MySQL MCP: ER_BAD_DB_ERROR Unknown database (fix MYSQL_DB or create it)

TL;DR: Auth worked, but the database named in MYSQL_DB does not exist on that MySQL server. Either create it with CREATE DATABASE, or fix the typo in the name. Then restart the client.

ER_BAD_DB_ERROR: Unknown database 'mydb'

Fix it

1. List the databases that actually exist, using the same credentials:

mysql -h YOUR_MYSQL_HOST -u appuser -p -e "SHOW DATABASES;"

Expected: a list. Check whether your intended name is there, exactly spelled.

1. If it is missing, create it:

CREATE DATABASE mydb CHARACTER SET utf8mb4;

1. If it exists under a slightly different name, fix MYSQL_DB in the client config env block. Common: mydb vs mydb_prod, singular vs plural.

1. Restart the MCP client.

Expected: the server connects to the database and tools work.

When to use this

• Auth succeeds (no access-denied error) but every call fails with Unknown database.
• You recently renamed things or copied the config from another environment.

When NOT to use this

• The error is ER_ACCESS_DENIED_ERROR. Fix credentials first.
• The error is ER_NO_SUCH_TABLE. The database exists; a table inside it does not.

Compatibility

• benborla/mcp-server-mysql (MYSQL_DB env config).
• MySQL 5.7, 8.x, MariaDB.

Why it happens

The MCP server passes MYSQL_DB straight to the MySQL driver as the default database. MySQL checks it at connect time and rejects unknown names. Configs copied between dev, staging, and prod often carry a stale database name while the host and credentials get updated.

Edge cases

• Database names are case-sensitive on Linux (filesystem-dependent). MyDB and mydb differ.
• If the database exists but the user lacks privileges on it, you get access denied, not unknown database. Different fix.
• Some setups expect the server to create the schema. It does not. Create it first, then connect.