A2A Multi-Turn Conversations

Before writing code

Fetch live docs:

1. Fetch https://a2a-protocol.org/latest/specification/ for multi-turn and input-required handling
2. Web-search site:github.com a2aproject A2A multi-turn input-required for multi-turn protocol details
3. Web-search site:github.com a2aproject a2a-samples multi-turn for conversation examples
4. Fetch SDK docs for multi-turn task handling patterns

Conceptual Architecture

What Multi-Turn Means in A2A

Multi-turn conversations allow agents to have iterative exchanges within a single task. The server agent can request more input from the client, creating a back-and-forth dialogue before reaching a final result.

The input-required State

The key mechanism for multi-turn is the input-required task state:

Client sends message → Task created (submitted)
Server processes → working
Server needs more info → input-required (with a message explaining what's needed)
Client sends follow-up → working (task continues with same taskId)
Server completes → completed

How It Works

1. Initial request: Client sends message/send or message/stream without taskId
2. Server processes: May transition to input-required if it needs clarification
3. Server response: Returns the task with input-required state and a message explaining what's needed
4. Client responds: Sends another message/send with the same taskId and the requested information
5. Server continues: Resumes processing with the additional context
6. Repeat or complete: May request more input or reach a terminal state

Context Preservation

Within a multi-turn task:

• All messages are associated with the same taskId
• The server maintains the full conversation history
• Each new message from the client adds to the context
• The server can reference previous messages when processing

Patterns

Clarification Loop

Agent asks for clarification when the request is ambiguous:

User: "Book a flight"
Agent: (input-required) "Where are you flying from and to? What dates?"
User: "NYC to London, March 15-22"
Agent: (input-required) "I found 3 options. Which do you prefer? [options]"
User: "Option 2"
Agent: (completed) "Booked! Confirmation #ABC123"

Progressive Refinement

Agent produces intermediate results and asks for feedback:

User: "Write a blog post about AI"
Agent: (input-required) "Here's an outline. Should I proceed? [outline]"
User: "Looks good, but add a section on safety"
Agent: (completed) "Here's the final post. [content]"

Human-in-the-Loop

Agent defers to a human for decisions it can't make:

User: "Process this insurance claim"
Agent: (input-required) "This claim requires manager approval for amounts over $10,000. Please confirm."
User: "Approved"
Agent: (completed) "Claim processed and approved."

Data Collection

Agent collects required information step by step:

User: "Set up my account"
Agent: (input-required) "What's your email address?"
User: "user@example.com"
Agent: (input-required) "What subscription plan? [Basic/Pro/Enterprise]"
User: "Pro"
Agent: (completed) "Account created with Pro plan."

Server Implementation

The server must:

• Maintain task state across multiple requests
• Store conversation history for context
• Return clear messages when requesting input
• Handle both new tasks and task continuations
• Validate that continued tasks exist and are in the right state

Client Implementation

The client must:

• Check response task state after each call
• Detect input-required and present the agent's message to the user (or auto-respond)
• Send follow-up messages with the same taskId
• Handle the possibility of multiple rounds
• Know when to give up (max turns, timeout)

Best Practices

• Include clear, specific messages when entering input-required — the client needs to know what to provide
• Set maximum turn limits to prevent infinite loops
• Preserve full context across turns — don't lose earlier parts of the conversation
• Use DataPart for structured input requests (e.g., forms, multiple-choice)
• Consider timeouts for tasks waiting in input-required state
• Log all turns for debugging multi-turn flows
• Handle the case where the client never responds (timeout → canceled or failed)

Fetch the specification for exact multi-turn handling rules, message continuation semantics, and state transition constraints before implementing.