a2a-spec 0.3.2

a2a-spec
Contract tests for multi-agent systems

a2a-spec finds the fields that one agent reads from another agent. It tests these fields in CI. The tests use recorded outputs, so they make no LLM calls and need no API keys.

The problem

Agent A sends data to agent B. You change the prompt of agent A, and its output changes. For example, agent A now sends urgency instead of priority.

Agent B reads priority with a default value. It does not stop with an error. It gives a wrong result, and no test fails.

What a2a-spec does

1. It reads the code of agent B and finds the fields that agent B uses.
2. It records the output of agent A one time and saves it as JSON files in git.
3. It tests the recorded output against the fields that agent B needs. You run these tests in pytest or in CI.

Terms

Get started

You need Python 3.11 or later and a git repository.

This example uses two agents. The producer triage_agent classifies a message. The consumer resolution_agent uses that result.

# agents/triage.py
async def triage_agent(data: dict) -> dict:
    text = data["message"].lower()
    category = "billing" if "charge" in text else "general"
    return {"category": category, "priority": "high" if "urgent" in text else "low"}

# agents/resolution.py
async def resolution_agent(ticket: dict) -> dict:
    escalate = ticket.get("priority", "low") == "high"
    return {"reply": f"We are checking your {ticket['category']} issue.", "escalate": escalate}

1. Install a2a-spec.

pip install a2a-spec

2. Create the project files.

a2aspec init

This command creates a2a-spec.yaml and the a2a_spec/ directory.

3. Remove the example spec and the example scenario.

rm a2a_spec/specs/example-spec.yaml a2a_spec/scenarios/example-scenarios.yaml

4. Add the producer to a2a_spec/adapters/__init__.py.

from a2a_spec import FunctionAdapter
from agents.triage import triage_agent

ADAPTERS = {
    "triage-agent": FunctionAdapter(fn=triage_agent, agent_id="triage-agent", version="1.0"),
}

5. Add inputs for the producer to a2a_spec/scenarios/support.yaml.

scenarios:
  - name: double_charge
    input:
      message: "I was charged twice. This is urgent."
  - name: question
    input:
      message: "How do I change my address?"

6. Record the output of the producer.

a2aspec record

The command calls the producer one time for each scenario:

    PASS triage-agent / double_charge recorded
    PASS triage-agent / question recorded
[done] 2 snapshots saved to ./a2a_spec/snapshots

7. Make the spec from the code of the consumer.

a2aspec infer --producer triage-agent --consumer resolution-agent \
  --source agents/resolution.py:resolution_agent

The command writes a2a_spec/specs/triage-agent-to-resolution-agent.yaml:

# Generated by 'a2aspec infer' from agents/resolution.py:resolution_agent and 2 triage-agent snapshot(s). Review before committing.
#   priority: required, read at line 2
#   category: required, read at line 3
spec:
  name: triage-agent-to-resolution-agent
  producer: triage-agent
  consumer: resolution-agent
  structural:
    type: object
    required: [priority, category]
    properties:
      priority: {type: string}
      category: {type: string}

Read the spec before you commit it. If the command shows a WARN line, read Limits.

8. Run the tests.

a2aspec test

PASS triage-agent-to-resolution-agent / double_charge
PASS triage-agent-to-resolution-agent / question
  2 passed  (total: 2)

9. Commit the files.

git add a2a-spec.yaml a2a_spec agents
git commit -m "test: add agent contracts"

The committed snapshots are the baseline for the next steps.

When you change an agent

1. Change the producer. For example, change its prompt, its model or its code. In this example, the producer now sends urgency instead of priority.

2. Record the output again.

a2aspec record

3. Compare the new output with the committed snapshots.

a2aspec diff

  triage-agent / double_charge:
    FAIL priority: Field 'priority' was removed
    FAIL urgency: Field 'urgency' was added

4. Run the tests.

a2aspec test

FAIL triage-agent-to-resolution-agent / double_charge
    [<root>] 'priority' is a required property

5. Correct the problem.

• If the change in the producer is an error, correct the producer. Then do steps 2 to 4 again.
• If the change is necessary, change the consumer. Then run a2aspec infer with --force and commit the new spec and snapshots.

