feat(FN-4067): add squash audit gate to merger with integration tests

Adds a squash-merge audit gate that verifies duplicate-cherry-pick commits and file-overlap losses after any squash merge into main, restoring the post-squash audit step to the merge workflow with test coverage for both the audit logic and the broader merger lifecycle.

Fusion-Task-Id: FN-4067

Fusion-Task-Lineage: 593aa917-5640-495b-b1ae-80c3eaa09d01

packages/engine/src/__tests__/executor-review-verdicts.test.ts

@@ -133,7 +133,7 @@ describe("TaskExecutor enginePaused soft pause (no agent termination)", () => {
       "Task marked complete with summary. All steps done. Moving to in-review.",
     );
     expect(watchdogSpy).toHaveBeenCalledWith("FN-001", "fn_task_done");
-    expect(store.updateTask).toHaveBeenCalledWith("FN-001", { paused: false, status: null });
+    expect(store.updateTask).toHaveBeenCalledWith("FN-001", { status: null });
     expect(store.moveTask).toHaveBeenCalledWith("FN-001", "in-review");
   });
 

packages/engine/src/__tests__/merger-merge-lifecycle.test.ts

@@ -121,6 +121,23 @@ vi.mock("../context-limit-detector.js", () => ({
   isContextLimitError: vi.fn(),
 }));
 
+vi.mock("../merger-squash-audit.js", () => ({
+  auditSquashMerge: vi.fn(async () => ({
+    squashSha: "mergedcommit123",
+    parentSha: "parent123",
+    squashSubject: "feat: squash merge",
+    lookback: 30,
+    branchSubjects: [],
+    recentMainSubjects: [],
+    duplicateSubjects: [],
+    touchedFiles: [],
+    touchedFileOverlaps: [],
+    findings: [],
+    issueCount: 0,
+    clean: true,
+  })),
+}));
+
 import {
   aiMergeTask,
   pushToRemoteAfterMerge,
@@ -149,11 +166,13 @@ import {
 } from "../merger.js";
 import { mergerLog } from "../logger.js";
 import { createFnAgent } from "../pi.js";
+import { auditSquashMerge } from "../merger-squash-audit.js";
 import { execSync, exec } from "node:child_process";
 import * as core from "@fusion/core";
 import { type TaskStore, type Task, type MergeResult, DEFAULT_SETTINGS } from "@fusion/core";
 
 const mockedCreateFnAgent = vi.mocked(createFnAgent);
+const mockedAuditSquashMerge = vi.mocked(auditSquashMerge);
 const mockedExecSync = vi.mocked(execSync);
 const mockedExec = vi.mocked(exec);
 const { existsSync: mockedExistsSyncRaw, readFileSync: mockedReadFileSyncRaw } = await import("node:fs");
@@ -2116,4 +2135,167 @@ describe("aiMergeTask — reset cleanup failure diagnostics", () => {
 
