[hermes-agent / A2A] outbound text parts advertise mediaType; strict A2A 1.0 peers reject

## Observed symptom A2A: outbound text parts advertise mediaType, which strict A2A 1.0 peers reject # A2A: outbound text parts advertise `mediaType`, which strict A2A 1.0 peers reject ## What happens `a2a_call` fails against an A2A 1.0 server that validates parts on member presence. The peer answers the whole request with a JSON-RPC `InvalidParams` error and Hermes surfaces it as: ``` Peer '<name>' returned an error: A2A server accepts text parts only ``` The message Hermes sends carries one part: ```json {"role": "ROLE_USER", "parts": [{"text": "…", "mediaType": "text/plain"}], "messageId": "…"} ``` ## Why it is wrong In the A2A 1.0 `Part` message, `mediaType` accompanies the file/data members; on a text part it is redundant. Receivers that discriminate parts by which members are present read a part that sets both `text` and `mediaType` as non-text and reject it. The official `a2 ## Literal error text ``` mediaType rejected by A2A 1.0 peers ``` ## Environment GitHub issue NousResearch/hermes-agent #127707; state=open ## Trigger / context # A2A: outbound text parts advertise `mediaType`, which strict A2A 1.0 peers reject ## What happens `a2a_call` fails against an A2A 1.0 server that validates parts on member presence. The peer answers the whole request with a JSON-RPC `InvalidParams` error and Hermes surfaces it as: ``` Peer '<name>' returned an error: A2A server accepts text parts only ``` The message Hermes sends carries one part: ```json {"role": "ROLE_USER", "parts": [{"text": "…", "mediaType": "text/plain"}], "messageId": "…"} ``` ## Why it is wrong In the A2A 1.0 `Part` message, `mediaType` accompanies the file/data members; on a text part it is redundant. Receivers that discriminate parts by which members are present read a part that sets both `text` and `mediaType` as non-text and reject it. The official `a2a-sdk` omits `mediaType` on text parts, so the same peer accepts a request from an SDK-built client and rejects the same request from Hermes — the field is the only difference. ## Where `plugins/platforms/a2a/protocol.py::text_part()`: ```python return {"text": text, "mediaType": "text/plain"} ``` It feeds text parts in messages, tasks and artifacts (three call sites in the same module). ## Reproduction 1. Run any A2A 1.0 server that discriminates parts on member presence (reproduced against a self-hosted implementation of the A2A 1.0 JSON-RPC binding; the server answers `InvalidParams` with `A2A server accepts text parts only`). 2. Configure it under `a2a_agents:` and enable the ## Source evidence - https://github.com/NousResearch/hermes-agent/issues/127707 (2026-10-02T09:32:26Z) — GitHub issue #127707 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:43:03.388295Z - scout_id: C9e6b1a4009 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.