fix(FN-XXX): harden windows path handling

packages/engine/src/__tests__/hybrid-executor.test.ts

@@ -433,5 +433,20 @@ describe("HybridExecutor", () => {
         executor.updateProject("non-existent", { maxConcurrent: 4 })
       ).rejects.toThrow("Runtime not found");
     });
+
+    it("reuses the registered project path when isolation mode changes without an explicit working directory", async () => {
+      const manager = mockProjectManagerInstances[0];
+      manager?.addProject.mockClear();
+
+      await executor.updateProject("proj_test123", { isolationMode: "child-process" });
+
+      expect(mockCentralCore.getProject).toHaveBeenCalledWith("proj_test123");
+      expect(manager?.removeProject).toHaveBeenCalledWith("proj_test123");
+      expect(manager?.addProject).toHaveBeenCalledWith(expect.objectContaining({
+        projectId: "proj_test123",
+        workingDirectory: "/tmp/test-project",
+        isolationMode: "child-process",
+      }));
+    });
   });
 });

packages/engine/src/__tests__/pi.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
-import { describeModel, compactSessionContext, COMPACTION_FALLBACK_INSTRUCTIONS, createFnAgent, promptWithFallback, type AgentOptions } from "../pi.js";
+import { describeModel, compactSessionContext, COMPACTION_FALLBACK_INSTRUCTIONS, createFnAgent, getProjectRootFromWorktree, promptWithFallback, type AgentOptions } from "../pi.js";
 import { createAgentSession, type AgentSession } from "@mariozechner/pi-coding-agent";
 import { piLog } from "../logger.js";
 
@@ -78,6 +78,18 @@ vi.mock("@mariozechner/pi-coding-agent", () => ({
 let resolveSessionSkillsMock: ReturnType<typeof vi.fn>;
 let createSkillsOverrideFromSelectionMock: ReturnType<typeof vi.fn>;
 
+describe("getProjectRootFromWorktree", () => {
+  it("detects POSIX worktree paths", () => {
+    expect(getProjectRootFromWorktree("/repo/.worktrees/fn-001")).toBe("/repo");
+    expect(getProjectRootFromWorktree("/repo/.worktrees/fn-001/src/file.ts")).toBe("/repo");
+  });
+
+  it("detects Windows worktree paths", () => {
+    expect(getProjectRootFromWorktree("C:\\repo\\.worktrees\\fn-001")).toBe("C:\\repo");
+    expect(getProjectRootFromWorktree("C:\\repo\\.worktrees\\fn-001\\src\\file.ts")).toBe("C:\\repo");
+  });
+});
+
 // Initialize mocks before first test
 beforeEach(() => {
   // Access mocks from the mocked module

packages/engine/src/hybrid-executor.ts

@@ -274,13 +274,19 @@ export class HybridExecutor extends EventEmitter<HybridExecutorEvents> {
         `Isolation mode changed for ${projectId}: ${currentMode} → ${config.isolationMode}`
       );
 
+      const project = await this.centralCore.getProject(projectId);
+      const workingDirectory = config.workingDirectory ?? project?.path;
+      if (!workingDirectory) {
+        throw new Error(`Project not found in CentralCore: ${projectId}`);
+      }
+
       // Stop old runtime
       await this.projectManager.removeProject(projectId);
 
       // Get the full current config
       const fullConfig: ProjectRuntimeConfig = {
         projectId,
-        workingDirectory: config.workingDirectory ?? "/tmp",
+        workingDirectory,
         isolationMode: config.isolationMode,
         maxConcurrent: config.maxConcurrent ?? 2,
         maxWorktrees: config.maxWorktrees ?? 4,

packages/engine/src/pi.ts

@@ -791,7 +791,7 @@ async function registerExtensionProviders(cwd: string, modelRegistry: ModelRegis
  *   `/project/.worktrees/fn-001/src/file.ts` → `/project`
  *   `/project` → null (not a worktree)
  */
-function getProjectRootFromWorktree(cwd: string): string | null {
+export function getProjectRootFromWorktree(cwd: string): string | null {
   // Match paths like:
   //   /project/.worktrees/task-id
   //   /project/.worktrees/task-id/src/file.ts