 // ── New Smart Conflict Resolution API Tests ────────────────────────────
 
+describe("aiMergeTask post-squash audit gate", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockedExistsSync.mockReturnValue(true);
+    mockedCreateFnAgent.mockResolvedValue({
+      session: {
+        prompt: vi.fn().mockResolvedValue(undefined),
+        dispose: vi.fn(),
+      },
+    } as any);
+    mockedAuditSquashMerge.mockResolvedValue({
+      squashSha: "mergedcommit123",
+      parentSha: "parent123",
+      squashSubject: "feat: squash merge",
+      lookback: 30,
+      branchSubjects: [],
+      recentMainSubjects: [],
+      duplicateSubjects: [],
+      touchedFiles: [],
+      touchedFileOverlaps: [],
+      findings: [],
+      issueCount: 0,
+      clean: true,
+    });
+  });
+
+  function setupAutoResolvedMergeExecSync() {
+    let squashCalls = 0;
+    mockedExecSync.mockImplementation((cmd: any) => {
+      const cmdStr = String(cmd);
+      if (cmdStr.includes("symbolic-ref --short HEAD")) return "main" as any;
+      if (cmdStr.includes("rev-parse --verify")) return Buffer.from("abc123");
+      if (cmdStr === "git rev-parse HEAD" || cmdStr.startsWith("git rev-parse HEAD ")) return "mergedcommit123" as any;
+      if (cmdStr.includes("git log")) return "- feat: something" as any;
+      if (cmdStr.includes("merge-base")) return Buffer.from("abc123");
+      if (cmdStr.includes("rev-list --count")) return "1\n" as any;
+      if (cmdStr.includes("git diff") && cmdStr.includes("--stat")) return "1 file changed" as any;
+      if (cmdStr.includes("merge --squash") && !cmdStr.includes("-X")) {
+        squashCalls += 1;
+        if (squashCalls === 2) {
+          const error = new Error("merge conflict");
+          (error as Error & { stdout?: string; stderr?: string }).stdout = "";
+          (error as Error & { stdout?: string; stderr?: string }).stderr = "CONFLICT";
+          throw error;
+        }
+        return Buffer.from("");
+      }
+      if (cmdStr.includes("diff --name-only --diff-filter=U")) return "pnpm-lock.yaml\n" as any;
+      if (cmdStr.includes("checkout --ours --") || cmdStr.includes("git add -- pnpm-lock.yaml")) return Buffer.from("");
+      if (cmdStr.includes("diff --cached --quiet")) return "1" as any;
+      if (cmdStr.includes("git commit ")) return Buffer.from("");
+      if (cmdStr.includes("show --shortstat")) return "3 files changed, 10 insertions(+), 2 deletions(-)" as any;
+      if (cmdStr.includes("branch -d") || cmdStr.includes("branch -D")) return Buffer.from("");
+      if (cmdStr.includes("worktree remove")) return Buffer.from("");
+      return Buffer.from("");
+    });
+  }
+
+  function createAuditStore() {
+    const store = createMockStore(
+      { id: "FN-050", worktree: "/tmp/root/.worktrees/KB-050" },
+      [{ id: "FN-050", worktree: "/tmp/root/.worktrees/KB-050", column: "in-review" } as Task],
+    );
+    (store.getSettings as ReturnType<typeof vi.fn>).mockResolvedValue({
+      ...DEFAULT_SETTINGS,
+      testCommand: "pnpm test",
+      mergeConflictStrategy: "ai-only",
+      worktreeRebaseBeforeMerge: false,
+      worktreeRebaseLocalBase: false,
+    });
+    return store;
+  }
+
+  it("moves the task to done when the post-squash audit is clean", async () => {
+    setupAutoResolvedMergeExecSync();
+    const store = createAuditStore();
