# Agent Note: PTC mode language dispatch and the Python SDK renderer

Status: implemented

English | [中文](2026-07-31-ptc-language-dispatch.zh.md)

## Problem

PTC mode generated one SDK flavor: TypeScript. `ToolRuntime` hard-coded `renderToolsSdk` for the `tools:sdk` section and `requireCodeRuntime` rejected any `ctx.codeRuntime.language !== 'typescript'`. Adding a CPython backend means a program's source language is no longer fixed: the same visible tool registry must project a Python SDK when a Python runtime is loaded, and the model-facing `run_code` schema strings ("Execute a Python program …") must match the SDK section's language so the model never sees a TypeScript instruction over a Python runtime.

This is the tool-facing half of the multi-language PTC mode split; the [code-runtime seam](../../../../packages/code-runtime/code-runtime/README.md) already carries `CodeRuntime.language`. This note owns only how `dsh-tools` dispatches on that field. The backend that implements `language: 'python'` is owned by its own note, delivered separately.

## Decision

Language selection is a lookup on `ctx.codeRuntime.language`, resolved lazily at prompt assembly, against two parallel tables in `dsh-tools`:

- `SDK_RENDERERS` (index.ts) maps a language to its `tools:sdk` renderer — `typescript → renderToolsSdk`, `python → renderToolsSdkPy`. The `tools:sdk` section reads the loaded runtime's language and picks the renderer; `requireCodeRuntime` rejects a `mode: ptc`/`both` runtime whose language is absent from the table, naming the known languages.
- `RUN_CODE_FLAVORS` (ptc.ts) maps a language to its two model-facing `run_code` strings (tool `description` and the `code` parameter description), so a language's SDK section and its transport schema always agree.

Both tables are read with `Object.hasOwn` before use so a language named `toString`/`constructor` cannot resolve an inherited `Object.prototype` member as a renderer. The two guards differ in reachability: `SDK_RENDERERS`' in-callback guard is unreachable because `requireCodeRuntime` validated the same `const` table earlier in the same callback (it carries a `/* v8 ignore */`), while `RUN_CODE_FLAVORS`' guard is the primary, publicly reachable rejection — any language absent from the flavor table hits it through `run_code`'s language-aware getters, which the public `schemas()` reaches without passing `requireCodeRuntime` first; the test reads one of those getters off the definition directly, under a language absent from both tables. A language present in `SDK_RENDERERS` but not `RUN_CODE_FLAVORS` is drift the shared `CodeSdkLanguage` `satisfies` pins reject at `typecheck`, so it is not an input either guard can see; what the guards still own is a mounted runtime reporting a language absent from both tables. Schema emission reads the runtime through `peekRuntime()` rather than `requireRuntime()`: `undefined` (no runtime mounted, reached by definition readers and `schemas()`, of which the doc-catalog harvest is the only shipped one and none of which feeds a model because assembly passes `requireCodeRuntime` first) degrades to the TypeScript flavor, whereas a mounted unknown language fails loud — this is NOT the silent fallback rejected below, which concerns emitting a wrong-language SDK for a real runtime. Adding a backend language is three parallel edits — a `CodeSdkLanguage` member and the two table entries — plus its renderer and the prose that names the well-known values instead of deriving them (the seam's `dsh-code-runtime` README pair, its `CodeRuntime.language` JSDoc, and the `docs/subsystems/code-runtime.md` pair; this package's own README pair and its `Config.mode` JSDoc — no gate checks any of it), with no `agent-loop` or registry-structure change.

`ptc.ts` depends only on the runtime Service Definition (`@deepseek-ai/dsh-code-runtime`), never on a concrete backend; dispatch is by `runtime.language` at run time. The tool layer is therefore independent of the Python protocol and backend — it needs only the service's `language` field.

### The Python SDK renderer

`py-types.ts` renders the same unified tool-schema vocabulary `jsonSchemaToTs` covers, targeting Python: `jsonSchemaToPy` emits a type expression per JSON-schema node, and `renderToolsSdkPy` assembles named `TypedDict`s for each visible tool's arguments and canonical output plus a `tools` object with usage instructions equivalent to the TypeScript flavor. Unsupported raw constructs degrade rather than throwing during assembly, matching the TypeScript renderer's contract. The output is deterministic — lexicographic tool order, byte-identical text for an unchanged tool set — so the prompt stays prefix-cache-friendly. Lexicographic means one ordered member stream: a tool whose name is not a legal attribute is listed as a `tools[name]` comment in its sorted position rather than partitioned to the end, matching how the TypeScript flavor quotes an exotic key in place. That stream forces one thing directly — comment lines are not statements, so a tool set that emits no method at all still needs an explicit `pass`. Three further rules are Python-specific rather than consequences of the ordering. The usage contract states that the declarations are static stubs and arguments are plain `dict`/`list` values: a `TypedDict` reads as a constructible class, so a model that writes `FooArgs(field=1)` gets a `NameError` — TypeScript's `interface` is visibly a type, and the TS flavor's "runs type-stripped" clause already covers it. A description becomes the method's docstring emitted as the FIRST statement of its body: above the `async def` the first one would document the `Tools` class and the rest would be dead expressions, leaving every method undocumented. And a `list[…]` chain degrades to `Any` past `MAX_LIST_NESTING`, because CPython's tokenizer rejects a line with more than 200 open brackets and the block must stay parseable Python — the same reason `docLines` escapes quotes and backslashes. `ts-types` needs neither: TypeScript attaches a leading `/** … */` to the member that follows it and bounds nesting nowhere in its grammar.

