MySQL+MCP+ER+BAD+DB+ERROR+Unknown+database+fix+MYSQL

Not specified by the original author.

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.

2. If it is missing, create it:

CREATE DATABASE mydb CHARACTER SET utf8mb4;

3. 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.

4. 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.

ERBADDBERROR: Unknown database is the standard MySQL error when the client requests a database that does not exist, documented in MySQL troubleshooting guides with the fix of creating the database first (CREATE DATABASE). The benborla/mcp-server-mysql server takes the database name from the MYSQLDB environment variable, so a typo or a not-yet-created database in that variable produces this exact error. Source: https://github.com/benborla/mcp-server-mysql

Not specified by the original author.

TL;DR: Access denied for user means the MySQL MCP server is sending the wrong username or password, or the user is not allowed to connect from the client's host. Test the exact credentials with the mysql CLI, fix MYSQL_USER / MYSQL_PASS in the client config env block, restart.

ER_ACCESS_DENIED_ERROR: Access denied for user 'appuser'@'YOUR_HOST' (using password value YES)

Fix it

1. Test the credentials outside the MCP layer with the exact same values:

mysql -h YOUR_MYSQL_HOST -u appuser -p'yourpassword' -e "SELECT 1;"

Expected: 1. If this fails, the problem is the credentials or grants, not the MCP server.

2. Check the host part. 'appuser'@'YOUR_HOST' and 'appuser'@'%' are different MySQL users. If the MCP server connects over TCP from another host, the user needs a matching host entry:

CREATE USER 'appuser'@'%' IDENTIFIED BY 'yourpassword';
GRANT SELECT ON mydb.* TO 'appuser'@'%';
FLUSH PRIVILEGES;

3. Update the client config env block:

{
  "env": {
    "MYSQL_HOST": "YOUR_MYSQL_HOST",
    "MYSQL_PORT": "3306",
    "MYSQL_USER": "appuser",
    "MYSQL_PASS": "yourpassword",
    "MYSQL_DB": "mydb"
  }
}

4. Restart the MCP client.

   Expected: the server connects and tools work.

When to use this

• Every tool call fails with ER_ACCESS_DENIED_ERROR.
• The mysql CLI with the same credentials also fails (proves it is credentials, not MCP).

When NOT to use this

• The error is ER_BAD_DB_ERROR: Unknown database. Auth worked; the database name is wrong.
• The error is ECONNREFUSED. The server is unreachable.
• The error mentions caching_sha2_password. That is an auth-plugin problem, different fix.

Compatibility

• benborla/mcp-server-mysql (MYSQL_HOST/PORT/USER/PASS/DB env config).
• MySQL 5.7, 8.x, MariaDB.

Why it happens

MySQL authenticates the pair of (username, host), not just the username. MCP configs get copied between machines, so the password is right but the host the server connects from does not match any grant. The (using password value YES) detail tells you a password was sent; the fix is aligning the user, password, and host grant.

Edge cases

• Special characters in MYSQL_PASS inside JSON need JSON escaping. A # or ! is fine; a \ or " needs escaping.
• MySQL 8 defaults to caching_sha2_password. Old clients fail with a plugin error, not access denied. If you see the plugin named, that is your problem instead.
• Skip-name-resolve servers: use IP addresses in host grants, not hostnames.

The benborla/mcp-server-mysql server authenticates with MYSQLUSER and MYSQLPASS environment variables. ERACCESSDENIED_ERROR: Access denied for user is the standard MySQL authentication failure, documented across MySQL troubleshooting guides with the fix of verifying the database username and password in the configuration and ensuring the MySQL server is running. For the MCP server the credentials live in the client config env block, not a .env file the server reads itself. Source: https://github.com/benborla/mcp-server-mysql

Not specified by the original author.

TL;DR: You exported MYSQL_PASS in your terminal, but the MCP server never saw it. MCP clients launch servers with a clean environment. Put every MYSQL_* variable in the client config's env block instead.

ER_ACCESS_DENIED_ERROR: Access denied for user 'appuser'@'host' (using password value NO)

(The tell is using password value NO even though you set one.)

Fix it

1. Move all five variables into the server's env block in your client config:

{
  "mcpServers": {
    "mysql": {
      "command": "npx",
      "args": ["-y", "mcp-server-mysql"],
      "env": {
        "MYSQL_HOST": "YOUR_MYSQL_HOST",
        "MYSQL_PORT": "3306",
        "MYSQL_USER": "appuser",
        "MYSQL_PASS": "yourpassword",
        "MYSQL_DB": "mydb"
      }
    }
  }
}

2. Fully quit and restart the client. Servers spawn at client launch.

   Expected: (using password value YES) in any subsequent error, and with correct values, a working connection.

When to use this

• The error says using password value NO but you definitely set a password somewhere.
• echo $MYSQL_PASS works in your terminal but the server acts like it is empty.
• You are on Windows, where GUI-launched servers do not inherit terminal env at all.

When NOT to use this

• The error says using password value YES and still denies access. The value is reaching the server but is wrong.
• The error is ECONNREFUSED or Unknown database. Those are host and database problems.

Compatibility

• benborla/mcp-server-mysql and any env-configured MCP database server.
• Claude Desktop, Claude Code, Cursor, Windsurf.

Why it happens

export affects your shell and its children. The MCP client is not your shell's child (especially GUI apps), so it never sees those variables, and the server it spawns inherits the client's clean environment. The env block exists precisely to bridge this gap.

Edge cases

• JSON-escape backslashes and double quotes inside passwords in the config file.
• If you use a secrets manager or 1Password-style env injection, make sure the injection applies to the client process, not just your terminal.
• After moving the vars, unset the shell exports to avoid confusion about which value is live.

The benborla/mcp-server-mysql server reads its configuration (MYSQLHOST, MYSQLPORT, MYSQLUSER, MYSQLPASS, MYSQLDB) from environment variables. MCP clients launch stdio servers with a clean environment that does not inherit interactive shell exports, so variables set via export in a terminal are not visible to the server process. The reliable channel is the env block in the MCP client configuration (claudedesktop_config.json, .mcp.json). This is the same documented behavior as other env-configured MCP database servers.