model: speak the two dialects natively and stop hosting a Model component

[weilueluo]

Brain speaks exactly two API shapes — OpenAI and Anthropic — natively, and knows nothing else. Its
model configuration is the dialect, the endpoint, the credential and the model name; a Model is no
longer an extension. Which provider names a platform resolves is that platform's policy.

Design note: aex-research/docs/model-provider-and-catalog.md (aexhq/aex-research#16).
Companion PRs: aexhq/aex#134 (Aex owns the catalog), aexhq/extensions#48 (the Model packages go).

Public surface

const session = await brain.sessions.create({
  model: {
    dialect: "openai",                       // or "anthropic"
    baseUrl: "https://api.openai.com/v1",
    name: "gpt-4.1-nano",
    apiKey: process.env.OPENAI_API_KEY!,
  },
  agentloop: pi(),
});

There is no unsupported-provider case at this layer: any endpoint speaking either shape works by
pointing the URL at it. That is the extension point, and it needs no component, no registry and
no catalog.

What this restores rather than rebuilds

The native codecs were never deleted — the reset left them in crates/brain/src/provider/, still
built into the dialect factory both compositions compose (brain_providers::default_factory), and
severed only from the session path, with view.rs hard-coding Dialect::OpenAiChat "until the
legacy provider codecs are deleted". The sealed prefix carries the dialect again and
(brain.provider_factory)(dialect) resolves the turn's provider.

Deleted with the world they served: contracts/model/v1, the published model.wit, the model
bindgen and its runtime/worker arms, the guest fixture, brain-providers/src/component.rs, and the
"model" component kind in the manifest and the TypeScript client. provider leaves the create,
the sealed prefix, the session view and the model.usage event.

Regressions carried across, not left to die with the packages

Each of these was a live defect in the last two days, and each now has a test here:

• an unset sampling field stays absent rather than serializing as null (both dialects)
• the sealed system prompt reaches the request (leading system message / system field)
• a non-2xx status keeps its status, body and retry hint
• a stream that dies mid-frame leaves its bytes pending, so EOF reports rather than looks clean
• reasoning_effort is refused at create by the Anthropic dialect that cannot send it

standalone_e2e now drives a scripted model endpoint over real HTTP in both shapes, asserting
per-session credentials, the system prompt and absent sampling fields on every round.

Endpoint convention

baseUrl is the endpoint's full API root, version segment included, and a dialect appends only its
operation path (/chat/completions, /messages). That is what models.dev publishes and what
providers that version differently — or not at all, like api.githubcopilot.com — actually need.

Cut clean

PrefixDoc loses provider and model_component with no tolerance window, so sessions sealed
before this do not hydrate: the development plane was reset for exactly this, and production has
never served a customer.

typed() moves here from @aexhq/model, beside the contracts whose extension-error it returns.

Gates

cargo fmt --all --check, cargo clippy --workspace --all-targets -D warnings,
cargo test --workspace (213 brain + protocol conformance + every brain-server gate including
standalone_e2e and component_tool_environment), cargo test -p brain-component-host (8/8 run
serially; the two frame-bound tests are timing-sensitive under local parallelism),
python tools/postprocess-generated.py --check, npm test (36), npm run package-smoke.

@aexhq/brain 0.6.0. 55 files, ~930 insertions against ~1260 deletions.

[weilueluo]

Parked as a draft: the owner moved the boundary again — Brain will speak the two dialects natively and stop treating Model as a component kind, and the catalog moves to Aex. Re-planning before this branch is rewritten; nothing here should merge as it stands.

[weilueluo]

Superseded by the merged provider normalization (#107) and the models.dev-driven registry (#114); conflicts with the post-reset main.