[Bug]: Clarify the Content-Type for errors for the HTTP+JSON transport.

[jmesnil]

What happened?

The spec does not specify what is the content-type for errors for the HTTP+JSON transport, application/json or application/problem+json.

Given that errors now follows https://google.aip.dev/193 instead of RFC 9457, I'd assume that application/json is the right content type for error responses

#1600 seems to be the cause of this missing information (the mention of application/problem+json was removed in this PR).

Note that there are still 2 error examples with application/problem+json in the spec:

• https://github.com/a2aproject/A2A/blob/173695755607e884aa9acf8ce4feed90e32727a1/docs/specification.md#64-version-negotiation-error
• https://github.com/a2aproject/A2A/blob/173695755607e884aa9acf8ce4feed90e32727a1/docs/specification.md#pagination-example

while another error example now uses application/json:

• https://github.com/a2aproject/A2A/blob/173695755607e884aa9acf8ce4feed90e32727a1/docs/specification.md#116-error-handling

If there is an agreement, I propose to:

• use application/son consistently for JSON-RPC and HTTP+JSON error examples

• Update

  A2A/docs/specification.md

  Line 2212 in 1736957

  - **Content-Type:** `application/json` for requests and responses

  :

  • Content-Type: application/json for requests and responses (including error responses)

• A2A/docs/specification.md

  Line 2729 in 1736957

  - **Content-Type:** `application/json` for requests and responses

  :

  • Content-Type: application/json for requests and responses (including error responses)

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[msaleme]

Thanks for calling this out. Since we moved to the AIP-193 error schema in #1600 the payload is a straight JSON blob (code/message/details) and no longer follows RFC 9457 problem+json conventions. That means the HTTP+JSON transport really should advertise Content-Type: application/json for success and error responses. JSON-RPC already requires application/json as well, so aligning the transport guidance keeps headers consistent across both entry points.

Unless maintainers object I can open a docs patch later today that:

• Updates sections 6.2 and 11.6 to spell out application/json "including error responses"
• Fixes the remaining examples that still show application/problem+json
• Adds a short note linking back to AIP-193 so implementers know why the change happened

Let me know if there is existing guidance I should carry forward.