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

Set the tool's `source:` to a looker-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

ValidateSource for looker-get-dimensions performs `source.(compatibleSource)` to guarantee the bound source can serve Looker LookML queries. A non-Looker source fails the assertion and this error is returned, preventing Invoke from running against an unusable source.

Source

Thrown at internal/tools/looker/lookergetdimensions/lookergetdimensions.go:107

// 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)
	}
	logger, err := util.LoggerFromContext(ctx)
	if err != nil {
		return nil, util.NewClientServerError("unable to get logger from ctx", http.StatusInternalServerError, err)
	}
	model, explore, err := lookercommon.ProcessFieldArgs(ctx, params)
	if err != nil {
		return nil, util.NewAgentError("error processing model or explore", err)
	}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Set the tool's `source:` to a looker-kind source.
Fix source name typos so the correct source resolves.
Implement compatibleSource for custom sources.
Validate config at startup to catch the mismatch early.

Example fix

// before
    kind: looker-get-dimensions
    source: bq-prod
// after
    kind: looker-get-dimensions
    source: looker-prod

Defensive patterns

Strategy: type-guard

Validate before calling

if err := tool.ValidateSource(src); err != nil {
    return fmt.Errorf("looker-get-dimensions needs a looker source: %w", err)
}

Type guard

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

Try / catch

if err := tool.ValidateSource(src); err != nil {
    log.Fatalf("source mismatch for looker tool: %v", err)
}

Prevention

Validate all tool/source bindings at server startup.
Bind looker-get-dimensions only to looker-kind sources.
Grep configs for stale source names after renames.
Implement compatibleSource fully on any custom Looker source.

When it happens

Trigger: Validation or the start of Invoke when the source bound to the looker-get-dimensions tool is a non-Looker kind (Postgres, HTTP, BigQuery, etc.).

Common situations: tools.yaml binds the tool to the wrong source; stale references after source renames; custom Source implementations lacking the Looker interface.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/adab880b81a89cb4. Report an issue: GitHub.