fix: preserve thread title when model returns placeholder

packages/provider-core/src/server/textGenerationOperations.ts

@@ -149,8 +149,12 @@ export function fromRunner(name: string, run: Runner): ProviderTextGeneration {
         instructions: input.instructions,
       }),
     });
+    const title = sanitizeThreadTitle(generated.title);
     return {
-      title: sanitizeThreadTitle(generated.title),
+      title:
+        title.toLowerCase() === "title" && input.previousTitle
+          ? sanitizeThreadTitle(input.previousTitle)
+          : title,
       ...(generated.needsRefinement ? { needsRefinement: true } : {}),
     };
   });

packages/provider-core/src/server/textGenerationUtils.test.ts

@@ -1,6 +1,10 @@
-import { describe, expect, it } from "vite-plus/test";
+import { ProviderInstanceId } from "@t3tools/contracts";
+import * as Effect from "effect/Effect";
+import { it } from "@effect/vitest";
+import { describe, expect } from "vite-plus/test";
 
-import { sanitizeCommitSubject } from "./textGenerationUtils.ts";
+import { decodeJsonReply, fromRunner } from "./textGenerationOperations.ts";
+import { sanitizeCommitSubject, sanitizeThreadTitle } from "./textGenerationUtils.ts";
 
 describe("sanitizeCommitSubject", () => {
   // regression: #19181 truncation split grapheme clusters at the subject limit
@@ -14,3 +18,46 @@ describe("sanitizeCommitSubject", () => {
     expect(sanitizeCommitSubject(`${prefix}${character} more`)).toBe(prefix);
   });
 });
+
+describe("sanitizeThreadTitle", () => {
+  it("normalizes generated titles", () => {
+    expect(sanitizeThreadTitle("  Implement issue #140  ")).toBe("Implement issue #140");
+  });
+});
+
+describe("generateThreadTitle", () => {
+  it.effect.each([
+    { generated: "title", previousTitle: "Implement issue #140", expected: "Implement issue #140" },
+    {
+      generated: ' "TITLE" ',
+      previousTitle: "  Implement issue #140  ",
+      expected: "Implement issue #140",
+    },
+    { generated: "title", previousTitle: undefined, expected: "title" },
+    { generated: "title", previousTitle: "", expected: "title" },
+    {
+      generated: "Improve title generation",
+      previousTitle: "Old title",
+      expected: "Improve title generation",
+    },
+  ])(
+    "returns $expected for $generated with previous title $previousTitle",
+    ({ generated, previousTitle, expected }) =>
+      Effect.gen(function* () {
+        const provider = fromRunner("test", (request) =>
+          decodeJsonReply(
+            request,
+            "test",
+            JSON.stringify({ title: generated, needsRefinement: true }),
+          ),
+        );
+        const result = yield* provider.generateThreadTitle({
+          cwd: "/repo",
+          message: "Implement issue #140",
+          modelSelection: { instanceId: ProviderInstanceId.make("codex"), model: "gpt-5" },
+          previousTitle,
+        });
+        expect(result).toEqual({ title: expected, needsRefinement: true });
+      }),
+  );
+});