docs: rebuild the documentation skill and standards (#2983)

packages/code-runtime/code-runtime-python/README.md

@@ -1,23 +1,100 @@
+---
+description: "fd-3 wire protocol between a Node host and a CPython subprocess for users and maintainers building or debugging the Python code-execution backend."
+kind: "package-library"
+---
+
 # @deepseek-ai/dsh-code-runtime-python
 
 English | [中文](README.zh.md)
 
-CPython-subprocess implementation of the [`@deepseek-ai/dsh-code-runtime`](../code-runtime/README.md) seam. Companion to [`@deepseek-ai/dsh-code-runtime-worker-thread`](../code-runtime-worker-thread/README.md); trades the Node worker thread for a fresh `python3` subprocess so model code is Python instead of TypeScript.
+## Summary
 
-The package owns the wire protocol for that seam: the host-side frame codec and the Python-side mirror of the same message vocabulary.
+`dsh-code-runtime-python` owns the versionless wire protocol between a Node host and a CPython subprocess for the [`dsh-code-runtime`](../code-runtime/README.md) seam: one JSON object per line on the child's fd 3, leaving stdout/stderr free for the program's own output. The package ships the host-side frame codec and hostile-frame validators (`src/protocol.ts`) plus the Python-side mirror of the same message vocabulary (`py/protocol.py`), so every consumer of the wire shares one vocabulary. It is the protocol layer for the Python backend — the package carries no subprocess execution path, so nothing here spawns `python3` outside the cross-language mirror test. The host treats every inbound frame as hostile, because model code has full access to fd 3 and can post anything through it.
 
-## Wire protocol
+## Table of Contents
 
-The host and the CPython subprocess exchange a versionless, JSON-lines protocol on the child's fd 3 — one JSON object per line, leaving stdout/stderr free for the program's own output. `src/protocol.ts` is the host side; `py/protocol.py` mirrors its message shapes and the shared truncation-marker text on the Python side.
+- [Use this package](#use-this-package)
+- [Understand the implementation](#understand-the-implementation)
+- [Further Exploration](#further-exploration)
+- [Model Experience](#model-experience)
+- [Known Limitations and Deferred Work](#known-limitations-and-deferred-work)
+- [Dev Note](#dev-note)
 
-- **fd 3, not stdout** — Node pins the channel positionally with `stdio: ['pipe','pipe','pipe','pipe']`; the Python bootstrap reads the same `PROTOCOL_FD` constant. JSON-lines framing.
-- **Host treats every inbound frame as hostile** — model code has full access to fd 3 and can post anything through it, so `validateChildFrame` shape-validates and REBUILDS each frame before the host reads it: forged extra fields never ride along, a non-number call id can never be echoed into a reply, and junk drops to `undefined` rather than throwing in the host's message handler. The Python side trusts host replies (the host is not model-controlled).
-- **Lossless-JSON crossing** — completion values and binding arguments cross as exact JSON. `encodeJsonPlain` serializes a `JSON.parse`-produced value without recursion, so a deep value below the byte budget crosses intact instead of dying on `JSON.stringify`'s stack limit; `checkDoneValue` meters a forged completion value's byte length AND number losslessness in one traversal that rejects an over-budget payload before the incremental work it would add (the enqueued children; strings and keys are metered by a non-allocating escaped-size scan, so the escaped copy is never materialized) — the frame's own width is already parsed and capped upstream by the host's fd-3 receive buffer, not re-bounded here; `hasUnsafeIntegerToken` reads the raw frame text to catch an integer token that `JSON.parse` would silently round; `hasNonLosslessNumber` rejects a non-finite or negative-zero number in unbounded `call.args`. Beyond-safe-range integral doubles serialize through `BigInt` digits so the exact integer crosses, not the rounded `String()` form.
-- **Shared truncation marker** — `logTruncationMarker(maxBytes)` produces byte-identical text on both sides, so a truncated log run reads the same however the cap was hit. The `log` frame's `truncated` flag distinguishes the child ledger's own marker from program output.
+-----
 
