What causes "invalid source for %q tool: source %q is not a compatible type" in googleapis/mcp-toolbox?

Set `source:` to a Postgres-family source (postgres, cloud-sql-postgres, alloydb-postgres).

invalid source for %q tool: source %q is not a compatible ty

Error message

invalid source for %q tool: source %q is not a compatible type

What it means

The postgres-list-query-stats tool's ValidateSource rejects the configured source because it lacks the compatibleSource methods (PostgresPool() *pgxpool.Pool, RunSQL). Query stats read pg_stat_statements-style data, which only Postgres-family sources can provide. The check happens when the tool is initialized and bound to the source.

Source

Thrown at internal/tools/postgres/postgreslistquerystats/postgreslistquerystats.go:140

	resp, err := source.RunSQL(ctx, listQueryStats, sliceParams)
	if err != nil {
		return nil, util.ProcessGeneralError(err)
	}
	return resp, nil
}

func (t Tool) GetSourceName() string {
	return t.Cfg.Source
}

func (t Tool) ToConfig() tools.ToolConfig {
	return t.Cfg
}

func (t Tool) ValidateSource(source sources.Source) error {
	_, ok := source.(compatibleSource)
	if !ok {
		return fmt.Errorf("invalid source for %q tool: source %q is not a compatible type", t.Cfg.Type, t.Cfg.Source)
	}
	return nil
}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Set `source:` to a Postgres-family source (postgres, cloud-sql-postgres, alloydb-postgres).
Ensure the source is defined in `sources:` with a Postgres `kind`.
Use the correct engine-specific tool if you are not on Postgres.

Example fix

# before
tools:
  query-stats:
    kind: postgres-list-query-stats
    source: my-sqlite
# after
tools:
  query-stats:
    kind: postgres-list-query-stats
    source: my-postgres

Defensive patterns

Strategy: validation

Validate before calling

src := myConfig.sources[tool.Source]
if _, ok := src.(interface {
    PostgresPool() *pgxpool.Pool
    RunSQL(context.Context, string, []any) (any, error)
}); !ok {
    return fmt.Errorf("postgres-list-query-stats requires a Postgres-family source, got %T", src)
}

Type guard

func isCompatiblePostgresSource(s sources.Source) bool {
    _, ok := s.(interface {
        PostgresPool() *pgxpool.Pool
        RunSQL(context.Context, string, []any) (any, error)
    })
    return ok
}

Prevention

Attach query-stats tools only to Postgres-family sources (pg_stat_statements is Postgres-only).
Verify source names exist in `sources:` after refactors.
Run a config validation/smoke test in CI.

When it happens

Trigger: A `postgres-list-query-stats` tool whose `source:` names a source of an incompatible kind (mysql, mssql, sqlite, http, etc.) or a nonexistent source key.

Common situations: Copy-pasting tool defs between database configs; referencing cloud-sql-mysql sources from postgres tools; typos in the source name.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/092bba4d7426859d. Report an issue: GitHub.