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@@ -1,10 +1,12 @@
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-# RFC 012: Optional Code Mode — model writes TypeScript against an SDK of all tools
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+# RFC: Optional Code Mode — model writes TypeScript against an SDK of all tools
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Status: proposed
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Status: proposed
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+<!-- XXX: legacy ADR/RFC body format, not yet normalized to a unified RFC template. -->
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+
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## Problem
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## Problem
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-Today the agent loop advertises every registered tool to the model as a native JSON-schema function definition. `ToolRegistry` feeds its schemas into `ctx.systemPrompt`, the loop puts them on `GenerateOptions.tools`, and the adapter serializes them to the provider's function-calling wire format. The model then invokes one `tool-call` block per step, the loop dispatches each call through `ctx.tools.execute()` **sequentially** (parallel tool execution is an explicit open TODO in `dsh-tools` and [docs/architecture.md](../architecture.md)), and **every** intermediate `tool-result` re-enters the model's context on the next request.
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+Today the agent loop advertises every registered tool to the model as a native JSON-schema function definition. `ToolRegistry` feeds its schemas into `ctx.systemPrompt`, the loop puts them on `GenerateOptions.tools`, and the adapter serializes them to the provider's function-calling wire format. The model then invokes one `tool-call` block per step, the loop dispatches each call through `ctx.tools.execute()` **sequentially** (parallel tool execution is an explicit open TODO in `dsh-tools` and [docs/architecture.md](../../architecture.md)), and **every** intermediate `tool-result` re-enters the model's context on the next request.
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For multi-step tool work this is token-heavy and serial. The model cannot compose tools — loop over a result set, branch on an intermediate value, fan out, post-process — without a full model round-trip per call, and each of those round-trips drags the entire intermediate result back into context whether the model needs it or not.
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For multi-step tool work this is token-heavy and serial. The model cannot compose tools — loop over a result set, branch on an intermediate value, fan out, post-process — without a full model round-trip per call, and each of those round-trips drags the entire intermediate result back into context whether the model needs it or not.
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@@ -14,9 +16,9 @@ This RFC proposes an **optional** Code Mode for the DeepSeek Harness, covering *
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## Proposal
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## Proposal
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-The design follows the codebase's capability-seam pattern ([ADR 0009](../adr/0009-capability-seams.md), the `bash` template) as a three-package split, plus one consumer plugin. Nothing in `dsh-session`, `dsh-agent`, `dsh-agent-loop`, `dsh-llm`, `dsh-tools`, or `dsh-system-prompt` changes.
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+The design follows the codebase's capability-seam pattern ([capability seams](../implemented/2026-06-13-capability-seams.md), the `bash` template) as a three-package split, plus one consumer plugin. Nothing in `dsh-session`, `dsh-agent`, `dsh-agent-loop`, `dsh-llm`, `dsh-tools`, or `dsh-system-prompt` changes.
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-**Prior art.** `@cloudflare/codemode` validates this shape directly and several of its decisions are adopted below. Its `Executor` interface is deliberately tiny — `execute(code, fns) → { result, error?, logs? }` — with a production `DynamicWorkerExecutor` (isolated Workers) and a six-line `NodeVMExecutor` example as two implementations behind it: exactly the interface/implementation split ADR 0009 prescribes. It generates TypeScript type definitions from tools for the model's context and runs the generated JavaScript in a sandbox, capturing console output alongside the return value. It normalizes model output into an async arrow function via AST parsing (acorn) and sanitizes tool names into valid JS identifiers (`my-tool` → `my_tool`, `delete` → `delete_`). It blocks outbound network by default. The transferable lessons — minimal executor contract, host-side type derivation, capture-output-and-return-value, name sanitization, AST-normalize the code, isolate by default — are folded into the design below. What does **not** transfer is the substrate: Cloudflare's isolation is Workers-specific; our equivalent hardened substrate is the deferred follow-up.