The standard that cap serves is grammatical validity, and the boundary is deliberate: a long `A | B | …` union is valid Python at any length and is left uncapped, even though CPython's `compile()` exhausts its C recursion walking the left-nested `BinOp` spine (measured on 3.9: 1,000 branches compile, 5,000 raise `RecursionError`). Nothing compiles this block — it is prompt text — so that limit costs nothing, whereas capping union length would retire the deep-chain tests that pin the walk's linear time and the class-name propagation cap. A future renderer that does need compilable output should flatten unions rather than truncate them.

`renderType` validates the whole schema once (`assertSupportedJsonSchema`) and then trusts it, wrapping the walk in one `try/catch` that degrades to `Any` — the same trusted-after-validation stance the sibling `ts-types` renderer takes at this typed same-process boundary ([Trust TypeScript at typed same-process boundaries](../../../../AGENTS.md)). It deliberately carries NO defenses against a schema whose accessors mutate between reads (post-validation cycles, TOCTOU on `const`/`enum`, self-referential functions): the input is a first-party registration (a `defineTool` literal or a raw registration) or a wire-derived plain JSON schema — the former is trusted per AGENTS.md, the latter is a `JSON.parse` product that physically cannot carry accessors, and `renderType` re-validates the whole tree on every call regardless — so such inputs are unreachable, and adding per-shape guards here would break symmetry with `ts-types` (which has none) for values the static interface forbids. `jsonSchemaToPy(schema: unknown)` accepts `unknown` and returns `Any` on a malformed schema — the Python counterpart of the TS flavor's `unknown` — but its contract is "degrade an unsupported schema", not "survive an adversarial mutating one".

## Alternatives considered

- **A `language` config field on `ToolRuntime`.** Deployment would then have two places to name the language (the loaded runtime and the tools config) that can disagree; the loaded runtime is the single source of truth, so the registry reads it rather than duplicating it.
- **Importing the Python backend into `ptc.ts` to detect it.** That would couple the tool layer to a concrete backend and force the protocol/backend PRs to land first. Runtime dispatch on `language` keeps the layer backend-agnostic and independently shippable.
- **A default renderer for an unknown language.** A silent fallback would emit a TypeScript SDK over, e.g., a Ruby runtime — the model would see instructions in the wrong language. Failing loud at assembly is the repository's misconfiguration stance.

## Consequences

Adding a backend language is three parallel edits — a `CodeSdkLanguage` member, an `SDK_RENDERERS` entry, and a `RUN_CODE_FLAVORS` entry — plus the renderer function the second points at, with no change to `agent-loop` or the registry structure. The two tables (`SDK_RENDERERS`, `RUN_CODE_FLAVORS`) must stay in step, and that invariant is checked statically rather than left to review: both are `satisfies`-checked against that one union, so a language added to one and not the other fails `typecheck`. This is the mechanical form the drift risk deserves — the runtime `Object.hasOwn` guards would catch it too, but only once a backend reporting that language ships: at the Consumer's integration point rather than where the drift was introduced — and never while no second backend exists. The tables keep their `Record<string, …>` declared type because `CodeRuntime.language` is an unconstrained `string`; the union pins what the harness ships, the guards reject what a runtime reports. What stays outside that check is the prose that names the well-known values instead of deriving them: `dsh-code-runtime`'s README pair, its `CodeRuntime.language` JSDoc, and the `docs/subsystems/code-runtime.md` pair at the seam, plus this package's own README pair and its `Config.mode` JSDoc. Earlier notes name the values as they stood at the time and are not on that list. Two separate reasons keep it ungated. Prose is not type-checked at all, wherever the union lives. And no type-level pin can stand in for it here: the Service Definition package must not import its Consumer's table, and `CodeRuntime.language` stays an unconstrained `string` by design, so moving the union into the Service Definition would not apply it either. A unit test pinning the two key sets equal was rejected in favor of this: it would buy the same check at the cost of a test-only export of two private tables, and would run later than the compiler does. Which of the two runtime failures surfaces depends on the entry point, for a language absent from both tables: assembly reports the missing renderer, because `wireSchemas` calls `requireCodeRuntime` before projecting, while the public `schemas()` reaches `run_code`'s language-aware getters first and reports the missing flavor. The tool layer stays free of any concrete backend dependency, so it ships and is testable ahead of the Python protocol and backend.

