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

Point `source:` at 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-roles tool's ValidateSource fails because the bound source does not implement compatibleSource (PostgresPool() *pgxpool.Pool and RunSQL). Only Postgres-family sources expose a pgx pool, so any other source kind is rejected. This surfaces at startup when tools are resolved against sources.

Source

Thrown at internal/tools/postgres/postgreslistroles/postgreslistroles.go:161

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

Point `source:` at a Postgres-family source (postgres, cloud-sql-postgres, alloydb-postgres).
Verify the source key exists in `sources:` and its kind is Postgres-based.
Swap in the correct tool for your engine if not on Postgres.

Example fix

# before
tools:
  list-roles:
    kind: postgres-list-roles
    source: mysql-prod
# after
tools:
  list-roles:
    kind: postgres-list-roles
    source: pg-prod

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-roles 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

Pair role-listing tools exclusively with Postgres kinds.
Keep one config file per database engine to prevent cross-engine tool reuse.
Validate configs at startup in CI.

When it happens

Trigger: A `postgres-list-roles` tool whose `source:` points to a non-Postgres source (mysql, mssql, sqlite, http, etc.) or to an undefined/misspelled source.

Common situations: Hand-maintained configs spanning engines; renamed sources; combining cloud-sql-mysql with postgres tools after copying a template.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/b3d3f7712d9fa73b. Report an issue: GitHub.