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-storage 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

Thrown by cloudstorage-read-object ValidateSource when the source passed to the tool does not implement its compatibleSource interface. This tool requires a Cloud Storage source; any other source type fails the assertion.

Source

Thrown at internal/tools/cloudstorage/cloudstoragereadobject/cloudstoragereadobject.go:117

// 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)
	}
	mapParams := params.AsMap()
	bucket := cloudstoragecommon.ResolveString(t.Cfg.Bucket, mapParams, bucketKey)
	if bucket == "" {
		return nil, util.NewAgentError(fmt.Sprintf("invalid or missing '%s' parameter; expected a non-empty string", bucketKey), nil)
	}
	object, ok := mapParams[objectKey].(string)
	if !ok || object == "" {
		return nil, util.NewAgentError(fmt.Sprintf("invalid or missing '%s' parameter; expected a non-empty string", objectKey), nil)
	}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's `source` at a cloud-storage kind source
Ensure the source name exists in the sources section of tools.yaml
Reload the server so validation runs against the corrected config

Example fix

// before
  read-object:
    kind: cloudstorage-read-object
    source: my-alloydb
// after
  read-object:
    kind: cloudstorage-read-object
    source: my-gcs

Defensive patterns

Strategy: type-guard

Validate before calling

if kind := sourceKinds[cfg.Source]; kind != "cloud-storage" {
    return fmt.Errorf("read-object needs a cloud-storage source; %q is %s", cfg.Source, kind)
}

Type guard

func toCloudStorageSource(s sources.Source) (sources.Source, bool) {
    if cs, ok := s.(compatibleSource); ok {
        return cs, true
    }
    return nil, false
}

Prevention

Map each tool kind to its allowed source kinds and validate pairs at startup
Avoid copy-pasting tool blocks without updating source
Run `toolbox validate` style checks in CI

When it happens

Trigger: Tool config's `source:` points at a non-cloud-storage source; ValidateSource invoked with a wrong sources.Source implementation in tests or code.

Common situations: Copy-pasted tool YAML retaining another source's name; renamed sources leaving stale references; wiring tools to database sources by mistake.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/d1c50f27baa42623. Report an issue: GitHub.