diff --git a/.changeset/fix-dm-thread-ts.md b/.changeset/fix-dm-thread-ts.md
new file mode 100644
index 000000000..e8d06e070
--- /dev/null
+++ b/.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`
diff --git a/packages/adapter-slack/src/index.test.ts b/packages/adapter-slack/src/index.test.ts
index 91cd09891..6bd2dfaf0 100644
--- a/packages/adapter-slack/src/index.test.ts
+++ b/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,
       })
     );
   });
@@ -2349,7 +2349,7 @@ describe("postEphemeral", () => {
     );
   });
 
-  it("omits thread_ts when empty", async () => {
+  it("normalizes empty threadTs to undefined", async () => {
     const adapter = createSlackAdapter({
       botToken: "xoxb-test-token",
       signingSecret: secret,
@@ -3303,7 +3303,7 @@ describe("postChannelMessage", () => {
     expect(client.chat.postMessage).toHaveBeenCalledWith(
       expect.objectContaining({
         channel: "C123",
-        thread_ts: "",
+        thread_ts: undefined,
       })
     );
   });
@@ -4982,3 +4982,61 @@ describe("reverse user lookup", () => {
     });
   });
 });
+
+// ============================================================================
+// Empty threadTs normalization tests
+// ============================================================================
+
+describe("stream with empty threadTs", () => {
+  it("throws ValidationError when threadTs is empty", async () => {
+    const adapter = createSlackAdapter({
+      botToken: "xoxb-test-token",
+      signingSecret: "test-signing-secret",
+      logger: mockLogger,
+    });
+
+    async function* emptyStream() {
+      yield "hello";
+    }
+
+    await expect(
+      adapter.stream("slack:C123:", emptyStream(), {
+        recipientUserId: "U123",
+        recipientTeamId: "T123",
+      })
+    ).rejects.toThrow(ValidationError);
+  });
+});
+
+describe("scheduleMessage with empty threadTs", () => {
+  it("normalizes empty threadTs to undefined", async () => {
+    const adapter = createSlackAdapter({
+      botToken: "xoxb-test-token",
+      signingSecret: "test-signing-secret",
+      logger: mockLogger,
+    });
+
+    mockClientMethod(
+      adapter,
+      "chat.scheduleMessage",
+      vi.fn().mockResolvedValue({
+        ok: true,
+        scheduled_message_id: "Q123",
+        post_at: Math.floor(Date.now() / 1000) + 3600,
+      })
+    );
+
+    const futureDate = new Date(Date.now() + 3600 * 1000);
+    await adapter.scheduleMessage("slack:C123:", "Scheduled msg", {
+      postAt: futureDate,
+    });
+
+    const client = getClient(adapter);
+    expect(client.chat.scheduleMessage).toHaveBeenCalledWith(
+      expect.objectContaining({
+        channel: "C123",
+        thread_ts: undefined,
+      })
+    );
+  });
+});
diff --git a/packages/adapter-slack/src/index.ts b/packages/adapter-slack/src/index.ts
index ebc419d4b..e7c7782b4 100644
--- a/packages/adapter-slack/src/index.ts
+++ b/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();