+
+    const result = await aiMergeTask(store, "/tmp/root", "FN-050");
+
+    expect(result.merged).toBe(true);
+    expect(mockedAuditSquashMerge).toHaveBeenCalledWith({
+      rootDir: "/tmp/root",
+      squashSha: "mergedcommit123",
+    });
+    expect(store.appendAgentLog).toHaveBeenCalledWith("FN-050", "post-squash audit clean", "text", undefined, "merger");
+    expect(store.moveTask).toHaveBeenCalledWith("FN-050", "done");
+  });
+
+  it("blocks completion and logs duplicate-subject findings", async () => {
+    setupAutoResolvedMergeExecSync();
+    mockedAuditSquashMerge.mockResolvedValue({
+      squashSha: "mergedcommit123",
+      parentSha: "parent123",
+      squashSubject: "feat: squash merge",
+      lookback: 30,
+      branchSubjects: ["feat: duplicate subject"],
+      recentMainSubjects: ["feat: duplicate subject"],
+      duplicateSubjects: [{ type: "duplicate-subject", subject: "feat: duplicate subject" }],
+      touchedFiles: ["src/example.ts"],
+      touchedFileOverlaps: [],
+      findings: [{ type: "duplicate-subject", subject: "feat: duplicate subject" }],
+      issueCount: 1,
+      clean: false,
+    });
+    const store = createAuditStore();
+
+    await expect(aiMergeTask(store, "/tmp/root", "FN-050")).rejects.toThrow(
+      "FN-050: post-squash audit blocked auto-completion for mergedco",
+    );
+
+    expect(store.moveTask).not.toHaveBeenCalled();
+    expect(store.appendAgentLog).toHaveBeenCalledWith(
+      "FN-050",
+      expect.stringContaining("post-squash audit blocked auto-completion"),
+      "tool_error",
+      expect.stringContaining("Duplicate-subject risks:\n- feat: duplicate subject"),
+      "merger",
+    );
+    expect(store.updateTask).toHaveBeenCalledWith("FN-050", { status: null });
+  });
+
+  it("blocks completion and logs touched-file-overlap findings", async () => {
+    setupAutoResolvedMergeExecSync();
+    mockedAuditSquashMerge.mockResolvedValue({
+      squashSha: "mergedcommit123",
+      parentSha: "parent123",
+      squashSubject: "feat: squash merge",
+      lookback: 30,
+      branchSubjects: ["feat: branch change"],
+      recentMainSubjects: [],
+      duplicateSubjects: [],
+      touchedFiles: ["src/shared.ts"],
+      touchedFileOverlaps: [{
+        type: "touched-file-overlap",
+        file: "src/shared.ts",
+        recentMainCommits: [{ sha: "abc1234", subject: "fix: recent main change" }],
+      }],
+      findings: [{
+        type: "touched-file-overlap",
+        file: "src/shared.ts",
+        recentMainCommits: [{ sha: "abc1234", subject: "fix: recent main change" }],
+      }],
+      issueCount: 1,
+      clean: false,
+    });
+    const store = createAuditStore();
+
+    await expect(aiMergeTask(store, "/tmp/root", "FN-050")).rejects.toThrow(
+      "FN-050: post-squash audit blocked auto-completion for mergedco",
+    );
+
+    expect(store.moveTask).not.toHaveBeenCalled();
+    expect(store.appendAgentLog).toHaveBeenCalledWith(
+      "FN-050",
+      expect.stringContaining("post-squash audit blocked auto-completion"),
+      "tool_error",
+      expect.stringContaining("Touched-file overlap risks:\n- src/shared.ts\n  - abc1234 fix: recent main change"),
+      "merger",
+    );
+    expect(store.updateTask).toHaveBeenCalledWith("FN-050", { status: null });
+  });
+});
+
 

