diff --git a/src/lib/db/providers.ts b/src/lib/db/providers.ts
index 4d95a6f484..a59e178cad 100644
--- a/src/lib/db/providers.ts
+++ b/src/lib/db/providers.ts
@@ -572,6 +572,61 @@ export async function updateProviderConnection(id: string, data: JsonRecord) {
   );
 }
 
+/**
+ * Atomic conditional clear of recoverable error state on a connection row.
+ *
+ * Returns true when the row was cleared, false when a concurrent writer
+ * (markAccountUnavailable, connectionRecovery tick, test, etc.) changed the
+ * row between the caller's snapshot read and this UPDATE — in which case the
+ * clear is skipped to preserve the freshest error state. Closes the TOCTOU
+ * window in the quota-recovery path.
+ *
+ * CAS token = (test_status, last_error_at, rate_limited_until).
+ * markAccountUnavailable always bumps last_error_at on every cooldown/error
+ * write, so an unchanged last_error_at reliably indicates no concurrent write.
+ */
+export async function clearConnectionErrorIfUnchanged(
+  id: string,
+  expected: {
+    testStatus: string | null | undefined;
+    lastErrorAt: string | null | undefined;
+    rateLimitedUntil: string | null | undefined;
+  }
+): Promise<boolean> {
+  const db = getDbInstance() as unknown as DbLike;
+  const result = db.prepare(
+    `
+    UPDATE provider_connections SET
+      test_status = 'active',
+      last_error = NULL,
+      last_error_at = NULL,
+      last_error_type = NULL,
+      last_error_source = NULL,
+      error_code = NULL,
+      rate_limited_until = NULL,
+      backoff_level = 0,
+      updated_at = ?
+    WHERE id = ?
+      AND IFNULL(test_status, '') = ?
+      AND IFNULL(last_error_at, '') = ?
+      AND IFNULL(rate_limited_until, '') = ?
+    `
+  ).run(
+    new Date().toISOString(),
+    id,
+    expected.testStatus ?? "",
+    expected.lastErrorAt ?? "",
+    expected.rateLimitedUntil ?? ""
+  );
+  const applied = (result.changes ?? 0) > 0;
+  if (applied) {
+    backupDbFile("pre-write");
+    invalidateDbCache("connections");
+    bumpProxyConfigGeneration();
+  }
+  return applied;
+}
+
 export async function deleteProviderConnection(id: string) {
   const db = getDbInstance() as unknown as DbLike;
   const existing = db.prepare("SELECT provider FROM provider_connections WHERE id = ?").get(id);
diff --git a/src/lib/localDb.ts b/src/lib/localDb.ts
index 366d899ed5..d911599723 100755
--- a/src/lib/localDb.ts
+++ b/src/lib/localDb.ts
@@ -12,6 +12,7 @@ export {
   getProviderConnectionById,
   createProviderConnection,
   updateProviderConnection,
+  clearConnectionErrorIfUnchanged,
   deleteProviderConnection,
   deleteProviderConnections,
   deleteProviderConnectionsByProvider,
diff --git a/src/lib/usage/providerLimits.ts b/src/lib/usage/providerLimits.ts
index cf02868f2d..9b529ca8ba 100644
--- a/src/lib/usage/providerLimits.ts
+++ b/src/lib/usage/providerLimits.ts
@@ -440,21 +440,37 @@ export async function maybeClearRecoveredQuotaState(
 
   if (!hasTransientState) return connection;
 
+  let cleared = true;
   try {
-    await clearRecoveredProviderState({
-      connectionId: connection.id,
-      testStatus: connection.testStatus,
-      lastError: connection.lastError ?? null,
-      rateLimitedUntil: connection.rateLimitedUntil ?? null,
-      errorCode: connection.errorCode ?? null,
-      lastErrorType: connection.lastErrorType ?? null,
-      lastErrorSource: connection.lastErrorSource ?? null,
-    });
+    const result = await clearRecoveredProviderState(
+      {
+        connectionId: connection.id,
+        testStatus: connection.testStatus,
+        lastError: connection.lastError ?? null,
+        rateLimitedUntil: connection.rateLimitedUntil ?? null,
+        errorCode: connection.errorCode ?? null,
+        lastErrorType: connection.lastErrorType ?? null,
+        lastErrorSource: connection.lastErrorSource ?? null,
+      },
+      {
+        testStatus: connection.testStatus ?? null,
+        lastErrorAt: connection.lastErrorAt ?? null,
+        rateLimitedUntil: connection.rateLimitedUntil ?? null,
+      }
+    );
+    cleared = result.applied;
   } catch (dbError) {
     console.warn("[ProviderLimits] Failed to clear recovered quota state:", dbError);
     return connection;
   }
 
