Implementation Plan: T5-P6-A4-WP1 ACP Session Contract Alignment Design Document (#4124)

## Summary

Create `docs/design/acp-session-contract.md` — a four-section normative
reference document mapping the autoskillit `CodingAgentBackend`
lifecycle to ACP (Agent Communication Protocol) session method
semantics. The document covers lifecycle mapping, recovery ladder,
capabilities translation, and Codex shim deviations. No code files are
modified. The `docs/design/README.md` index is updated to include the
new document.

Closes #4053

## Implementation Plan

Plan file:
`/home/talon/projects/autoskillit-runs/impl-20260627-031355-771942/.autoskillit/temp/make-plan/t5_p6_a4_wp1_acp_session_contract_alignment_plan_2026-06-27_031500.md`

🤖 Generated with [Claude Code](https://claude.com/claude-code) via
AutoSkillit
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steps=prepare_pr,run_arch_lenses,compose_pr,annotate_pr_diff,review_pr
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## Token Usage Summary

| Step | Model | count | uncached | output | cache_read | peak_ctx |
turns | cache_write | time |

|------|-------|-------|----------|--------|------------|----------|-------|-------------|------|
| plan* | opus[1m] | 1 | 5.6k | 10.7k | 1.1M | 77.9k | 41 | 64.8k | 6m
26s |
| verify* | sonnet | 1 | 70 | 9.9k | 354.4k | 59.4k | 27 | 40.9k | 5m
27s |
| implement* | MiniMax-M3 | 1 | 57.6k | 17.3k | 989.1k | 0 | 58 | 0 | 4m
47s |
| fix* | sonnet | 1 | 158 | 4.9k | 880.6k | 56.4k | 42 | 37.9k | 3m 11s
|
| audit_impl* | sonnet | 1 | 244 | 8.9k | 289.4k | 50.9k | 18 | 39.9k |
5m 9s |
| prepare_pr* | MiniMax-M3 | 1 | 42.6k | 1.8k | 111.3k | 0 | 11 | 0 |
38s |
| compose_pr* | MiniMax-M3 | 1 | 36.4k | 1.7k | 213.1k | 0 | 14 | 0 |
44s |
| **Total** | | | 142.7k | 55.3k | 4.0M | 77.9k | | 183.5k | 26m 24s |

\* *Step used a non-Anthropic provider; caching behavior may differ.*

## Token Efficiency

| Step | LoC Changed | cache_read/LoC | cache_write/LoC | output/LoC |
|------|-------------|----------------|-----------------|------------|
| plan | 0 | — | — | — |
| verify | 0 | — | — | — |
| implement | 378 | 2616.6 | 0.0 | 45.9 |
| fix | 2 | 440318.0 | 18937.5 | 2464.5 |
| audit_impl | 0 | — | — | — |
| prepare_pr | 0 | — | — | — |
| compose_pr | 0 | — | — | — |
| **Total** | **380** | 10447.3 | 482.8 | 145.5 |

## Model Usage Breakdown

| Model | steps | uncached | output | cache_read | cache_write | time |
|-------|-------|----------|--------|------------|-------------|------|
| opus[1m] | 1 | 5.6k | 10.7k | 1.1M | 64.8k | 6m 26s |
| sonnet | 3 | 472 | 23.7k | 1.5M | 118.6k | 13m 48s |
| MiniMax-M3 | 3 | 136.6k | 20.8k | 1.3M | 0 | 6m 9s |

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Trecek

docs/design/README.md

@@ -6,3 +6,4 @@ Design specifications for planned features and skills.
 |---|---|
 | [env-setup-design.md](env-setup-design.md) | Design spec for the dedicated `setup-environment` skill — Docker vs micromamba-host decision tree, structured output tokens, and recipe integration |
 | [recording-replay-accepted-degradations.md](recording-replay-accepted-degradations.md) | Accepted degradations in P8 recording/replay: Claude PTY cassette format incompatibility with Codex replay, and unchanged Claude session recording path |
+| [acp-session-contract.md](acp-session-contract.md) | Normative reference for P6-A3-WP1: ACP session method mapping for the `CodingAgentBackend` lifecycle, recovery ladder from `RetryReason` to `session/resume`/`session/load`/`session/new`, capabilities translation of all 41 `BackendCapabilities` fields, and Codex shim deviations |