packages/engine/src/__tests__/merger-squash-audit.test.ts

@@ -0,0 +1,126 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { execSync, spawnSync } from "node:child_process";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { auditSquashMerge } from "../merger-squash-audit.js";
+
+const hasGit = spawnSync("git", ["--version"], { stdio: "pipe" }).status === 0;
+const describeIfGit = hasGit ? describe : describe.skip;
+
+function git(repo: string, command: string): string {
+  return execSync(command, { cwd: repo, encoding: "utf-8", stdio: ["pipe", "pipe", "pipe"] }).trim();
+}
+
+function write(repo: string, relativePath: string, content: string): void {
+  writeFileSync(join(repo, relativePath), content, "utf-8");
+}
+
+describeIfGit("auditSquashMerge", () => {
+  const repos: string[] = [];
+
+  afterEach(() => {
+    for (const repo of repos.splice(0)) {
+      rmSync(repo, { recursive: true, force: true });
+    }
+  });
+
+  function setupRepo(): string {
+    const repo = mkdtempSync(join(tmpdir(), "fusion-merger-squash-audit-"));
+    repos.push(repo);
+    git(repo, "git init -b main");
+    git(repo, 'git config user.email "test@example.com"');
+    git(repo, 'git config user.name "Test User"');
+    write(repo, "README.md", "init\n");
+    git(repo, "git add README.md && git commit -m 'init'");
+    return repo;
+  }
+
+  function createSquashCommit(repo: string, bodyLines: string[]): string {
+    const body = bodyLines.map((line) => `- ${line}`).join("\n");
+    git(repo, `git commit -m ${JSON.stringify("feat: squash merge")} -m ${JSON.stringify(body)}`);
+    return git(repo, "git rev-parse HEAD");
+  }
+
+  it("returns a clean result when no duplicate subjects or recent-main file overlaps exist", async () => {
+    const repo = setupRepo();
+
+    git(repo, "git checkout -b feature/clean");
+    write(repo, "feature.txt", "branch-only\n");
+    git(repo, "git add feature.txt && git commit -m 'feat: branch clean change'");
+
+    git(repo, "git checkout main");
+    write(repo, "main-only.txt", "recent main\n");
+    git(repo, "git add main-only.txt && git commit -m 'chore: recent main touch'");
+
+    git(repo, "git merge --squash feature/clean");
+    const squashSha = createSquashCommit(repo, ["feat: branch clean change"]);
+
+    const findings = await auditSquashMerge({ rootDir: repo, squashSha, lookback: 10 });
+
+    expect(findings.clean).toBe(true);
+    expect(findings.issueCount).toBe(0);
+    expect(findings.duplicateSubjects).toEqual([]);
+    expect(findings.touchedFileOverlaps).toEqual([]);
+  });
+
+  it("reports duplicate branch subjects that match recent main commits", async () => {
+    const repo = setupRepo();
+
+    git(repo, "git checkout -b feature/dupe");
+    write(repo, "branch.txt", "feature\n");
+    git(repo, "git add branch.txt && git commit -m 'feat: duplicate subject'");
+
+    git(repo, "git checkout main");
+    write(repo, "main.txt", "main\n");
+    git(repo, "git add main.txt && git commit -m 'feat: duplicate subject'");
+
+    git(repo, "git merge --squash feature/dupe");
+    const squashSha = createSquashCommit(repo, ["feat: duplicate subject"]);
+
+    const findings = await auditSquashMerge({ rootDir: repo, squashSha, lookback: 10 });
+
+    expect(findings.clean).toBe(false);
+    expect(findings.duplicateSubjects).toEqual([
+      { type: "duplicate-subject", subject: "feat: duplicate subject" },
+    ]);
+    expect(findings.issueCount).toBe(1);
+  });
+
+  it("reports touched-file overlaps with recent main commits", async () => {
+    const repo = setupRepo();
+
+    write(repo, "shared.txt", "alpha\nbeta\ngamma\n");
+    git(repo, "git add shared.txt && git commit -m 'chore: add shared file'");
+
+    git(repo, "git checkout -b feature/overlap");
+    write(repo, "shared.txt", "alpha-branch\nbeta\ngamma\n");
+    git(repo, "git add shared.txt && git commit -m 'feat: branch edits shared file'");
+
+    git(repo, "git checkout main");
+    write(repo, "shared.txt", "alpha\nbeta\ngamma-main\n");
+    git(repo, "git add shared.txt && git commit -m 'fix: main edits shared file'");
+
+    git(repo, "git merge --squash feature/overlap");
+    const squashSha = createSquashCommit(repo, ["feat: branch edits shared file"]);
+
+    const findings = await auditSquashMerge({ rootDir: repo, squashSha, lookback: 10 });
+
+    expect(findings.clean).toBe(false);
+    expect(findings.duplicateSubjects).toEqual([]);
+    expect(findings.touchedFileOverlaps).toHaveLength(1);
+    expect(findings.touchedFileOverlaps[0]).toMatchObject({
+      type: "touched-file-overlap",
+      file: "shared.txt",
+    });
+    expect(findings.touchedFileOverlaps[0].recentMainCommits).toEqual(
+      expect.arrayContaining([
+        {
+          sha: expect.any(String),
+          subject: "fix: main edits shared file",
+        },
+      ]),
+    );
+    expect(findings.issueCount).toBe(1);
+  });
+});

packages/engine/src/index.ts

@@ -31,6 +31,15 @@ export {
   type MergerOptions,
   type AutostashOrphanRecord,
 } from "./merger.js";
+export {
+  auditSquashMerge,
+  formatSquashAuditReport,
+  type SquashAuditFindings,
+  type SquashAuditFinding,
+  type SquashAuditDuplicateSubjectFinding,
+  type SquashAuditTouchedFileOverlapFinding,
+  type SquashAuditRecentMainCommit,
+} from "./merger-squash-audit.js";
 export { reviewStep, type ReviewType, type ReviewVerdict, type ReviewResult, type ReviewOptions } from "./reviewer.js";
 export { createFnAgent, promptWithFallback, describeModel, setHostExtensionPaths, getHostExtensionPaths, type AgentOptions, type AgentResult } from "./pi.js";
 

