FN-8901: restore duplicate conflict responses

Require matching named proposal claims before a deterministic duplicate can be reused.

- Restore 409 duplicate responses for ordinary or unrelated proposal intake.
- Preserve duplicate reuse for matching trusted recommendation claims.
- Cover active and terminal canonical task cases.
- Add a patch changeset for the intake fix.

Files changed:
 .changeset/fn-8901-deterministic-duplicate-409.md  |  7 ++++
 .../__tests__/routes-tasks-near-duplicate.test.ts  | 47 ++++++++++++++++++++++
 .../src/routes/register-task-workflow-routes.ts    | 11 ++++-
 3 files changed, 64 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-8901

Fusion-Task-Lineage: f9a41ceb-46aa-4589-9fdc-7f1cc35a01e1

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

.changeset/fn-8901-deterministic-duplicate-409.md

@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+summary: Restore duplicate conflict responses for ordinary task intake.
+category: fix
+dev: createTaskThroughGuardedIntake now requires a named proposalClaimId before trusted duplicate reuse.

packages/dashboard/src/__tests__/routes-tasks-near-duplicate.test.ts

@@ -268,6 +268,53 @@ describe("routes /api/tasks near duplicate", () => {
     expect(match.reason).toBeUndefined();
   });
 
+  it.each([
+    ["no proposal claim", undefined],
+    ["an unrelated proposal claim", "recommendation:FN-1:rec-1"],
+  ])("blocks an ordinary deterministic duplicate with %s", async (_label, proposalClaimId) => {
+    const title = routeIncoming.title;
+    const description = routeIncoming.description;
+    const fingerprint = core.computeContentFingerprint({ title, description }) as string;
+    const { app, tasks } = buildApp([
+      mkTask({
+        id: "FN-8002",
+        title,
+        description,
+        column: "todo",
+        proposalClaimId,
+        source: { sourceType: "api", sourceMetadata: { contentFingerprint: fingerprint } },
+      }),
+    ]);
+
+    const res = await performRequest(app, "POST", "/api/tasks", JSON.stringify({ title, description }), { "content-type": "application/json" });
+
+    expect(res.status).toBe(409);
+    const match = (res.body as { details: { matches: Array<{ id: string; deterministic?: boolean; reason?: string }> } }).details.matches[0];
+    expect(match).toMatchObject({ id: "FN-8002", deterministic: true });
+    expect(match.reason).toBeUndefined();
+    expect(tasks).toHaveLength(1);
+  });
+
+  it.each(["done", "archived"] as const)("allows an ordinary deterministic create when the canonical is %s", async (column) => {
+    const title = routeIncoming.title;
+    const description = routeIncoming.description;
+    const fingerprint = core.computeContentFingerprint({ title, description }) as string;
+    const { app, tasks } = buildApp([
+      mkTask({
+        id: "FN-8003",
+        title,
+        description,
+        column,
+        source: { sourceType: "api", sourceMetadata: { contentFingerprint: fingerprint } },
+      }),
+    ]);
+
+    const res = await performRequest(app, "POST", "/api/tasks", JSON.stringify({ title, description }), { "content-type": "application/json" });
+
+    expect(res.status).toBe(201);
+    expect(tasks).toHaveLength(2);
+  });
+
   it("fails open when intent extraction throws", async () => {
     const extractorSpy = vi.spyOn(core, "extractIntentSignature").mockImplementation(() => {
       throw new Error("boom");

packages/dashboard/src/routes/register-task-workflow-routes.ts

@@ -1910,7 +1910,11 @@ export function registerTaskWorkflowRoutes(ctx: ApiRoutesContext, deps: TaskWork
         if (
           deterministicGuard.action === "duplicate"
           && deterministicGuard.existing
-          && deterministicGuard.existing.proposalClaimId === trusted?.proposalClaimId
+          && typeof trusted?.proposalClaimId === "string"
+          && trusted.proposalClaimId.length > 0
+          && typeof deterministicGuard.existing.proposalClaimId === "string"
+          && deterministicGuard.existing.proposalClaimId.length > 0
+          && deterministicGuard.existing.proposalClaimId === trusted.proposalClaimId
         ) {
           /*
           FNXC:TaskRecommendations 2026-08-08-05:27:
@@ -1918,6 +1922,11 @@ export function registerTaskWorkflowRoutes(ctx: ApiRoutesContext, deps: TaskWork
           A second process can observe its newly-created child while checking the normal content
           guard; reuse that immutable same-recommendation winner and repair the parent link rather
           than surfacing the ordinary duplicate conflict reserved for distinct recommendations.
+
+          FNXC:TaskRecommendations 2026-08-12-00:58:
+          Reuse is reserved for a named proposal claim on both the trusted request and canonical.
+          Comparing absent ids made every ordinary deterministic duplicate return 200 and skipped
+          the duplicate-blocker classification that preserves legitimate done or archived creates.
           */
           const trustedCreateResult = await trusted?.onCreated?.(deterministicGuard.existing);
           res.status(200).json(trusted?.responseForCreated?.(deterministicGuard.existing, trustedCreateResult) ?? deterministicGuard.existing);