FN-5644: default AI merge commit body to summarize changes

Fusion-Task-Id: FN-5644

Fusion-Task-Lineage: 5cf0133b-2a14-4bb4-a1c4-2d5bef5793bc

.changeset/fn-5644-merge-commit-summary-default-docs.md

@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+Update `useAiMergeCommitSummary` docs/JSDoc to match the intended default of `true`, including that merge commit summaries include a subject plus body summary (narrative + bullets + diff-stat).
+
+Also fixes AI merge-mode prompt guidance so AI-authored squash commits include a summarized body instead of subject-only commit messages.

docs/settings-reference.md

@@ -388,7 +388,7 @@ Default notes:
 | `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 and agent/tool-created tasks. |
-| `useAiMergeCommitSummary` | `boolean` | `false` | Use AI-generated merge commit summaries instead of raw step-commit subject lists. |
+| `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. |
 | `titleSummarizerFallbackProvider` | `string` | `undefined` | Fallback provider for title summarization. |

packages/core/src/__tests__/settings-defaults.test.ts

@@ -74,6 +74,11 @@ describe("settings defaults invariants", () => {
     expect("githubCloseSourceIssueOnDone" in DEFAULT_GLOBAL_SETTINGS).toBe(false);
   });
 
+  it("defaults AI merge commit summaries to enabled", () => {
+    // FN-5642/FN-5644 intentionally default this on for subject + body summary coverage.
+    expect(DEFAULT_PROJECT_SETTINGS.useAiMergeCommitSummary).toBe(true);
+  });
+
   describe("recycleWorktrees default", () => {
     it("keeps recycleWorktrees explicitly false in project defaults", () => {
       expect(DEFAULT_PROJECT_SETTINGS.recycleWorktrees).toBe(false);

packages/core/src/types.ts

@@ -3378,8 +3378,9 @@ export interface ProjectSettings {
    *  Default: false. */
   autoSummarizeTitles?: boolean;
   /** When true, merge commit messages include an AI-generated summary of the
-   *  changes instead of just listing step commit subjects. Uses the title
+   *  changes instead of just listing step commit subjects. Body composition
-   *  summarizer model. Default: false. */
+   *  includes a narrative line, bullet summary, and `git diff --stat` when
+   *  available. Uses the title summarizer model. Default: true. */
   useAiMergeCommitSummary?: boolean;
   /** AI model provider for title summarization (when autoSummarizeTitles is enabled).
    *  Must be set together with `titleSummarizerModelId`. Falls back to planningProvider,

packages/dashboard/app/components/SettingsModal.tsx

@@ -3583,7 +3583,7 @@ export function SettingsModal({
                 AI merge commit summaries
               </label>
               <small>
-                When enabled, merge commit messages will include an AI-generated summary of the changes instead of just listing step commit subjects. Uses the title summarization model.
+                When enabled, merge commit messages include an AI-generated subject plus body summary (narrative + bullets + diff-stat) instead of just listing step commit subjects. Uses the title summarization model.
               </small>
             </div>
 

packages/engine/src/__tests__/merger-ai.test.ts

@@ -8,6 +8,7 @@ import {
   landSquash,
   parseReviewVerdict,
   buildMergeSystemPrompt,
+  buildMergePrompt,
   buildReviewSystemPrompt,
   REVIEW_VERDICT_MARKER,
   AiMergeBlockedError,
@@ -116,9 +117,11 @@ describe("parseReviewVerdict", () => {
     expect(buildMergeSystemPrompt().toLowerCase()).toContain("conflict");
   });
 