packages/engine/src/merger-squash-audit.ts

@@ -0,0 +1,187 @@
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+
+const execFileAsync = promisify(execFile);
+const DEFAULT_LOOKBACK = 30;
+const GIT_OUTPUT_MAX_BUFFER = 10 * 1024 * 1024;
+
+export interface SquashAuditRecentMainCommit {
+  sha: string;
+  subject: string;
+}
+
+export interface SquashAuditDuplicateSubjectFinding {
+  type: "duplicate-subject";
+  subject: string;
+}
+
+export interface SquashAuditTouchedFileOverlapFinding {
+  type: "touched-file-overlap";
+  file: string;
+  recentMainCommits: SquashAuditRecentMainCommit[];
+}
+
+export type SquashAuditFinding = SquashAuditDuplicateSubjectFinding | SquashAuditTouchedFileOverlapFinding;
+
+export interface SquashAuditFindings {
+  squashSha: string;
+  parentSha: string;
+  squashSubject: string;
+  lookback: number;
+  branchSubjects: string[];
+  recentMainSubjects: string[];
+  duplicateSubjects: SquashAuditDuplicateSubjectFinding[];
+  touchedFiles: string[];
+  touchedFileOverlaps: SquashAuditTouchedFileOverlapFinding[];
+  findings: SquashAuditFinding[];
+  issueCount: number;
+  clean: boolean;
+}
+
+export async function auditSquashMerge({
+  rootDir,
+  squashSha,
+  lookback = DEFAULT_LOOKBACK,
+}: {
+  rootDir: string;
+  squashSha: string;
+  lookback?: number;
+}): Promise<SquashAuditFindings> {
+  const normalizedLookback = normalizeLookback(lookback);
+  const parentSha = await git(rootDir, ["rev-parse", `${squashSha}^`]);
+  const squashSubject = await git(rootDir, ["log", "-1", "--format=%s", squashSha]);
+  const branchSubjects = normalizeLines(await git(rootDir, ["log", "-1", "--format=%b", squashSha]))
+    .map((line) => line.replace(/^- /, "").trim())
+    .filter(Boolean);
+
+  const recentMainCommits = await listRecentMainCommits(rootDir, parentSha, normalizedLookback);
+  const recentMainSubjects = recentMainCommits.map((entry) => entry.subject);
+
+  const duplicateSubjects = branchSubjects
+    .filter((subject) => recentMainSubjects.includes(subject))
+    .map((subject) => ({ type: "duplicate-subject", subject }) satisfies SquashAuditDuplicateSubjectFinding);
+
+  const touchedFiles = normalizeLines(await git(rootDir, ["diff", "--name-only", parentSha, squashSha]));
+  const touchedFileOverlaps: SquashAuditTouchedFileOverlapFinding[] = [];
+
+  for (const file of touchedFiles) {
+    const overlappingCommits: SquashAuditRecentMainCommit[] = [];
+    for (const commit of recentMainCommits) {
+      const touchedInCommit = await git(rootDir, ["diff-tree", "--no-commit-id", "--name-only", "-r", commit.sha, "--", file]);
+      if (normalizeLines(touchedInCommit).includes(file)) {
+        overlappingCommits.push({ sha: commit.shortSha, subject: commit.subject });
+      }
+    }
+
+    if (overlappingCommits.length > 0) {
+      touchedFileOverlaps.push({
+        type: "touched-file-overlap",
+        file,
+        recentMainCommits: overlappingCommits,
+      });
+    }
+  }
+
+  const findings: SquashAuditFinding[] = [...duplicateSubjects, ...touchedFileOverlaps];
+
+  return {
+    squashSha,
+    parentSha,
+    squashSubject,
+    lookback: normalizedLookback,
+    branchSubjects,
+    recentMainSubjects,
+    duplicateSubjects,
+    touchedFiles,
+    touchedFileOverlaps,
+    findings,
+    issueCount: findings.length,
+    clean: findings.length === 0,
+  };
+}
+
+export function formatSquashAuditReport(findings: SquashAuditFindings): string {
+  const lines: string[] = [
+    `Auditing squash: ${findings.squashSha} — ${findings.squashSubject}`,
+    `Parent (main before squash): ${findings.parentSha}`,
+    `Lookback window on main: ${findings.lookback} commits`,
+    "",
+    "=== Duplicate-cherry-pick risk ===",
+  ];
+
+  if (findings.duplicateSubjects.length === 0) {
+    lines.push("(none — no branch commit subjects match recent main commits)", "");
+  } else {
+    lines.push(
+      "WARN: branch contains commits whose subjects match recent main commits.",
+      "Auto-resolve may have picked the older side, dropping refinements.",
+      "Action: diff each main commit below against HEAD and confirm its",
+      "net contribution survived. Restore anything dropped as a follow-up.",
+      "",
+      ...findings.duplicateSubjects.map((entry) => `  - ${entry.subject}`),
+      "",
+    );
+  }
+
+  lines.push(`=== Touched-file overlap (${findings.touchedFiles.length} files in squash) ===`);
+  if (findings.touchedFileOverlaps.length === 0) {
+    lines.push("(none — squash touches files no recent main commit touched)", "");
+  } else {
+    lines.push(
+      "Files the squash touched that also have recent main activity.",
+      "Action: for each commit below, verify its changes still appear",
+      "in HEAD. Reapply any silently dropped changes on the same branch.",
+      "",
+    );
+    for (const overlap of findings.touchedFileOverlaps) {
+      lines.push(`  ${overlap.file}`);
+      for (const commit of overlap.recentMainCommits) {
+        lines.push(`    - ${commit.sha}  ${commit.subject}`);
+      }
+    }
+    lines.push("");
+  }
+
+  lines.push(`Audit complete. ${findings.issueCount} item(s) for the calling agent to review.`);
+  return lines.join("\n");
+}
+
+async function git(rootDir: string, args: string[]): Promise<string> {
+  const { stdout } = await execFileAsync("git", args, {
+    cwd: rootDir,
+    encoding: "utf-8",
+    maxBuffer: GIT_OUTPUT_MAX_BUFFER,
+  });
+  return stdout.trim();
+}
+
+function normalizeLines(value: string): string[] {
+  const trimmed = value.trim();
+  if (!trimmed) return [];
+  return trimmed.split("\n").map((line) => line.trim()).filter(Boolean);
+}
+
+async function listRecentMainCommits(rootDir: string, parentSha: string, lookback: number): Promise<Array<{ sha: string; shortSha: string; subject: string }>> {
+  const entries = normalizeLines(await git(rootDir, ["log", `--format=%H~%h~%s`, `-n`, String(lookback), parentSha]));
+  return entries
+    .map((entry) => {
+      const [sha, shortSha, ...subjectParts] = entry.split("~");
+      const subject = subjectParts.join("~").trim();
+      if (!sha?.trim() || !shortSha?.trim() || !subject) {
+        return null;
+      }
+      return {
+        sha: sha.trim(),
+        shortSha: shortSha.trim(),
+        subject,
+      };
+    })
+    .filter((entry): entry is { sha: string; shortSha: string; subject: string } => entry !== null);
+}
+
+function normalizeLookback(value: number | undefined): number {
+  if (!Number.isFinite(value) || !value || value < 1) {
+    return DEFAULT_LOOKBACK;
+  }
+  return Math.trunc(value);
+}

