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 source of kind cloudhealthcare in tools.yaml

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 on the cloudhealthcare-retrieve-rendered-dicom-instance tool asserts that the provided source implements the tool's compatibleSource interface. Any source that is not a Cloud Healthcare source fails the assertion and produces this error. It is the tool's way of rejecting misconfigured source bindings before Invoke runs.

Source

Thrown at internal/tools/cloudhealthcare/cloudhealthcareretrieverendereddicominstance/cloudhealthcareretrieverendereddicominstance.go:106

// validate interface
var _ tools.Tool = 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
}

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)
	}
	storeID, err := common.ValidateAndFetchStoreID(params, source.AllowedDICOMStores())
	if err != nil {
		return nil, util.NewAgentError("failed to validate store ID", err)
	}
	var tokenStr string
	if source.UseClientAuthorization() {
		tokenStr, err = accessToken.ParseBearerToken()
		if err != nil {
			return nil, util.NewClientServerError("error parsing access token", http.StatusUnauthorized, err)

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's source at a source of kind cloudhealthcare in tools.yaml
Run the server startup and read the ValidateSource failure to identify which source name is wrong
Confirm the referenced source initializes to *cloudhealthcare.Source (implements UseClientAuthorization/AllowedDICOMStores)

Example fix

// before (tools.yaml)
source: cloudsql-postgres-prod
// after
source: healthcare-fhir-dicom-store

Defensive patterns

Strategy: type-guard

Validate before calling

// in tools.yaml, ensure kinds line up:
// tool kind: cloudhealthcare-retrieve-rendered-dicom-instance
// source kind: cloudhealthcare

Type guard

func isHealthcareSource(s sources.Source) bool {
	_, ok := s.(cloudhealthcare.Source)
	return ok
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
	if strings.Contains(err.Error(), "not a compatible type") {
		log.Fatalf("tool %s must bind a cloudhealthcare source", tool.GetName())
	}
	return err
}

Prevention

Check source kind matches tool family before referencing it
Avoid renaming sources that tools depend on without updating tool configs
Run integration config validation locally before merge

When it happens

Trigger: Any call path that runs ValidateSource (typically during server startup/initialization) with a source other than an internal/sources/cloudhealthcare Source bound to this tool.

Common situations: tools.yaml binding the tool to a wrong-kind source (e.g. a Cloud SQL or postgres source); refactoring that renamed the healthcare source so another source is picked up; using a copied config where the source kind was changed but the tool reference wasn't.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/43769af9fb2b5922. Report an issue: GitHub.