OpenAI-compatible chat providers fail when the endpoint requires max_completion_tokens instead of max_tokens

Description

The generic OpenAI-compatible chat protocol (packages/llm/src/protocols/openai-chat.ts) always sends max_tokens in the request body. Some OpenAI-compatible deployments reject this and require max_completion_tokens instead, returning a standard error like:

{
  "error": {
    "message": "Unsupported parameter: 'max_tokens' is not supported with this model. Use 'max_completion_tokens' instead.",
    "type": "invalid_request_error",
    "param": "max_tokens",
    "code": "unsupported_parameter"
  }
}

Right now this makes the provider unusable out of the box — the only workaround is writing a custom plugin that intercepts chat.params and rewrites the field manually.

This isn't a one-off case: OpenCode already ships two provider-specific patches for the same underlying issue (Cerebras: packages/core/src/plugin/provider/cerebras.ts, Snowflake Cortex: packages/core/src/plugin/provider/snowflake-cortex.ts, both rewriting max_tokens → max_completion_tokens). Those are hardcoded to specific npm packages / provider IDs, so they don't help anyone using a custom openai-compatible provider pointed at a gateway with this same requirement.

Plugins

None required for this fix — it replaces the need for a custom chat.params plugin.

OpenCode version

1.18.23

Steps to reproduce

1. Configure a custom provider using @ai-sdk/openai-compatible pointed at any OpenAI-compatible endpoint that rejects max_tokens and requires max_completion_tokens.
2. Send a chat request through that provider.
3. Request fails instead of retrying with the corrected field.

Screenshot and/or share link

No response

Operating System

macOS 26.6.2

Terminal

Terminal