diff --git a/packages/pi-fff/src/aux-finders.ts b/packages/pi-fff/src/aux-finders.ts
index 8bec5b75b..6b1fd4713 100644
--- a/packages/pi-fff/src/aux-finders.ts
+++ b/packages/pi-fff/src/aux-finders.ts
@@ -128,6 +128,14 @@ export function resolveAuxRoot(
   return null;
 }
 
+export function isOutsideWorkspaceRelativePath(relativePath: string): boolean {
+  return (
+    path.isAbsolute(relativePath) ||
+    relativePath === ".." ||
+    relativePath.startsWith(`..${path.sep}`)
+  );
+}
+
 // Decide whether a `path` parameter should route to the workspace finder or
 // to an aux finder. Accepts absolute paths, `~`-prefixed paths, and relative
 // paths escaping the workspace (`../other-project`); everything is resolved
@@ -147,7 +155,7 @@ export function routePathConstraint(
     candidate = path.resolve(cwd, candidate);
   }
   const rel = path.relative(cwd, candidate);
-  if (rel !== ".." && !rel.startsWith(`..${path.sep}`)) return null;
+  if (!isOutsideWorkspaceRelativePath(rel)) return null;
   return resolveAuxRoot(candidate);
 }
 
diff --git a/packages/pi-fff/test/aux-finders.test.ts b/packages/pi-fff/test/aux-finders.test.ts
index 0b829bf92..7237aa030 100644
--- a/packages/pi-fff/test/aux-finders.test.ts
+++ b/packages/pi-fff/test/aux-finders.test.ts
@@ -3,6 +3,7 @@ import fs from "node:fs";
 import os from "node:os";
 import path from "node:path";
 import {
+  isOutsideWorkspaceRelativePath,
   resolveAuxRoot,
   rootCovers,
   routePathConstraint,
@@ -28,6 +29,14 @@ describe("routePathConstraint", () => {
     expect(route).toEqual({ root: "/tmp", suffix: "" });
   });
 
+  if (process.platform === "win32") {
+    test("treats a cross-volume relative result as outside the workspace", () => {
+      const rel = path.win32.relative("D:\\workspace", "C:\\target");
+      expect(rel).toBe("C:\\target");
+      expect(isOutsideWorkspaceRelativePath(rel)).toBe(true);
+    });
+  }
+
   test("splits glob suffix from existing dir prefix", () => {
     const route = routePathConstraint("/tmp/**/*.ts", cwd);
     expect(route).toEqual({ root: "/tmp", suffix: "**/*.ts" });