fix(eventqueue): add JSON marshal/unmarshal to Message so events survive roundtrip (fixes #349)

[kuangmi-bit]

What

Add custom MarshalJSON/UnmarshalJSON to eventqueue.Message so that its Event field (an a2a.Event interface) correctly survives a JSON roundtrip.

Why

eventqueue.Message is the core message type flowing through the event queue between AgentExecutor and the A2A server stack. When this type is serialized to JSON (e.g., for persistence, external event buses, or debugging), the Event field — which is an a2a.Event interface — cannot be deserialized by Go's default encoding/json.

Fix

Delegate to a2a.StreamResponse for the Event field, reusing the same polymorphic JSON wrapper pattern (event struct with omitempty fields for each concrete type) already established in a2a/core.go.

func (m Message) MarshalJSON() ([]byte, error) {
    return json.Marshal(messageJSON{
        Event:       a2a.StreamResponse{Event: m.Event},
        TaskVersion: m.TaskVersion,
        Protocol:    m.Protocol,
    })
}

Test

Added queue_test.go with roundtrip tests covering all four event types:

• TaskStatusUpdateEvent
• TaskArtifactUpdateEvent
• Message
• Task

All 38 packages pass with -race.

Closes #349

[gemini-code-assist]

Code Review

This pull request implements custom JSON marshaling and unmarshaling for the Message struct in a2asrv/eventqueue/queue.go to ensure that the a2a.Event interface survives a JSON roundtrip. It also introduces comprehensive unit tests in a2asrv/eventqueue/queue_test.go to verify the roundtrip behavior across different event types. The review feedback suggests adding defensive checks in both MarshalJSON and UnmarshalJSON to handle nil event scenarios and prevent potential runtime panics or invalid states.

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[gemini-code-assist]

If the incoming JSON is missing the event field or contains an invalid event type, wrapper.Event.Event will be nil. Assigning this to m.Event results in a nil interface, which will cause a panic when methods are called on it. Additionally, we should defensively check if the receiver m is nil to prevent a panic during assignment.

func (m *Message) UnmarshalJSON(data []byte) error {
	if m == nil {
		return errors.New("eventqueue: UnmarshalJSON on nil Message pointer")
	}
	var wrapper messageJSON
	if err := json.Unmarshal(data, &wrapper); err != nil {
		return err
	}
	if wrapper.Event.Event == nil {
		return errors.New("eventqueue: message event cannot be nil")
	}
	m.Event = wrapper.Event.Event
	m.TaskVersion = wrapper.TaskVersion
	m.Protocol = wrapper.Protocol
	return nil
}

[gemini-code-assist]

If m.Event is nil, marshaling will proceed with a nil event, which represents an invalid state and could lead to issues downstream. Consider adding a defensive check to return an error if m.Event is nil.

func (m Message) MarshalJSON() ([]byte, error) {
	if m.Event == nil {
		return nil, errors.New("eventqueue: cannot marshal Message with nil Event")
	}
	return json.Marshal(messageJSON{
		Event:       a2a.StreamResponse{Event: m.Event},
		TaskVersion: m.TaskVersion,
		Protocol:    m.Protocol,
	})
}

[nahapetyan-serob]

This makes sense, please address this and other gemini-code-assist comment

[nahapetyan-serob]

2026

[nahapetyan-serob]

This makes sense, please address this and other gemini-code-assist comment

[kuangmi-bit]

Addressed review feedback:

• MarshalJSON now returns ErrNilEvent when Event is nil instead of producing invalid JSON
• UnmarshalJSON now returns ErrNilEvent when the incoming JSON has no valid event field
• Added TestMessageNilEventError covering both paths

All existing tests still pass. PTAL @nahapetyan-serob