fix: preserve subagent thread routing (#1241)

Thanks @gnarco.

Co-authored-by: gnarco <gnarco@users.noreply.github.com>

src/auto-reply/reply/agent-runner-execution.ts

@@ -210,6 +210,8 @@ export async function runAgentTurnWithFallback(params: {
             sessionKey: params.sessionKey,
             messageProvider: params.sessionCtx.Provider?.trim().toLowerCase() || undefined,
             agentAccountId: params.sessionCtx.AccountId,
+            messageTo: params.sessionCtx.OriginatingTo ?? params.sessionCtx.To,
+            messageThreadId: params.sessionCtx.MessageThreadId ?? undefined,
             // Provider threading context for tool auto-injection
             ...buildThreadingToolContext({
               sessionCtx: params.sessionCtx,

src/auto-reply/reply/agent-runner-memory.ts

@@ -106,6 +106,8 @@ export async function runMemoryFlushIfNeeded(params: {
           sessionKey: params.sessionKey,
           messageProvider: params.sessionCtx.Provider?.trim().toLowerCase() || undefined,
           agentAccountId: params.sessionCtx.AccountId,
+          messageTo: params.sessionCtx.OriginatingTo ?? params.sessionCtx.To,
+          messageThreadId: params.sessionCtx.MessageThreadId ?? undefined,
           // Provider threading context for tool auto-injection
           ...buildThreadingToolContext({
             sessionCtx: params.sessionCtx,

src/auto-reply/reply/followup-runner.ts

@@ -145,6 +145,8 @@ export function createFollowupRunner(params: {
               sessionKey: queued.run.sessionKey,
               messageProvider: queued.run.messageProvider,
               agentAccountId: queued.run.agentAccountId,
+              messageTo: queued.originatingTo,
+              messageThreadId: queued.originatingThreadId,
               sessionFile: queued.run.sessionFile,
               workspaceDir: queued.run.workspaceDir,
               config: queued.run.config,

src/auto-reply/reply/get-reply-inline-actions.ts

@@ -168,6 +168,8 @@ export async function handleInlineActions(params: {
         agentSessionKey: sessionKey,
         agentChannel: channel,
         agentAccountId: (ctx as { AccountId?: string }).AccountId,
+        agentTo: ctx.OriginatingTo ?? ctx.To,
+        agentThreadId: ctx.MessageThreadId ?? undefined,
         agentDir,
         workspaceDir,
         config: cfg,

src/auto-reply/reply/route-reply.test.ts

@@ -255,6 +255,22 @@ describe("routeReply", () => {
     );
   });
 
+  it("uses threadId as threadTs for Slack when replyToId is missing", async () => {
+    mocks.sendMessageSlack.mockClear();
+    await routeReply({
+      payload: { text: "hi" },
+      channel: "slack",
+      to: "channel:C123",
+      threadId: "1710000000.9999",
+      cfg: {} as never,
+    });
+    expect(mocks.sendMessageSlack).toHaveBeenCalledWith(
+      "channel:C123",
+      "hi",
+      expect.objectContaining({ threadTs: "1710000000.9999" }),
+    );
+  });
+
   it("sends multiple mediaUrls (caption only on first)", async () => {
     mocks.sendMessageSlack.mockClear();
     await routeReply({

src/auto-reply/reply/route-reply.ts

@@ -100,6 +100,11 @@ export async function routeReply(params: RouteReplyParams): Promise<RouteReplyRe
     return { ok: false, error: "Reply routing aborted" };
   }
 
+  const resolvedReplyToId =
+    replyToId ??
+    (channelId === "slack" && threadId != null && threadId !== "" ? String(threadId) : undefined);
+  const resolvedThreadId = channelId === "slack" ? null : (threadId ?? null);
+
   try {
     // Provider docking: this is an execution boundary (we're about to send).
     // Keep the module cheap to import by loading outbound plumbing lazily.
@@ -110,8 +115,8 @@ export async function routeReply(params: RouteReplyParams): Promise<RouteReplyRe
       to,
       accountId: accountId ?? undefined,
       payloads: [normalized],
-      replyToId: replyToId ?? null,
-      threadId: threadId ?? null,
+      replyToId: resolvedReplyToId ?? null,
+      threadId: resolvedThreadId,
       abortSignal,
       mirror: params.sessionKey
         ? {

src/auto-reply/reply/session.ts

@@ -221,16 +221,20 @@ export async function initSessionState(params: {
   const lastChannelRaw = (ctx.OriginatingChannel as string | undefined) || baseEntry?.lastChannel;
   const lastToRaw = (ctx.OriginatingTo as string | undefined) || ctx.To || baseEntry?.lastTo;
   const lastAccountIdRaw = (ctx.AccountId as string | undefined) || baseEntry?.lastAccountId;
+  const lastThreadIdRaw =
+    (ctx.MessageThreadId as string | number | undefined) || baseEntry?.lastThreadId;
   const deliveryFields = normalizeSessionDeliveryFields({
     deliveryContext: {
       channel: lastChannelRaw,
       to: lastToRaw,
       accountId: lastAccountIdRaw,
+      threadId: lastThreadIdRaw,
     },
   });
   const lastChannel = deliveryFields.lastChannel ?? lastChannelRaw;
   const lastTo = deliveryFields.lastTo ?? lastToRaw;
   const lastAccountId = deliveryFields.lastAccountId ?? lastAccountIdRaw;
+  const lastThreadId = deliveryFields.lastThreadId ?? lastThreadIdRaw;
   sessionEntry = {
     ...baseEntry,
     sessionId,
@@ -261,6 +265,7 @@ export async function initSessionState(params: {
     lastChannel,
     lastTo,
     lastAccountId,
+    lastThreadId,
   };
   const metaPatch = deriveSessionMetaPatch({
     ctx: sessionCtxForState,