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

Point the tool's 'source' field at a source declared with kind: valkey

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

Valkey tools assert that the attached source implements the compatibleSource interface (a Valkey client-backed source). ValidateSource returns this error when the tool is bound to a source of any other type, so the tool never runs against a non-Valkey backend.

Source

Thrown at internal/tools/valkey/valkey.go:99

// 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)
	}

	// Replace parameters
	commands, err := replaceCommandsParams(t.Cfg.Commands, t.Cfg.Parameters, params)
	if err != nil {
		return nil, util.NewAgentError("error replacing commands' parameters", err)
	}
	res, err := source.RunCommand(ctx, commands)
	if err != nil {
		return nil, util.ProcessGeneralError(err)

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's 'source' field at a source declared with kind: valkey
If the backend is Redis, use the corresponding redis tool kind instead of the valkey one
Confirm the source initializes successfully before the tool binds to it

Example fix

# before
sources:
  cache:
    kind: redis
    address: localhost:6379
tools:
  valkey-cmd:
    kind: valkey-execute-sql
    source: cache
# after
sources:
  cache:
    kind: valkey
    address: localhost:6379
tools:
  valkey-cmd:
    kind: valkey-execute-sql
    source: cache

Defensive patterns

Strategy: validation

Validate before calling

kind, _ := getSourceKind(toolsYAML, tool.Source)
if kind != "valkey" {
    return fmt.Errorf("tool %s requires a valkey source, got %s", tool.Name, kind)
}

Type guard

func asValkeySource(s sources.Source) (compatibleSource, bool) {
    cs, ok := s.(compatibleSource)
    return cs, ok
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
    // rebind tool to a kind: valkey source and reload
}

Prevention

Don't assume redis sources work with valkey tools; declare kind: valkey explicitly
Name sources by kind in tools.yaml
Run a toolbox config preflight check in CI

When it happens

Trigger: tools.yaml pairing a valkey tool with a source whose kind is not valkey (e.g. redis, postgres); calling ValidateSource or Invoke with a mismatched sources.Source implementation.

Common situations: Assuming Redis and Valkey sources are interchangeable; copy-pasted configs where the source name points to a different kind; refactors that changed the source kind but not the tool bindings.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/47ec046417189d2b. Report an issue: GitHub.