fix(FN-XXX): harden windows path handling

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

@@ -0,0 +1,38 @@
+import { describe, expect, it } from "vitest";
+import { formatPiExtensionSource } from "../pi-extensions.js";
+
+describe("formatPiExtensionSource", () => {
+  it("formats project-relative extension paths", () => {
+    expect(
+      formatPiExtensionSource(
+        "fusion-project",
+        "/repo/.fusion/extensions/tooling/index.ts",
+        "/repo",
+        "/Users/alice",
+      ),
+    ).toBe("fusion-project: .fusion/extensions/tooling/index.ts");
+  });
+
+  it("formats home-relative Windows extension paths", () => {
+    expect(
+      formatPiExtensionSource(
+        "fusion-global",
+        "C:\\Users\\alice\\.fusion\\agent\\extensions\\tooling\\index.ts",
+        "C:\\repo",
+        "C:\\Users\\alice",
+      ),
+    ).toBe("fusion-global: ~/.fusion/agent/extensions/tooling/index.ts");
+  });
+
+  it("does not treat prefix-matching sibling paths as inside the project or home", () => {
+    const projectLookalike = "C:\\repo-two\\.fusion\\extensions\\tooling\\index.ts";
+    const homeLookalike = "C:\\Users\\alice-dev\\.fusion\\agent\\extensions\\tooling\\index.ts";
+
+    expect(
+      formatPiExtensionSource("fusion-project", projectLookalike, "C:\\repo", "C:\\Users\\alice"),
+    ).toBe(`fusion-project: ${projectLookalike}`);
+    expect(
+      formatPiExtensionSource("fusion-global", homeLookalike, "C:\\repo", "C:\\Users\\alice"),
+    ).toBe(`fusion-global: ${homeLookalike}`);
+  });
+});

packages/core/src/__tests__/setup-test-isolation.test.ts

@@ -8,10 +8,12 @@ const TEMP_HOME_PREFIX = "fn-test-home-";
 describe("test isolation setup", () => {
   it("process.env.HOME is overridden to a temp directory", () => {
     const home = process.env.HOME;
+    const userProfile = process.env.USERPROFILE;
 
     expect(home).toBeDefined();
     expect(home).toContain(tmpdir());
     expect(home).toContain(TEMP_HOME_PREFIX);
+    expect(userProfile).toBe(home);
   });
 
   it("homedir() resolves to the temp HOME", () => {

packages/core/src/pi-extensions.ts

@@ -1,6 +1,6 @@
 import { existsSync, mkdirSync, readdirSync, readFileSync, statSync, writeFileSync } from "node:fs";
 import { homedir } from "node:os";
-import { basename, join, relative, resolve, sep } from "node:path";
+import { basename, isAbsolute, join, relative, resolve, sep, win32 } from "node:path";
 
 const FUSION_DISABLED_EXTENSIONS_KEY = "fusionDisabledExtensions";
 
@@ -276,13 +276,28 @@ export function reconcileClaudeCliPaths(
   return filtered;
 }
 
+function getDisplayPathWithinRoot(root: string, targetPath: string): string | null {
+  const usesWindowsPaths = /^[A-Za-z]:[\\/]/.test(root) || /^[A-Za-z]:[\\/]/.test(targetPath) || root.includes("\\") || targetPath.includes("\\");
+  const pathApi = usesWindowsPaths ? win32 : { relative, isAbsolute, sep };
+  const rel = pathApi.relative(root, targetPath);
+  if (rel === "") {
+    return "";
+  }
+  if (!rel || rel === ".." || rel.startsWith(`..${pathApi.sep}`) || pathApi.isAbsolute(rel)) {
+    return null;
+  }
+  return rel.split(pathApi.sep).join("/");
+}
+
 export function formatPiExtensionSource(source: PiExtensionSource, extensionPath: string, cwd: string, home?: string): string {
   const homeDir = getHomeDir(home);
   const projectRoot = resolvePiExtensionProjectRoot(cwd);
-  const relativePath = extensionPath.startsWith(homeDir)
-    ? `~${extensionPath.slice(homeDir.length)}`
-    : extensionPath.startsWith(projectRoot)
-      ? relative(projectRoot, extensionPath).split(sep).join("/")
+  const relativeToHome = getDisplayPathWithinRoot(homeDir, extensionPath);
+  const relativeToProject = getDisplayPathWithinRoot(projectRoot, extensionPath);
+  const relativePath = relativeToHome !== null
+    ? relativeToHome.length > 0 ? `~/${relativeToHome}` : "~"
+    : relativeToProject !== null
+      ? relativeToProject || "."
       : extensionPath;
   return `${source}: ${relativePath}`;
 }