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

Bind the tool to a cloudstorage source (the `source:` value must match a cloudstorage entry in `sources:`).

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

cloudstorage-get-bucket-metadata's ValidateSource rejects any source not implementing its compatibleSource interface. The tool can only be bound to a Cloud Storage source; binding it to any other source type fails at startup with this message naming the tool and configured source.

Source

Thrown at internal/tools/cloudstorage/cloudstoragegetbucketmetadata/cloudstoragegetbucketmetadata.go:107

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)
	}

	attrs, err := source.GetBucketMetadata(ctx, bucket)
	if err != nil {
		return nil, cloudstoragecommon.ProcessGCSError(err)

View on GitHub (pinned to 8cc6e09de2)

Solutions

Bind the tool to a cloudstorage source (the `source:` value must match a cloudstorage entry in `sources:`).
Delete or move the tool to the correct source's toolset.
Verify source key spelling in the YAML.

Example fix

# before
source: alloydb-source
# after
source: gcs-source

Defensive patterns

Strategy: validation

Validate before calling

if _, ok := src.(compatibleSource); !ok {
    return fmt.Errorf("get-bucket-metadata requires a cloudstorage source, got %T", src)
}

Type guard

func isCompatible(s sources.Source) bool { _, ok := s.(compatibleSource); return ok }

Prevention

Match each tool kind to its source kind in config review
Rename sources with search-and-replace across the whole config
Run toolbox startup validation in CI

When it happens

Trigger: Tool config `source` references a source whose Go type fails the `source.(compatibleSource)` type assertion during ValidateSource.

Common situations: Tool defined under the wrong source in YAML; refactoring source names; reusing a generic tools.yaml across projects with different sources.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/f21797282dfc757f. Report an issue: GitHub.