How to set a default value for env vars in GenAI Toolbox/mcp-toolbox configs?

Question

GenAI Toolbox (mcp-toolbox) prebuilt configs reference environment variables like ${MSSQL_PORT}, but there was no way to give an optional parameter a fallback value when the variable is not set. This issue asks for a syntax such as ${MSSQL_PORT:5432} so configs can have sensible defaults. Is that syntax now supported?

Originally asked on GitHub: https://github.com/googleapis/mcp-toolbox/issues/1001

AgentArmorHQ team · Accepted Answer

**Status: fixed.** Support for `${ENV_VARIABLE:default_value}` was added in PR #1222, and the issue is closed as completed. No release version was mentioned in the thread, so check the PR for the version it landed in.

In a prebuilt config, write the fallback directly in the env-var placeholder:

```yaml
mssql-source:
  kind: mssql
  host: ${MSSQL_HOST}
  port: ${MSSQL_PORT:5432}
```

If `MSSQL_PORT` is not set, `5432` is used. The same `${ENV_VARIABLE:default_value}` syntax works for other environment variables in configs, so optional parameters no longer have to be set explicitly.

General advice: the fallback literal is stored in your config file, so use it only for non-secret defaults. Do not put passwords or tokens in the default value.

How to set a default value for env vars in GenAI Toolbox/mcp-toolbox configs?

1 answer