# OpenAI-Compatible Chat Integration

## Purpose

This document defines the outbound HTTP behavior implemented by Promptkit's
internal OpenAI-compatible model client. The
[internal model-client document](../internal/llm.md) owns implementation flow,
errors, and test ownership. The root Promptkit engine uses this client by
default unless a consumer injects another implementation. The
[framework format reference](../formats.md) owns the profile and prompt values
that produce these outbound settings.

## Endpoint And Method

Generation sends an HTTP `POST` with `Content-Type: application/json`.
Before the client is called, the engine resolves framework, backend, profile,
and request values into one execution target. Endpoint configuration is trimmed
and must be an absolute HTTP or HTTPS URL with a host and without user
information, a query, or a fragment. A non-empty endpoint from the target
overrides the client's configured base URL. The final selected endpoint is
validated again before transport.

The completion URL is composed through parsed URL path operations. Nested base
paths are retained, repeated trailing slashes are normalized, and the result
has exactly one appended `/chat/completions` suffix. Generation fails before
sending when neither source supplies a valid endpoint.

The target's backend ID is routing metadata for prepared values, results, and
injected clients. The built-in client does not derive the URL from that ID and
does not serialize it in the provider request.

## Authentication

A non-empty API key supplied directly on the execution target takes
precedence. Otherwise, when an API-key environment-variable name is supplied,
the client reads that variable and requires a non-empty value. The selected
key is sent as `Authorization: Bearer <key>`. No authorization header is sent
when neither mechanism is configured.

The target contains the already resolved environment-variable name: an
explicit request override takes precedence over profile metadata, which takes
precedence over the backend default. Only the name reaches prepared metadata;
the environment value is read just before the provider call and is never added
to the JSON body.

## Request Body

The request body always contains `model` and `messages`. The execution
target's model takes precedence over the client's configured model, and one
must be available.

Each ordinary message contains its `role` and string `content`. A
cache-controlled message instead uses a text content block containing `type`,
`text`, and `cache_control`; an empty cache-control TTL is omitted.

The effective direct or prompt-rendered session ID is trimmed, limited to 256
Unicode code points, and sent when nonempty as top-level `session_id`. It is
never also sent as a session header.

The client conditionally includes:

- `temperature`, `max_tokens`, and `top_p` only when selected by a profile or
  runtime override, including an explicit runtime zero; they are absent when
  unspecified;
- non-empty `service_tier` and effective `reasoning_effort`; an explicitly
  disabled reasoning setting is empty and therefore omitted; and
- `response_format` for JSON Schema structured output, including its name,
  strict flag, and schema document.

The engine resolves backend, profile, and request extra-parameter maps by
whole-map replacement rather than key merging. The resulting effective map is
then merged directly into the top-level body after JSON serialization is
verified. Empty keys and collisions with these reserved fields are rejected
before any provider call:

- `model`
- `session_id`
- `messages`
- `temperature`
- `max_tokens`
- `top_p`
- `service_tier`
- `reasoning_effort`
- `response_format`

`backend_id`, `api_key_env`, and resolved credential values are not provider
request fields.

## Response Handling

Any 2xx response is decoded as an OpenAI-compatible chat response. The client
returns the first choice's non-empty message content and maps prompt,
completion, total, cached, and cache-write token counts.

Invalid JSON, absent choices, and empty first-choice content are malformed
responses. For a non-2xx status, the error includes the status code but never
the provider response body.

An outbound `http.Client.Do` failure retains both Promptkit's request-failure
identity and the exact transport error for `errors.Is` and `errors.As` checks.
The rendered error does not include the selected endpoint, request headers,
request content, credentials, or provider body.

## Timeout And Cancellation

Timeouts are layered:

- the caller context remains the outer cancellation boundary;
- a positive generation timeout adds a request context deadline;
- zero adds no generation-specific deadline;
- a negative generation timeout is invalid; and
- the cloned `http.Client` supplies the whole-request transport cap, retaining
  a positive supplied-client timeout or applying the configured/default
  timeout when the supplied value is not positive.

The earliest applicable caller, generation, or transport deadline controls the
request. Caller cancellation retains `context.Canceled`; caller, generation,
and whole-request timeout failures retain `context.DeadlineExceeded`, together
with the request-failure identity. Constructing the internal client does not
mutate a supplied `http.Client`.