diff --git a/src/oauth/store.ts b/src/oauth/store.ts
index 287aa38df..478f9c73f 100644
--- a/src/oauth/store.ts
+++ b/src/oauth/store.ts
@@ -286,15 +286,18 @@ function normalizeCredential(cred: unknown): OAuthCredentials | null {
 }
 
 /**
- * Stable short account id. MUST be deterministic for a given credential: legacy
- * single-credential stores are re-normalized on EVERY load without being persisted,
+ * Stable collision-resistant account id. MUST be deterministic for a given credential:
+ * legacy single-credential stores are re-normalized on EVERY load without being persisted,
  * so a time-salted id would differ between two loads (getAccountSet vs
  * getAccountCredential), surfacing as a spurious OAuthLoginRequiredError and making
  * refresh persists silently miss the account (rotated refresh token lost).
+ *
+ * Keep 128 bits of SHA-256 rather than the historical 32-bit prefix. Existing persisted
+ * account ids are read as-is; only newly-derived ids and legacy normalization use this width.
  */
 function newAccountId(cred: OAuthCredentials): string {
   const identity = cred.accountId ?? cred.email ?? cred.refresh;
-  return createHash("sha256").update(identity).digest("hex").slice(0, 8);
+  return createHash("sha256").update(identity).digest("hex").slice(0, 32);
 }
 
 function normalizeAccount(value: unknown): ProviderAccount | null {
diff --git a/tests/oauth-account-id-collision.test.ts b/tests/oauth-account-id-collision.test.ts
new file mode 100644
index 000000000..3ff6e31cc
--- /dev/null
+++ b/tests/oauth-account-id-collision.test.ts
@@ -0,0 +1,100 @@
+import { afterEach, beforeEach, describe, expect, test } from "bun:test";
+import { existsSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+import {
+  resetHardenedStateForTests,
+  setIcaclsRunnerForTests,
+} from "../src/lib/windows-secret-acl";
+import {
+  getAccountSet,
+  getCredential,
+  saveCredential,
+} from "../src/oauth/store";
+
+const TEST_DIR = join(import.meta.dir, ".tmp-oauth-account-id-collision-test");
+let previousOpencodexHome: string | undefined;
+
+const COLLIDING_ACCOUNT_A = "account-collision-16138";
+const COLLIDING_ACCOUNT_B = "account-collision-28806";
+
+describe("OAuth account id collision hardening", () => {
+  beforeEach(() => {
+    previousOpencodexHome = process.env.OPENCODEX_HOME;
+    if (existsSync(TEST_DIR)) rmSync(TEST_DIR, { recursive: true });
+    mkdirSync(TEST_DIR, { recursive: true });
+    process.env.OPENCODEX_HOME = TEST_DIR;
+    resetHardenedStateForTests();
+    setIcaclsRunnerForTests(() => ({
+      success: true,
+      exitCode: 0,
+      timedOut: false,
+      stdout: "",
+    }));
+  });
+
+  afterEach(() => {
+    setIcaclsRunnerForTests(null);
+    resetHardenedStateForTests();
+    if (previousOpencodexHome === undefined) delete process.env.OPENCODEX_HOME;
+    else process.env.OPENCODEX_HOME = previousOpencodexHome;
+    if (existsSync(TEST_DIR)) rmSync(TEST_DIR, { recursive: true });
+  });
+
+  test("distinct identities that collide on the historical 32-bit prefix get distinct slots", async () => {
+    // sha256(account-collision-16138) and sha256(account-collision-28806) both
+    // start with da1e26d2. The historical 8-hex id therefore aliased these
+    // two accounts and made active-account lookup return the wrong credential.
+    await saveCredential("anthropic", {
+      access: "access-a",
+      refresh: "refresh-a",
+      expires: Date.now() + 3600_000,
+      accountId: COLLIDING_ACCOUNT_A,
+    });
+    await saveCredential("anthropic", {
+      access: "access-b",
+      refresh: "refresh-b",
+      expires: Date.now() + 3600_000,
+      accountId: COLLIDING_ACCOUNT_B,
+    });
+
+    const set = getAccountSet("anthropic");
+    expect(set).not.toBeNull();
+    expect(set!.accounts).toHaveLength(2);
+    expect(new Set(set!.accounts.map(account => account.id)).size).toBe(2);
+    expect(set!.accounts.every(account => account.id.length === 32)).toBe(true);
+    expect(getCredential("anthropic")?.accountId).toBe(COLLIDING_ACCOUNT_B);
+    expect(getCredential("anthropic")?.access).toBe("access-b");
+  });
+
+  test("existing persisted 32-bit account ids remain valid and are not rewritten", async () => {
+    const authPath = join(TEST_DIR, "auth.json");
+    writeFileSync(authPath, JSON.stringify({
+      anthropic: {
+        activeAccountId: "deadbeef",
+        accounts: [{
+          id: "deadbeef",
+          credential: {
+            access: "old-access",
+            refresh: "old-refresh",
+            expires: Date.now() + 3600_000,
+            accountId: "existing-account",
+          },
+        }],
+      },
+    }));
+
+    expect(getAccountSet("anthropic")?.activeAccountId).toBe("deadbeef");
+
+    await saveCredential("anthropic", {
+      access: "rotated-access",
+      refresh: "rotated-refresh",
+      expires: Date.now() + 7200_000,
+      accountId: "existing-account",
+    });
+
+    const set = getAccountSet("anthropic");
+    expect(set?.activeAccountId).toBe("deadbeef");
+    expect(set?.accounts[0]?.id).toBe("deadbeef");
+    expect(getCredential("anthropic")?.access).toBe("rotated-access");
+  });
+});