diff --git a/extensions/thread-ownership/index.test.ts b/extensions/thread-ownership/index.test.ts
index b195c9e9581..27bafe3fbc9 100644
--- a/extensions/thread-ownership/index.test.ts
+++ b/extensions/thread-ownership/index.test.ts
@@ -122,6 +122,30 @@ describe("thread-ownership plugin", () => {
       );
     });
 
+    it("canonicalizes non-canonical Slack targets when shared conversationId is missing", async () => {
+      vi.mocked(globalThis.fetch).mockResolvedValue(
+        new Response(JSON.stringify({ owner: "test-agent" }), { status: 200 }),
+      );
+
+      const result = await hooks.message_sending(
+        {
+          content: "hello",
+          replyToId: "1234.5678",
+          to: "channel:C123",
+        },
+        { channelId: "slack", conversationId: "" },
+      );
+
+      expect(result).toBeUndefined();
+      expect(globalThis.fetch).toHaveBeenCalledWith(
+        "http://localhost:8750/api/v1/ownership/C123/1234.5678",
+        expect.objectContaining({
+          method: "POST",
+          body: JSON.stringify({ agent_id: "test-agent" }),
+        }),
+      );
+    });
+
     it("cancels when thread owned by another agent", async () => {
       vi.mocked(globalThis.fetch).mockResolvedValue(
         new Response(JSON.stringify({ owner: "other-agent" }), { status: 409 }),
@@ -194,6 +218,29 @@ describe("thread-ownership plugin", () => {
       expect(globalThis.fetch).not.toHaveBeenCalled();
     });
 
+    it("canonicalizes inbound non-canonical metadata without shared conversation context", async () => {
+      await hooks.message_received(
+        {
+          content: "Hey @TestBot help me",
+          threadId: "9999.0003",
+          metadata: { channelId: "channel:C456" },
+        },
+        { channelId: "slack", conversationId: "" },
+      );
+
+      const result = await hooks.message_sending(
+        {
+          content: "Sure!",
+          replyToId: "9999.0003",
+          to: "C456",
+        },
+        { channelId: "slack", conversationId: "C456" },
+      );
+
+      expect(result).toBeUndefined();
+      expect(globalThis.fetch).not.toHaveBeenCalled();
+    });
+
     it("ignores @-mentions on non-slack channels", async () => {
       // Use a unique thread key so module-level state from other tests doesn't interfere.
       await hooks.message_received(
diff --git a/extensions/thread-ownership/index.ts b/extensions/thread-ownership/index.ts
index 03739a1c5c2..ab75ed22992 100644
--- a/extensions/thread-ownership/index.ts
+++ b/extensions/thread-ownership/index.ts
@@ -24,6 +24,16 @@ function resolveThreadToken(value: unknown): string {
   return typeof value === "string" || typeof value === "number" ? String(value) : "";
 }
 
+function resolveSlackConversationId(value: unknown): string {
+  const raw = normalizeOptionalString(value) ?? "";
+  if (!raw) {
+    return "";
+  }
+  const trimmed = raw.trim();
+  const match = /^(?:slack:)?channel:(.+)$/i.exec(trimmed);
+  return match?.[1]?.trim() || trimmed;
+}
+
 function cleanExpiredMentions(): void {
   const now = Date.now();
   for (const [key, ts] of mentionedThreads) {
@@ -81,8 +91,8 @@ export default definePluginEntry({
         resolveThreadToken(event.metadata?.threadId) ||
         resolveThreadToken(event.metadata?.threadTs);
       const channelId =
-        normalizeOptionalString(ctx.conversationId) ||
-        normalizeOptionalString(event.metadata?.channelId) ||
+        resolveSlackConversationId(ctx.conversationId) ||
+        resolveSlackConversationId(event.metadata?.channelId) ||
         "";
       if (!threadTs || !channelId) {
         return;
@@ -108,9 +118,9 @@ export default definePluginEntry({
         resolveThreadToken(event.metadata?.threadId) ||
         resolveThreadToken(event.metadata?.threadTs);
       const channelId =
-        normalizeOptionalString(ctx.conversationId) ||
-        normalizeOptionalString(event.metadata?.channelId) ||
-        normalizeOptionalString(event.to) ||
+        resolveSlackConversationId(ctx.conversationId) ||
+        resolveSlackConversationId(event.metadata?.channelId) ||
+        resolveSlackConversationId(event.to) ||
         "";
       if (!threadTs || !channelId) {
         return undefined;