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

Attach a dataplex source to the tool in the YAML config

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 dataplex-lookup-context tool requires a source implementing its compatibleSource interface (LookupContext method). ValidateSource returns this error when the injected source is not that type. It guards against mismatched source/tool wiring in the toolbox configuration.

Source

Thrown at internal/tools/dataplex/dataplexlookupcontext/dataplexlookupcontext.go:104

// 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)
	}
	paramsMap := params.AsMap()
	resourcesSlice, err := parameters.ConvertAnySliceToTyped(paramsMap["resources"].([]any), "string")
	if err != nil {
		return nil, util.NewAgentError(fmt.Sprintf("can't convert resources to array of strings: %s", err), err)
	}
	resources := resourcesSlice.([]string)

	if len(resources) == 0 {
		err := fmt.Errorf("resources cannot be empty")

View on GitHub (pinned to 8cc6e09de2)

Solutions

Attach a dataplex source to the tool in the YAML config
Check the source name referenced by the tool exists and is of kind dataplex
Run the server and verify startup validation passes for this tool

Example fix

// before
source: cloud-sql-postgres
// after
source: dataplex

Defensive patterns

Strategy: type-guard

Validate before calling

if _, ok := src.(interface {
	LookupContext(ctx context.Context, name string, resources []string) (*dataplex.GoogleCloudDataplexV1LookupContextResponse, error)
}); !ok {
	return fmt.Errorf("source %T does not support LookupContext", src)
}

Type guard

func isDataplexLookupContextSource(s sources.Source) bool {
	_, ok := s.(interface {
		LookupContext(ctx context.Context, name string, resources []string) (*dataplex.GoogleCloudDataplexV1LookupContextResponse, error)
	})
	return ok
}

Try / catch

err := tool.ValidateSource(src)
if err != nil {
	// incompatible source: correct the tool->source mapping and reinit
	return fmt.Errorf("rebind tool to a dataplex source: %w", err)
}

Prevention

Keep one dataplex source and reference it explicitly by name in each dataplex tool
Validate config with startup checks before serving traffic
Avoid copying tool blocks across integrations without updating source

When it happens

Trigger: ValidateSource is called with a sources.Source that does not implement compatibleSource (LookupContext(ctx, name, resources)) for dataplex-lookup-context.

Common situations: Tool config points at a wrong source (e.g. a database source instead of dataplex); hand-edited YAML with a typo in the source name resolving to another source.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/9f00b9bf4f0f98d5. Report an issue: GitHub.