fix(FN-756): reset recycled worktree baselines

packages/engine/src/executor.test.ts

@@ -1318,6 +1318,48 @@ describe("TaskExecutor worktree pool integration", () => {
     expect(pool.size).toBe(0);
   });
 
+  it("overwrites baseCommitSha when starting from a pooled worktree", async () => {
+    const pool = new WorktreePool();
+    pool.release("/tmp/test/.worktrees/idle-wt");
+    mockedExistsSync.mockImplementation((p) => p === "/tmp/test/.worktrees/idle-wt");
+
+    mockedExecSync.mockImplementation((cmd: any) => {
+      if (String(cmd) === "git rev-parse HEAD") {
+        return "newbase123\n" as any;
+      }
+      return "" as any;
+    });
+
+    const store = createMockStore();
+    store.getTask.mockResolvedValue({
+      id: "FN-020",
+      title: "Test",
+      description: "Test",
+      column: "in-progress",
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      prompt: "# test\n## Steps\n### Step 0: Preflight\n- [ ] check",
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+      baseCommitSha: "stale-base",
+    });
+    store.getSettings.mockResolvedValue({
+      maxConcurrent: 2,
+      maxWorktrees: 4,
+      pollIntervalMs: 15000,
+      groupOverlappingFiles: false,
+      autoMerge: false,
+      recycleWorktrees: true,
+    });
+
+    const executor = new TaskExecutor(store, "/tmp/test", { pool });
+    await executor.execute(makeTask());
+
+    expect(store.updateTask).toHaveBeenCalledWith("FN-020", { baseCommitSha: "newbase123" });
+  });
+
   it("creates fresh worktree when pool is empty", async () => {
     const pool = new WorktreePool();
     // Pool is empty

packages/engine/src/executor.ts

@@ -503,9 +503,10 @@ export class TaskExecutor {
         worktreePath = await this.createWorktree(branchName, worktreePath, task.id);
       }
 
-      // Capture the base commit SHA for diff computation
-      // This is done after worktree creation when we're on the new branch
-      if (!task.baseCommitSha) {
+      // Capture the base commit SHA for diff computation whenever a task
+      // starts with a newly assigned worktree. Recycled worktrees must
+      // overwrite any prior task baseline instead of inheriting it.
+      if (!isResume) {
         try {
           const baseCommitSha = execSync("git rev-parse HEAD", {
             cwd: worktreePath,

packages/engine/src/worktree-pool.test.ts

@@ -138,6 +138,11 @@ describe("WorktreePool", () => {
     it("creates branch from main with force-reset", () => {
       pool.prepareForTask("/tmp/wt", "fusion/fn-042");
 
+      expect(mockedExecSync).toHaveBeenCalledWith(
+        "git checkout --detach main",
+        expect.objectContaining({ cwd: "/tmp/wt" }),
+      );
+
       const checkoutCall = mockedExecSync.mock.calls.find(
         (c) => typeof c[0] === "string" && (c[0] as string).includes("checkout -B"),
       );
@@ -157,6 +162,11 @@ describe("WorktreePool", () => {
     it("creates branch from custom startPoint when provided", () => {
       pool.prepareForTask("/tmp/wt", "fusion/fn-042", "fusion/fn-041");
 
+      expect(mockedExecSync).toHaveBeenCalledWith(
+        "git checkout --detach fusion/fn-041",
+        expect.objectContaining({ cwd: "/tmp/wt" }),
+      );
+
       const checkoutCall = mockedExecSync.mock.calls.find(
         (c) => typeof c[0] === "string" && (c[0] as string).includes("checkout -B"),
       );
@@ -176,6 +186,7 @@ describe("WorktreePool", () => {
       // Should still run clean and branch creation
       const calls = mockedExecSync.mock.calls.map((c) => c[0]);
       expect(calls).toContain("git clean -fd");
+      expect(calls).toContain("git checkout --detach main");
       expect(calls).toContain('git checkout -B "fusion/fn-001" main');
     });
 

packages/engine/src/worktree-pool.ts

@@ -108,7 +108,8 @@ export class WorktreePool {
    * Steps performed:
    * 1. `git checkout -- .` — discard tracked file modifications
    * 2. `git clean -fd` — remove untracked files (but not .gitignore'd caches)
-   * 3. `git checkout -B <branchName> <startPoint>` — create/reset branch from start point
+   * 3. `git checkout --detach <startPoint>` — move HEAD to the latest base commit
+   * 4. `git checkout -B <branchName> <startPoint>` — create/reset branch from start point
    *
    * Returns the actual branch name used. This may differ from `branchName`
    * when conflict recovery generates a suffixed name (e.g., `kb/fn-042-2`).
@@ -129,8 +130,13 @@ export class WorktreePool {
     // Remove untracked files (but not .gitignore'd build caches)
     execSync("git clean -fd", { cwd: worktreePath, stdio: "pipe" });
 
-    // Create or force-reset the branch from the start point (or main)
     const base = startPoint || "main";
+    execSync(`git checkout --detach ${base}`, {
+      cwd: worktreePath,
+      stdio: "pipe",
+    });
+
+    // Create or force-reset the branch from the start point (or main)
     const checkoutCmd = `git checkout -B "${branchName}" ${base}`;
     try {
       execSync(checkoutCmd, {