FN-6305: allow title summarization for long descriptions

Title summarization now accepts oversized task descriptions while bounding the model prompt.

- Remove the 2000-character validation ceiling for summarize-title requests.
- Truncate long descriptions only at model-input time with a dedicated exported cap.
- Update route/API docs and tests for long-description behavior.
- Add a patch changeset for the published Fusion package.

Files changed:
 .changeset/fn-6305-title-summarize-any-length.md   |  5 +++
 packages/core/src/__tests__/ai-summarize.test.ts   | 43 ++++++++++++++++++----
 packages/core/src/ai-summarize.ts                  | 34 +++++++++++------
 packages/core/src/index.ts                         |  1 +
 packages/dashboard/app/api/legacy.ts               |  2 +-
 .../src/__tests__/routes-planning.test.ts          | 25 +++++++++++++
 packages/dashboard/src/routes.ts                   |  2 +-
 7 files changed, 92 insertions(+), 20 deletions(-)

Fusion-Task-Id: FN-6305
Fusion-Task-Lineage: 69a83173-ac85-489f-b4cc-d1a70b6d25eb

.changeset/fn-6305-title-summarize-any-length.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": minor
+---
+
+Title summarization now accepts descriptions of any length by truncating the model input to a bounded prompt instead of rejecting descriptions over 2000 characters.

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

@@ -22,6 +22,7 @@ import {
   MERGE_COMMIT_SUMMARIZE_SYSTEM_PROMPT,
   COMMIT_BODY_SYSTEM_PROMPT,
   MAX_DESCRIPTION_LENGTH,
+  MAX_TITLE_SUMMARIZE_INPUT_LENGTH,
   MIN_DESCRIPTION_LENGTH,
   MAX_TITLE_LENGTH,
   MAX_MERGE_COMMIT_SUMMARY_LENGTH,
@@ -53,6 +54,7 @@ describe("ai-summarize", () => {
     it("should have correct length limits", () => {
       expect(MIN_DESCRIPTION_LENGTH).toBe(201);
       expect(MAX_DESCRIPTION_LENGTH).toBe(2000);
+      expect(MAX_TITLE_SUMMARIZE_INPUT_LENGTH).toBe(4000);
       expect(MAX_TITLE_LENGTH).toBe(60);
     });
 
@@ -89,21 +91,21 @@ describe("ai-summarize", () => {
       expect(() => validateDescription(desc)).toThrow("at least 201 characters");
     });
 
-    it("should throw for description too long", () => {
-      const desc = "a".repeat(2001);
-      expect(() => validateDescription(desc)).toThrow(ValidationError);
-      expect(() => validateDescription(desc)).toThrow("not exceed 2000 characters");
-    });
-
     it("should accept description at minimum boundary", () => {
       const desc = "a".repeat(201);
       expect(validateDescription(desc)).toBe(desc);
     });
 
-    it("should accept description at maximum boundary", () => {
+    it("should accept description at historical maximum boundary", () => {
       const desc = "a".repeat(2000);
       expect(validateDescription(desc)).toBe(desc);
     });
+
+    it("should accept descriptions longer than the historical maximum", () => {
+      const desc = "a".repeat(5000);
+      expect(() => validateDescription(desc)).not.toThrow();
+      expect(validateDescription(desc)).toBe(desc);
+    });
   });
 
   // ── Rate Limiting ──────────────────────────────────────────────────────────
@@ -206,6 +208,33 @@ describe("ai-summarize", () => {
       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(() =>
+        Promise.resolve({
+          session: {
+            prompt,
+            dispose: vi.fn(),
+            state: {
+              messages: [
+                { role: "assistant", content: "Summarize long description" },
+              ],
+            },
+          },
+        })
+      );
+      const description = "a".repeat(MAX_TITLE_SUMMARIZE_INPUT_LENGTH) + "tail".repeat(250);
+
+      const title = await summarizeTitle(description, "/tmp");
+
+      expect(title).toBe("Summarize long description");
+      expect(prompt).toHaveBeenCalledTimes(1);
+      const promptText = prompt.mock.calls[0][0] as string;
+      expect(promptText).toContain("…(truncated)");
+      expect(promptText).not.toContain("tail");
+      expect(promptText.length).toBeLessThanOrEqual(MAX_TITLE_SUMMARIZE_INPUT_LENGTH + 250);
+    });
+
     it("strips chatty preamble + markdown from AI response (FN-3057 regression)", async () => {
       // Reproduces the FN-3057 incident: model wrote a chat-style reply
       // ("Created **FN-3058** with the full spec…") that was sliced mid-word

packages/core/src/ai-summarize.ts

@@ -8,7 +8,7 @@
  * Features:
  * - Rate limiting per IP (10 requests per hour)
  * - Dynamic import of @fusion/engine for AI agent creation
- * - Text length validation (201-2000 characters)
+ * - Text length validation (minimum 201 characters; model input is truncated)
  */
 
 import { getFnAgent, type AgentMessage } from "./ai-engine-loader.js";
@@ -32,9 +32,21 @@ Your ONLY job is to create a concise title (max 60 characters) that summarizes t
 - Maximum 60 characters
 - Focus on the main goal or deliverable of the task`;
 
-/** Maximum description length in characters */
+/**
+ * Historical maximum accepted description length in characters.
+ *
+ * @deprecated Title summarization now accepts descriptions of any length;
+ * use MAX_TITLE_SUMMARIZE_INPUT_LENGTH for the bounded model-input cap.
+ */
 export const MAX_DESCRIPTION_LENGTH = 2000;
 
