googleapis/mcp-toolbox · error

unknown prompt type: %q

Error message

unknown prompt type: %q

What it means

DecodeConfig resolves a prompt's `kind` (resourceType) against the promptRegistry of registered prompt factories, defaulting to "custom" when kind is empty. It throws this error when the kind is set to a value that has no registered factory, so the prompt config cannot be decoded.

Source

Thrown at internal/prompts/prompts.go:55

	if _, exists := promptRegistry[resourceType]; exists {
		// Prompt with this type already exists, do not overwrite.
		return false
	}
	promptRegistry[resourceType] = factory
	return true
}

// DecodeConfig looks up the registered factory for the given type and uses it
// to decode the prompt configuration.
func DecodeConfig(ctx context.Context, resourceType, name string, decoder *yaml.Decoder) (PromptConfig, error) {
	factory, found := promptRegistry[resourceType]
	if !found && resourceType == "" {
		resourceType = "custom"
		factory, found = promptRegistry[resourceType]
	}

	if !found {
		return nil, fmt.Errorf("unknown prompt type: %q", resourceType)
	}

	promptConfig, err := factory(ctx, name, decoder)
	if err != nil {
		return nil, fmt.Errorf("unable to parse prompt %q as resourceType %q: %w", name, resourceType, err)
	}
	return promptConfig, nil
}

type PromptConfig interface {
	PromptConfigType() string
	Initialize() (Prompt, error)
}

type Prompt interface {
	GetDesc() string
	GetArguments() Arguments
	SubstituteParams(parameters.ParamValues) (any, error)

View on GitHub (pinned to 8cc6e09de2)

Solutions

1. Remove the `kind:` field or set it to `custom` to use the default prompt factory.
2. Check spelling/casing of kind against the registered prompt types (e.g. the keys in promptRegistry).
3. Ensure the package providing the custom prompt factory is imported so its init() registers it.
4. Upgrade the toolbox binary if the kind is only supported in newer versions.

Example fix

// before
prompts:
  my-prompt:
    kind: gcp_prompt
    description: ...
// after
prompts:
  my-prompt:
    kind: custom
    description: ...

knownKinds := map[string]bool{"": true, "custom": true /* + registered kinds */}
if !knownKinds[promptCfg.Kind] {
	return fmt.Errorf("prompt kind %q unknown; use 'custom' or omit kind", promptCfg.Kind)
}

pc, err := prompts.DecodeConfig(ctx, name, decoder)
if err != nil {
	if strings.Contains(err.Error(), "unknown prompt type") {
		return fmt.Errorf("set prompts kind to 'custom' (or omit it); %w", err)
	}
	return err
}

When it happens

Trigger: A prompt in tools.yaml declares `kind: mydb_prompt` (or any non-empty kind other than registered types like "custom"), and no factory for that kind was registered via the registry's Register call at init time.

Common situations: Typo in kind (e.g. "Custom" with capital C); copying a kind from a tools/config section into a prompts section where it isn't valid; using a kind from an extension not imported into the binary so its init never ran; version mismatch where the kind was added in a newer release.

Related errors

• error parsing argument: %w
• invalid role %q: must be 'user' or 'assistant'
• unable to parse prebuilt tool configuration for '%s': %w
• prebuilt source tool for '%s' not found. %s
• error substituting params for message: %w

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/3251844678d94a76. Report an issue: GitHub.