diff --git a/src/domain/localPath.test.ts b/src/domain/localPath.test.ts
index 1bf3290d91..0cc8721870 100644
--- a/src/domain/localPath.test.ts
+++ b/src/domain/localPath.test.ts
@@ -1,22 +1,27 @@
-import { isAbsolute } from "node:path";
+import { isAbsolute, win32 } from "node:path";
 
 import { expect, it } from "vitest";
 
-import { absoluteLocalPathStringSchema } from "./localPath.js";
+import {
+  absoluteLocalPathStringSchema,
+  windowsAbsolutePathPattern,
+} from "./localPath.js";
+
+const PATHS = [
+  "/absolute/path",
+  "/",
+  "relative/path",
+  "C:\\absolute\\path",
+  "c:/absolute/path",
+  "C:relative/path",
+  "\\\\server\\share\\path",
+  "\\rooted\\path",
+  "//server/share/path",
+  "file:///absolute/path",
+];
 
 it("matches the host path module for absolute local paths", () => {
-  for (const path of [
-    "/absolute/path",
-    "/",
-    "relative/path",
-    "C:\\absolute\\path",
-    "c:/absolute/path",
-    "C:relative/path",
-    "\\\\server\\share\\path",
-    "\\rooted\\path",
-    "//server/share/path",
-    "file:///absolute/path",
-  ]) {
+  for (const path of PATHS) {
     expect(absoluteLocalPathStringSchema.safeParse(path).success).toBe(
       isAbsolute(path),
     );
@@ -26,3 +31,17 @@ it("matches the host path module for absolute local paths", () => {
     absoluteLocalPathStringSchema.safeParse("/absolute/with\0nul").success,
   ).toBe(false);
 });
+
+it("keeps the Windows absolute-path pattern valid in every client mode", () => {
+  // Annex B and `u` accept an unescaped `/` inside a class; `v` does not, and a
+  // client that compiles advertised patterns rejects the complete tools request.
+  for (const flags of ["", "u", "v"])
+    expect(
+      () => new RegExp(windowsAbsolutePathPattern.source, flags),
+      flags,
+    ).not.toThrow();
+  for (const path of PATHS)
+    expect(windowsAbsolutePathPattern.test(path), path).toBe(
+      win32.isAbsolute(path),
+    );
+});
diff --git a/src/domain/localPath.ts b/src/domain/localPath.ts
index f6035536e7..8b4a109fd0 100644
--- a/src/domain/localPath.ts
+++ b/src/domain/localPath.ts
@@ -19,9 +19,17 @@ export const nulFreeStringSchema = z
 /** A selected local filesystem path: nonempty and free of NUL bytes. */
 export const localPathStringSchema = nulFreeStringSchema.min(1);
 
+/**
+ * A Windows absolute path: rooted, UNC, or drive-qualified. The classes spell
+ * backslash and slash as `\x5C` and `\x2F` because `v` mode rejects an
+ * unescaped `/` inside a class, and Windows hosts advertise this pattern.
+ */
+export const windowsAbsolutePathPattern =
+  /^(?:[\x5C\x2F]|[A-Za-z]:[\x5C\x2F])/u;
+
 /** A caller-selected path that is absolute on the host running REA. */
 export const absoluteLocalPathStringSchema = localPathStringSchema.regex(
-  process.platform === "win32" ? /^(?:[\\/]|[A-Za-z]:[\\/])/u : /^\//u,
+  process.platform === "win32" ? windowsAbsolutePathPattern : /^\//u,
   "must be an absolute local filesystem path",
 );