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 `kind: serverless-spark` 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

The serverlessspark-get-session tool throws this in its ValidateSource method when the bound source does not satisfy the tool's `compatibleSource` interface (a Serverless Spark source able to create session clients). The library validates tool/source compatibility at startup so misconfiguration is caught before any request. A type assertion failure means the named source exists but is of a different kind than the tool requires.

Source

Thrown at internal/tools/serverlessspark/serverlesssparkgetsession/serverlesssparkgetsession.go:128

	res, err := source.GetSession(ctx, name)
	if err != nil {
		return nil, util.ProcessGcpError(err)
	}
	return res, nil
}

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
}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's `source` at a `kind: serverless-spark` source in tools.yaml.
Verify source name spelling and that the source block still exists with the serverless-spark kind.
Remove or replace the tool if it no longer matches your intended source type.

Example fix

# before
tools:
  get-session:
    kind: serverless-spark-get-session
    source: mysql-src
# after
tools:
  get-session:
    kind: serverless-spark-get-session
    source: spark-src  # kind: serverless-spark

Defensive patterns

Strategy: validation

Validate before calling

tools:
  get-session:
    kind: serverless-spark-get-session
    source: spark-src  # source must be kind: serverless-spark

Type guard

func isServerlessSparkSource(s sources.Source) bool {
    _, ok := s.(serverlessspark.Source)
    return ok
}

Prevention

Validate the tools file at CI time by launching the toolbox binary against it.
Use distinct, descriptive source names to avoid binding tools to the wrong source.
Keep tool definitions close to their source definitions in the YAML for reviewability.

When it happens

Trigger: A `serverless-spark-get-session` tool whose `source` field names a source whose concrete Go type is not the Serverless Spark source, so `source.(compatibleSource)` returns ok=false.

Common situations: tools.yaml with multiple sources where the tool references the wrong one; a refactored config where the Spark source was renamed/deleted and the name now resolves to another source; hand-edited YAML merging two example configs.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/2fc0cc2f4b86c5df. Report an issue: GitHub.