Refactor/split sdk python agentfield agent into mixins

[Luffy2208]

Summary

Refactored sdk/python/agentfield/agent.py (~4500 LOC) by splitting it into multiple concern-based mixins while preserving all existing behavior and public APIs.

This change is part of the "god files" refactor epic and improves maintainability, readability, and contributor experience by organizing logically grouped functionality into separate modules.

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What was moved where

agent_schema.py → _AgentSchemaMixin

• _types_to_json_schema
• _type_to_json_schema
• _validate_handler_input

agent_discovery.py → _AgentDiscoveryMixin

• _handle_discovery
• _build_callback_discovery_payload
• _apply_discovery_response

agent_serverless.py → _AgentServerlessMixin

• handle_serverless

agent_vc.py → _AgentVCMixin

• _initialize_did_system
• _populate_execution_context_with_did
• _agent_vc_default
• _set_reasoner_vc_override
• _set_skill_vc_override
• _effective_component_vc_setting
• _should_generate_vc
• _build_agent_metadata
• _build_vc_metadata
• _generate_vc_async

agent_pause.py

• _PauseManager class (moved as-is)

---

Agent Composition

The Agent class now composes these via mixins:

class Agent(
    FastAPI,
    _AgentSchemaMixin,
    _AgentDiscoveryMixin,
    _AgentVCMixin,
    _AgentServerlessMixin,
):
    ...

---

Testing

• ./scripts/test-all.sh
• Additional verification:

  uv run pytest sdk/python/tests/

  All tests passed ✅

---

Test Changes

No existing tests were modified.
Added monkeypatch overrides in test/helper.py:

monkeypatch.setattr("agentfield.agent_vc.DIDManager", _FakeDIDManager)
monkeypatch.setattr("agentfield.agent_vc.VCGenerator", _FakeVCGenerator)

---

Checklist

• I updated documentation where applicable.
• I added or updated tests (or none were needed).
• I updated CHANGELOG.md (or this change does not warrant a changelog entry).

---

Related Issues

Fixes #411

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Python	7.9 KB	-13%	0.33 µs	-6%	✓	✓

✓ No regressions detected

[github-actions]

📊 Coverage gate

Thresholds from .coverage-gate.toml: per-surface ≥ 86%, aggregate ≥ 88%, max per-surface regression ≤ 1.0 pp, max aggregate regression ≤ 0.50 pp.

Surface	Current	Baseline	Δ
control-plane	87.20%	87.30%	↓ -0.10 pp	🟡
sdk-go	90.70%	90.70%	→ +0.00 pp	🟢
sdk-python	93.63%	93.63%	↑ +0.00 pp	🟢
sdk-typescript	92.56%	92.56%	→ +0.00 pp	🟢
web-ui	90.02%	90.01%	↑ +0.01 pp	🟢
aggregate	88.98%	89.01%	↓ -0.03 pp	🟡

✅ Gate passed

No surface regressed past the allowed threshold and the aggregate stayed above the floor.

[github-actions]

📐 Patch coverage gate

Threshold: 80% on lines this PR touches vs origin/main (from .coverage-gate.toml:thresholds.min_patch).

Surface	Touched lines	Patch coverage	Status
control-plane	0	—	➖ no changes
sdk-go	0	—	➖ no changes
sdk-python	0	—	➖ no changes
sdk-typescript	0	—	➖ no changes
web-ui	0	—	➖ no changes

✅ Patch gate passed

Every surface whose lines were touched by this PR has patch coverage at or above the threshold.

[AbirAbbas]

CI is failing — ruff catches an unused import asyncio in agent_vc.py:1. Remove that and you should be green.