counterpart

A2A lets one AI agent hand work to another. counterpart mocks the agent on the other end, including the ways it can go wrong:

async def test_carrier_reports_success_with_no_price(mock_agent):
    peer = mock_agent("false_success")     # reports done, returns garbage

    task = await peer.ask("Quote 2 pallets LA to Dallas", contract={"price": float})

    assert task.status == "completed"   # the peer said it finished
    assert task.contract_violated      # what it sent back was unusable

Why counterpart

If your agent hands work to an agent somebody else runs, you cannot test it without them. counterpart stands in for that agent, and it also checks what comes back, because a task reaching completed only means the peer stopped working. Every one of these arrives as a completed task and the protocol is satisfied by all of them:

{"message": "Your quote is ready!"}          # reads fine, contains no price
{"price": "call for rate"}                   # a sentence where you needed a number
{"price": 1420.0, "account": "AC-99812"}     # right shape, somebody else's account
{}                                           # nothing at all

A contract is you writing down what you were expecting, so the test fails at the point the peer lets you down instead of three functions later or in an invoice. {"price": float} is the shorthand form. For real checks, pass a pydantic model and add named predicates, with strict=True when you do not want "1420.00" silently coerced to a number:

contract = (
    Contract("freight quote")
    .returns(Quote, strict=True)
    .require(price_positive=lambda q: q.price > 0)
)

The mock runs inside your test process. No socket, no port, nothing on the network. mock_agent is the pytest fixture; MockAgent is the same object without pytest, and its .serve() binds a real address when you need to point another process at it. It works in both directions too: wrap() turns your own agent into an A2A server so the mock can call it and hold your answers to the same contract.

If your agents are three functions in the same process, you do not need any of this.

Personas

Six ship with the package. All deterministic, none needing an LLM.

Writing your own takes one class and one call. There is no DSL.

from counterpart.core import Complete, Progress
from counterpart.personas import register

class HalfAnswer:
    def respond(self, turn, ctx):
        return [Progress("working"), Complete(result={"partial": True})]

register("half_answer", HalfAnswer)

Installation

pip install counterpart

Installing the package is the whole setup. The mock_agent fixture and the async configuration ship with it, so there is no conftest.py to write. Python 3.11+.

There is also a CLI for poking a live agent: counterpart check <url> scores it against the spec, section by section, and counterpart attack <url> sends deliberately malformed requests and reports what came back. For the full rulebook use the official a2a-tck.

Examples

Five runnable scenarios in examples, each in a domain where the counterparty genuinely belongs to somebody else:

• Freight procurement: pick the cheapest usable quote, not the cheapest string that parses.
• Freight edge cases: 22 ways a quote is unusable, 14 that look wrong and are fine. Checking them changes which carrier gets booked, at $592.50 more.
• Prior authorization: 25 insurer replies that all report success, including the standard's own example of a pending decision that does.
• Satellite downlink: twelve plans that all validate and each describes a different physical reality. Radians where the field says degrees.
• Chaos multihop: four hops, real sockets, twenty concurrent users, corruption injected at the bottom and caught at the top.

Every example tests both halves: each unusable answer is caught, and each legitimate answer is left alone. The second half is harder. All four bugs found in these contracts were false positives, checks rejecting good answers, and a checker that flags good data gets switched off.

Documentation

• docs/spec-notes.md maps A2A v1.0 to the code, citing the spec throughout.
• docs/prior-art.md covers what already exists and where this fits.
• docs/roadmap.md lists what is out of scope, and limits_probe.py demonstrates it: no production monitoring, no agent identity, and a contract sees the payload, not the clock.

Contributing

Version 0.1.9, Apache-2.0, the API will change. If a peer's reply has fooled something you built, open an issue with the shape of the payload. That is the most useful contribution this project can get.