Anthropic

Configure Anthropic (Claude) in GoModel and understand how reasoning effort maps to Claude’s adaptive thinking and effort control.

Anthropic setup is just an API key. This page exists for one quirk: how GoModel maps the OpenAI-style reasoning.effort knob onto Claude’s native thinking and effort controls, which differ by model generation.

Configure

ANTHROPIC_API_KEY=sk-ant-...
# ANTHROPIC_BASE_URL=https://api.anthropic.com/v1   # optional override
# ANTHROPIC_DEFAULT_MAX_TOKENS=4096                 # injected when callers omit max_tokens

Or in config.yaml:

providers:
  anthropic:
    type: anthropic
    api_key: "${ANTHROPIC_API_KEY}"

Anthropic’s /v1/messages requires max_tokens on every request. GoModel injects ANTHROPIC_DEFAULT_MAX_TOKENS (default 4096) when a caller omits it, keeping the OpenAI-compatible surface lenient.

Claude subscription (OAuth token)

GoModel also accepts a Claude subscription OAuth token as the Anthropic credential. Generate one with claude setup-token (requires a Claude subscription — Pro, Max, Team, or Enterprise — and the Claude Code CLI) and set it as the provider key:

ANTHROPIC_API_KEY=sk-ant-oat01-...

Tokens with the sk-ant-oat prefix are detected automatically: GoModel sends them as Authorization: Bearer with the oauth-2025-04-20 beta instead of x-api-key. No extra configuration is needed.

Anthropic authorizes subscription OAuth tokens only for Claude Code traffic. Use this to route your own Claude Code through GoModel (see the Claude Code guide); requests from other clients are rejected upstream with “This credential is only authorized for use with Claude Code”. Endpoints outside the Claude Code surface (such as model listing) may also be rejected — if provider model discovery fails, configure the models list for the provider explicitly.

Reasoning effort mapping

GoModel accepts the OpenAI-shaped "reasoning": {"effort": "..."} object as well as the Chat Completions string form "reasoning_effort": "..." (a non-empty reasoning.effort wins when both are present; an empty object falls back to the string form) and translates them to Claude’s native controls. The five accepted levels are low, medium, high, xhigh, and max; values are matched case-insensitively and any other value is downgraded to low and logged. The translation depends on whether the model supports adaptive thinking.

Model generation	Thinking config	Effort destination
Adaptive — claude-fable-5, claude-fable-5-1, claude-mythos-5, claude-mythos-5-1, claude-opus-5, claude-sonnet-5, claude-opus-4-8, claude-opus-4-7, claude-opus-4-6, claude-sonnet-4-6 (and dated snapshots of each)	thinking: {type: "adaptive"}	output_config.effort (passed through)
Legacy (everything else, e.g. claude-opus-4-5, claude-3-5-sonnet)	thinking: {type: "enabled", budget_tokens: N}	mapped to a token budget

Adaptive routing is an explicit allowlist, not a version comparison. New model IDs are treated as legacy until added to the list. For pre-4.7 models the legacy fallback keeps working via budget_tokens; models from Opus 4.7 onward reject budget_tokens outright, so a new adaptive-only model ID fails with an upstream 400 until it is added to the allowlist.

For legacy models the effort string maps to a thinking budget; max_tokens is bumped above the budget when needed. xhigh and max are adaptive-only levels, so on legacy models they are capped at the high budget rather than inflating max_tokens past what those models can emit:

Effort	Budget tokens
low	5000
medium	10000
high / xhigh / max	20000

Omit reasoning to leave thinking at the model’s default. GoModel only sets thinking: {type: "adaptive"} when you pass reasoning.effort (or reasoning_effort). Without it, Opus 4.6 to 4.8 and Sonnet 4.6/5 do not engage extended thinking, while Fable 5/5.1, Mythos 5/5.1, and Opus 5 think adaptively on their own (see the always-on note below). Effort is a separate control that governs overall token spend (text and tool calls) whether or not thinking is engaged, and Anthropic defaults it to high when unset. It is a behavioral signal for depth and verbosity, not a hard budget — actual usage varies per request and is bounded by max_tokens.

Effort levels are model-gated upstream: xhigh is available on Fable 5/5.1, Opus 5, Sonnet 5, and Opus 4.8/4.7; max on those plus Opus 4.6 and Sonnet 4.6. GoModel forwards the level you send; Anthropic rejects it with a 400 if the target model does not support it. Manual budget_tokens thinking is rejected from Opus 4.7 onward, which is why GoModel uses adaptive thinking for those models.

On Fable 5/5.1, Mythos 5/5.1, and Opus 5 thinking is always on, whether or not you send reasoning; reasoning.effort only tunes its depth. The tokens it spends are reported as usage.completion_reasoning_tokens in Chat Completions responses. The reasoning text itself is not returned; only the token count is.

Sampling parameters

Anthropic removed temperature and top_p from Fable 5/5.1, Mythos 5/5.1, Opus 5, Sonnet 5, and Opus 4.8/4.7 — any value, including the OpenAI SDK default of temperature: 1, is rejected upstream with a 400. GoModel drops both fields for those models and logs the discarded values, so clients that always send a temperature keep working. Older models still receive them as sent.

Independently of the model, when extended thinking is engaged Anthropic requires temperature = 1. GoModel drops any other temperature value (and logs it) rather than failing the request.

Forced tool choice on Fable 5.1

Fable 5.1 and Mythos 5.1 accept only tool_choice: "auto" and "none"; forcing a call with "required" or {"type": "function", ...} returns a 400 from Anthropic. GoModel follows Anthropic’s documented replacement: the choice is downgraded to auto and an instruction is appended to the system prompt — “You must respond by calling one of the provided tools.” for required, or “You must respond by calling the tool named <name>.” for a named function. The downgrade is logged. parallel_tool_calls: false is still honored. Fable 5 and every other Claude model keep forced tool use unchanged.

The instruction is strong guidance, not a hard guarantee: the model can still answer in text. If you relied on forced tool choice to obtain JSON, check finish_reason and retry, or use the native passthrough with Anthropic’s structured outputs instead.

Native passthrough

To send Claude-native request fields that have no OpenAI-compatible equivalent (for example inline mid-task system entries in the messages array), use the passthrough route /p/anthropic/messages, which forwards the body verbatim.