packages/engine/src/merger.ts

@@ -70,6 +70,7 @@ import type { ToolDefinition } from "@mariozechner/pi-coding-agent";
 import { Type } from "typebox";
 import { createRunAuditor, generateSyntheticRunId, type EngineRunContext } from "./run-audit.js";
 import { createWebFetchTool } from "./agent-tools.js";
+import { auditSquashMerge, type SquashAuditFindings } from "./merger-squash-audit.js";
 
 /** Conflict type classification for merge conflict resolution */
 export type ConflictType =
@@ -475,6 +476,17 @@ export class MergeAbortedError extends Error {
   }
 }
 
+export class SquashAuditError extends Error {
+  constructor(
+    taskId: string,
+    public readonly squashSha: string,
+    public readonly findings: SquashAuditFindings,
+  ) {
+    super(buildSquashAuditBlockingMessage(taskId, squashSha, findings));
+    this.name = "SquashAuditError";
+  }
+}
+
 export function throwIfAborted(signal: AbortSignal | undefined, taskId: string): void {
   if (!signal?.aborted) return;
   throw new MergeAbortedError(`Merge aborted for ${taskId}: engine shutdown requested`);
@@ -4143,6 +4155,46 @@ function quoteArg(value: string): string {
   return `"${value.replace(/(["\\$`])/g, "\\$1")}"`;
 }
 
