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

Set the tool's source field to a cassandra source in tools.yaml

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

Thrown by the cassandracql Tool.ValidateSource when the sources.Source passed in does not implement the tool's compatibleSource interface. This fails fast with a message identifying the tool and the offending source name before any CQL is executed.

Source

Thrown at internal/tools/cassandra/cassandracql/cassandracql.go:106

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
}

// Invoke implements tools.Tool.
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()
	newStatement, err := parameters.ResolveTemplateParams(t.Cfg.TemplateParameters, t.Cfg.Statement, paramsMap)
	if err != nil {
		return nil, util.NewAgentError("unable to extract template params", err)
	}

	newParams, err := parameters.GetParams(t.Cfg.Parameters, paramsMap)
	if err != nil {

View on GitHub (pinned to 8cc6e09de2)

Solutions

Set the tool's source field to a cassandra source in tools.yaml
Verify the referenced source name exists and is a Cassandra-compatible source
Restart the toolbox to reload the configuration
If invoking programmatically, pass the Cassandra source instance satisfying compatibleSource

Example fix

// before (tools.yaml)
sources:
  my-db:
    kind: postgres
    uri: ...
tools:
  cql-query:
    kind: cassandra-cql
    source: my-db
// after
sources:
  my-cassandra:
    kind: cassandra
    hosts: [127.0.0.1]
tools:
  cql-query:
    kind: cassandra-cql
    source: my-cassandra

Defensive patterns

Strategy: type-guard

Validate before calling

// Pre-check the source before running the cql tool
if err := tool.ValidateSource(mySource); err != nil {
    return fmt.Errorf("bad source-tool pairing for %s: %w", toolName, err)
}

Type guard

func isCompatibleCassandraSource(s sources.Source) (cassandracql.CompatibleSource, bool) {
    cs, ok := s.(cassandracql.CompatibleSource)
    return cs, ok
}

Prevention

Point cassandra-cql tools only at cassandra sources
Validate config at startup; the toolbox fails fast on incompatible pairs
After renaming/deleting sources, update all tool references
Write the error message's source name down — it names the offending source

When it happens

Trigger: Invoke/ValidateSource is called with a source whose concrete type fails the type assertion to cassandracql's compatibleSource — typically the tool's source field in tools.yaml references a non-Cassandra source.

Common situations: Pointing a cassandra-cql tool at a Postgres/MySQL/other source; renaming sources and missing a reference; programmatically pairing tools with sources of the wrong kind.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/677e721f0f526f4a. Report an issue: GitHub.