+  if (!cleared) {
+    // CAS miss — a concurrent writer (markAccountUnavailable, etc.) updated
+    // the row between our read and the clear. Return the original snapshot;
+    // the next read from DB will surface the fresh state.
+    return connection;
+  }
+
   return {
     ...connection,
     testStatus: "active",
diff --git a/src/sse/services/auth.ts b/src/sse/services/auth.ts
index 0158be94d7..6c07cf7b66 100644
--- a/src/sse/services/auth.ts
+++ b/src/sse/services/auth.ts
@@ -4,6 +4,7 @@ import {
   getProviderNodes,
   validateApiKey,
   updateProviderConnection,
+  clearConnectionErrorIfUnchanged,
   getSettings,
   getCachedSettings,
 } from "@/lib/localDb";
@@ -2266,11 +2267,41 @@ export async function clearAccountError(
   log.info("AUTH", `Account ${connectionId.slice(0, 8)} error cleared`);
 }
 
+/**
+ * Optional CAS token. When provided, the clear is performed via an atomic
+ * conditional UPDATE (clearConnectionErrorIfUnchanged) that aborts if the row
+ * was written by a concurrent path between the caller's snapshot read and this
+ * clear. Closes the TOCTOU window in the quota-recovery path. When omitted,
+ * the clear is unconditional (preserves existing post-success-call behavior).
+ */
+export interface RecoveredStateExpectation {
+  testStatus: string | null;
+  lastErrorAt: string | null;
+  rateLimitedUntil: string | null;
+}
+
 export async function clearRecoveredProviderState(
-  credentials: Partial<RecoverableConnectionState> | null
-) {
-  if (!credentials?.connectionId) return;
+  credentials: Partial<RecoverableConnectionState> | null,
+  expectedState?: RecoveredStateExpectation
+): Promise<{ applied: boolean }> {
+  if (!credentials?.connectionId) return { applied: false };
+  if (expectedState) {
+    const applied = await clearConnectionErrorIfUnchanged(
+      credentials.connectionId,
+      expectedState
+    );
+    if (!applied) {
+      log.info(
+        "AUTH",
+        `Skipped recovery clear for ${credentials.connectionId.slice(0, 8)} — state changed concurrently (CAS miss)`
+      );
+      return { applied: false };
+    }
+    log.info("AUTH", `Account ${credentials.connectionId.slice(0, 8)} error cleared (CAS)`);
+    return { applied: true };
+  }
   await clearAccountError(credentials.connectionId, credentials);
+  return { applied: true };
 }
 