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+**Prior art.** `@cloudflare/codemode` validates this shape directly and several of its decisions are adopted below. Its `Executor` interface is deliberately tiny — `execute(code, fns) → { result, error?, logs? }` — with a production `DynamicWorkerExecutor` (isolated Workers) and a six-line `NodeVMExecutor` example as two implementations behind it: exactly the interface/implementation split [the capability-seam pattern](../implemented/2026-06-13-capability-seams.md) prescribes. It generates TypeScript type definitions from tools for the model's context and runs the generated JavaScript in a sandbox, capturing console output alongside the return value. It normalizes model output into an async arrow function via AST parsing (acorn) and sanitizes tool names into valid JS identifiers (`my-tool` → `my_tool`, `delete` → `delete_`). It blocks outbound network by default. The transferable lessons — minimal executor contract, host-side type derivation, capture-output-and-return-value, name sanitization, AST-normalize the code, isolate by default — are folded into the design below. What does **not** transfer is the substrate: Cloudflare's isolation is Workers-specific; our equivalent hardened substrate is the deferred follow-up.
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**Prompt-budget tradeoff (Code Mode is not unconditionally cheaper).** Deriving the SDK types host-side costs no extra *discovery* round-trip, but the generated `.d.ts` is injected into the system prompt (§3a), so the type definitions themselves **do** consume context — and for an all-tools SDK that cost scales with every registered tool and can be comparable to, or larger than, the native JSON schemas it replaces. Code Mode's saving is on the **output/result** side (the model curates what comes back; intermediate results never re-enter context) and on **round-trips** (compose many calls in one program), not on the input-side tool description. The net win is workload-dependent: it pays off for multi-call, large-intermediate-result workflows and can cost *more* for a single call against a large tool surface. The `.d.ts` section is a prefix-stable prompt prefix, so prompt caching amortizes its per-turn cost across a session; the RFC notes that caching is what keeps the injected SDK affordable, and that a deployment with a very large tool surface should weigh the SDK size against native schemas rather than assume Code Mode is strictly cheaper.
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**Prompt-budget tradeoff (Code Mode is not unconditionally cheaper).** Deriving the SDK types host-side costs no extra *discovery* round-trip, but the generated `.d.ts` is injected into the system prompt (§3a), so the type definitions themselves **do** consume context — and for an all-tools SDK that cost scales with every registered tool and can be comparable to, or larger than, the native JSON schemas it replaces. Code Mode's saving is on the **output/result** side (the model curates what comes back; intermediate results never re-enter context) and on **round-trips** (compose many calls in one program), not on the input-side tool description. The net win is workload-dependent: it pays off for multi-call, large-intermediate-result workflows and can cost *more* for a single call against a large tool surface. The `.d.ts` section is a prefix-stable prompt prefix, so prompt caching amortizes its per-turn cost across a session; the RFC notes that caching is what keeps the injected SDK affordable, and that a deployment with a very large tool surface should weigh the SDK size against native schemas rather than assume Code Mode is strictly cheaper.
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@@ -27,7 +29,7 @@ The design follows the codebase's capability-seam pattern ([ADR 0009](../adr/000
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- a readonly `safe: boolean` on the `CodeRuntime` service — `false` for an unsandboxed stub, `true` only for a real isolating substrate; consumers gate on it (§2).
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- a readonly `safe: boolean` on the `CodeRuntime` service — `false` for an unsandboxed stub, `true` only for a real isolating substrate; consumers gate on it (§2).
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- `SdkBinding = { namespace: string; fns: Record<string, (args: unknown) => Promise<unknown>> }`
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- `SdkBinding = { namespace: string; fns: Record<string, (args: unknown) => Promise<unknown>> }`
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-Per the "explicit > implicit at seams" convention, the request spells out every field the runtime acts on; defaulting (e.g. an output cap, a timeout derived from `signal`) is the implementation's explicit job, not a hidden `?? default` inside `run()`. The split into interface + implementation is justified under ADR 0009 because there is **genuinely more than one planned implementation** — the node:vm stub *and* the hardened substrate (a real isolate, or the generated program run as a sandboxed process through the existing `ctx.bash` seam) that is scheduled follow-up work, not speculative optionality. ADR 0009 warns against splitting preemptively when only one implementation is conceivable; here a second is not just conceivable but required before any untrusted use, so the seam earns its keep.
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+Per the "explicit > implicit at seams" convention, the request spells out every field the runtime acts on; defaulting (e.g. an output cap, a timeout derived from `signal`) is the implementation's explicit job, not a hidden `?? default` inside `run()`. The split into interface + implementation is justified under [the capability-seam pattern](../implemented/2026-06-13-capability-seams.md) because there is **genuinely more than one planned implementation** — the node:vm stub *and* the hardened substrate (a real isolate, or the generated program run as a sandboxed process through the existing `ctx.bash` seam) that is scheduled follow-up work, not speculative optionality. The capability-seam pattern warns against splitting preemptively when only one implementation is conceivable; here a second is not just conceivable but required before any untrusted use, so the seam earns its keep.
