Description

When chaining steps in composite tools, templates cannot access nested fields in backend tool outputs. This is because the HTTP backend client ignores StructuredContent.

Current Behavior

The backend HTTP client processes tool results here (pkg/vmcp/client/client.go#L474-L500):

for i, content := range result.Content {
    if textContent, ok := mcp.AsTextContent(content); ok {
        resultMap["text"] = textContent.Text  
    } else if imageContent, ok := mcp.AsImageContent(content); ok {
        resultMap[key] = imageContent.Data
    } else {
        logger.Debugf("Unsupported content type...")  
    }
}
// result.StructuredContent ignored entirely.

If a backend tool returns:

{"message": "hello", "count": 42}

The step output becomes:

{"text": "{\"message\": \"hello\", \"count\": 42}"}

Expected Behavior

Based on the documentation, Templates like {{.steps.step1.output.message}} should work for accessing nested fields in tool results.

Impact

• Step chaining is limited: Can only access .steps.stepID.output.text, not .steps.stepID.output.someField.someNestedField

Suggested Solution

The schema for step step.output in the composite step templating becomes:

{
    structuredContent any 
    text string
   // other text_i fields, etc. as the code currently performs above.
}

Alternatively, if structuredContent is present, then that simply is the output and the other content is ignored.

Regardless, we'll want to backfill testing to ensure references to structured data between tool steps is working as intended.

Related

• Discovered while implementing Composite Tools Supports Default Return Values #2989 (defaultResults for skipped/failed steps)