+<a id="use-this-package"></a>
+## Use this package
+
+Choose this package when you build or consume the CPython code-runtime wire: implement the Python backend or the host that drives it, or debug a Python code run's framing. The package is the wire protocol intended for a CPython code-runtime provider — such a provider runs each model program in a fresh `python3 -I` subprocess — and this package supplies the protocol both sides speak, so its exports are the single TS-side source of truth for the wire.
+
+### What you get
+
+The package re-exports the host-side protocol vocabulary from `src/index.ts`: `validateChildFrame` (rebuilds every inbound frame before the host reads it), the lossless-JSON codec and meters (`encodeJsonPlain`, `checkDoneValue`, `hasUnsafeIntegerToken`, `hasNonLosslessNumber`), and `logTruncationMarker` (the shared truncation-marker text). The Python side mirrors the message shapes as `TypedDict`s in `py/protocol.py` and re-declares the two surfaces both sides execute against — `PROTOCOL_FD = 3` and the marker text.
+
+### The wire
+
+Frames travel on the child's fd 3 as JSON-lines — one object per line — so stdout/stderr stay clear for the program's own output. Child → host: `boot-ack`, `call`, `log`, `done`. Host → child: `boot` (first frame, carrying every cap and the namespace declarations), `run` (after `boot-ack`, carrying only the program body), and one `reply` per `call`. A forged frame can carry both `value` and `error` on `done`, so a consumer must check `error` first and ignore `value` when it is set.
+
+### What can go wrong
+
+Host-side validation drops junk without throwing, so a malformed or forged frame never crashes the host process: `validateChildFrame` returns `undefined` for anything that does not rebuild cleanly, a non-number call id can never be echoed into a reply, and forged extra fields never ride along. A completion value that is not lossless JSON, or that exceeds the configured byte budget, is rejected explicitly (`non-lossless` / `over-budget`) rather than silently rounded or truncated.
+
+-----
+
+<a id="understand-the-implementation"></a>
+## Understand the implementation
+
+<details>
+<summary>Implementation internals — click to expand</summary>
+
+This section explains the design behind the wire protocol; observable behavior is fully covered in [Use this package](#use-this-package).
+
+### Design concept
+
+The protocol assumes one direction of trust: the host treats every inbound frame as hostile (model code can forge anything on fd 3) and REBUILDS it field by field before reading; the Python side trusts host replies, because the host is not model-controlled. The package is deliberately the protocol layer only — the Python-side JSON codec lives in the backend's bootstrap, not in `py/protocol.py`, so the mirror stays the pure wire-vocabulary counterpart of `src/protocol.ts`.
+
+### Wire contract
+
+The frames are `boot` / `run` (host → child) and `boot-ack` / `call` / `log` / `done` plus one `reply` per call (child → host). The `log` frame's `truncated` flag marks the frame that IS the child ledger's truncation marker, so the host stops capturing at the same point the child did instead of inferring it from its own budget. `done.error.kind` is one of `exception`, `invalid-output`, `output-limit`; wall/CPU budgets, aborts, and substrate death are observed host-side, not carried as frames.
+
+### Lossless JSON crossing
+
+Completion values and binding arguments cross as exact JSON: values serialize without recursion, so a deep payload below the byte budget survives instead of dying on `JSON.stringify`'s stack limit, and integral doubles beyond the safe range cross as exact digits rather than silently rounded tokens; the meters in [`src/protocol.ts`](src/protocol.ts) enforce byte budgets and number losslessness before anything else reads the payload.
+
+### Mirror alignment
+
+`tests/protocol-mirror.e2e.ts` spawns a real `python3` and asserts, against `src/protocol.ts`, both `PROTOCOL_FD` / the truncation-marker text and each `TypedDict`'s required/optional wire field set in `py/protocol.py`, so a renamed or dropped field — or one side making a field optional the other requires — fails the test. Field *types* are not compared across the language boundary; that residue stays with review plus the backend's real-subprocess suite.
+
+### Source map
+
+| File | Role |
+|---|---|
+| [`src/index.ts`](src/index.ts) | Plugin entry: re-exports the protocol vocabulary for every consumer of the wire |
+| [`src/protocol.ts`](src/protocol.ts) | Host side: frame codec, hostile-frame validators, lossless-JSON meters, shared marker text |
+| [`py/protocol.py`](py/protocol.py) | Python side: `PROTOCOL_FD`, `TypedDict` frame mirrors, `log_truncation_marker` |
+| [`tests/protocol-mirror.e2e.ts`](tests/protocol-mirror.e2e.ts) | Cross-language mirror test against a real `python3` |
+| [`src/invariant.ts`](src/invariant.ts) | Invariant companion (no runtime invariant; the package registers no mutable data relation) |
+
+</details>
+
+-----
+
+<a id="further-exploration"></a>
+## Further Exploration
+
+Read these when the protocol contract is not enough. They move from the seam definition to the protocol's design record and the companion backend.
+
+- [Code runtime seam](../code-runtime/README.md) — the abstract contract the Python backend implements.
+- [fd-3 protocol Agent Note](../../../.agents/notes/implemented/architecture/2026-07-31-code-runtime-python-fd3-protocol.md) — design rationale, wire contract, and the mirror-alignment decision.
+- [Worker-thread backend](../code-runtime-worker-thread/README.md) — the shipped TypeScript sibling, the model for the Python backend's behavior.
+- [Code runtime subsystem reference](../../../docs/subsystems/code-runtime.md) — request/result vocabulary, bindings, and failure taxonomy.
+
+-----
+
+<a id="model-experience"></a>
 ## Model Experience
 