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**Backends can differ by language/runtime, not only by trust level.** The two implementations above (unsafe stub vs. hardened substrate) differ along the *trust* axis while staying TypeScript/JS, but nothing in the `CodeRuntime` contract — a program string plus a set of named async SDK bindings in, and a `{ result, logs, error? }` out — is bound to one source language. The same seam can host backends that differ along the *language* axis, executing a program written in something other than TypeScript. Two illustrative directions:
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**Backends can differ by language/runtime, not only by trust level.** The two implementations above (unsafe stub vs. hardened substrate) differ along the *trust* axis while staying TypeScript/JS, but nothing in the `CodeRuntime` contract — a program string plus a set of named async SDK bindings in, and a `{ result, logs, error? }` out — is bound to one source language. The same seam can host backends that differ along the *language* axis, executing a program written in something other than TypeScript. Two illustrative directions:
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@@ -36,7 +38,7 @@ Per the "explicit > implicit at seams" convention, the request spells out every
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These are illustrations of the seam's reach, **not commitments** — the MVP ships only the TypeScript path. The honest caveat is that the *execution* contract is language-agnostic but the *presentation* is not: the SDK-generation pipeline below (§3a and the `jsonSchemaToTs` codegen, which emits a TypeScript `.d.ts`) is TypeScript-specific, so a non-TS backend pairs the shared `CodeRuntime` contract with its own language-appropriate SDK generator and system-prompt section (a `.pyi` stub and Python usage instructions for the Python backend, AssemblyScript-flavored types for that one). The runtime seam is reused as-is; only the codegen/prompt half is per-language.
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These are illustrations of the seam's reach, **not commitments** — the MVP ships only the TypeScript path. The honest caveat is that the *execution* contract is language-agnostic but the *presentation* is not: the SDK-generation pipeline below (§3a and the `jsonSchemaToTs` codegen, which emits a TypeScript `.d.ts`) is TypeScript-specific, so a non-TS backend pairs the shared `CodeRuntime` contract with its own language-appropriate SDK generator and system-prompt section (a `.pyi` stub and Python usage instructions for the Python backend, AssemblyScript-flavored types for that one). The runtime seam is reused as-is; only the codegen/prompt half is per-language.
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-**2. Implementation package `packages/code-runtime-vm/`** — a new package `@deepseek-ai/dsh-code-runtime-vm`, the `node:vm` reference stub. It type-erases the model's TypeScript via the compiler's `transpileModule` (or sucrase) — the types exist only to guide the model; the runtime is plain JS — then wraps the body in an async IIFE for top-level `await` (Cloudflare's `NodeVMExecutor` does literally `new AsyncFunction("codemode", "return await (${code})()")`), runs it in a `vm.Context` whose globals are a capturing `console` and the SDK namespace objects, awaits the IIFE, and captures the return value, the buffered logs, and any thrown error (as `error: string`). It applies an **output cap** (truncate captured logs) and a **timeout tied to `request.signal`**. These caps limit blast radius; **they are not a security boundary**. node:vm is **not** isolation: withholding `require`/`process` does not contain anything (code escapes via `constructor`/prototype reflection), and per [AGENTS.md](../../AGENTS.md) the harness must never hand model output the ambient environment.
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+**2. Implementation package `packages/code-runtime-vm/`** — a new package `@deepseek-ai/dsh-code-runtime-vm`, the `node:vm` reference stub. It type-erases the model's TypeScript via the compiler's `transpileModule` (or sucrase) — the types exist only to guide the model; the runtime is plain JS — then wraps the body in an async IIFE for top-level `await` (Cloudflare's `NodeVMExecutor` does literally `new AsyncFunction("codemode", "return await (${code})()")`), runs it in a `vm.Context` whose globals are a capturing `console` and the SDK namespace objects, awaits the IIFE, and captures the return value, the buffered logs, and any thrown error (as `error: string`). It applies an **output cap** (truncate captured logs) and a **timeout tied to `request.signal`**. These caps limit blast radius; **they are not a security boundary**. node:vm is **not** isolation: withholding `require`/`process` does not contain anything (code escapes via `constructor`/prototype reflection), and per [AGENTS.md](../../../AGENTS.md) the harness must never hand model output the ambient environment.
