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

Point the tool's `source` at a source with kind `falkordb`

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 the falkordb-cypher tool asserts the bound source implements the tool's private `compatibleSource` interface (a FalkorDB source). Any other source kind fails the assertion, so the tool reports the source is not a compatible type and will not serve Cypher execution against it.

Source

Thrown at internal/tools/falkordb/falkordbcypher/falkordbcypher.go:100

// 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()
	resp, err := source.RunQuery(ctx, "", t.Cfg.Statement, paramsMap, false, false)
	if err != nil {
		return nil, util.ProcessGeneralError(err)
	}
	return resp, nil
}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's `source` at a source with kind `falkordb`
Verify the sources section: the referenced source must be the FalkorDB one
Remove or rename duplicate source names to avoid ambiguity
Rebuild/restart the toolbox and re-check tool loading

Example fix

// before (tools.yaml)
tools:
  cypher:
    kind: falkordb-cypher
    source: postgres-src
// after
tools:
  cypher:
    kind: falkordb-cypher
    source: falkordb-src

Defensive patterns

Strategy: validation

Validate before calling

// falkordb-cypher requires a falkordb source:
if src.Kind != "falkordb" {
    return fmt.Errorf("tool %s needs kind=falkordb, got %s", toolName, src.Kind)
}

Type guard

if _, ok := src.(falkordbcypher.CompatibleSource); !ok {
    return fmt.Errorf("source %T is not a FalkorDB source", src)
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
    log.Fatalf("rebind %s to the falkordb source: %v", toolName, err)
}

Prevention

Verify the `source:` line of every graph tool points at the falkordb source
Keep graph and relational sources in clearly separated config sections
Test tool loading after any source rename or backend migration
Use unique, engine-prefixed source names in shared configs

When it happens

Trigger: A `falkordb-cypher` tool bound via its `source` field to a source defined with a non-FalkorDB kind (Postgres, MySQL, Elasticsearch, etc.).

Common situations: Reusing a shared tools.yaml where the source name collides with another engine's source; switching graph backends without updating tool source bindings; typos in the source name resolving to the wrong entry.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/5d7d71c451511007. Report an issue: GitHub.