fix(slack): guard Slack API calls against empty threadTs to fix invalid_thread_ts

Preserve the intentional empty threadTs for top-level DMs (added in #39
for openDM() subscription matching) while preventing Slack API errors.

Instead of changing the threadTs logic, normalize empty threadTs to
undefined at the entry of each method that calls Slack APIs (postMessage,
postEphemeral, scheduleMessage, stream). This way:

- openDM() subscription matching continues to work (empty threadTs)
- Slack API calls receive undefined instead of "" (no invalid_thread_ts)

The stream method throws ValidationError on empty threadTs (matching
startTyping's early-return pattern) so TypeScript narrows correctly
without any `as string` casts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: chechunhsu

.changeset/fix-dm-thread-ts.md

@@ -0,0 +1,5 @@
+---
+"@chat-adapter/slack": patch
+---
+
+Fix DM messages failing with `invalid_thread_ts` by guarding Slack API calls with `threadTs || undefined`

packages/adapter-slack/src/index.test.ts

@@ -2253,7 +2253,7 @@ describe("postMessage", () => {
     expect(client.chat.postMessage).toHaveBeenCalledWith(
       expect.objectContaining({
         channel: "C123",
-        thread_ts: "",
+        thread_ts: undefined,
       })
     );
   });
@@ -3303,7 +3303,7 @@ describe("postChannelMessage", () => {
     expect(client.chat.postMessage).toHaveBeenCalledWith(
       expect.objectContaining({
         channel: "C123",
-        thread_ts: "",
+        thread_ts: undefined,
       })
     );
   });

packages/adapter-slack/src/index.ts

@@ -2161,14 +2161,16 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
     _message: AdapterPostableMessage
   ): Promise<RawMessage<unknown>> {
     const message = await this.resolveMessageMentions(_message, threadId);
-    const { channel, threadTs } = this.decodeThreadId(threadId);
+    const { channel, threadTs: rawThreadTs } = this.decodeThreadId(threadId);
+    // Normalize empty threadTs to undefined to avoid Slack API "invalid_thread_ts" errors
+    const threadTs = rawThreadTs || undefined;
 
     try {
       // Check for files to upload
       const files = extractFiles(message);
       if (files.length > 0) {
         // Upload files first (they're shared to the channel automatically)
-        await this.uploadFiles(files, channel, threadTs || undefined);
+        await this.uploadFiles(files, channel, threadTs);
 
         // If message only has files (no text/card), return early
         const hasText =
@@ -2302,7 +2304,8 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
     _message: AdapterPostableMessage
   ): Promise<EphemeralMessage> {
     const message = await this.resolveMessageMentions(_message, threadId);
-    const { channel, threadTs } = this.decodeThreadId(threadId);
+    const { channel, threadTs: rawThreadTs } = this.decodeThreadId(threadId);
+    const threadTs = rawThreadTs || undefined;
 
     try {
       // Check if message contains a card
@@ -2323,7 +2326,7 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
         const result = await this.client.chat.postEphemeral(
           this.withToken({
             channel,
-            thread_ts: threadTs || undefined,
+            thread_ts: threadTs,
             user: userId,
             text: fallbackText,
             blocks,
@@ -2356,7 +2359,7 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
         const result = await this.client.chat.postEphemeral(
           this.withToken({
             channel,
-            thread_ts: threadTs || undefined,
+            thread_ts: threadTs,
             user: userId,
             text: tableResult.text,
             blocks: tableResult.blocks,
@@ -2392,7 +2395,7 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
       const result = await this.client.chat.postEphemeral(
         this.withToken({
           channel,
-          thread_ts: threadTs || undefined,
+          thread_ts: threadTs,
           user: userId,
           text,
         })
@@ -2420,7 +2423,8 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
     options: { postAt: Date }
   ): Promise<ScheduledMessage> {
     const message = await this.resolveMessageMentions(_message, threadId);
-    const { channel, threadTs } = this.decodeThreadId(threadId);
+    const { channel, threadTs: rawThreadTs } = this.decodeThreadId(threadId);
+    const threadTs = rawThreadTs || undefined;
     const postAtUnix = Math.floor(options.postAt.getTime() / 1000);
 
     if (postAtUnix <= Math.floor(Date.now() / 1000)) {
@@ -2456,7 +2460,7 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
         const result = await this.client.chat.scheduleMessage({
           token,
           channel,
-          thread_ts: threadTs || undefined,
+          thread_ts: threadTs,
           post_at: postAtUnix,
           text: fallbackText,
           blocks,
@@ -2498,7 +2502,7 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
       const result = await this.client.chat.scheduleMessage({
         token,
         channel,
-        thread_ts: threadTs || undefined,
+        thread_ts: threadTs,
         post_at: postAtUnix,
         text,
         unfurl_links: false,
@@ -2929,7 +2933,16 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
         "Slack streaming requires recipientUserId and recipientTeamId in options"
       );
     }
-    const { channel, threadTs } = this.decodeThreadId(threadId);
+    const { channel, threadTs: rawThreadTs } = this.decodeThreadId(threadId);
+    // Normalize empty threadTs to undefined to avoid Slack API "invalid_thread_ts" errors
+    const threadTs = rawThreadTs || undefined;
+    if (!threadTs) {
+      this.logger.debug("Slack: stream skipped - no thread context");
+      throw new ValidationError(
+        "slack",
+        "Slack streaming requires a valid thread context (non-empty threadTs)"
+      );
+    }
     this.logger.debug("Slack: starting stream", { channel, threadTs });
 
     const token = this.getToken();