+function shouldRunPostSquashAudit(result: MergeResult, mergeWasEmpty: boolean, isEmptyCommit: boolean, commitSha?: string): boolean {
+  if (mergeWasEmpty || isEmptyCommit || !commitSha) {
+    return false;
+  }
+  return (result.autoResolvedCount ?? 0) > 0 || result.attemptsMade === 3;
+}
+
+function buildSquashAuditBlockingMessage(taskId: string, squashSha: string, findings: SquashAuditFindings): string {
+  const riskParts: string[] = [];
+  if (findings.duplicateSubjects.length > 0) {
+    riskParts.push(`${findings.duplicateSubjects.length} duplicate-subject risk${findings.duplicateSubjects.length === 1 ? "" : "s"}`);
+  }
+  if (findings.touchedFileOverlaps.length > 0) {
+    riskParts.push(`${findings.touchedFileOverlaps.length} touched-file overlap risk${findings.touchedFileOverlaps.length === 1 ? "" : "s"}`);
+  }
+  const summary = riskParts.length > 0 ? riskParts.join(", ") : `${findings.issueCount} audit finding(s)`;
+  return `${taskId}: post-squash audit blocked auto-completion for ${squashSha.slice(0, 8)} (${summary})`;
+}
+
+function formatSquashAuditAgentLog(findings: SquashAuditFindings): string {
+  const lines: string[] = [];
+  if (findings.duplicateSubjects.length > 0) {
+    lines.push("Duplicate-subject risks:");
+    for (const duplicate of findings.duplicateSubjects) {
+      lines.push(`- ${duplicate.subject}`);
+    }
+  }
+  if (findings.touchedFileOverlaps.length > 0) {
+    if (lines.length > 0) lines.push("");
+    lines.push("Touched-file overlap risks:");
+    for (const overlap of findings.touchedFileOverlaps) {
+      lines.push(`- ${overlap.file}`);
+      for (const commit of overlap.recentMainCommits) {
+        lines.push(`  - ${commit.sha} ${commit.subject}`);
+      }
+    }
+  }
+  return lines.join("\n");
+}
+
 /**
  * Resolve a non-empty commit body for fallback merge commits. Used by sites
  * that would otherwise emit `-m ""` when the branch's commit log is empty
@@ -6065,6 +6117,27 @@ export async function aiMergeTask(
     // attemptWithSideStrategy return true without committing when nothing
     // was staged. The recorded HEAD then has nothing to do with this task.
     const recordedSha = (isEmptyCommit || mergeWasEmpty) ? undefined : commitSha;
+
+    const auditSha = recordedSha;
+    if (auditSha && shouldRunPostSquashAudit(result, mergeWasEmpty, isEmptyCommit, auditSha)) {
+      const auditFindings = await auditSquashMerge({
+        rootDir,
+        squashSha: auditSha,
+      });
+      if (!auditFindings.clean) {
+        const auditError = new SquashAuditError(taskId, auditSha, auditFindings);
+        await store.appendAgentLog(
+          taskId,
+          auditError.message,
+          "tool_error",
+          formatSquashAuditAgentLog(auditFindings),
+          "merger",
+        );
+        await store.updateTask(taskId, { status: null });
+        throw auditError;
+      }
+      await store.appendAgentLog(taskId, "post-squash audit clean", "text", undefined, "merger");
+    }
     if (isEmptyCommit) {
       mergerLog.warn(
         `${taskId}: local squash produced an empty commit (${commitSha?.slice(0, 8)}) — branch likely contained dupes of main. Skipping commitSha; recovery will backfill when real commit lands.`,
@@ -6138,6 +6211,9 @@ export async function aiMergeTask(
       "merger",
     );
   } catch (err: any) {
+    if (err instanceof SquashAuditError || err?.name === "SquashAuditError") {
+      throw err;
+    }
     mergerLog.warn(`${taskId}: failed to collect/store merge details: ${err.message}`);
   }