A2A responses fail validation due to strict 'extra inputs not permitted'

[bokelley]

Problem

When calling tools via A2A protocol, responses that include a message field fail Pydantic validation because the response models use extra='forbid'.

Reproduction

uvx adcp https://adcp-test-agent.fly.dev get_products '{"brief": "test campaign"}' --protocol a2a --auth "ci-test-token" --json

Error

Failed to parse response: Response doesn't match expected schema GetProductsResponse: 1 validation error for GetProductsResponse
message
  Extra inputs are not permitted [type=extra_forbidden, input_value='No products matched your requirements.', input_type=str]

Comparison with TypeScript Client

The TypeScript client (@adcp/client) handles this gracefully - it extracts the message field separately and displays it as "Protocol Message":

npx @adcp/client https://adcp-test-agent.fly.dev get_products '{"brief": "test campaign"}' --protocol a2a --auth "ci-test-token"
# ✅ SUCCESS
# Protocol Message: No products matched your requirements.
# Response: { "products": [] }

Suggested Fix

Either:

1. Change response models to use extra='ignore' instead of extra='forbid'
2. Or extract known protocol-level fields (like message) before validation

Environment

• adcp Python SDK: v2.16.0
• Target AdCP Spec: 2.5.0