Do not record the snapshots again only to make a test pass. Do not delete snapshots or remove fields from a spec for this purpose.

Run the tests with pytest

a2a-spec has a pytest plugin. Add these lines to pyproject.toml:

[tool.pytest.ini_options]
a2a_config = "a2a-spec.yaml"
testpaths = ["tests", "a2a_spec/specs"]

pytest then runs one test for each snapshot:

$ pytest -v
a2a_spec/specs/triage-agent-to-resolution-agent.yaml::double_charge PASSED   [ 50%]
a2a_spec/specs/triage-agent-to-resolution-agent.yaml::question PASSED        [100%]

The usual pytest options work, for example -k, -x and --junitxml. To run the tests one time without the configuration, use pytest --a2a.

To test a live output in your own test, use the a2a_check fixture. This example is an async test, so it needs an async plugin such as pytest-asyncio.

async def test_triage_live(a2a_check):
    output = await triage_agent({"message": "I was charged twice"})
    a2a_check("triage-agent-to-resolution-agent", output)

Run the tests in CI

The tests use the committed snapshots. CI needs no API keys.

# .github/workflows/contracts.yml
name: Agent contracts
on: [push, pull_request]

jobs:
  contracts:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v6
      - uses: actions/setup-python@v6
        with:
          python-version: "3.12"
      - run: pip install a2a-spec
      - run: a2aspec test

If your specs have semantic rules, install a2a-spec[semantic] instead.

Write a spec by hand

A spec can have three types of rules:

spec:
  name: triage-to-resolution
  version: "1.0"
  producer: triage-agent
  consumer: resolution-agent
  structural:
    type: object
    required: [category]
    properties:
      category: {type: string, enum: [billing, general]}
  semantic:
    - rule: summary_matches_message
      method: embedding_similarity
      source_field: message     # field in the producer input
      target_field: summary     # field in the producer output
      threshold: 0.5
  policy:
    - rule: no_card_numbers
      method: regex
      patterns: ['\b\d{4}[- ]?\d{4}[- ]?\d{4}[- ]?\d{4}\b']

Semantic rules need pip install "a2a-spec[semantic]". See Writing specs for all options.

Connect your agents

For an A2A agent, give the base URL of the agent. The adapter reads the Agent Card, finds the JSON-RPC interface and sends the scenario input. It records the agent version from the Agent Card in each snapshot.

from a2a_spec import A2AAdapter

ADAPTERS = {
    "triage-agent": A2AAdapter(url="https://triage.example.com", agent_id="triage-agent"),
}

See Writing adapters for all options.

Use with coding agents

a2a-spec contains an Agent Skill. The skill tells a coding agent how to use a2a-spec. It also tells the agent not to make a failed test pass by recording the snapshots again.

a2aspec skill --dir path/to/skills

This command writes path/to/skills/a2a-spec/SKILL.md. Export the skill again after you update a2a-spec.

Limits

• a2aspec infer reads Python consumers only. It finds data["key"], data.get("key") and "key" in data for top-level keys.
• If the consumer gives its input to another function, a2aspec infer cannot follow it. The command then shows a WARN line. Add the missing fields to the spec by hand.
• a2aspec diff compares with the snapshots in git. Commit the snapshots before you use it.
• A2AAdapter supports A2A 1.0 with JSON-RPC. It does not support streaming or file parts.
• Semantic rules use a local embedding model. The semantic extra installs PyTorch.

Commands

See CLI reference for all options.

Documentation

• Writing specs
• Writing adapters
• CLI reference
• CI integration
• Concepts
• Architecture
• Example project

Contributing

See CONTRIBUTING.md.

License

Apache 2.0. See LICENSE.