fix(FN-6079): resolve task tools from merge worktrees

packages/core/src/__tests__/pi-extensions.test.ts

@@ -1,9 +1,14 @@
 import { describe, expect, it } from "vitest";
-import { mkdtempSync, mkdirSync, rmSync } from "node:fs";
+import { mkdtempSync, mkdirSync, realpathSync, rmSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
+import { execSync } from "node:child_process";
 import { getProjectRootFromWorktree, resolvePiExtensionProjectRoot } from "../pi-extensions.js";
 
+function git(cwd: string, args: string): string {
+  return execSync(`git ${args}`, { cwd, encoding: "utf8", stdio: ["ignore", "pipe", "pipe"] }).trim();
+}
+
 describe("getProjectRootFromWorktree", () => {
   it("detects POSIX worktree paths", () => {
     expect(getProjectRootFromWorktree("/repo/.worktrees/fn-001")).toBe("/repo");
@@ -32,6 +37,35 @@ describe("getProjectRootFromWorktree", () => {
       }),
     ).toBe("/tmp");
   });
+
+  it("detects arbitrary Git linked worktree paths when the parent has Fusion metadata", () => {
+    const root = mkdtempSync(join(tmpdir(), "fn-6079-root-"));
+    const worktreeRoot = mkdtempSync(join(tmpdir(), "fusion-ai-merge-fn-6079-"));
+    try {
+      const expectedRoot = realpathSync(root);
+      git(root, "init -q -b main");
+      git(root, "config user.email test@example.com");
+      git(root, "config user.name Test");
+      mkdirSync(join(root, ".fusion"), { recursive: true });
+      writeFileSync(join(root, "base.txt"), "base\n");
+      git(root, "add -A");
+      git(root, "commit -q -m base");
+      git(root, `worktree add --detach ${JSON.stringify(worktreeRoot)} HEAD`);
+      mkdirSync(join(worktreeRoot, "subdir"), { recursive: true });
+
+      expect(getProjectRootFromWorktree(worktreeRoot)).toBe(expectedRoot);
+      expect(getProjectRootFromWorktree(join(worktreeRoot, "subdir"))).toBe(expectedRoot);
+      expect(resolvePiExtensionProjectRoot(worktreeRoot)).toBe(expectedRoot);
+    } finally {
+      try {
+        git(root, `worktree remove --force ${JSON.stringify(worktreeRoot)}`);
+      } catch {
+        // best effort cleanup
+      }
+      rmSync(worktreeRoot, { recursive: true, force: true });
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
 });
 
 describe("@fusion/core export surface", () => {

packages/core/src/pi-extensions.ts

@@ -1,6 +1,7 @@
 import { existsSync, mkdirSync, readdirSync, readFileSync, statSync, writeFileSync } from "node:fs";
 import { homedir } from "node:os";
-import { basename, isAbsolute, join, relative, resolve, sep, win32 } from "node:path";
+import { basename, dirname, isAbsolute, join, relative, resolve, sep, win32 } from "node:path";
+import { spawnSync } from "node:child_process";
 
 const FUSION_DISABLED_EXTENSIONS_KEY = "fusionDisabledExtensions";
 
@@ -66,9 +67,42 @@ export function getProjectRootFromWorktree(
     }
   }
 
+  const gitLinkedWorktreeRoot = getProjectRootFromGitLinkedWorktree(cwd);
+  if (gitLinkedWorktreeRoot) {
+    return gitLinkedWorktreeRoot;
+  }
+
   return null;
 }
 
+function getProjectRootFromGitLinkedWorktree(cwd: string): string | null {
+  const resolvedCwd = resolve(cwd);
+  const commonDir = spawnSync("git", ["rev-parse", "--git-common-dir"], {
+    cwd: resolvedCwd,
+    encoding: "utf8",
+    stdio: ["ignore", "pipe", "ignore"],
+  });
+  const gitDir = spawnSync("git", ["rev-parse", "--git-dir"], {
+    cwd: resolvedCwd,
+    encoding: "utf8",
+    stdio: ["ignore", "pipe", "ignore"],
+  });
+  if (commonDir.status !== 0 || gitDir.status !== 0) {
+    return null;
+  }
+
+  const resolvedCommonDir = resolve(resolvedCwd, commonDir.stdout.trim());
+  const resolvedGitDir = resolve(resolvedCwd, gitDir.stdout.trim());
+  if (!resolvedCommonDir || !resolvedGitDir || resolvedCommonDir === resolvedGitDir) {
+    return null;
+  }
+
+  const parentRoot = resolvedCommonDir.endsWith(`${sep}.git`)
+    ? dirname(resolvedCommonDir)
+    : resolvedCommonDir;
+  return existsSync(join(parentRoot, ".fusion")) ? parentRoot : null;
+}
+
 export function resolvePiExtensionProjectRoot(cwd: string): string {
   const worktreeProjectRoot = getProjectRootFromWorktree(cwd);
   if (worktreeProjectRoot && existsSync(join(worktreeProjectRoot, ".fusion"))) {