Fast tier & thinking effort

Two request settings are worth exposing in a chat UI: the Codex service tier (Fast mode) and the reasoning effort, usually shown as a "thinking level". Browser code cannot rewrite proxied bodies, so the SDK exposes both as narrow headers. The server validates each value and writes it into the request.

Fast tier from the browser

Codex Fast mode is service_tier: "fast" on supported models. Send the SDK header on any AI SDK call:

import { streamText } from "ai";

const result = streamText({
  model: chatgpt("gpt-5.5"),
  prompt: "Refactor this helper.",
  headers: {
    "x-login-with-chatgpt-service-tier": "fast",
  },
});

Accepted values are auto, default, flex, priority, and fast. Anything else returns 400. If upstream rejects Fast for that session or model, the handler retries once without service_tier. The request still streams, and the response carries x-login-with-chatgpt-service-tier-fallback: auto so your UI can show that Fast did not apply.

Fast is account- and model-dependent. Show the toggle only for models that support it, and tell users it consumes their included usage faster:

const supportsFast = model === "gpt-5.5" || model === "gpt-5.4";

Thinking effort from the browser

The reasoning header works the same way and lands in reasoning.effort:

const result = streamText({
  model: chatgpt("gpt-5.5"),
  prompt: "Review this migration plan.",
  headers: {
    "x-login-with-chatgpt-reasoning-effort": "high",
  },
});

Accepted values are none, low, medium, high, and xhigh. Invalid values return 400. When nothing is sent, the default is medium. For a user-facing control, stick to low / medium / high; the outer values are model-dependent and worth testing before you offer them.

Defaults on the server

Both settings can also be fixed server-side instead of per request, on the handler for proxied traffic or on the direct provider for server-side calls:

const auth = createChatGPTHandler({
  secret: process.env.LWC_SECRET,
  reasoningEffort: "high",   // default for proxied requests
  serviceTier: "fast",       // applied when the body has no service_tier
});

const chatgpt = createChatGPT({
  credentials: tokens,
  serviceTier: "fast",
  reasoningEffort: "high",
});

Header values from the browser win over these defaults, because the header is written into the body before normalization fills the gaps.