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**The unsafe-runtime guard is enforceable, not a README warning.** Because a README caveat is not a control — and AGENTS.md's "never hand model output ambient authority" is a hard rule, not advice — the design makes the danger refuse to run by construction. Two layers:
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**The unsafe-runtime guard is enforceable, not a README warning.** Because a README caveat is not a control — and AGENTS.md's "never hand model output ambient authority" is a hard rule, not advice — the design makes the danger refuse to run by construction. Two layers:
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@@ -61,11 +63,11 @@ These are illustrations of the seam's reach, **not commitments** — the MVP shi
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**Sub-call CallIds.** Real tool calls dispatched from inside `run_code` need ids, but `CallId` is normally provider-issued (a branded string for correlating a call with its result — only brand-wrapped via `CallId()`, with no generator and no documented session-global-uniqueness guarantee). The plugin mints deterministic sub-ids scoped to the parent: `` `${exec.callId}:code:${n}` `` with a per-run counter `n`. These are unique within one `run_code` run (assuming the parent `callId` is unique, which the provider guarantees per turn); the `code/dispatch` event additionally carries the session log's `seq` so the UI and persistence can order and disambiguate globally without relying on the id alone. `ToolExecution.agent` is optional; the normal loop always supplies it (and with it `exec.agent.session`, the log `code/dispatch` appends to). A `run_code` execution arriving without `exec.agent` still runs (sub-calls propagate `agent: undefined`, exactly as the loop's own contract allows) but **skips session-log observability** — with no session to append to, those direct runs are simply not logged.
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**Sub-call CallIds.** Real tool calls dispatched from inside `run_code` need ids, but `CallId` is normally provider-issued (a branded string for correlating a call with its result — only brand-wrapped via `CallId()`, with no generator and no documented session-global-uniqueness guarantee). The plugin mints deterministic sub-ids scoped to the parent: `` `${exec.callId}:code:${n}` `` with a per-run counter `n`. These are unique within one `run_code` run (assuming the parent `callId` is unique, which the provider guarantees per turn); the `code/dispatch` event additionally carries the session log's `seq` so the UI and persistence can order and disambiguate globally without relying on the id alone. `ToolExecution.agent` is optional; the normal loop always supplies it (and with it `exec.agent.session`, the log `code/dispatch` appends to). A `run_code` execution arriving without `exec.agent` still runs (sub-calls propagate `agent: undefined`, exactly as the loop's own contract allows) but **skips session-log observability** — with no session to append to, those direct runs are simply not logged.
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-**Observability without context cost.** Each sub-dispatch emits a session event **declared by the `dsh-code-mode` plugin itself** via `SessionEventMap` declaration merging (the map is merge-extensible precisely so plugins can add events without touching `dsh-session`). Shape: `code/dispatch` with `{ parentCallId, subCallId, name, arguments (or redacted), isError, summary }`, ordered by the session log's own `seq`. `deriveMessages()` does **not** translate it into a model message — an unknown event type falls through its `default`, per the merge-extensible-union convention — so the UI and persistence ([RFC 009](009-session-persistence-and-resumability.md)) can render every sub-call while the model's context only ever receives the single `run_code` tool-result. Because the event lives in the plugin, this adds no core change.
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+**Observability without context cost.** Each sub-dispatch emits a session event **declared by the `dsh-code-mode` plugin itself** via `SessionEventMap` declaration merging (the map is merge-extensible precisely so plugins can add events without touching `dsh-session`). Shape: `code/dispatch` with `{ parentCallId, subCallId, name, arguments (or redacted), isError, summary }`, ordered by the session log's own `seq`. `deriveMessages()` does **not** translate it into a model message — an unknown event type falls through its `default`, per the merge-extensible-union convention — so the UI and persistence ([session persistence](../implemented/2026-06-14-session-persistence.md)) can render every sub-call while the model's context only ever receives the single `run_code` tool-result. Because the event lives in the plugin, this adds no core change.