-  it("merge system prompt enforces new-breakage verification + uses the editable merger prompt", () => {
+  it("merge system prompt enforces new-breakage verification + commit body summary guidance", () => {
     expect(buildMergeSystemPrompt().toLowerCase()).toContain("type-check");
     expect(buildMergeSystemPrompt()).toMatch(/new failure/i);
+    expect(buildMergeSystemPrompt()).toMatch(/bullet list of key changes/i);
+    expect(buildMergeSystemPrompt()).toMatch(/Files changed:/i);
     // A custom 'merger' role prompt is incorporated as the base, while the hard
     // rules (verification + trailers) are still appended.
     const cfg = {
@@ -129,6 +132,23 @@ describe("parseReviewVerdict", () => {
     expect(p).toContain("CUSTOM MERGER PERSONA");
     expect(p).toContain("Verify before committing");
   });
+
+  it("merge prompt requires subject, body summary, and diff-stat in commit message", () => {
+    const prompt = buildMergePrompt({
+      taskId: "FN-1",
+      branch: "fusion/fn-1",
+      integrationBranch: "main",
+      tipSha: "0123456789abcdef0123456789abcdef01234567",
+      taskTitle: "Do the thing",
+      includeTaskId: true,
+      trailers: ["Fusion-Task-Id: FN-1"],
+    });
+    expect(prompt).toMatch(/Build a merge body from the staged squash diff/i);
+    expect(prompt).toMatch(/bullet list of key changes/i);
+    expect(prompt).toMatch(/Files changed:/i);
+    expect(prompt).toMatch(/git diff --stat/i);
+    expect(prompt).toMatch(/git commit -m "FN-1: <concise imperative summary of the squashed changes>" -m/i);
+  });
 });
 
 describe("runAiMerge", () => {

packages/engine/src/merger-ai.ts

@@ -225,6 +225,10 @@ export function buildMergeSystemPrompt(agentPrompts?: AgentPromptsConfig): strin
     "  - The subject line must CONCISELY SUMMARIZE the squashed changes in",
     "    imperative mood (e.g. \"add X\", \"fix Y\") based on the actual diff — do",
     "    not just restate the task title.",
+    "  - The commit BODY must include:",
+    "      1) one short narrative summary line,",
+    "      2) a bullet list of key changes, and",
+    "      3) a `Files changed:` section populated from `git diff --stat`.",
     "  - Include the task-id prefix and the trailer lines EXACTLY as given in the",
     "    task instructions (they associate the commit with the board task).",
   ].filter((l) => l !== undefined).join("\n");
@@ -253,11 +257,15 @@ export function buildMergePrompt(input: {
     "Steps:",
     `  1. Run: git merge --squash ${input.branch}`,
     "  2. If there are conflicts, resolve them (favor the task's intent), then `git add` the resolved files.",
-    "  3. Commit the staged result as a SINGLE commit whose subject summarizes the",
+    "  3. Build a merge body from the staged squash diff:",
-    `     actual changes${input.taskTitle ? ` (task title hint: ${JSON.stringify(input.taskTitle)})` : ""}, including the required trailers:`,
+    "       - one short narrative summary line",
-    `       git commit -m ${subjectShape}${trailerArgs}`,
+    "       - bullet list of key changes",
+    "       - `Files changed:` + the output of `git diff --stat`",
+    "  4. Commit the staged result as a SINGLE commit whose subject summarizes the",
+    `     actual changes${input.taskTitle ? ` (task title hint: ${JSON.stringify(input.taskTitle)})` : ""}, including the body above and required trailers:`,
+    `       git commit -m ${subjectShape} -m "<narrative + bullet list + Files changed: ...>"${trailerArgs}`,
     "     Keep the trailer line(s) verbatim — they link the commit to the board task.",
-    "  4. Verify `git log --oneline ${tip}..HEAD` shows exactly one new commit and `git status` is clean.".replace("${tip}", short(input.tipSha)),
+    "  5. Verify `git log --oneline ${tip}..HEAD` shows exactly one new commit and `git status` is clean.".replace("${tip}", short(input.tipSha)),
     "",
     "If `git merge --squash` reports the branch is already up to date (nothing to",
     "merge), do nothing and leave HEAD unchanged.",