore::empty();
+        let root = store.append_node(None, mk_msg("user", "root"));
+        let c1 = store.append_node(Some(root), mk_msg("assistant", "c1"));
+        let c2 = store.append_node(Some(root), mk_msg("user", "c2"));
+        let _d1 = store.append_node(Some(c1), mk_msg("assistant", "d1"));
+        let children = store.children_of(root);
+        assert_eq!(children.len(), 2);
+        assert_eq!(children[0].id, c1);
+        assert_eq!(children[1].id, c2);
+    }
+
+    #[test]
+    fn store_fork_at_node_detects_fork() {
+        let mut store = ChatHistoryStore::empty();
+        let root = store.append_node(None, mk_msg("user", "root"));
+        let c1 = store.append_node(Some(root), mk_msg("assistant", "c1"));
+        let c2 = store.append_node(Some(root), mk_msg("user", "c2"));
+        let c3 = store.append_node(Some(root), mk_msg("user", "c3"));
+        // c1 is position 1 of 3 siblings
+        assert_eq!(store.fork_at_node(c1), Some((1, 3)));
+        assert_eq!(store.fork_at_node(c2), Some((2, 3)));
+        assert_eq!(store.fork_at_node(c3), Some((3, 3)));
+        // root has no parent, so no fork
+        assert!(store.fork_at_node(root).is_none());
+    }
+
+    #[test]
+    fn store_fork_at_node_returns_none_for_single_child() {
+        let mut store = ChatHistoryStore::empty();
+        let root = store.append_node(None, mk_msg("user", "root"));
+        let _only = store.append_node(Some(root), mk_msg("assistant", "only"));
+        assert!(store.fork_at_node(2).is_none());
+    }
+
+    #[test]
+    fn store_leaves_returns_leaf_ids() {
+        let mut store = ChatHistoryStore::empty();
+        let root = store.append_node(None, mk_msg("user", "root"));
+        let c1 = store.append_node(Some(root), mk_msg("assistant", "c1"));
+        let c2 = store.append_node(Some(root), mk_msg("user", "c2"));
+        let _d1 = store.append_node(Some(c1), mk_msg("assistant", "d1"));
+        let leaves = store.leaves();
+        assert_eq!(leaves.len(), 2);
+        assert!(leaves.contains(&c2));
+        assert!(leaves.contains(&_d1));
+        assert!(!leaves.contains(&root));
+        assert!(!leaves.contains(&c1));
+    }
+
+    #[test]
+    fn store_parent_of_returns_parent() {
+        let mut store = ChatHistoryStore::empty();
+        let root = store.append_node(None, mk_msg("user", "root"));
+        let child = store.append_node(Some(root), mk_msg("assistant", "child"));
+        let parent = store.parent_of(child).unwrap();
+        assert_eq!(parent.id, root);
+        assert!(store.parent_of(root).is_none());
+    }
+
+    #[test]
+    fn store_next_id_increments() {
+        let mut store = ChatHistoryStore::empty();
+        assert_eq!(store.next_id(), 1);
+        store.append_node(None, mk_msg("user", "a"));
+        assert_eq!(store.next_id(), 2);
+        store.append_node(Some(1), mk_msg("assistant", "b"));
+        assert_eq!(store.next_id(), 3);
+    }
+
+    #[test]
+    fn store_append_node_sets_parent() {
+        let mut store = ChatHistoryStore::empty();
+        let parent = store.append_node(None, mk_msg("user", "parent"));
+        let child = store.append_node(Some(parent), mk_msg("assistant", "child"));
+        assert_eq!(store.nodes.len(), 2);
+        assert_eq!(store.nodes[1].parent_id, Some(parent));
+        assert_eq!(store.nodes[1].id, child);
+    }
 }

src/ai/mod.rs

@@ -25,12 +25,12 @@ pub use daily_summary_job::{
     generate_daily_summaries, strip_summary_boilerplate,
 };
 pub use handlers::{
-    cancel_generation_handler, cancel_turn_handler, chat_history_handler, chat_rewind_handler,
-    chat_stream_handler, chat_turn_handler, delete_insight_handler, export_training_data_handler,
-    generate_agentic_insight_handler, generate_insight_handler, generation_status_handler,
-    get_all_insights_handler, get_available_models_handler, get_insight_handler,
-    get_insight_history_handler, get_openrouter_models_handler, rate_insight_handler,
-    turn_async_handler, turn_replay_handler,
+    cancel_generation_handler, cancel_turn_handler, chat_branches_handler, chat_history_handler,
+    chat_rewind_handler, chat_stream_handler, chat_switch_branch_handler, chat_turn_handler,
+    delete_insight_handler, export_training_data_handler, generate_agentic_insight_handler,
+    generate_insight_handler, generation_status_handler, get_all_insights_handler,
+    get_available_models_handler, get_insight_handler, get_insight_history_handler,
+    get_openrouter_models_handler, rate_insight_handler, turn_async_handler, turn_replay_handler,
 };
 pub use insight_generator::InsightGenerator;
 pub use llamacpp::LlamaCppClient;

src/main.rs

@@ -377,6 +377,8 @@ fn main() -> std::io::Result<()> {
                 .service(ai::chat_stream_handler)
                 .service(ai::chat_history_handler)
                 .service(ai::chat_rewind_handler)
+                .service(ai::chat_branches_handler)
+                .service(ai::chat_switch_branch_handler)
                 .service(ai::turn_async_handler)
                 .service(ai::turn_replay_handler)
                 .service(ai::cancel_turn_handler)