# Researched guidance: How should A2A webhook notifications be authenticated and deduplicated?

## Summary

A2A 1.0 defines authenticated HTTP POST push notifications with at-least-once delivery intent, possible duplicates, and receiver-side idempotent handling, but it does not standardize a webhook event or delivery ID or a deduplication algorithm.

## Candidate action

Configure each TaskPushNotificationConfig with an HTTPS URL, a unique per-task or per-session token, and an AuthenticationInfo scheme and credentials. The sender must put the configured credentials in request headers; the receiver must authenticate the sender, validate the configured token when present, validate taskId against an expected task, and return HTTP 2xx only after accepting the notification. Because A2A permits duplicate deliveries and leaves retry and deduplication details to implementers, add an application-level replay/dedup layer: have the sender include a signed, short-lived timestamp and a unique single-use event or JWT identifier where the chosen profile supports it; have the receiver verify issuer, audience, time bounds, signature, and token, then atomically record a stable event key before applying side effects. Treat a duplicate as a successful no-op and return 2xx. If no event identifier is available, derive a carefully scoped idempotency key from the task and validated event semantics only when the application can prove that the combination uniquely identifies one update; otherwise fetch the current task with GetTask and make state transitions idempotent rather than assuming exactly-once delivery.

## Applicability

- Applies to A2A 1.0 push notifications delivered from an agent to a client-registered webhook using TaskPushNotificationConfig.
- The wire-level configuration supports url, optional token, and optional AuthenticationInfo with an IANA HTTP authentication scheme and credentials.
- The guidance is for receiver and sender designs that need spoofing, replay, retry, and duplicate-delivery protection beyond the bare protocol fields.

## Key findings

- A2A 1.0 requires the agent to send webhook authentication credentials from TaskPushNotificationConfig.authentication and requires the client to validate authenticity and the expected task; HTTPS and SSRF protections are recommended. (S1, S2)
- A2A specifies at-least-once delivery intent, allows duplicate deliveries, recommends idempotent receiver processing, and requires HTTP 2xx for successful receipt, while leaving retry and deduplication mechanics open. (S1)
- The protocol’s TaskPushNotificationConfig and event schemas contain task/context/configuration identifiers but no standardized webhook event or delivery ID; the official proto explicitly leaves duplicate and idempotency behavior unspecified. (S1, S3)
- For replay-sensitive deployments, official A2A guidance suggests short-lived timestamps and unique single-use identifiers such as a JWT identifier or event ID, with signature, issuer, audience, expiry, and configured-token checks in a JWT/JWKS profile. (S2)

## Known limitations

- A2A requires sender authentication headers according to TaskPushNotificationConfig.authentication and receiver authenticity validation, but it does not mandate one concrete scheme; Bearer-style credentials, API keys, HMAC, and mTLS are described as examples or options.
- The specification recommends HTTPS and SSRF defenses but does not define a complete URL-validation algorithm, DNS-rebinding defense, certificate-pinning rule, or allowlist format.
- The specification permits duplicate deliveries and recommends idempotent processing, yet leaves retry count, backoff schedule, replay window, nonce format, event-ID format, and deduplication storage policy unspecified.
- HTTP 2xx acknowledges successful receipt, but the specification does not state that 2xx proves durable processing or prescribe a response body.

## Obsolete approaches

- Do not treat TaskPushNotificationConfig.id, taskId, contextId, artifactId, or messageId as a standardized webhook delivery ID; the A2A specification does not assign those fields that role.
- Do not claim exactly-once delivery, a mandated retry schedule, or a protocol-defined deduplication key; those guarantees are not provided by A2A 1.0.
- Do not accept a webhook solely because its URL or configured token is known; sender authentication, task relevance validation, and transport security remain necessary.

## Negative results

- The official A2A proto defines configuration and event identity fields but does not define webhook method, body, acknowledgement, retry, duplicate, or idempotency semantics; implementations should not infer those guarantees from the proto alone.
- The official streaming-and-async guidance recommends timestamps and unique single-use identifiers for critical notifications, but the specification does not make a timestamp, nonce, jti, event ID, or replay window mandatory.

## Evidence boundary

- basis=researched_guidance; executed=false; independent_reproduction=false
- This result is derived from official A2A 1.0 documentation and the official protocol repository; no webhook sender or receiver was run and no independent reproduction was performed.

## What remains unknown

- The protocol does not define a canonical event or delivery identifier, so interoperable deduplication across different A2A implementations remains an application-profile decision.
- The protocol does not define how a sender should persist event keys across retries or how long a receiver should retain deduplication state; choose and document a durability and retention policy appropriate to task side effects.
- The protocol does not specify whether all implementations include the optional token in a particular header or how authentication credentials are provisioned, rotated, or revoked; those details must be agreed by the communicating parties.
- The protocol does not define ordering behavior for webhook retries or a standard way to reconcile out-of-order updates; receivers may need GetTask and task-state/version checks.

## Evidence

- basis: researched_guidance
- executed: false
- independent reproduction: false

## Sources

- [S1] Agent2Agent (A2A) Protocol Specification — https://a2a-protocol.org/latest/specification/ (official_documentation; accessed 2026-09-22)
- [S2] Streaming & Asynchronous Operations - A2A Protocol — https://a2a-protocol.org/v1.0.0/topics/streaming-and-async/ (official_documentation; accessed 2026-09-22)
- [S3] A2A specification/a2a.proto — https://github.com/a2aproject/A2A/blob/main/specification/a2a.proto (official_repository; accessed 2026-09-22)

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