diff --git a/src/codex/history-manifest.ts b/src/codex/history-manifest.ts
index 01e649b306..047dd036c4 100644
--- a/src/codex/history-manifest.ts
+++ b/src/codex/history-manifest.ts
@@ -54,8 +54,23 @@ export type CodexHistoryManifestValidation =
       readonly scope: "manifest" | "entry-shape" | "entry-provenance";
     };
 
+/**
+ * Strips Windows extended-length and device prefixes (e.g. \\?\, \\.\, //?/, //./, \\?\UNC\, //?/UNC/)
+ * to allow deterministic path comparison across tools. Pure string operation independent of host OS.
+ */
+export function stripWindowsPathPrefix(path: string): string {
+  if (/^(\\\\(\?|\.)\\UNC\\|\/\/(\?|\.)\/UNC\/)/i.test(path)) {
+    return "\\\\" + path.slice(8);
+  }
+  if (/^(\\\\(\?|\.)\\|\/\/(\?|\.)\/)/.test(path)) {
+    return path.slice(4);
+  }
+  return path;
+}
+
 function codexHistoryPathIdentity(path: string): string {
-  const canonical = resolve(path);
+  const stripped = stripWindowsPathPrefix(path);
+  const canonical = resolve(stripped);
   return process.platform === "win32" ? canonical.toLowerCase() : canonical;
 }
 
diff --git a/tests/codex-integration/codex-history-provider.test.ts b/tests/codex-integration/codex-history-provider.test.ts
index 972245846f..53f298bac3 100644
--- a/tests/codex-integration/codex-history-provider.test.ts
+++ b/tests/codex-integration/codex-history-provider.test.ts
@@ -4,6 +4,7 @@ import { join } from "node:path";
 import { Database } from "bun:sqlite";
 import { afterEach, describe, expect, setDefaultTimeout, test } from "bun:test";
 import { classifyRecoverableHistoryError, countPendingOpencodexHistory, historyBackupPathFor, isRecoverableHistoryError, migrateHistoryToOpenai, restoreLegacyOpenaiHistory, restoredUserEventFor, setAfterNoopPendingCountForTests, setAfterStrictHistoryRolloutAppendForTests, setBeforeHistoryApplyTransactionForTests, setBeforeHistoryBackupConsumeForTests, setBeforeStrictHistoryRolloutAppendForTests, setHistoryDbBusyTimeoutForTests, snapshotCodexHistoryNoop, syncCodexHistoryProvider, withHistoryRetry } from "../../src/codex/history-provider";
+import { sameCodexHistoryPath, stripWindowsPathPrefix } from "../../src/codex/history-manifest";
 import { INVALID_HISTORY_BACKUP_FIXTURES, validHistoryBackupFixture } from "../helpers/codex-history-manifest-fixtures";
 import { preflightCodexHistoryInjection, setHistoryAppendHooksForTests } from "../../src/codex/history-provider";
 
@@ -1513,4 +1514,36 @@ describe("Design B migration helpers", () => {
     expect(db.query("SELECT model_provider FROM threads WHERE id = 'thread-1'").get()).toEqual({ model_provider: "openai" });
     db.close();
   });
+
+  test("stripWindowsPathPrefix normalizes Windows extended prefixes platform-independently", () => {
+    expect(stripWindowsPathPrefix("\\\\?\\C:\\Users\\test\\state.db")).toBe("C:\\Users\\test\\state.db");
+    expect(stripWindowsPathPrefix("\\\\.\\C:\\Users\\test\\state.db")).toBe("C:\\Users\\test\\state.db");
+    expect(stripWindowsPathPrefix("//?/C:/Users/test/state.db")).toBe("C:/Users/test/state.db");
+    expect(stripWindowsPathPrefix("//./C:/Users/test/state.db")).toBe("C:/Users/test/state.db");
+    expect(stripWindowsPathPrefix("\\\\?\\UNC\\server\\share\\path")).toBe("\\\\server\\share\\path");
+    expect(stripWindowsPathPrefix("\\\\?\\unc\\server\\share\\path")).toBe("\\\\server\\share\\path");
+    expect(stripWindowsPathPrefix("//?/UNC/server/share/path")).toBe("\\\\server/share/path");
+    expect(stripWindowsPathPrefix("C:\\standard\\path")).toBe("C:\\standard\\path");
+    expect(stripWindowsPathPrefix("/unix/style/path")).toBe("/unix/style/path");
+    expect(stripWindowsPathPrefix("")).toBe("");
+  });
+
+  test("sameCodexHistoryPath normalizes Windows extended-length and device path prefixes", () => {
+    if (process.platform !== "win32") return;
+    const std = "C:\\Users\\test\\state.db";
+    const ext = "\\\\?\\C:\\Users\\test\\state.db";
+    const dev = "\\\\.\\C:\\Users\\test\\state.db";
+    const fwd = "//?/C:/Users/test/state.db";
+    expect(sameCodexHistoryPath(std, ext)).toBe(true);
+    expect(sameCodexHistoryPath(ext, std)).toBe(true);
+    expect(sameCodexHistoryPath(std, dev)).toBe(true);
+    expect(sameCodexHistoryPath(std, fwd)).toBe(true);
+  });
+
+  test("sameCodexHistoryPath normalizes Windows UNC paths with extended prefix", () => {
+    if (process.platform !== "win32") return;
+    const unc = "\\\\server\\share\\rollout.jsonl";
+    const extUnc = "\\\\?\\UNC\\server\\share\\rollout.jsonl";
+    expect(sameCodexHistoryPath(unc, extUnc)).toBe(true);
+  });
 });