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

Change the tool's `source:` field to a Postgres source defined in `sources:`

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

This error is thrown by the postgres-replication-stats tool's ValidateSource method when the source named in the tool's YAML config does not implement the tool's `compatibleSource` interface (a Postgres-backed source exposing a pgx pool). MCP Toolbox tools are decoupled from concrete source types, so at startup each tool type-asserts its configured source; a failed assertion means the tool was wired to a source it cannot use.

Source

Thrown at internal/tools/postgres/postgresreplicationstats/postgresreplicationstats.go:135

	resp, err := source.RunSQL(ctx, replicationStats, 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

Change the tool's `source:` field to a Postgres source defined in `sources:`
Confirm the source's kind implements PostgresPool()/the required methods
Validate the config locally before deploying

Example fix

# before
kind: postgres-replication-stats
source: mysql-replica
# after
kind: postgres-replication-stats
source: postgres-prod

Defensive patterns

Strategy: validation

Validate before calling

if _, ok := any(myPostgresSource).(interface {
	PostgresPool() *pgxpool.Pool
	RunSQL(context.Context, string, []any) (any, error)
}); !ok {
	log.Fatalf("source is not compatible with postgres-replication-stats")
}

Type guard

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

Prevention

Match tool kind prefix to source kind (postgres-* -> kind: postgres)
Validate tools.yaml in CI with the toolbox binary
Rename sources atomically: update sources.yaml and all tool references in one change
Add a startup smoke test that initializes the full config

When it happens

Trigger: Server startup/config validation when a tool of `kind: postgres-replication-stats` references a `source:` whose registered source does not satisfy the compatibleSource interface (e.g. redis, mysql, http sources).

Common situations: Copy-pasted tool entries with stale `source:` names; pointing Postgres tools at non-Postgres sources; env-specific configs pointing to the wrong source.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/c54da1aca12c7d1c. Report an issue: GitHub.