fix(ai): map Anthropic max_tokens to OpenAI max_completion_tokens

[dimakis]

Summary

• The anthropic_to_openai request transform maps Anthropic's max_tokens to OpenAI's max_tokens in Chat Completions requests
• GPT-5.x and reasoning models (o1, o3) reject max_tokens, returning: "Unsupported parameter: 'max_tokens' is not supported with this model. Use 'max_completion_tokens' instead."
• One-line fix: emit max_completion_tokens instead, which is backwards-compatible with older models

Context

OpenAI has three different token limit parameters across their APIs:

API	Parameter	Endpoint
Anthropic Messages	max_tokens	/v1/messages
OpenAI Chat Completions	max_completion_tokens	/v1/chat/completions
OpenAI Responses	max_output_tokens	/v1/responses

The max_completion_tokens parameter was introduced with the o1 reasoning models to distinguish between reasoning tokens and output tokens. GPT-5.x models continued this convention, rejecting the legacy max_tokens entirely.

This fix addresses the Chat Completions path only. The Responses API (/v1/responses) uses max_output_tokens and would need separate translation logic.

Evidence

Direct API error (GPT-5.5 via Chat Completions through praxis-proxy):

"Unsupported parameter: 'max_tokens' is not supported with this model.
 Use 'max_completion_tokens' instead."

OpenAI API reference (developers.openai.com/api/docs/api-reference/evals):

The max_completion_tokens parameter sets the maximum number of tokens that can be included in the generated output from the model.

Backwards compatibility: max_completion_tokens is accepted by both legacy models (GPT-4.1, GPT-3.5-turbo) and current frontier models (GPT-5.x, o3). Safe to apply unconditionally.

Verification

Tested locally with praxis-proxy proxying Anthropic-format requests to OpenAI:

Test	Model	Result
Non-streaming	gpt-4.1-mini	Pass
Streaming SSE	gpt-4.1-mini	Pass
Tool calling	gpt-4.1-mini	Pass
Non-streaming	gpt-5.5	Pass (failed before fix)
Streaming SSE	gpt-5.5	Pass

Existing unit tests updated and passing (14/14).

Test plan

• Unit tests pass (14/14)
• Manual curl: GPT-5.5 non-streaming through proxy
• Manual curl: GPT-5.5 streaming through proxy
• Manual curl: GPT-4.1-mini backwards compat

[praxis-bot-app]

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