diff --git a/packages/server/review-workspace.test.ts b/packages/server/review-workspace.test.ts
index bf6acef53..6a2a3da0f 100644
--- a/packages/server/review-workspace.test.ts
+++ b/packages/server/review-workspace.test.ts
@@ -24,6 +24,7 @@ import { spawnSync } from "node:child_process";
 import {
   aggregateWorkspacePatch,
   buildLocalWorkspaceReview,
+  isRepoRelative,
   prefixPatchPaths,
   resolveWorkspaceFilePath,
   discoverWorkspaceRepoPaths,
@@ -1127,6 +1128,16 @@ describe("review-workspace", () => {
       expect(workspace.normalizeAnnotationPath(join(api, "src/file.ts"))).toBe("api/src/file.ts");
     });
 
+    it("treats cross-drive relative() results as escaping the repo", () => {
+      expect(isRepoRelative("src/file.ts")).toBe(true);
+      expect(isRepoRelative("nested/deep/file.ts")).toBe(true);
+      expect(isRepoRelative("../outside.ts")).toBe(false);
+      expect(isRepoRelative("")).toBe(false);
+      // On Windows, path.relative returns the target's absolute path when the
+      // base is on a different drive; after normalization that is "L:/...".
+      expect(isRepoRelative("L:/repos/project/src/file.ts")).toBe(false);
+    });
+
     it("keeps requested Git-only workspace modes available when another child repo fails detection", async () => {
       const root = makeTempDir("plannotator-workspace-partial-failure-");
       const api = join(root, "api");
diff --git a/packages/server/review-workspace.ts b/packages/server/review-workspace.ts
index 4dc8803ed..ff8fa45f0 100644
--- a/packages/server/review-workspace.ts
+++ b/packages/server/review-workspace.ts
@@ -15,6 +15,7 @@ import {
 
 export {
   WorkspaceReviewSession,
+  isRepoRelative,
   mapRepoDiffTypeToWorkspaceMode,
   mapWorkspaceModeToRepoDiffType,
   resolveWorkspaceInitialDiffType,
diff --git a/packages/shared/review-workspace.ts b/packages/shared/review-workspace.ts
index 21689d369..23e22aff3 100644
--- a/packages/shared/review-workspace.ts
+++ b/packages/shared/review-workspace.ts
@@ -224,6 +224,14 @@ function aggregateRepos(repos: WorkspaceRepoRuntimeState[]): WorkspaceDiffSnapsh
   };
 }
 
+// Guards a normalizeWorkspacePath(relative(...)) result: on Windows,
+// path.relative cannot relativize across drives and returns the target's
+// absolute path instead (after backslash normalization, e.g. "L:/other/..."),
+// which must be treated as escaping the repo the same as "..".
+export function isRepoRelative(rel: string): boolean {
+  return Boolean(rel) && !rel.startsWith("..") && !rel.startsWith("/") && !/^[A-Za-z]:/.test(rel);
+}
+
 function normalizeAgentPath(root: string, repos: WorkspaceRepoRuntimeState[], filePath: string): string {
   const normalized = normalizeWorkspacePath(filePath);
   if (resolveWorkspaceFilePath(repos, normalized)) return normalized;
@@ -231,13 +239,13 @@ function normalizeAgentPath(root: string, repos: WorkspaceRepoRuntimeState[], fi
   const sorted = [...repos].sort((a, b) => b.cwd.length - a.cwd.length);
   for (const repo of sorted) {
     const rel = normalizeWorkspacePath(relative(repo.cwd, filePath));
-    if (rel && !rel.startsWith("..") && !rel.startsWith("/")) {
+    if (isRepoRelative(rel)) {
       return `${normalizeWorkspacePath(repo.label)}/${rel}`;
     }
   }
 
   const rootRel = normalizeWorkspacePath(relative(root, filePath));
-  if (rootRel && !rootRel.startsWith("..") && !rootRel.startsWith("/")) {
+  if (isRepoRelative(rootRel)) {
     if (resolveWorkspaceFilePath(repos, rootRel)) return rootRel;
   }
 
@@ -246,7 +254,7 @@ function normalizeAgentPath(root: string, repos: WorkspaceRepoRuntimeState[], fi
     return `${normalizeWorkspacePath(changedRepos[0].label)}/${normalized}`;
   }
 
-  if (rootRel && !rootRel.startsWith("..") && !rootRel.startsWith("/")) return rootRel;
+  if (isRepoRelative(rootRel)) return rootRel;
   return normalized;
 }