-Indirectly, through Code Mode in [`dsh-tools`](../../core/tools/README.md), which renders this backend's exact completion value when it fits (or an explicit `invalid-output` / `output-limit` failure), plus the exact `[dsh-code-runtime-python] log capture truncated at <maxLogBytes> bytes` log marker, into a retained `run_code` result.
+Indirectly, through Code Mode in `dsh-tools`, which renders the program's completion value or failure into a retained `run_code` result.
 
 #### KV Cache effect
 
@@ -25,5 +102,20 @@ No direct invalidation; the named consumer owns any request-prefix changes.
 
 ## Known Limitations and Deferred Work
 
-- **The cross-language guard covers the runtime-executed surfaces and the frame field shapes** — `tests/protocol-mirror.e2e.ts` spawns a real `python3` and asserts, against `src/protocol.ts`, both `PROTOCOL_FD` / the log truncation marker text AND each `TypedDict`'s required/optional wire field set in `py/protocol.py`. What it does not compare is the field *types* (e.g. that `cpuSeconds` is an `int` on both sides): comparing type declarations across TypeScript and Python has no mechanical equivalent here, so a type-level drift is still caught by review plus the backend's real-subprocess suite rather than this package's tests.
+<a id="known-limitations-and-deferred-work"></a>
+
+
+These limits define what the package does and does not cover; they are current package constraints, not a task backlog.
+
+- **The cross-language guard covers the executed surfaces and the frame field shapes, not the field types** — the mirror e2e compares required/optional field sets, not that `cpuSeconds` is an `int` on both sides; comparing type declarations across TypeScript and Python has no mechanical equivalent here, so a type-level drift is caught by review plus the backend's real-subprocess suite.
 - **`src/index.ts` exports the protocol vocabulary only** — the package carries no subprocess execution path and no Python-side JSON codec, so nothing here spawns `python3` outside the mirror test.
+
+<a id="dev-note"></a>
+### Dev Note
+
+<details>
+<summary>Working context for maintainers — click to expand</summary>
+
+None.
+
+</details>