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**SDK codegen.** A pure `jsonSchemaToTs(schema)` in `code-mode` maps the JSON-schema subset the `defineTool` DSL produces (object/string/number/boolean/array, `properties`, `required[]`, `enum` → string-literal union, nested objects, array `items`) to a TS type literal. It is **total**: any unsupported construct (`$ref`, `oneOf`/`anyOf`, `integer`, `null`, `additionalProperties`, or any raw MCP shape it does not recognize) degrades to `unknown` without throwing — it never crashes codegen. Typing is best-effort, not a guarantee, because MCP tools accept arbitrary JSON Schema and `ToolSchema.parameters` is typed only as `Record<string, unknown>`. Because `ToolSchema.name` is an arbitrary string (not necessarily a valid TS identifier), the SDK is generated as a **namespace with quoted access** (e.g. `tools["some-mcp-tool"](args)`) plus safe camelCase aliases where the name is a clean identifier; alias collisions and TS reserved words fall back to quoted-only access (no duplicate alias emitted). This mirrors Cloudflare's `sanitizeToolName`. `run_code` itself is filtered out of the SDK. The MVP surfaces text content only; image and other block types in sub-results are deferred (noted as a limitation).
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**SDK codegen.** A pure `jsonSchemaToTs(schema)` in `code-mode` maps the JSON-schema subset the `defineTool` DSL produces (object/string/number/boolean/array, `properties`, `required[]`, `enum` → string-literal union, nested objects, array `items`) to a TS type literal. It is **total**: any unsupported construct (`$ref`, `oneOf`/`anyOf`, `integer`, `null`, `additionalProperties`, or any raw MCP shape it does not recognize) degrades to `unknown` without throwing — it never crashes codegen. Typing is best-effort, not a guarantee, because MCP tools accept arbitrary JSON Schema and `ToolSchema.parameters` is typed only as `Record<string, unknown>`. Because `ToolSchema.name` is an arbitrary string (not necessarily a valid TS identifier), the SDK is generated as a **namespace with quoted access** (e.g. `tools["some-mcp-tool"](args)`) plus safe camelCase aliases where the name is a clean identifier; alias collisions and TS reserved words fall back to quoted-only access (no duplicate alias emitted). This mirrors Cloudflare's `sanitizeToolName`. `run_code` itself is filtered out of the SDK. The MVP surfaces text content only; image and other block types in sub-results are deferred (noted as a limitation).
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-**Concurrency — serialized by default (the binding must enforce it).** The SDK functions are async, so a model writing `await Promise.all([tools.a(...), tools.b(...)])` would *start both* immediately, and each would call `ctx.tools.execute` right away — i.e. the binding shape makes concurrent dispatch the **default**, not an opt-in. Because the tool contract carries **no concurrency-safety metadata today** (parallel tool execution and a concurrency-safety hint are an open TODO in both `dsh-tools` and [docs/architecture.md](../architecture.md): "phase 1 executes tool calls sequentially"), concurrent dispatch through a not-yet-hardened tool may race. So a prose "may serialize" is not sufficient. **Decision: the MVP SDK bindings enforce serialization** — each `run_code` invocation owns a per-run dispatch queue, and every `invoke()` chains onto it (`tail = tail.then(() => ctx.tools.execute(...))`), so even `Promise.all` over SDK calls executes them one at a time in submission order. This is a hard acceptance criterion, with a test that issues `Promise.all([...])` from a program and asserts the underlying `ctx.tools.execute` calls did **not** overlap (e.g. a probe tool records enter/exit and the test asserts no interleaving). The `.d.ts` may *describe* the model-visible functions as async (they are), but the implementation guarantees serial execution. Lifting serialization is deferred: only once a tool can declare itself read-only / concurrency-safe does the binding allow those specific tools to overlap. The same per-run queue is where the before/after abort checks (§3c) live, so an aborted run drains no further queued dispatches.
