Last updated October 10, 2026

Codex Claude: Use Claude 5.5 Models in Codex

This guide shows you how to add Claude models to the Codex model picker with Codex Router. The credential-isolated Anthropic API provider now includes Claude Opus 5.5, Sonnet 5.5, Fable 5.1 and Haiku 5.5 alongside Opus 4.8, forwarding requests through the Anthropic Messages protocol.

ClaudeAnthropic API

What you can add

Picker label	Model ID	Authentication
Claude Opus 5.5 (API)	`anthropic-api/claude-opus-5.5`	Separately billed Anthropic API key
Claude Sonnet 5.5 (API)	`anthropic-api/claude-sonnet-5.5`	Separately billed Anthropic API key
Claude Fable 5.1 (API)	`anthropic-api/claude-fable-5.1`	Separately billed Anthropic API key
Claude Haiku 5.5 (API)	`anthropic-api/claude-haiku-5.5`	Separately billed Anthropic API key
Claude Opus 4.8 (API)	`anthropic-api/claude-opus-4.8`	Separately billed Anthropic API key

The four new 5.x API routes declare a 1M-token context window, text and image input, and auto-compaction at 900K. They have not been live-probed by upstream. Command Code, OpenRouter, Nous Research and Venice carry separate Claude routes with their own credentials and billing.

Requirements

The Codex App or CLI, plus the usual prerequisites (see the installation guide).
An Anthropic API key. Anthropic API access is a separate account and billing system from ChatGPT, Kimi, DeepSeek and xAI — a ChatGPT or other key cannot authenticate Claude requests.

Enable the provider

./bin/model-router codex provider-key anthropic-api set
./bin/model-router codex providers enable anthropic-api

The set command opens a hidden prompt with terminal echo disabled. The key is stored in a per-user protected file (mode 600 on POSIX, a current-user ACL on Windows) and diagnostics report only presence and source. On Windows, use ./codex-router.ps1 codex provider-key anthropic-api set.

Never paste an Anthropic key into chat, a command argument, or a tracked file. The router sends it only to the configured Anthropic endpoint, and only for requests that select an Anthropic model.

Verify the integration

./bin/model-router codex doctor
./bin/model-router codex providers

anthropic-api must report SHOW and ready. Then fully quit Codex, reopen it, create a new task, and check the picker for the 5.x models. You can inspect the startup catalog with codex debug models. For a live, quota-consuming check:

./bin/test-model 'anthropic-api/claude-opus-4.8' --live --yes

How Claude requests behave

The router translates Codex’s Responses requests to Anthropic Messages through its credential-isolating forwarder, preserving streaming and tool calls. The new 5.x routes advertise low, medium, high, xhigh and max effort. Opus 5.5 and Haiku 5.5 default to medium; Sonnet 5.5 and Fable 5.1 default to high. The forwarder passes the selected effort to Anthropic’s output_config.effort.

Common pitfalls

Missing picker entry usually means missing credentials. Confirm anthropic-api reports ready, then restart Codex. See models missing from the picker.
Anthropic keys do not work elsewhere. The router keeps each provider’s key in its own file, and an Anthropic key cannot authenticate any other provider (or vice versa).
Provider policy risk. Anthropic may restrict or block proxy traffic. Routing Claude through a third-party router is at your own risk; check Anthropic’s terms before relying on it.
Effort levels depend on your Codex build. On older Codex builds, unsupported effort values are republished at the nearest supported tier, so the picker may show fewer levels than Anthropic supports.