BoundaryML/baml · error

unexpected type for id: %T

Error message

unexpected type for id: %T

What it means

Thrown by httpResponse.RequestId() when the FFI CallMethod("id") succeeded but the decoded result is not a Go string. The %T verb reports the actual decoded type. This indicates the core returned the id in an unexpected representation (e.g. an integer or wrapped object) relative to what the Go bindings expect.

Source

Thrown at engine/language_client_go/pkg/rawobjects_http_response.go:35

}

func (h *httpResponse) ObjectType() cffi.BamlObjectType {
	return cffi.BamlObjectType_OBJECT_HTTP_RESPONSE
}

func (h *httpResponse) pointer() int64 {
	return h.RawObject.Pointer()
}

func (h *httpResponse) RequestId() (string, error) {
	result, err := raw_objects.CallMethod(h, "id", nil)
	if err != nil {
		return "", fmt.Errorf("failed to get id: %w", err)
	}

	id, ok := result.(string)
	if !ok {
		return "", fmt.Errorf("unexpected type for id: %T", result)
	}

	return id, nil
}

func (h *httpResponse) Status() (int64, error) {
	result, err := raw_objects.CallMethod(h, "status", nil)
	if err != nil {
		return 0, fmt.Errorf("failed to get status: %w", err)
	}

	status, ok := result.(int64)
	if !ok {
		return 0, fmt.Errorf("unexpected type for status: %T", result)
	}

	return status, nil
}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Upgrade the Go bindings and BAML core to the same release so the id is decoded as a string
2. Check the %T in the error to identify the actual decoded type and search pkg/cffi for the corresponding decoder
3. Fall back to LLMCall.RequestId() which reads http_request_id if the response id is unusable
4. Report to BAML maintainers with the %T value if versions are already aligned

Example fix

id, err := resp.RequestId()
if err != nil {
    // guard against non-string decode
    return fmt.Errorf("request id not usable: %w", err)
}

if resp == nil { return errors.New("nil HTTPResponse") }

id, err := resp.RequestId()
if err != nil {
    if strings.Contains(err.Error(), "unexpected type") {
        return "", fmt.Errorf("id decode drift, check versions: %w", err)
    }
    return "", err
}

When it happens

Trigger: Calling HTTPResponse.RequestId() when the core's id field decodes to a non-string CFFI value — typically after a version mismatch where the id representation changed between the core and Go bindings.

Common situations: Upgraded BAML core with older Go bindings (or vice versa); event-log data produced by a different BAML version; corrupted decoding of the id payload.

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 class property builders: %T
• unexpected type for class property builder: %T
• unexpected type for class property type: %T
• unexpected type for body: %T
• unexpected type for status: %T

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/8aba08463ed5167b. Report an issue: GitHub.