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+**Concurrency — serialized by default (the binding must enforce it).** The SDK functions are async, so a model writing `await Promise.all([tools.a(...), tools.b(...)])` would *start both* immediately, and each would call `ctx.tools.execute` right away — i.e. the binding shape makes concurrent dispatch the **default**, not an opt-in. Because the tool contract carries **no concurrency-safety metadata today** (parallel tool execution and a concurrency-safety hint are an open TODO in both `dsh-tools` and [docs/architecture.md](../../architecture.md): "phase 1 executes tool calls sequentially"), concurrent dispatch through a not-yet-hardened tool may race. So a prose "may serialize" is not sufficient. **Decision: the MVP SDK bindings enforce serialization** — each `run_code` invocation owns a per-run dispatch queue, and every `invoke()` chains onto it (`tail = tail.then(() => ctx.tools.execute(...))`), so even `Promise.all` over SDK calls executes them one at a time in submission order. This is a hard acceptance criterion, with a test that issues `Promise.all([...])` from a program and asserts the underlying `ctx.tools.execute` calls did **not** overlap (e.g. a probe tool records enter/exit and the test asserts no interleaving). The `.d.ts` may *describe* the model-visible functions as async (they are), but the implementation guarantees serial execution. Lifting serialization is deferred: only once a tool can declare itself read-only / concurrency-safe does the binding allow those specific tools to overlap. The same per-run queue is where the before/after abort checks (§3c) live, so an aborted run drains no further queued dispatches.
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**Tool visibility tiers (design intentionally skipped).** A natural extension is to mark each tool with a *visibility tier*: some tools "direct-call eligible" (still offered as native wire tools alongside `run_code`), some "code-mode only" (reachable solely from within a `run_code` program, never on the wire), and the default "both." This would let a deployment keep a few high-frequency or approval-gated tools as direct calls while routing the long tail through Code Mode, or hide composition-only primitives from the native surface entirely. This RFC notes the possibility but **intentionally skips the detailed design** — the per-tool metadata, how it interacts with the `agent/request` enforcement in 3b, and the presentation split in 3a are left to a follow-up. The MVP is the simple two-state model: Code Mode on (everything via `run_code`) or off (everything native).
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**Tool visibility tiers (design intentionally skipped).** A natural extension is to mark each tool with a *visibility tier*: some tools "direct-call eligible" (still offered as native wire tools alongside `run_code`), some "code-mode only" (reachable solely from within a `run_code` program, never on the wire), and the default "both." This would let a deployment keep a few high-frequency or approval-gated tools as direct calls while routing the long tail through Code Mode, or hide composition-only primitives from the native surface entirely. This RFC notes the possibility but **intentionally skips the detailed design** — the per-tool metadata, how it interacts with the `agent/request` enforcement in 3b, and the presentation split in 3a are left to a follow-up. The MVP is the simple two-state model: Code Mode on (everything via `run_code`) or off (everything native).
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@@ -73,7 +75,7 @@ These are illustrations of the seam's reach, **not commitments** — the MVP shi
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## Alternatives
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## Alternatives
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-**Result elision / summarization over native tool-calling (the narrower route).** The Problem has two halves — context bloat (every intermediate `tool-result` re-enters context) and serial composition (one tool call per round-trip). The context-bloat half can be addressed *without* any code-execution runtime: keep provider tool-calling exactly as it is, and add a plugin on the `agent/request` waterfall (or a compaction pass akin to RFC 009's session work) that elides or summarizes older `tool-result` blocks before they re-enter the model's context — drop them past a window, replace large payloads with a digest, or keep only the blocks the model still references. This is strictly less invasive than Code Mode: no new runtime seam, no model-written programs, no new safety surface. It is the right tool if context growth is the only pain.
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+**Result elision / summarization over native tool-calling (the narrower route).** The Problem has two halves — context bloat (every intermediate `tool-result` re-enters context) and serial composition (one tool call per round-trip). The context-bloat half can be addressed *without* any code-execution runtime: keep provider tool-calling exactly as it is, and add a plugin on the `agent/request` waterfall (or a compaction pass akin to [the session-persistence work](../implemented/2026-06-14-session-persistence.md)) that elides or summarizes older `tool-result` blocks before they re-enter the model's context — drop them past a window, replace large payloads with a digest, or keep only the blocks the model still references. This is strictly less invasive than Code Mode: no new runtime seam, no model-written programs, no new safety surface. It is the right tool if context growth is the only pain.
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It is insufficient for the **composition / round-trip** half, which is the decisive reason this RFC does not stop there. Elision still pays one model round-trip per tool call: a loop over N items is N turns, a branch on an intermediate value is a turn to fetch then a turn to act, and post-processing (filter, join, reduce) either happens in the model's head over full payloads or not at all. Code Mode collapses all of that into one program — the loop, the branch, the join run in the runtime, and only the curated result returns. Elision also cannot express fan-out or data-dependent control flow; it only shrinks what comes back. So the two are complementary, not competing: elision could even layer *under* Code Mode for the residual native-tool paths. The RFC chooses Code Mode because the round-trip/composition cost is the larger structural limit, and accepts the new code-execution surface as the price — which is exactly why the execution substrate is gated behind the enforceable safety guard (§2) and the hardened backend is a hard prerequisite for untrusted use.
