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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.

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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.

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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 generationThinking configEffort destination
Adaptive — claude-fable-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:
EffortBudget tokens
low5000
medium10000
high / xhigh / max20000
Omit reasoning to leave thinking off. On the adaptive models above, GoModel only sets thinking: {type: "adaptive"} when you pass reasoning.effort; without it those models do not engage extended thinking. 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 only available on Fable 5 and Opus 4.8/4.7, and max on Fable 5, Opus 4.8/4.7/4.6, 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 on Fable 5 and Opus 4.7/4.8, which is why GoModel uses adaptive thinking for those models.
When extended thinking is engaged, Anthropic requires temperature = 1. GoModel drops any other temperature value (and logs it) rather than failing the request.

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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.