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

Set the tool's `source` to the FalkorDB source defined in the same file

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 falkordb-listgraphs asserts the provided sources.Source implements `compatibleSource` (FalkorDB). A source of another type fails the assertion and the tool returns this error, refusing to list graphs from an incompatible backend.

Source

Thrown at internal/tools/falkordb/falkordblistgraphs/falkordblistgraphs.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)
	}

	graphs, err := source.FalkorDBClient().ListGraphs()
	if err != nil {
		return nil, util.ProcessGeneralError(err)
	}
	return map[string]any{"graphs": graphs}, nil
}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Set the tool's `source` to the FalkorDB source defined in the same file
Cross-check every tool's source reference against the sources map
Use distinct source names per engine to prevent accidental rebinding
Restart the toolbox and verify the tool validates cleanly

Example fix

// before (tools.yaml)
tools:
  list-graphs:
    kind: falkordb-listgraphs
    source: redis-src
// after
tools:
  list-graphs:
    kind: falkordb-listgraphs
    source: falkordb-src

Defensive patterns

Strategy: validation

Validate before calling

if src.Kind != "falkordb" {
    return fmt.Errorf("listgraphs tool requires falkordb source, got kind=%s", src.Kind)
}

Type guard

if _, ok := src.(falkordblistgraphs.CompatibleSource); !ok {
    return fmt.Errorf("%T is not usable by falkordb-listgraphs", src)
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
    return fmt.Errorf("rebind %s to falkordb source: %w", toolName, err)
}

Prevention

Keep one FalkorDB source and reference it consistently across falkordb-* tools
Watch for env-specific config overrides silently swapping the source
Diff source names between environments when promoting configs
Run startup validation after every config change

When it happens

Trigger: Binding a `falkordb-listgraphs` tool to a source entry whose kind is not `falkordb` in tools.yaml.

Common situations: Multi-source configs where the listgraphs tool was copied from another engine's section; environment-specific overrides swapping the source to a different database; typo'd source names falling through to a similarly named source.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/88c6a3d1ea008760. Report an issue: GitHub.