FN-8070: preserve task title input language

Ensure auto-generated task titles use the task description's language.

- Detect description language and add a confident language hint to title prompts
- Cover multilingual title generation and stale-model fallback behavior
- Document the language-preserving title-summary setting and add a patch changeset

Files changed:
 .changeset/fn-8070-title-summary-input-language.md |  7 ++
 docs/settings-reference.md                         |  2 +-
 packages/core/src/__tests__/ai-summarize.test.ts   | 87 ++++++++++++++++++----
 packages/core/src/ai-summarize.ts                  | 18 +++++
 4 files changed, 98 insertions(+), 16 deletions(-)

Fusion-Task-Id: FN-8070

Fusion-Task-Lineage: e003e2e0-eb0c-487e-83de-8a9b37ee1c02

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

.changeset/fn-8070-title-summary-input-language.md

@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+summary: Auto-summarized task titles now match the language of the task description.
+category: fix
+dev: The existing ai-summarize prompt uses a synchronous input-language hint with no extra model calls.

docs/settings-reference.md

@@ -674,7 +674,7 @@ GitLab configuration examples: leave both URL fields blank for GitLab.com (`http
 | `memoryBackupRetention` | `number` | `14` | Number of memory backups to retain. |
 | `memoryBackupDir` | `string` | `".fusion/backups/memory"` | Relative memory backup directory path. |
 | `memoryBackupScope` | `"project" \| "agents" \| "all"` | `"all"` | Backup scope: project memory, agent memory, or both. |
-| `autoSummarizeTitles` | `boolean` | `false` | Auto-generate titles for long untitled descriptions across dashboard/API task creation. Agent-created tasks from `fn_task_create` and `fn_delegate_task` always request summarization for untitled tasks, regardless of this setting. |
+| `autoSummarizeTitles` | `boolean` | `false` | Auto-generate titles for long untitled descriptions across dashboard/API task creation. Generated titles match the operator's input language from the task description. Agent-created tasks from `fn_task_create` and `fn_delegate_task` always request summarization for untitled tasks, regardless of this setting. |
 | `useAiMergeCommitSummary` | `boolean` | `true` | Use AI-generated merge commit summaries (subject + bullet body + diff-stat) instead of raw step-commit subject lists. |
 | `titleSummarizerProvider` | `string` | `undefined` | Provider for title summarization. |
 | `titleSummarizerModelId` | `string` | `undefined` | Model ID for title summarization. |

packages/core/src/__tests__/ai-summarize.test.ts

@@ -48,7 +48,8 @@ describe("ai-summarize", () => {
   // ── Constants ──────────────────────────────────────────────────────────────
 
   describe("constants", () => {
-    it("should have correct system prompt", () => {
+    it("should require titles in the description language", () => {
+      expect(SUMMARIZE_SYSTEM_PROMPT).toContain("SAME language as the task description");
       expect(SUMMARIZE_SYSTEM_PROMPT).toContain("max 60 characters");
       expect(SUMMARIZE_SYSTEM_PROMPT).toContain("title summarization");
     });
@@ -168,6 +169,7 @@ describe("ai-summarize", () => {
     it("should return null for descriptions <= 200 characters", async () => {
       const result = await summarizeTitle("Short description", "/tmp");
       expect(result).toBeNull();
+      expect(getFnAgentMock).not.toHaveBeenCalled();
     });
 
     it("should throw AiServiceError when AI service cannot process request", async () => {
@@ -187,6 +189,50 @@ describe("ai-summarize", () => {
       ).rejects.toThrow(AiServiceError);
     });
 