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It is insufficient for the **composition / round-trip** half, which is the decisive reason this RFC does not stop there. Elision still pays one model round-trip per tool call: a loop over N items is N turns, a branch on an intermediate value is a turn to fetch then a turn to act, and post-processing (filter, join, reduce) either happens in the model's head over full payloads or not at all. Code Mode collapses all of that into one program — the loop, the branch, the join run in the runtime, and only the curated result returns. Elision also cannot express fan-out or data-dependent control flow; it only shrinks what comes back. So the two are complementary, not competing: elision could even layer *under* Code Mode for the residual native-tool paths. The RFC chooses Code Mode because the round-trip/composition cost is the larger structural limit, and accepts the new code-execution surface as the price — which is exactly why the execution substrate is gated behind the enforceable safety guard (§2) and the hardened backend is a hard prerequisite for untrusted use.
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@@ -81,12 +83,12 @@ It is insufficient for the **composition / round-trip** half, which is the decis
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## Plan
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## Plan
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-1. Scaffold the interface package `packages/code-runtime/` per [the cookbook](../cookbook/adding-a-package.md): abstract `CodeRuntime extends Service` (`super(ctx, 'codeRuntime')`) with a readonly `safe: boolean`, the `declare module 'cordis'` ctx key, the `CodeRunRequest`/`CodeRunResult`/`SdkBinding` vocabulary, method contracts documented in JSDoc (what `run` captures, abort semantics, that an error is a result field not a throw, what `safe` means). HMR-safety test (dispose the contributing fiber, assert `ctx.codeRuntime` is gone).
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+1. Scaffold the interface package `packages/code-runtime/` per [the cookbook](../../cookbook/adding-a-package.md): abstract `CodeRuntime extends Service` (`super(ctx, 'codeRuntime')`) with a readonly `safe: boolean`, the `declare module 'cordis'` ctx key, the `CodeRunRequest`/`CodeRunResult`/`SdkBinding` vocabulary, method contracts documented in JSDoc (what `run` captures, abort semantics, that an error is a result field not a throw, what `safe` means). HMR-safety test (dispose the contributing fiber, assert `ctx.codeRuntime` is gone).
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2. Scaffold the implementation package `packages/code-runtime-vm/`: the node:vm stub — `safe = false`, a constructor that **throws unless given `{ unsafe: true }`**, transpile/type-erase, async-IIFE wrap, capturing `console`, SDK globals, return-value/logs/error capture, output cap, signal-tied timeout. Tests for output capture, return value, error-as-field, abort, the constructor refusal without `unsafe`, and a README documenting the "not a sandbox, trusted-only" caveat prominently.
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2. Scaffold the implementation package `packages/code-runtime-vm/`: the node:vm stub — `safe = false`, a constructor that **throws unless given `{ unsafe: true }`**, transpile/type-erase, async-IIFE wrap, capturing `console`, SDK globals, return-value/logs/error capture, output cap, signal-tied timeout. Tests for output capture, return value, error-as-field, abort, the constructor refusal without `unsafe`, and a README documenting the "not a sandbox, trusted-only" caveat prominently.
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3. Scaffold the consumer plugin `packages/code-mode/`: `jsonSchemaToTs` codegen with namespace/quoted-access + alias handling (unit tests, including non-identifier MCP names and unsupported-shape → `unknown`); the registered lazy `ctx.systemPrompt.section()` carrying the SDK `.d.ts`; the `agent/request` listener (`prepend: true`) collapsing `request.tools` to `[run_code]` after `await next()`; the **unsafe-runtime gate** (refuse to register `run_code` when `ctx.codeRuntime.safe === false` unless `allowUnsafeRuntime` is set); the `run_code` tool with the dispatch bridge (per-run serialization queue, deterministic sub-call ids, before/after abort checks, `CodeRunError` on error results); and the `code/dispatch` event declared here via `SessionEventMap` merge. Declare `inject = ['tools', 'systemPrompt', 'codeRuntime']`.