The cost is that the Python branch of both tables is unreachable in the shipped tree: `CodeRuntime.language` is set by the loaded backend, the only published backend is `dsh-code-runtime-worker-thread` (`'typescript'`), and the registry reads the loaded runtime rather than a config field, so no assembled application can select `renderToolsSdkPy` or `PYTHON_FLAVOR`. The model-visible surface is therefore unchanged by this note's work until a backend reporting `'python'` is published, and this change's coverage is unit-level — the renderer output plus the dispatch and rejection paths. The keyless snapshot for the Python model interface belongs to the change that publishes that backend, because only there does a real `cordis.yml` over published plugins produce a Python assembly; a snapshot example that mounted a fixture runtime here would assert against a test double, which [docs/testing.md](../../../../docs/testing.md) rejects as a substitute for the assembled application transcript.

Two runtime contracts the Python SDK text asserts are owed by that same backend PR. First, the instructions tell the model that exactly `tools` and `ToolCallError` are bound and that the declared `TypedDict` classes are not, so the backend must inject those two names — with `ToolCallError.toolName` populated per the seam's `errorClass` contract — and must NOT bind the declared class names into the program's globals; injecting them "helpfully" would make the SDK text false. Second, the language has to be bound to the request: `requireCodeRuntime` resolves `ctx.codeRuntime` separately at assembly and at `run_code` execution, so a reload that swapped the runtime between those two points would hand a program written against one flavor to the other. The split is finer than those two points — `run_code`'s `description` and `parameters` getters each call `resolveFlavor(peekRuntime())`, and `schemaOf` destructures both, so one projection reads the runtime twice; both reads are for `run_code`'s own schema, since the getters are installed on that one definition and every other definition carries plain data properties. A reload between those two reads yields a single schema whose two halves name different languages. Neither is reachable here — one published backend means both reads return the same flavor and no program ever runs against this renderer's output — and the cross-language rejection is not testable until a second language exists.

Third, that PR owns the CPython floor, and with it the renderer's Unicode-table skew. Four expressions read the running engine's tables (Node 22.23.1: Unicode 17.0) while the interpreter uses its own (CPython 3.9.6: 13.0.0): `isBareIdentifier`'s `IDENTIFIER`, and `camelCase`'s split set, head test, and `toUpperCase()`. An interpreter older than the engine is the failing direction — the engine emits a character its tokenizer refuses, taking the whole block down — and it arrives by three independent paths. Through the predicate, a bare method or field name carrying a character added between the two versions — to `XID_Start` at its head, or to `XID_Continue` in any tail position, the middle of a name included. Through `camelCase`'s XID reads, a class name, which reaches emitted text whenever any object shape in the tool's schema declares a `TypedDict`, and which the predicate's verdict on the tool name does not gate: `zz-` plus U+1E4D0 never reaches the predicate's skew, since the `-` rejects it outright, yet it still declares `class Zz𞓐xArgs`. Through the case mapping, a class name derived from a tool the predicate accepted — a different table and a wider window than XID membership: U+019B is XID_Start and NFKC-stable, so `async def ƛ` compiles on 3.9.6, but Node uppercases it to U+A7DC (unassigned there; CPython's own `.upper()` is the identity) and `class ꟜArgs` fails with `invalid non-printable character U+A7DC`. The exposure window is the characters and mappings that changed between the two versions, so the PR that names a supported CPython range must decide explicitly between accepting it and pinning all four read points to tables for that floor — pinning the predicate alone leaves both class-name paths open. Nothing here can decide it: the floor does not exist yet, and a table pinned to a guess would be a deployment-varying constant with no configurability behind it. A second axis rides along with the floor and is not one of the four: the names and syntax the block would evaluate at definition time. `TypedDict` needs 3.8, the PEP 585 builtin generics `dict[str, Any]` and `list[…]` need 3.9, an `A | B` annotation 3.10, and `NotRequired` 3.11. These are not parse failures — the block parses on any version, which is the standard the `MAX_LIST_NESTING` cap serves — but definition-time evaluation failures, and nothing in the product evaluates this text. Recording them with the read points keeps "parseable on the supported range" from being read as "executable on it".