 type AuthRequestHeaders = Headers | Record<string, string | string[] | undefined>;
diff --git a/tests/unit/provider-limits-recovery.test.ts b/tests/unit/provider-limits-recovery.test.ts
index f719d19090..ae25b84ab5 100644
--- a/tests/unit/provider-limits-recovery.test.ts
+++ b/tests/unit/provider-limits-recovery.test.ts
@@ -201,3 +201,104 @@ test("error-only quota response does not clear transient state", async () => {
   assert.equal(updated.testStatus, "unavailable", "transient state should not be cleared on error");
   assert.equal(updated.lastErrorType, "rate_limited");
 });
+
+test("CAS primitive clears when expected state matches", async () => {
+  const created = await createGlmConnectionWithTransientCooldown();
+  const connectionId = (created as { id: string }).id;
+  const before = (await providersDb.getProviderConnectionById(connectionId)) as Record<
+    string,
+    unknown
+  >;
+
+  const applied = await providersDb.clearConnectionErrorIfUnchanged(connectionId, {
+    testStatus: (before.testStatus as string) ?? null,
+    lastErrorAt: (before.lastErrorAt as string) ?? null,
+    rateLimitedUntil: (before.rateLimitedUntil as string) ?? null,
+  });
+
+  assert.equal(applied, true, "CAS UPDATE should apply when expected state matches");
+  const after = (await providersDb.getProviderConnectionById(connectionId)) as Record<
+    string,
+    unknown
+  >;
+  assert.equal(after.testStatus, "active");
+  assert.equal(after.rateLimitedUntil, undefined);
+  assert.equal(after.backoffLevel, 0);
+});
+
+test("CAS primitive aborts when state changed concurrently", async () => {
+  const created = await createGlmConnectionWithTransientCooldown();
+  const connectionId = (created as { id: string }).id;
+  const before = (await providersDb.getProviderConnectionById(connectionId)) as Record<
+    string,
+    unknown
+  >;
+
+  // Simulate a concurrent markAccountUnavailable writing a fresh error state.
+  const newLastErrorAt = new Date(Date.now() + 1000).toISOString();
+  const newRateLimitedUntil = new Date(Date.now() + 120_000).toISOString();
+  await providersDb.updateProviderConnection(connectionId, {
+    lastErrorAt: newLastErrorAt,
+    rateLimitedUntil: newRateLimitedUntil,
+    lastError: "fresh 429",
+    errorCode: 429,
+    backoffLevel: 3,
+  });
+
+  const applied = await providersDb.clearConnectionErrorIfUnchanged(connectionId, {
+    testStatus: (before.testStatus as string) ?? null,
+    lastErrorAt: (before.lastErrorAt as string) ?? null,
+    rateLimitedUntil: (before.rateLimitedUntil as string) ?? null,
+  });
+
+  assert.equal(applied, false, "CAS UPDATE should abort when state changed");
+  const after = (await providersDb.getProviderConnectionById(connectionId)) as Record<
+    string,
+    unknown
+  >;
+  assert.equal(after.testStatus, "unavailable", "fresh mark should be preserved");
+  assert.equal(after.backoffLevel, 3, "fresh backoff level should be preserved");
+  assert.equal(after.lastError, "fresh 429");
+});
+
+test("quota recovery path does NOT overwrite a concurrent mark (TOCTOU closed)", async () => {
+  const created = await createGlmConnectionWithTransientCooldown();
+  const connectionId = (created as { id: string }).id;
+  const snapshotBeforeClear = (await providersDb.getProviderConnectionById(
+    connectionId
+  )) as Record<string, unknown>;
+  const expectedLastErrorAt = (snapshotBeforeClear.lastErrorAt as string) ?? null;
+
+  // Mock fetch so that DURING the quota fetch (between read and clear), a
+  // concurrent mark writes a fresh error state. This deterministically
+  // reproduces the TOCTOU window the CAS primitive is meant to close.
+  const concurrentMarkFetch = (() => {
+    // Simulate concurrent markAccountUnavailable writing fresh state.
+    providersDb.updateProviderConnection(connectionId, {
+      lastErrorAt: new Date(Date.now() + 1000).toISOString(),
+      rateLimitedUntil: new Date(Date.now() + 120_000).toISOString(),
+      lastError: "fresh concurrent 429",
+      errorCode: 429,
+      backoffLevel: 3,
+    });
+    return glmQuotaResponse();
+  }) as typeof fetch;
+
+  await withMockedFetch(concurrentMarkFetch, async () => {
+    await providerLimits.fetchAndPersistProviderLimits(connectionId, "manual");
+  });
+
+  const after = (await providersDb.getProviderConnectionById(connectionId)) as Record<
+    string,
+    unknown
+  >;
+  // Recovery should have aborted (CAS miss) — fresh mark must survive.
+  assert.notEqual(
+    after.lastErrorAt,
+    expectedLastErrorAt,
+    "fresh lastErrorAt must not be overwritten by recovery clear"
+  );
+  assert.equal(after.testStatus, "unavailable", "fresh testStatus must survive");
+  assert.equal(after.backoffLevel, 3, "fresh backoff level must survive");
+  assert.equal(after.lastError, "fresh concurrent 429");
+});