"cannot execute in a read-only transaction": you are on the replica

"cannot execute ... in a read-only transaction": wrong endpoint

Symptom

Reads work, every write fails: cannot execute INSERT INTO ... in a read-only transaction (SQLSTATE 25006).

Cause

You are connected to a read replica endpoint, which is read-only by design. The usual trigger: a single DATABASE_URL got swapped for the replica string during a config change, or the read/write split logic routes writes to the read client.

Confirm

1. Compare the connected hostname against the Console: replica endpoints are separate from the primary.
2. SHOW transaction_read_only; returns on.

Fix

• Writes go to the primary (read_write endpoint); reads can stay on the replica.
• Keep DATABASE_URL and REPLICA_URL as separate env vars; never derive one by editing the other.
• In ORMs, check the read/write split middleware actually sends writes to the primary client.

Verify

A write followed by a read-your-write on the primary succeeds; replica reads still work.

Neon's connection error docs (verified live 2026-09-26) list 'cannot execute ... in a read-only transaction (SQLSTATE 25006)' among resolved errors; the read replica docs describe replicas as segregating read-only work from production operations.