"no more connections allowed": you hit the pooler client cap

"no more connections allowed": diagnose and fix

Symptom

Connections rejected with no more connections allowed even though Postgres itself is not at max_connections.

Cause

This is PgBouncer's max_client_conn cap: 10,000 maximum client connections to the pooler. A related, softer limit: each user+database pair gets default_pool_size (90% of max_connections) active backend slots and queues beyond that with a 2-minute timeout. So either one tenant is flooding, or the app genuinely holds 10k+ client connections.

Confirm

1. Are you on the pooled string (-pooler)? This error only comes from the pooler.
2. Estimate client connections: instances x pool size per instance. Serverless with high per-instance pools is the classic cause.
3. Check whether it is one user/database pair (queueing, slow) or global (immediate rejection).

Fix

• Lower per-instance pool sizes; most apps need far fewer than they configure.
• Fix connection leaks: every checkout must be released, on all code paths including errors.
• For genuinely huge fleets, split traffic across read replicas or more projects.
• Raising compute size raises default_pool_size (90% of max_connections) for the queueing case, but does not raise the 10,000 client cap.

Verify

Client connection count stays under the cap at peak; queueing disappears from the pooler.

Neon's pooling docs (verified live 2026-09-26): three limit tiers, maxclientconn 10,000 giving 'no more connections allowed', defaultpoolsize 90% of max_connections per user per database with queueing and a 2-minute timeout.