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

Point the tool at a source of kind `mongodb`

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

Tool.ValidateSource for mongodb-aggregate asserts the attached source implements the MongoDB compatibleSource interface (which provides the Mongo client/database). Any other source kind is rejected with this error since the tool cannot execute a pipeline against it.

Source

Thrown at internal/tools/mongodb/mongodbaggregate/mongodbaggregate.go:123

// 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()

	collection, tbErr := mongodbcommon.ResolveCollection(t.Cfg.Collection, paramsMap)
	if tbErr != nil {
		return nil, tbErr
	}

	pipelineString, err := parameters.PopulateTemplateWithJSON("MongoDBAggregatePipeline", t.Cfg.PipelinePayload, paramsMap)
	if err != nil {

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool at a source of kind `mongodb`
Verify the source name exists under `sources:` in tools.yaml
Validate the full config locally before deployment

Example fix

// before
aggregate:
  kind: mongodb-aggregate
  source: my-postgres
// after
aggregate:
  kind: mongodb-aggregate
  source: my-mongodb

Defensive patterns

Strategy: validation

Validate before calling

if err := aggregateTool.ValidateSource(src); err != nil {
    return fmt.Errorf("mongodb-aggregate %q: %w", name, err)
}

Prevention

Bind mongodb tools only to `kind: mongodb` sources
Dry-start the toolbox to surface ValidateSource errors early
Keep a naming convention mapping source kind in the name

When it happens

Trigger: Binding mongodb-aggregate to a non-MongoDB source (e.g. postgres, http); ValidateSource runs at startup when tools are wired to sources.

Common situations: Wrong source name in the tool's `source` field; source replaced by a different kind; copy-paste errors in multi-source configs.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/fa2b7e4642e74d39. Report an issue: GitHub.