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

Point the tool's `source` at a cloud-sql (admin) kind source.

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

ValidateSource for cloudsql-clone-instance asserts the source implements the tool's compatibleSource interface (a Cloud SQL Admin source). A source of any other Go type fails the assertion, returning this error. The tool needs Cloud SQL Admin capabilities (project/instance access) that only the compatible source provides.

Source

Thrown at internal/tools/cloudsql/cloudsqlcloneinstance/cloudsqlcloneinstance.go:102

}

// Tool represents the clone-instance tool.
type Tool struct {
	tools.BaseTool[Config]
}

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
}

// Invoke executes the tool's logic.
func (t Tool) Invoke(ctx context.Context, s sources.Source, params parameters.ParamValues, accessToken tools.AccessToken) (any, util.ToolboxError) {
	source, ok := s.(compatibleSource)
	if !ok {
		return nil, util.NewClientServerError("source used is not compatible with the tool", http.StatusInternalServerError, nil)
	}
	paramsMap := params.AsMap()

	project, ok := paramsMap["project"].(string)
	if !ok {
		return nil, util.NewAgentError(fmt.Sprintf("error casting 'project' parameter: %v", paramsMap["project"]), nil)
	}
	sourceInstanceName, ok := paramsMap["sourceInstanceName"].(string)
	if !ok {

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's `source` at a cloud-sql (admin) kind source.
Run config validation before starting the server to catch the mismatch.
For custom wrappers, implement the full compatibleSource interface.
Verify the source kind under `sources:` actually supports instance cloning operations.

Example fix

// before
  clone-instance:
    kind: cloud-sql-clone-instance
    source: my-postgres
// after
  clone-instance:
    kind: cloud-sql-clone-instance
    source: cloud-sql-admin

Defensive patterns

Strategy: validation

Validate before calling

// Go: validate the Cloud SQL source before tool use
if err := cloneTool.ValidateSource(src); err != nil {
    return fmt.Errorf("clone-instance requires a cloud-sql admin source: %w", err)
}

Type guard

func asCloudSQLSource(s sources.Source) (compatibleSource, bool) {
    cs, ok := s.(compatibleSource)
    return cs, ok
}

Try / catch

if err := cloneTool.ValidateSource(src); err != nil {
    log.Fatalf("cloud-sql-clone-instance misconfigured: %v", err)
}

Prevention

Distinguish the Cloud SQL Admin source from engine connection sources in tools.yaml and bind clone-instance to the former.
Validate bindings at startup and in CI.
Keep custom source wrappers implementing compatibleSource with compile-time assertions.
After version upgrades, confirm the compatibleSource interface hasn't gained methods.

When it happens

Trigger: Config validation or Invoke-time checks where the clone-instance tool is bound to a non-Cloud SQL source, e.g. a plain postgres/mysql source instead of cloud-sql admin, or a custom source missing the interface methods.

Common situations: Binding the tool to the database connection source rather than the Cloud SQL Admin source; renaming sources and updating tool bindings incorrectly; interface changes after a toolbox upgrade.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/7154c709ce6d4b45. Report an issue: GitHub.