+/**
+ * Maximum input length for title summarization. Descriptions can be very large;
+ * we truncate before sending so the prompt stays bounded while preserving the
+ * long-input API behavior.
+ */
+export const MAX_TITLE_SUMMARIZE_INPUT_LENGTH = 4000;
+
 /** Minimum description length for summarization in characters */
 export const MIN_DESCRIPTION_LENGTH = 201;
 
@@ -178,17 +190,13 @@ export function validateDescription(description: unknown): string {
     throw new ValidationError("description must be a string");
   }
 
-  // Validate description length
+  // Validate description length floor. There is intentionally no upper bound:
+  // runTitleSummarizer truncates model input before prompting.
   if (description.length < MIN_DESCRIPTION_LENGTH) {
     throw new ValidationError(
       `description must be at least ${MIN_DESCRIPTION_LENGTH} characters for summarization`
     );
   }
-  if (description.length > MAX_DESCRIPTION_LENGTH) {
-    throw new ValidationError(
-      `description must not exceed ${MAX_DESCRIPTION_LENGTH} characters`
-    );
-  }
 
   return description;
 }
@@ -246,12 +254,16 @@ async function runTitleSummarizer(
     // Wrap the user-supplied description in a delimiter so the model treats it
     // as content to summarize, not as instructions to follow. Belt-and-suspenders
     // alongside the system-prompt guardrails and the engine's readonly tool
-    // isolation.
+    // isolation. Truncate before prompt construction so arbitrarily long task
+    // descriptions cannot produce unbounded model input.
+    const truncatedDescription = description.length > MAX_TITLE_SUMMARIZE_INPUT_LENGTH
+      ? description.slice(0, MAX_TITLE_SUMMARIZE_INPUT_LENGTH) + "\n…(truncated)"
+      : description;
     const wrappedPrompt =
       "Summarize the following task description into a title (≤60 chars). " +
       "Output ONLY the title text on a single line. Do not call any tools.\n\n" +
       "<description>\n" +
-      description +
+      truncatedDescription +
       "\n</description>";
     await agentResult.session.prompt(wrappedPrompt);
 
@@ -317,7 +329,7 @@ async function runTitleSummarizer(
 
 /**
  * Summarize a task description into a concise title using AI.
- * @param description - The task description to summarize (must be 201-2000 chars)
+ * @param description - The task description to summarize (must be >200 chars; model input is truncated)
  * @param rootDir - Project root directory for AI agent context
  * @param provider - Optional AI model provider (e.g., "anthropic")
  * @param modelId - Optional AI model ID (e.g., "claude-sonnet-4-5")

packages/core/src/index.ts

@@ -1006,6 +1006,7 @@ export {
   MAX_COMMIT_SUBJECT_LENGTH,
   DEFAULT_COMMIT_SUBJECT_TIMEOUT_MS,
   MAX_DESCRIPTION_LENGTH,
+  MAX_TITLE_SUMMARIZE_INPUT_LENGTH,
   MIN_DESCRIPTION_LENGTH,
   MAX_TITLE_LENGTH,
   MAX_MERGE_COMMIT_SUMMARY_LENGTH,

packages/dashboard/app/api/legacy.ts

@@ -6439,7 +6439,7 @@ export interface SummarizeTitleResponse {
 }
 
 /** Summarize a task description into a concise title using AI.
- * @param description - The task description to summarize (must be 201-2000 chars)
+ * @param description - The task description to summarize (must be >200 chars; model input is truncated)
  * @param provider - Optional AI model provider (e.g., "anthropic")
  * @param modelId - Optional AI model ID (e.g., "claude-sonnet-4-5")
  * @param projectId - Optional project ID for scoped settings resolution

packages/dashboard/src/__tests__/routes-planning.test.ts

@@ -3757,6 +3757,31 @@ describe("POST /api/ai/summarize-title", () => {
     );
   });
 
+  it("accepts descriptions longer than 2000 characters", async () => {
+    const fusionCore = await import("@fusion/core");
+    const summarizeTitleSpy = vi
+      .spyOn(fusionCore, "summarizeTitle")
+      .mockResolvedValueOnce("Generated title");
+
+    const description = "x".repeat(5000);
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      "/api/ai/summarize-title",
+      JSON.stringify({ description }),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual({ title: "Generated title" });
+    expect(summarizeTitleSpy).toHaveBeenCalledWith(
+      description,
+      "/test/project",
+      undefined,
+      undefined,
+    );
+  });
+
   it("emits structured diagnostics for unexpected summarize failures", async () => {
     const diagnostics = captureDiagnostics();
     const fusionCore = await import("@fusion/core");

packages/dashboard/src/routes.ts

@@ -1848,6 +1848,7 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
    * Returns: { title: string }
    *
    * Generates a concise title (≤60 characters) from descriptions longer than 200 characters.
+   * Long descriptions are accepted; core truncates model input before prompting.
    * Rate limited: 10 requests per hour per IP
    */
   router.post("/ai/summarize-title", async (req, res) => {
@@ -1863,7 +1864,6 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
         summarizeTitle,
         validateDescription,
         MIN_DESCRIPTION_LENGTH,
-        MAX_DESCRIPTION_LENGTH: _MAX_DESCRIPTION_LENGTH,
         RateLimitError: _RateLimitError4,
         ValidationError: _ValidationError2,
         AiServiceError: _AiServiceError2,