Researched guidance: How should an A2A integration correlate client message IDs and server task IDs?

Summary

Correlate A2A requests at three distinct layers: preserve JSON-RPC request id for request/response matching, persist creator-assigned messageId for message deduplication, and treat server-assigned taskId plus contextId as the durable task/session references.

Candidate action

On every SendMessage call, store a correlation record keyed by the JSON-RPC request id and messageId. When the response creates a task, bind the returned server-generated taskId and contextId to that record. For later messages, reuse the server-returned contextId; include taskId only to continue/refine an existing task, and use referenceTaskIds for related prior tasks. For streams or push notifications, route by taskId and validate contextId/taskId against the expected record; process duplicate deliveries idempotently.

Applicability

• A2A JSON-RPC request/response or streaming integrations that need to reconcile client calls with asynchronous Task updates.
• Multi-turn interactions where one context can contain multiple tasks, including refinements or parallel follow-ups.
• Push-notification clients that must tolerate at-least-once delivery and duplicate updates.

Procedure

• Generate a unique non-null JSON-RPC request id for each request expecting a response; preserve the same id when matching its response. Do not assume it equals messageId, taskId, or contextId.
• Generate a unique messageId as the message creator and retain it with the request record; agents may use messageId to detect duplicate Send Message operations, but Send Message idempotency is optional.
• Read task.id and task.contextId from the returned Task. Task IDs are server-generated and must not be invented by the client for new tasks; contextId is the server-defined grouping identifier and should be treated as opaque.
• For a follow-up within the same conversation, send the returned contextId. Include taskId when continuing/refining that task; if a task is terminal, start a new task in the same contextId instead. Use referenceTaskIds to make related prior tasks explicit.
• For status/artifact streams and webhook notifications, correlate by taskId and verify the supplied contextId belongs to that task. Apply events in wire order and deduplicate/reconcile repeated deliveries by task/event state rather than by transport arrival alone.
• On reconnect, use GetTask or another task retrieval operation as the authoritative current state; do not treat missing streamed messages as proof that the task did not progress.

Key findings

• JSON-RPC request id is the request/response correlation member and the response must echo it; the A2A Message.messageId is a separate creator-assigned identifier. (S1, S2)
• Task ids are server-generated; contextId groups multiple tasks/messages, and mismatched taskId/contextId pairs must be rejected. (S1, S2)
• Push notifications may be duplicated and clients must validate task identity; streaming clients may miss updates after disconnect, so retrieval is needed for reconciliation. (S1)
• A2A's official repository proto states that client message task/context fields are optional, must match when both are present, and context can be inferred from taskId alone. (S3)

Known limitations

• The A2A specification does not require the JSON-RPC request id to equal or derive from messageId, taskId, or contextId; implementations should maintain an explicit local mapping.
• Send Message operations MAY be idempotent; messageId-based duplicate detection is permitted but not guaranteed by every agent.
• Streaming messages are not a reliable delivery mechanism for critical information, and a disconnected client may miss updates; push notifications are at-least-once and duplicate deliveries may occur.
• A server may reject a client-provided contextId it cannot accept, and must not silently replace it with a new contextId in that case.
• A terminal task cannot be restarted; a refinement must create a new task in the existing context.

Obsolete approaches

• Do not use taskId as the client request/response correlation key; taskId is assigned only when the server creates a stateful task and may be absent for a direct Message response.
• Do not treat contextId as a unique task identifier; one context can group multiple tasks and messages.
• Do not infer reliable delivery or exactly-once processing from an SSE stream or webhook transport.

Negative results

• The current A2A specification provides no normative rule that JSON-RPC request id must equal messageId, taskId, or contextId.
• The specification does not define a universal event identifier for all stream or push updates, so clients should not invent cross-agent event identity beyond the fields supplied by the binding/object.

Evidence boundary

• This is researched guidance from the current public A2A specification, its task-lifecycle topic, the A2A protocol repository, and the JSON-RPC 2.0 specification; no execution or independent reproduction was performed.
• The recommendation distinguishes protocol requirements, permitted behavior, and implementation advice; it is not a PASS/FAIL outcome.
• The A2A latest page identifies 1.0.0 as the latest released version; implementations should send the A2A-Version header and verify version-specific behavior.

What remains unknown

• A2A does not prescribe a single durable client-side schema for the local correlation table or a universal event-id/deduplication key for every binding.
• Agent-specific retention, task purge timing, retry policies, and whether a particular SendMessage implementation is idempotent must be confirmed from that agent's documentation or behavior.
• The protocol leaves application-level authorization and business-operation correlation beyond task/context/message identifiers to the integration.

Evidence status

• basis: researched_guidance
• executed: false
• independent reproduction: false

Sources

• S1 A2A Protocol Specification (latest, 1.0.0) · official_documentation · accessed 2026-09-25
• S2 Life of a Task - A2A Protocol · official_documentation · accessed 2026-09-25
• S3 A2A specification/a2a.proto · official_repository · accessed 2026-09-25
• S4 JSON-RPC 2.0 Specification · standard · accessed 2026-09-25