LINE error 400: The maximum number of access tokens has already been issued

Issuing a channel access token v2.1 (POST https://api.line.me/oauth2/v2.1/token) failed because the channel already holds the maximum number of live tokens. The reference states the cap plainly: "You can issue up to 30 tokens per channel for channel access tokens v2.1. If you reach the maximum limit, additional requests of issuing channel access tokens are blocked. An expired channel access token isn't counted as issued."

Your JWT assertion is fine - a bad assertion produces an invalid_client error instead (see invalid_client). What's wrong is the token lifecycle. v2.1 tokens can be valid for up to 30 days, and code that mints a fresh token on every process start, every request, or every deploy accumulates live tokens quickly. Once 30 are outstanding, issuance stops until some expire or you revoke them.

Short-lived channel access tokens also allow 30 per channel but behave differently at the cap: issuing one more revokes the oldest token instead of failing, so the over-issuing bug shows up there as random 401s rather than this 400. LINE's channel access token guide says it directly: don't reissue a token each time you use a channel. The fix is never to wait it out blindly: revoke what you don't need, then change your code so it reuses a token for its lifetime.

// When the maximum number of channel access tokens that can be issued is reached (400 Bad Request)
{
  "message": "The maximum number of access tokens has already been issued"
}

Design token handling as a single-writer process: one job issues, stores, and rotates the token, and everything else reads it. Record each token's key_id so you can revoke precisely. Alert when the count of live key IDs passes a threshold well below 30, so a runaway loop is caught before issuance is blocked in production.