+    it.each([
+      ["French", "Nous devons améliorer la création des tâches afin que les opérateurs puissent suivre leurs demandes plus facilement. ".repeat(3), "Français"],
+      ["Spanish", "Necesitamos mejorar la creación de tareas para que los operadores puedan seguir sus solicitudes con mayor facilidad. ".repeat(3), "Español"],
+      ["Chinese", "我们需要改进任务创建流程，让操作员能够更轻松地跟踪他们的请求并完成日常工作。".repeat(8), "简体中文"],
+      ["Korean", "운영자가 요청을 더 쉽게 추적하고 일상 업무를 완료할 수 있도록 작업 생성 과정을 개선해야 합니다. ".repeat(4), "한국어"],
+    ])("instructs %s descriptions to retain their language", async (_language, description, languageHint) => {
+      const prompt = vi.fn().mockResolvedValue(undefined);
+      const createFnAgent = vi.fn().mockResolvedValue({
+        session: {
+          prompt,
+          dispose: vi.fn(),
+          state: {
+            messages: [{ role: "assistant", content: "Generated task title" }],
+          },
+        },
+      });
+      getFnAgentMock.mockResolvedValue(createFnAgent);
+
+      await expect(summarizeTitle(description, "/tmp")).resolves.toBe("Generated task title");
+
+      const agentOptions = createFnAgent.mock.calls[0][0] as { systemPrompt: string };
+      const wrappedPrompt = prompt.mock.calls[0][0] as string;
+      expect(agentOptions.systemPrompt).toContain("SAME language as the task description");
+      expect(wrappedPrompt).toContain("SAME language as the task description");
+      expect(wrappedPrompt).toContain(`Likely language: ${languageHint}`);
+      expect(wrappedPrompt).toContain("<description>");
+    });
+
+    it("keeps English descriptions in English without forcing another language", async () => {
+      const prompt = vi.fn().mockResolvedValue(undefined);
+      getFnAgentMock.mockResolvedValue(() => Promise.resolve({
+        session: {
+          prompt,
+          dispose: vi.fn(),
+          state: { messages: [{ role: "assistant", content: "Improve task creation" }] },
+        },
+      }));
+      const description = "We need to improve task creation so operators can follow requests more easily during their daily work. ".repeat(3);
+
+      await expect(summarizeTitle(description, "/tmp")).resolves.toBe("Improve task creation");
+      expect(SUMMARIZE_SYSTEM_PROMPT).toContain("SAME language as the task description");
+      expect(prompt.mock.calls[0][0]).toContain("Likely language: English");
+    });
+
     it("returns sanitized title when AI responds cleanly", async () => {
       const prompt = vi.fn().mockResolvedValue(undefined);
       getFnAgentMock.mockResolvedValue(() =>
@@ -207,24 +253,24 @@ describe("ai-summarize", () => {
       // Verify wrapped prompt was sent (prompt-injection mitigation)
       expect(prompt).toHaveBeenCalledTimes(1);
       expect(prompt.mock.calls[0][0]).toContain("<description>");
+      expect(prompt.mock.calls[0][0]).toContain("SAME language as the task description");
       expect(prompt.mock.calls[0][0]).toContain("Do not call any tools");
     });
 
     it("summarizes long descriptions with bounded prompt input", async () => {
       const prompt = vi.fn().mockResolvedValue(undefined);
-      getFnAgentMock.mockResolvedValue(() =>
+      const createFnAgent = vi.fn().mockResolvedValue({
-        Promise.resolve({
+        session: {
-          session: {
+          prompt,
-            prompt,
+          dispose: vi.fn(),
-            dispose: vi.fn(),
+          state: {
-            state: {
+            messages: [
-              messages: [
+              { role: "assistant", content: "Summarize long description" },
-                { role: "assistant", content: "Summarize long description" },
+            ],
-              ],
-            },
           },
-        })
+        },
-      );
+      });
+      getFnAgentMock.mockResolvedValue(createFnAgent);
       const description = "a".repeat(MAX_TITLE_SUMMARIZE_INPUT_LENGTH) + "tail".repeat(250);
 
       const title = await summarizeTitle(description, "/tmp");
@@ -234,6 +280,9 @@ describe("ai-summarize", () => {
       const promptText = prompt.mock.calls[0][0] as string;
       expect(promptText).toContain("…(truncated)");
       expect(promptText).not.toContain("tail");
+      expect(promptText).toContain("SAME language as the task description");
+      expect((createFnAgent.mock.calls[0][0] as { systemPrompt: string }).systemPrompt)
+        .toContain("SAME language as the task description");
       expect(promptText.length).toBeLessThanOrEqual(MAX_TITLE_SUMMARIZE_INPUT_LENGTH + 250);
     });
 
