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

Point `source:` at a `kind: serverless-spark` source.

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-list-sessions tool throws this from ValidateSource when the source attached to the tool is not a Serverless Spark source implementing `compatibleSource`. The error surfaces during configuration loading/initialization because Toolbox validates each tool against its source before serving, ensuring the tool can construct the Dataproc session client it needs.

Source

Thrown at internal/tools/serverlessspark/serverlesssparklistsessions/serverlesssparklistsessions.go:132

	res, err := source.ListSessions(ctx, pageSize, pt, filter)
	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 `source:` at a `kind: serverless-spark` source.
Correct the source name typo or restore the missing serverless-spark source entry.
Regenerate/validate the tools.yaml with the Toolbox dev UI to confirm every tool/source pairing passes ValidateSource.

Example fix

# before
tools:
  list-sessions:
    kind: serverless-spark-list-sessions
    source: dataproc-src
# after
tools:
  list-sessions:
    kind: serverless-spark-list-sessions
    source: serverless-spark-src

Defensive patterns

Strategy: validation

Validate before calling

tools:
  list-sessions:
    kind: serverless-spark-list-sessions
    source: spark-src  # kind: serverless-spark required

Type guard

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

Prevention

Verify tool/source pairings after any YAML restructure or merge.
Use the toolbox dev UI to load configs interactively and see validation errors immediately.
Keep integration-specific tools in separate config files to reduce cross-binding mistakes.

When it happens

Trigger: Configuring `serverless-spark-list-sessions` with a `source` that resolves to a different source implementation, causing `source.(compatibleSource)` to fail in serverlesssparklistsessions.go.

Common situations: Wrong source name after copying a tool definition from another integration's docs; sources section reorganized and the Spark source key changed; YAML anchors/merging binding one source to many mismatched tools.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/32bbc1484232afe0. Report an issue: GitHub.