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3. Scaffold the consumer plugin `packages/code-mode/`: `jsonSchemaToTs` codegen with namespace/quoted-access + alias handling (unit tests, including non-identifier MCP names and unsupported-shape → `unknown`); the registered lazy `ctx.systemPrompt.section()` carrying the SDK `.d.ts`; the `agent/request` listener (`prepend: true`) collapsing `request.tools` to `[run_code]` after `await next()`; the **unsafe-runtime gate** (refuse to register `run_code` when `ctx.codeRuntime.safe === false` unless `allowUnsafeRuntime` is set); the `run_code` tool with the dispatch bridge (per-run serialization queue, deterministic sub-call ids, before/after abort checks, `CodeRunError` on error results); and the `code/dispatch` event declared here via `SessionEventMap` merge. Declare `inject = ['tools', 'systemPrompt', 'codeRuntime']`.
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4. Tests: HMR-safety (dispose removes the tool, the section, and the listener); a waterfall test that the wire tool list is exactly `[run_code]` (spy adapter, asserting via `agent/request` and optionally `llm/stream`); an integration test that a program calling two tools returns only its printed/returned output (verify the world, not the self-report); a **serialization test** that `Promise.all([...])` over SDK calls does not overlap the underlying `ctx.tools.execute` invocations (a probe tool records enter/exit; assert no interleaving); `deriveMessages()` ignores `code/dispatch`; abort mid-program stops further dispatches; `CodeRunError` surfaces as `isError: true`; and the **unsafe-runtime refusal test** (§3, the VM-guard): with the unsafe flag unset, a non-mock agent's `run_code` is refused; with it set, the program runs.
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4. Tests: HMR-safety (dispose removes the tool, the section, and the listener); a waterfall test that the wire tool list is exactly `[run_code]` (spy adapter, asserting via `agent/request` and optionally `llm/stream`); an integration test that a program calling two tools returns only its printed/returned output (verify the world, not the self-report); a **serialization test** that `Promise.all([...])` over SDK calls does not overlap the underlying `ctx.tools.execute` invocations (a probe tool records enter/exit; assert no interleaving); `deriveMessages()` ignores `code/dispatch`; abort mid-program stops further dispatches; `CodeRunError` surfaces as `isError: true`; and the **unsafe-runtime refusal test** (§3, the VM-guard): with the unsafe flag unset, a non-mock agent's `run_code` is refused; with it set, the program runs.
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5. Wire an example: `examples/coding-agent-code-mode` (or a config flag on the existing example) loading the trio. Running it against the node:vm stub requires both opt-ins (`VmCodeRuntime({ unsafe: true })` and `code-mode`'s `allowUnsafeRuntime`); the example sets them explicitly and comments why, or uses a mock model — a real model never reaches the unsandboxed stub without those deliberate flags. Add a `pnpm run demo:*` entry.
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5. Wire an example: `examples/coding-agent-code-mode` (or a config flag on the existing example) loading the trio. Running it against the node:vm stub requires both opt-ins (`VmCodeRuntime({ unsafe: true })` and `code-mode`'s `allowUnsafeRuntime`); the example sets them explicitly and comments why, or uses a mock model — a real model never reaches the unsandboxed stub without those deliberate flags. Add a `pnpm run demo:*` entry.
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-6. Docs: update [docs/architecture.md](../architecture.md) (a `ctx.codeRuntime` row in the service map, a Code Mode note under the tool pipeline / capability seams sections); add a [cookbook](../cookbook/) note on writing a `CodeRuntime` backend; and **file the follow-up RFC for the hardened execution substrate** (the isolate/sandboxed-process design, the additional-language backends sketched in §1 — AssemblyScript/WASM, Python — with their per-language SDK generators, plus the tool-visibility-tier design skipped here). Append the `| 012 | … | proposed |` row to [the RFC index](README.md).
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+6. Docs: update [docs/architecture.md](../../architecture.md) (a `ctx.codeRuntime` row in the service map, a Code Mode note under the tool pipeline / capability seams sections); add a [cookbook](../../cookbook/) note on writing a `CodeRuntime` backend; and **file the follow-up RFC for the hardened execution substrate** (the isolate/sandboxed-process design, the additional-language backends sketched in §1 — AssemblyScript/WASM, Python — with their per-language SDK generators, plus the tool-visibility-tier design skipped here). On landing, move this file to `implemented/` and update its row in [the RFC index](../README.md).
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## Risks
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## Risks
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