@@ -289,6 +338,7 @@ describe("ai-summarize", () => {
 
     it("retries stale configured model ids with automatic resolution and logs the stale id", async () => {
       const warnSpy = vi.spyOn(console, "warn").mockImplementation(() => {});
+      const fallbackPrompt = vi.fn().mockResolvedValue(undefined);
       const createFnAgent = vi
         .fn()
         .mockRejectedValueOnce(new Error(
@@ -297,7 +347,7 @@ describe("ai-summarize", () => {
         ))
         .mockResolvedValueOnce({
           session: {
-            prompt: vi.fn().mockResolvedValue(undefined),
+            prompt: fallbackPrompt,
             dispose: vi.fn(),
             state: {
               messages: [{ role: "assistant", content: "Fix pi upgrade regressions" }],
@@ -305,9 +355,10 @@ describe("ai-summarize", () => {
           },
         });
       getFnAgentMock.mockResolvedValue(createFnAgent);
+      const description = "Nous devons corriger les régressions de mise à niveau afin que les opérateurs puissent poursuivre leur travail sans interruption. ".repeat(3);
 
       const title = await summarizeTitle(
-        "a".repeat(201),
+        description,
         "/tmp",
         "fireworksai",
         "accounts/fireworks/routers/kimi-k2p5-turbo",
@@ -322,6 +373,12 @@ describe("ai-summarize", () => {
         defaultProvider: expect.any(String),
         defaultModelId: expect.any(String),
       }));
+      const primaryOptions = createFnAgent.mock.calls[0][0] as { systemPrompt: string };
+      const fallbackOptions = createFnAgent.mock.calls[1][0] as { systemPrompt: string };
+      expect(primaryOptions.systemPrompt).toContain("SAME language as the task description");
+      expect(fallbackOptions.systemPrompt).toContain("SAME language as the task description");
+      expect(fallbackPrompt.mock.calls[0][0]).toContain("SAME language as the task description");
+      expect(fallbackPrompt.mock.calls[0][0]).toContain("Likely language: Français");
       expect(warnSpy).toHaveBeenCalledWith(expect.stringContaining("fireworksai/accounts/fireworks/routers/kimi-k2p5-turbo"));
       warnSpy.mockRestore();
     });

packages/core/src/ai-summarize.ts

@@ -12,6 +12,7 @@
  */
 
 import { getFnAgent, type AgentMessage } from "./ai-engine-loader.js";
+import { detectContentLanguage, localeDisplayName } from "./detect-content-language.js";
 import { DANGLING_TAIL_STOPWORDS, stripDanglingTail, stripEmptyPlaceholders } from "./task-title-id-drift.js";
 
 // ── Constants ───────────────────────────────────────────────────────────────
@@ -29,6 +30,7 @@ Your ONLY job is to create a concise title (max 60 characters) that summarizes t
 
 ## Style
 - Clear, descriptive, actionable, professional
+- Write the title in the SAME language as the task description content. For example, a French description requires a French title.
 - Maximum 60 characters
 - Focus on the main goal or deliverable of the task`;
 
@@ -218,6 +220,21 @@ function formatConfiguredModel(provider?: string, modelId?: string): string {
   return provider && modelId ? `${provider}/${modelId}` : "unknown configured model";
 }
 
+/*
+ * FNXC:TitleSummaryInputLanguage 2026-07-16-00:00:
+ * When existing autoSummarizeTitles or per-request summarize behavior invokes this shared summarizer,
+ * the generated title must match the operator's description language without another model call.
+ * Detection only adds a medium-or-higher confidence hint; matching the description remains the rule.
+ */
+function buildTitleLanguageInstruction(description: string): string {
+  const detected = detectContentLanguage(description);
+  if (detected.locale !== "unknown" && detected.confidence !== "low") {
+    return `Write the title in the SAME language as the task description. Likely language: ${localeDisplayName(detected.locale)}.`;
+  }
+
+  return "Write the title in the SAME language as the task description.";
+}
+
 async function runTitleSummarizer(
   createFnAgent: NonNullable<Awaited<ReturnType<typeof getFnAgent>>>,
   description: string,
@@ -264,6 +281,7 @@ async function runTitleSummarizer(
       : description;
     const wrappedPrompt =
       "Summarize the following task description into a title (≤60 chars). " +
+      buildTitleLanguageInstruction(description) + " " +
       "Output ONLY the title text on a single line. Do not call any tools.\n\n" +
       "<description>\n" +
       truncatedDescription +