Matrix: tighten fallback resolution and ACP lookup

src/infra/outbound/message-action-params.ts

@@ -83,11 +83,6 @@ function normalizeMatrixThreadTarget(raw: string): string | undefined {
   return normalized || undefined;
 }
 
-function normalizeMatrixDirectUserTarget(raw: string): string | undefined {
-  const normalized = normalizeMatrixThreadTarget(raw);
-  return normalized?.startsWith("@") ? normalized : undefined;
-}
-
 export function resolveMatrixAutoThreadId(params: {
   to: string;
   toolContext?: ChannelThreadingToolContext;
@@ -101,15 +96,11 @@ export function resolveMatrixAutoThreadId(params: {
   if (!target || !currentChannel) {
     return undefined;
   }
+  // Matrix user:@ targets resolve to a DM room at send time, which can differ
+  // from the current room after DM recreation or stale m.direct ordering.
+  // Only auto-thread when the explicit room target already matches.
   if (target.toLowerCase() !== currentChannel.toLowerCase()) {
-    const directTarget = normalizeMatrixDirectUserTarget(params.to);
-    const currentDirectUserId = normalizeMatrixDirectUserTarget(context.currentDirectUserId ?? "");
-    if (!directTarget || !currentDirectUserId) {
-      return undefined;
-    }
-    if (directTarget.toLowerCase() !== currentDirectUserId.toLowerCase()) {
-      return undefined;
-    }
+    return undefined;
   }
   return context.currentThreadTs;
 }

src/infra/outbound/message-action-runner.threading.test.ts

@@ -309,7 +309,7 @@ describe("runMessageAction threading auto-injection", () => {
     expect(call?.ctx?.params?.threadId).toBe("$explicit");
   });
 
-  it("injects threadId for matching Matrix dm user target", async () => {
+  it("skips threadId for Matrix dm user targets until the resolved room matches", async () => {
     mockHandledSendAction();
 
     const call = await runThreadingAction({
@@ -322,8 +322,8 @@ describe("runMessageAction threading auto-injection", () => {
       toolContext: defaultMatrixDmToolContext,
     });
 
-    expect(call?.threadId).toBe("$thread");
-    expect(call?.ctx?.params?.threadId).toBe("$thread");
+    expect(call?.threadId).toBeUndefined();
+    expect(call?.ctx?.params?.threadId).toBeUndefined();
   });
 
   it("skips threadId for different Matrix dm user target", async () => {

src/plugin-sdk/matrix.ts

@@ -115,6 +115,10 @@ export {
   resolveMatrixLegacyFlatStoragePaths,
   sanitizeMatrixPathSegment,
 } from "../infra/matrix-storage-paths.js";
+export {
+  requiresExplicitMatrixDefaultAccount,
+  resolveMatrixDefaultOrOnlyAccountId,
+} from "../infra/matrix-account-selection.js";
 export {
   hasActionableMatrixMigration,
   hasPendingMatrixMigration,