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

Point the tool at a Postgres-based source kind (postgres, cloud-sql-postgres, alloydb-postgres, etc.).

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-database-overview tool's ValidateSource asserts the source is a Postgres-compatible source via its compatibleSource interface. A non-Postgres source bound to this tool yields this error before the overview queries execute.

Source

Thrown at internal/tools/postgres/postgresdatabaseoverview/postgresdatabaseoverview.go:130

	resp, err := source.RunSQL(ctx, databaseOverviewStatement, 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 at a Postgres-based source kind (postgres, cloud-sql-postgres, alloydb-postgres, etc.).
Confirm the source name exists in the sources section and is Postgres-compatible.
Programmatic usage: pass the Postgres source instance.

Example fix

# before
  overview:
    kind: postgres-database-overview
    source: my-mysql
# after
  overview:
    kind: postgres-database-overview
    source: my-postgres

Defensive patterns

Strategy: type-guard

Validate before calling

if err := tool.ValidateSource(src); err != nil { /* rebind source in tools.yaml */ }

Type guard

func isPostgresSource(s sources.Source) bool {
    type pgSrc interface{ SourceType() string }
    if p, ok := s.(pgSrc); ok {
        switch p.SourceType() {
        case "postgres", "cloud-sql-postgres", "alloydb-postgres", "cloud-sql-mysql":
            return p.SourceType() != "cloud-sql-mysql"
        }
    }
    return false
}

Try / catch

if err := t.ValidateSource(src); err != nil {
    return fmt.Errorf("postgres-database-overview requires a Postgres source: %w", err)
}

Prevention

Only bind postgres tools to Postgres-family source kinds.
Run startup-time validation of the full config.
Keep naming conventions that encode engine type in source names (e.g. my-postgres).

When it happens

Trigger: A postgres-database-overview tool referencing a MySQL/Oracle/MongoDB or other non-Postgres source; passing an incompatible sources.Source at runtime.

Common situations: Template reuse across configs; switching a source's engine kind without updating dependent tools; source-name typos.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/7bb6a4f4a3678e0e. Report an issue: GitHub.