Claude Code with Opus 5.5: setup, cost and settings

TL;DR Point Claude Code at apimodels.app with ANTHROPIC_BASE_URL=https://api.apimodels.app, your key in ANTHROPIC_AUTH_TOKEN and ANTHROPIC_MODEL=claude-opus-5-5. You pay per token at $2.40 input / $12 output per million, 40% below Anthropic’s $4 / $20, with no subscription. Opus 5.5 has no free API tier anywhere; per-token billing is the most affordable way to use it occasionally.

Opus 5.5 is available in Claude Code from launch day, and Anthropic’s launch numbers explain why people want it there: 66.4% on Terminal-Bench 4.0 and 57.8% on CursorBench 4.0, both ahead of Fable 5.1. Claude Code speaks the Anthropic Messages API, so any gateway that serves /v1/messages can run it. apimodels.app does, with prompt caching passed through.

What a session costs

Claude Code sends the whole working context on every turn, so input tokens dominate and prompt caching matters. Opus 5.5 cache reads list at $0.20 per million and cache writes at $5; on apimodels.app they are $0.12 and $3.00. As a rough guide, a turn that re-sends 60,000 cached tokens and writes 2,000 tokens of code costs about $0.007 for the cached input plus $0.024 for the output on apimodels.app.

Per 1M tokens	Anthropic list	apimodels.app
Input	$4	$2.40
Output	$20	$12
Cache read	$0.20	$0.12
Cache write	$5	$3.00

Settings worth knowing

Thinking is always on for Opus 5.5; you control depth with effort (low, medium, high, xhigh, max), and the default is medium. Medium is right for most edits; raise it for large refactors or debugging you have already failed at once. Reasoning tokens bill as output, so higher effort costs more per turn.

Coming from Opus 5: four things break. Thinking cannot be disabled, forcing a specific tool with tool_choice returns an error, thinking blocks are bound to the model that produced them, and the old computer_20251124 tool is no longer accepted. Claude Code itself handles all four; custom agents built on the API need to check them.

Quick start

macOS / Linux (~/.zshrc or ~/.bashrc)

export ANTHROPIC_BASE_URL="https://api.apimodels.app"
export ANTHROPIC_AUTH_TOKEN="sk_…your_apimodels_key…"
export ANTHROPIC_MODEL="claude-opus-5-5"
# then: source ~/.zshrc && claude

Windows (PowerShell)

[Environment]::SetEnvironmentVariable("ANTHROPIC_BASE_URL", "https://api.apimodels.app", "User")
[Environment]::SetEnvironmentVariable("ANTHROPIC_AUTH_TOKEN", "sk_…your_apimodels_key…", "User")
[Environment]::SetEnvironmentVariable("ANTHROPIC_MODEL", "claude-opus-5-5", "User")
# open a new terminal, then run: claude

cURL

curl https://api.apimodels.app/v1/messages \
  -H "x-api-key: $APIMODELS_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-opus-5-5",
    "max_tokens": 16000,
    "messages": [{"role": "user", "content": "Review this function for race conditions: ..."}]
  }'

FAQ

Can I use Opus 5.5 in Claude Code without an Anthropic subscription?

Yes. Set ANTHROPIC_BASE_URL to https://api.apimodels.app, put your apimodels key in ANTHROPIC_AUTH_TOKEN and set ANTHROPIC_MODEL to claude-opus-5-5. You pay per token, $2.40 / $12 per million, with no monthly fee.

Is Opus 5.5 free?

Not through any API we know of: Anthropic lists it at $4 / $20 per million tokens and resellers charge per token too. The most affordable way to use it occasionally is per-token billing with no subscription; on apimodels.app a short coding turn usually costs a few cents. Pages promising free unlimited Opus 5.5 access are worth treating with suspicion.

Does prompt caching work through apimodels.app?

Yes. Cache-control markers are passed through to the upstream, and cache reads bill at $0.12 per million, a twentieth of the input price. Claude Code sets them automatically.

Should I use Opus 5.5 or Sonnet 5.5 in Claude Code?

Anthropic reports Sonnet 5.5 at 70.6% on Terminal-Bench 4.0 against Opus 5.5’s 66.4%, at half the list price, so for well-scoped edits Sonnet 5.5 is a strong default. Opus 5.5 is the better choice for large, ambiguous changes. On apimodels.app Sonnet 5.5 is $1.20 / $6 per million: set ANTHROPIC_MODEL to claude-sonnet-5-5.

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