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

Point the tool's `source:` field at a source whose kind is postgres, cloud-sql-postgres, alloydb-postgres, or another Postgres-family source (PostgresPool/RunSQL implementer).

googleapis/mcp-toolbox · error

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-list-locks tool's ValidateSource when the source attached to the tool does not implement the compatibleSource interface (PostgresPool() *pgxpool.Pool and RunSQL(context.Context, string, []any) (any, error)). Toolbox validates every tool/source pairing at startup so failures surface early instead of at invocation time. It means the tool is wired to a source kind (e.g. mysql, http) that cannot serve Postgres queries.

Source

Thrown at internal/tools/postgres/postgreslistlocks/postgreslistlocks.go:139

	resp, err := source.RunSQL(ctx, listLocks, 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 the tool's `source:` field at a source whose kind is postgres, cloud-sql-postgres, alloydb-postgres, or another Postgres-family source (PostgresPool/RunSQL implementer).
Confirm the referenced source is defined in the `sources:` section and its `kind` is a Postgres type.
If you intended a different database, use the equivalent tool for that source (e.g. mysql-list-tables) instead of the postgres tool.
Regenerate from the matching prebuilt config (e.g. postgres prebuilt.yaml) rather than hand-editing cross-source.

Example fix

# before
sources:
  my-db:
    kind: mysql
    ...
tools:
  list-locks:
    kind: postgres-list-locks
    source: my-db
# after
sources:
  my-db:
    kind: postgres
    ...
tools:
  list-locks:
    kind: postgres-list-locks
    source: my-db

Defensive patterns

Strategy: validation

Validate before calling

kind, _ := myConfig.sources[tool.Source].(sources.Source)
pgSrc, ok := kind.(interface {
    PostgresPool() *pgxpool.Pool
    RunSQL(context.Context, string, []any) (any, error)
})
if !ok {
    return fmt.Errorf("source %q for tool %q must be a Postgres-family source", tool.Source, tool.Kind)
}

Type guard

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

Prevention

Keep each tool's `source:` pointing at a source of the matching engine family (postgres tools -> postgres kinds).
Validate the toolbox yaml with `toolbox --prebuilt postgres` templates or a startup smoke test before deploying.
Never share a single tools block across mysql/postgres configs; generate per-engine configs.
Add a CI check that parses the yaml and asserts source kinds against tool kinds.

When it happens

Trigger: Running `toolbox` (or AddTool/GetTool resolution) with a tools yaml where a `postgres-list-locks` tool references a `source:` whose registered source type does not satisfy compatibleSource — e.g. source kind mysql, mssql, cloud-sql-mysql, alloydb-mysql, sqlite, or any non-Postgres source.

Common situations: Copy-pasting a tool definition into a different source's config without changing `source:`; typo'd or unregistered `sourceKind` falling back to the wrong source; using cloud-sql-mysql with postgres tools; prebuilt config mixing when hand-editing.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/4cf2baa7239da57b. Report an issue: GitHub.