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

Point the tool's `source:` field at a source with `kind: sqlite`.

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

Same pattern as the Spanner tool: sqlite-execute-sql's ValidateSource asserts that the bound source implements compatibleSource (an SQLite source exposing *sql.DB). Any non-SQLite source fails the assertion and yields this error, preventing cross-engine tool/source pairing.

Source

Thrown at internal/tools/sqlite/sqliteexecutesql/sqliteexecutesql.go:101

// 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)
	}
	sqlStr, ok := params.AsMap()["sql"].(string)
	if !ok {
		return nil, util.NewAgentError("missing or invalid 'sql' parameter", nil)
	}
	if sqlStr == "" {
		return nil, util.NewAgentError("sql parameter cannot be empty", nil)
	}

	// Log the query executed for debugging.

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's `source:` field at a source with `kind: sqlite`.
Add/fix the sqlite source declaration in `sources:`.
Validate tool-to-source engine compatibility at config load time.

Example fix

// before
source: "my-postgres"
// after
source: "my-sqlite"

Defensive patterns

Strategy: type-guard

Validate before calling

yq '.tools[] | select(.kind == "sqlite-execute-sql") | .source' tools.yaml
# cross-check the referenced source's kind is sqlite:
yq '.sources[<name>].kind' tools.yaml

Type guard

func isSQLiteSource(s sources.Source) bool {
    _, ok := s.(sqliteexecutesql.CompatibleSource)
    return ok
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
    return fmt.Errorf("bind %q to a sqlite source: %w", toolName, err)
}

Prevention

Name sources clearly by engine (my-sqlite, my-pg) and reference exactly.
Run `toolbox --tools-file tools.yaml` (validate/dry-run) before deploying.
Re-grep source refs after renaming any source.
Avoid template variables that could swap source names between engines.

When it happens

Trigger: Tool of kind `sqlite-execute-sql` bound via `source:` to a source whose kind is not `sqlite`; passing a foreign sources.Source implementation directly to Invoke/ValidateSource.

Common situations: Reusing one shared tools.yaml across projects where source names collide; renaming the sqlite source but leaving the tool pointing at a different engine's source; dynamic tool wiring in Go code.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/100a1a57ca3f0c8b. Report an issue: GitHub.