What causes "source is not compatible with the tool" in googleapis/mcp-toolbox?

Set the tool's source to a bigtable-kind source in tools.yaml

source is not compatible with the tool

Error message

source is not compatible with the tool

What it means

The bigtable-list-instances tool's ValidateSource returns this error when the provided sources.Source does not satisfy its compatibleSource interface (the Bigtable source that can list instances). The assertion fails for any source type other than the Bigtable source or a build where its method set changed. It is raised before any Bigtable admin API call is attempted.

Source

Thrown at internal/tools/bigtable/bigtablelistinstances/bigtablelistinstances.go:92

			allParameters,
		),
	}, nil
}

var _ tools.Tool = Tool{}

type Tool struct {
	tools.BaseTool[Config]
}

func (t Tool) GetSourceName() string {
	return t.Cfg.Source
}

func (t Tool) ValidateSource(src sources.Source) error {
	_, ok := src.(compatibleSource)
	if !ok {
		return fmt.Errorf("source is not compatible with the tool")
	}
	return nil
}

func (t Tool) ToConfig() tools.ToolConfig {
	return t.Cfg
}

func (t Tool) Invoke(ctx context.Context, src sources.Source, params parameters.ParamValues, accessToken tools.AccessToken) (any, util.ToolboxError) {
	source, ok := src.(compatibleSource)
	if !ok {
		return nil, util.NewClientServerError("source used is not compatible with the tool", http.StatusInternalServerError, nil)
	}

	instances, err := source.ListInstances(ctx)
	if err != nil {
		return nil, util.ProcessGcpError(err)
	}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Set the tool's source to a bigtable-kind source in tools.yaml
Verify the source implements the expected ListInstances capability
Align toolbox and source versions and rebuild
Validate configuration at startup to surface mismatches immediately

Example fix

// before
source: bigquery-warehouse
// after
source: bigtable-instance-source

Defensive patterns

Strategy: validation

Validate before calling

if err := tool.ValidateSource(mySource); err != nil {
    return fmt.Errorf("bigtable-list-instances needs a bigtable source: %w", err)
}

Type guard

func isBigtableInstanceSource(s sources.Source) bool {
    _, ok := s.(interface{ ListInstances(context.Context) (any, error) })
    return ok
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
    log.Fatalf("wrong source for bigtable-list-instances: %v", err)
}

Prevention

Check that tools.yaml tool entries reference bigtable sources
Validate config at startup before accepting requests
Avoid hand-editing kind: fields in shared configs
Pin the toolbox version used across environments

When it happens

Trigger: ValidateSource is invoked, or the tool is executed through the server, with a non-Bigtable source (e.g. postgres, alloydb, bigquery) or an incompatible custom source.

Common situations: Wrong source referenced in the tool's tools.yaml entry; source kind edited by mistake; version mismatch after a toolbox upgrade altered the internal source interface.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/86e031f592673bc5. Report an issue: GitHub.