diff --git a/packages/coding-agent/src/core/extensions/builtin/rules/changes.md b/packages/coding-agent/src/core/extensions/builtin/rules/changes.md
index 85e797ee2e..d2c10a8886 100644
--- a/packages/coding-agent/src/core/extensions/builtin/rules/changes.md
+++ b/packages/coding-agent/src/core/extensions/builtin/rules/changes.md
@@ -7,7 +7,8 @@ Vendored from [`code-yeongyu/pi-rules`](https://github.com/code-yeongyu/pi-rules
 - Imports rewritten by `scripts/vendor-transform.mjs`: `@mariozechner/pi-tui` -> `@earendil-works/pi-tui`; `@mariozechner/pi-coding-agent` symbols -> `../../types.ts` (and `Theme` -> `modes/interactive/theme/theme.ts`); relative `.js` import suffixes -> `.ts`.
 - `ui/dynamic-border.ts` and `ui/rules-banner.ts`: constructor parameter properties (`private readonly …`) -> explicit fields + constructor assignment (senpi's root tsconfig is `erasableSyntaxOnly`; parameter properties are disallowed).
 - Runtime dep `picomatch` (+ `@types/picomatch`) added to `package.json`.
-- No behavior changes. Registers `/rules` and `/reload-rules` and discovers rule files from `.sisyphus/rules`, `.claude/rules`, `.cursor/rules`, `.github/instructions`, `AGENTS.md`, `CLAUDE.md`.
+- `rules/project-root.ts`: `findProjectRoot` stops the marker walk when `dirname()` stops progressing, fixing an infinite synchronous loop for targets on a different Windows drive than cwd (or UNC shares). Behavior fix also submitted upstream: https://github.com/code-yeongyu/pi-rules/pull/19 — drop this adaptation once a fixed pi-rules release is re-vendored.
+- No other behavior changes. Registers `/rules` and `/reload-rules` and discovers rule files from `.sisyphus/rules`, `.claude/rules`, `.cursor/rules`, `.github/instructions`, `AGENTS.md`, `CLAUDE.md`.
 
 ## Conflict zones
 
diff --git a/packages/coding-agent/src/core/extensions/builtin/rules/rules/project-root.ts b/packages/coding-agent/src/core/extensions/builtin/rules/rules/project-root.ts
index 2c3f387309..bf2c4decc4 100644
--- a/packages/coding-agent/src/core/extensions/builtin/rules/rules/project-root.ts
+++ b/packages/coding-agent/src/core/extensions/builtin/rules/rules/project-root.ts
@@ -25,6 +25,13 @@ export function findProjectRoot(startPath: string, markers: ReadonlyArray<string
 			return null;
 		}
 
-		currentDirectory = dirname(currentDirectory);
+		const parentDirectory = dirname(currentDirectory);
+		// Cross-drive / UNC walk: dirname() of a drive root returns the root itself,
+		// and a root on a different drive than cwd never equals resolve("/"), so the
+		// walk would spin forever without this guard.
+		if (parentDirectory === currentDirectory) {
+			return null;
+		}
+		currentDirectory = parentDirectory;
 	}
 }
diff --git a/packages/coding-agent/test/suite/regressions/0000-rules-find-project-root-cross-drive.test.ts b/packages/coding-agent/test/suite/regressions/0000-rules-find-project-root-cross-drive.test.ts
new file mode 100644
index 0000000000..a15f8567c6
--- /dev/null
+++ b/packages/coding-agent/test/suite/regressions/0000-rules-find-project-root-cross-drive.test.ts
@@ -0,0 +1,36 @@
+import { win32 } from "node:path";
+import { describe, expect, it, vi } from "vitest";
+import { findProjectRoot } from "../../../src/core/extensions/builtin/rules/rules/project-root.ts";
+
+const { cwdRoot, startPath } = vi.hoisted(() => ({
+	cwdRoot: "E:\\",
+	startPath: "C:\\workspace\\plain\\nested",
+}));
+
+vi.mock("node:fs", () => ({
+	existsSync: (path: string) => path === startPath,
+	statSync: () => ({ isDirectory: () => true }),
+}));
+
+vi.mock("node:path", async (importOriginal) => {
+	const path = await importOriginal<typeof import("node:path")>();
+	return {
+		...path,
+		dirname: path.win32.dirname,
+		join: path.win32.join,
+		resolve: (input: string) => (input === "/" ? cwdRoot : path.win32.resolve(input)),
+	};
+});
+
+describe("rules findProjectRoot cross-drive termination", () => {
+	it("#given startPath on a different drive than cwd (Windows cross-drive) #when finding root #then terminates with null instead of looping forever", () => {
+		// given
+		expect(win32.dirname("C:\\")).toBe("C:\\");
+
+		// when
+		const result = findProjectRoot(startPath, [".nonexistent-marker-senpi"]);
+
+		// then
+		expect(result).toBeNull();
+	});
+});