BoundaryML/baml · error

unexpected type for http response: %T

Error message

unexpected type for http response: %T

What it means

LLMCall.HttpResponse() asserts the runtime result implements HTTPResponse; this error fires when the deserialized FFI value has a different type (%T reveals it). It means the runtime returned an unexpected payload shape for the http_response attribute — a Go-binding/runtime contract mismatch. A nil result is handled earlier and returns (nil, nil), so this only fires on a present-but-wrong-typed value.

Source

Thrown at engine/language_client_go/pkg/rawobjects_llm_call.go:90

		return nil, fmt.Errorf("unexpected type for http request: %T", result)
	}

	return request, nil
}

func (l *llmCall) HttpResponse() (HTTPResponse, error) {
	result, err := raw_objects.CallMethod(l, "http_response", nil)
	if err != nil {
		return nil, fmt.Errorf("failed to get http response: %w", err)
	}

	if result == nil {
		return nil, nil
	}

	response, ok := result.(HTTPResponse)
	if !ok {
		return nil, fmt.Errorf("unexpected type for http response: %T", result)
	}

	return response, nil
}

func (l *llmCall) Usage() (Usage, error) {
	result, err := raw_objects.CallMethod(l, "usage", nil)
	if err != nil {
		return nil, fmt.Errorf("failed to get usage: %w", err)
	}

	if result == nil {
		return nil, nil
	}

	usage, ok := result.(Usage)
	if !ok {
		return nil, fmt.Errorf("unexpected type for usage: %T", result)

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Upgrade and regenerate BAML bindings so both sides agree on the response object type.
2. Inspect the %T value to see what was actually returned and report persistent mismatches.
3. Handle the error gracefully in tracing/logging code paths.
4. Ensure you never swapped request/response objects when storing call metadata.

Example fix

// before
resp, err := call.HttpResponse()
if err != nil { return err }

// after
resp, err := call.HttpResponse()
if err != nil {
    log.Printf("http response type mismatch: %v", err)
    resp = nil
}

func isHTTPResponse(v interface{}) (baml.HTTPResponse, bool) {
    resp, ok := v.(baml.HTTPResponse)
    return resp, ok
}

resp, err := call.HttpResponse()
if err != nil {
    log.Printf("http response type mismatch: %v", err)
    return nil
}

When it happens

Trigger: raw_objects.CallMethod(l, "http_response", nil) returns a non-nil value failing the response.(HTTPResponse) assertion at rawobjects_llm_call.go:88-91 — e.g. an HTTPRequest object or a raw CFFI wrapper.

Common situations: Version skew between Go bindings and native runtime changing how http_response is typed; a runtime bug returning a request object where a response was expected; objects created by a different runtime instance.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• unexpected type for client name: %T
• unexpected type for provider: %T
• unexpected type for http request: %T
• unexpected type for usage: %T
• unexpected type for class property builders: %T

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/188cc8d604ae7f0c. Report an issue: GitHub.