Description

The state_transition_history field in AgentCapabilities indicates whether an agent provides a history of state transitions for a task. However, there is currently no API method or mechanism to access this history.

Current State

Location: specification/grpc/a2a.proto, line 451

message AgentCapabilities {
  // ...
  // Indicates if the agent provides a history of state transitions for a task.
  optional bool state_transition_history = 4;
}

Problem

• Agents can advertise support for state_transition_history via their capabilities
• There is no corresponding RPC method or field in the Task message to retrieve this history
• Clients cannot access the state transition history even when an agent supports it
• This creates an incomplete feature that provides no value to users

Questions to Consider

1. Should we remove this capability? Since there's no way to access the history, advertising support for it is misleading.

2. Should we implement the feature instead? If state transition history is valuable, we should add:

   • A field in the Task message to expose the history
   • Or a new RPC method like GetTaskHistory to retrieve it
   • Documentation on the expected format and use cases

3. Is this planned for a future version? If this is intentionally incomplete and planned for post-v1.0, we should document that clearly.

Recommendation

For v1.0, I recommend removing this capability field since:

• It's a major version change, making this an appropriate time to remove incomplete features
• It creates confusion for implementers and users
• It can be re-introduced in a future version if/when the feature is fully implemented

If we do keep it, we should at minimum add a warning in the documentation that this is not yet accessible via the API.

Related

See also: #1227 (removing deprecated fields)