diff --git a/src/helpers.ts b/src/helpers.ts
index fc2070e..e9a6b13 100644
--- a/src/helpers.ts
+++ b/src/helpers.ts
@@ -8,7 +8,7 @@
 
 import { z } from "zod";
 import { existsSync } from "node:fs";
-import { resolve } from "node:path";
+import { isAbsolute, resolve } from "node:path";
 
 /* eslint-disable @typescript-eslint/no-explicit-any */
 
@@ -81,24 +81,34 @@ export function applyModelDefaults(
 
 /**
  * Normalize and validate a directory path:
- *  - Resolves to absolute (handles "..", trailing slashes, etc.)
- *  - Rejects relative paths (must start with /)
+ *  - Resolves to absolute (handles "..", ".", trailing slashes, and
+ *    converts relative inputs against `process.cwd()`)
+ *  - Confirms the resolved path is absolute for the current platform
  *  - Validates that the path exists on disk
  *
+ * Accepts both POSIX ("/home/user/my-project") and Windows
+ * ("C:\\Users\\me\\my-project", "\\\\server\\share") absolute paths via
+ * the platform-aware `resolve` + `isAbsolute` from `node:path`.
+ *
  * Returns the normalized path, or undefined if input was undefined.
  * Throws a descriptive Error on validation failure.
  */
 export function normalizeDirectory(directory?: string): string | undefined {
   if (!directory) return undefined;
 
-  // Resolve to absolute (handles "..", ".", trailing slashes)
+  // Resolve to an absolute, platform-appropriate form. `resolve` handles
+  // "..", ".", trailing slashes, and will convert a relative input against
+  // `process.cwd()`.
   const normalized = resolve(directory);
 
-  // Must be an absolute path
-  if (!normalized.startsWith("/")) {
+  // Defensive check: `resolve` guarantees an absolute path on every
+  // supported platform, but we verify via the platform-aware `isAbsolute`
+  // so callers get a clear error if that assumption is ever violated.
+  if (!isAbsolute(normalized)) {
     throw new Error(
       `Invalid directory: "${directory}" is not an absolute path. ` +
-        `Provide a full path like "/home/user/my-project".`,
+        `Provide a full path like "/home/user/my-project" (POSIX) or ` +
+        `"C:\\\\Users\\\\me\\\\my-project" (Windows).`,
     );
   }
 
diff --git a/tests/helpers.test.ts b/tests/helpers.test.ts
index bb718c4..b942a84 100644
--- a/tests/helpers.test.ts
+++ b/tests/helpers.test.ts
@@ -927,6 +927,29 @@ describe("normalizeDirectory", () => {
     const result = normalizeDirectory("/tmp");
     expect(result).toBe("/tmp");
   });
+
+  it("accepts the process working directory on any platform", () => {
+    // Platform-agnostic regression: process.cwd() is always absolute for the
+    // current platform, so normalizeDirectory must accept it regardless of OS.
+    const result = normalizeDirectory(process.cwd());
+    expect(result).toBeDefined();
+  });
+
+  // Regression for Windows drive-letter paths.
+  // Before the fix, normalizeDirectory required the resolved path to start
+  // with "/", which meant Windows absolute paths like "C:\\Users\\..." were
+  // rejected. The fix switches to the platform-aware `path.isAbsolute`, so
+  // this test guards the Windows path on a Windows CI runner.
+  it.runIf(process.platform === "win32")(
+    "accepts Windows drive-letter absolute paths",
+    () => {
+      const cwd = process.cwd(); // e.g. "C:\\Users\\runner\\work\\..."
+      expect(cwd).toMatch(/^[A-Za-z]:\\/);
+      const result = normalizeDirectory(cwd);
+      expect(result).toBeDefined();
+      expect(result).toMatch(/^[A-Za-z]:\\/);
+    },
+  );
 });
 
 // ─── diagnoseError (via toolError) ───────────────────────────────────────