A refresh-token race can cause intermittent invalid_grant errors when two workers use the same token and the authorization server rotates it. Serialize refreshes for each credential, reread the latest token after coordination, and atomically save the complete response. The error alone does not prove a race: expiry, revocation, client mismatch, and provider session rules can produce similar failures.
What invalid_grant means
invalid_grant is a broad token-endpoint error, not a specific diagnosis. It means the authorization server did not accept the grant presented in the request; the reason may be a stale refresh token, an expired or revoked grant, or a mismatch between the token and the client making the request.
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Refresh tokens are used to obtain new access tokens, but OAuth does not require every authorization server to issue them or give them the same lifetime and revocation behavior. When a server does issue a replacement refresh token, RFC 6749 says the client must discard the old one and replace it with the new one. If the response is lost or the client fails to persist the replacement, later refresh attempts may use stale state.
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- Workers A and B read the same credential record, which contains refresh token R1.
- Both decide the access token needs refreshing and send R1 to the token endpoint.
- Worker A succeeds and receives a new access token and replacement refresh token R2. It saves the response.
- Worker B’s request may now fail because R1 is no longer valid.
If the authorization server detects reuse of a rotated token, it may treat the second request as replay and revoke the active refresh token as well. In that case, even the worker that received R2 may find that its token no longer works. RFC 9700, the OAuth 2.0 Security Best Current Practice published in January 2025, describes this trade-off: the server cannot tell which party made the invalid reuse, so it may revoke the active token.
This sequence depends on provider behavior. Not every server rotates refresh tokens on every exchange, and OAuth does not define a universal grace period for concurrent requests. Where rotation or one-time use applies, allowing multiple workers to refresh the same credential concurrently is unsafe.
How to make refreshes safe across workers
Coordinate by the credential or grant being refreshed—not just by process—so that workers on different hosts cannot independently submit the same token. OAuth standards describe token replacement and replay risks, but do not prescribe a particular lock implementation.
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- Acquire coordination for the credential. Use a distributed lock, lease, or database-backed compare-and-set that covers every instance capable of refreshing that credential.
- Reread the stored state. A different worker may have completed a refresh while this one waited. If the current access token remains valid with an appropriate safety margin, use it rather than sending another refresh request.
- Send one refresh request. Use the provider’s required client authentication. Keep refresh tokens confidential in storage and transit; RFC 6749 says they should be shared only between the authorization server and the client to which they were issued.
- Atomically persist the complete result. Save the access token, any replacement refresh token, expiry metadata, and the credential’s version together. A worker must not overwrite a newer record with a response based on an older snapshot.
- Release coordination after durable storage. Use a bounded lease or equivalent recovery mechanism so a worker crash does not leave the credential permanently blocked.
- Handle ambiguous timeouts cautiously. If the request may have reached the server but the response was lost, the old token might already have been rotated. Do not assume replaying it is harmless; use the provider’s recovery guidance.
- Log state transitions without secrets. Record the provider, credential version, attempt ID, lock owner, outcome, error and subtype, and whether a replacement token was saved. Never log raw access tokens, refresh tokens, client secrets, or authorization headers.
What to check when the error persists
- Expired or revoked grant: A user or automated security process may have revoked access, or the provider may impose expiration or inactivity limits. Google’s OAuth guidance recommends handling invalidation or expiration, including asking the user to sign in again when appropriate.
- Provider session policy: Google documents session-control expiry as a possible cause of refresh failure and may return the provider-specific subtype
invalid_rapt. Do not assume that subtype or behavior applies to other authorization servers. - Client or environment mismatch: Verify that the refresh token is being used with the client identity and authentication method associated with its grant. Check tenant, application configuration, and environment as relevant.
- Stale or overwritten token state: Inspect the credential record’s version and update history. Confirm that a returned replacement token was saved and that no later write restored an older token.
- Provider-specific reuse rules: Check the current documentation for token rotation, replay detection, recovery after a lost response, and any concurrency grace period. The behavior cannot safely be inferred from another provider’s policy.
How to respond to invalid_grant
First check whether another worker refreshed the same credential around the time of the failure, and compare the record version before and after those attempts. Then check the provider’s response body and documented error subtype, the credential’s update history, and whether the client identity matches the grant.
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