v2 (opencode2): max_tokens never sent for @ai-sdk/openai-compatible models, limit.output ignored, thinking turns truncate at provider default #47398

Description

On the v2 CLI (opencode2, @opencode-ai/cli beta) requests to @ai-sdk/openai-compatible models go out without max_tokens, regardless of limit.output in the provider config. Stable opencode 1.18.20 with the same config sends max_tokens: 32000.

Because the field is missing, the upstream applies its own default budget. In my setup (DeepSeek V4 with thinking enabled, behind an OpenAI-compatible proxy) the effective cap was 4096 tokens: long-thinking turns spend the whole budget on reasoning and finish with finish_reason: length and empty content, at exactly 4096 output tokens. Same symptom as #46595 (Bedrock), different route.

Captured outbound body from the beta (sanitized, messages/tools omitted):

{
  "model": "combo-deepseek-v4-flash",
  "stream": true,
  "stream_options": { "include_usage": true },
  "store": false,
  "prompt_cache_key": "ses_…"
}

Same session on opencode/1.18.20:

{ "model": "combo-deepseek-v4-flash", "stream": true, "max_tokens": 32000, … }

Where it goes wrong, as far as I can tell:

• packages/core/src/session/runner/llm.ts builds LLM.request({ model, http, providerOptions, system, messages, tools, toolChoice }) with no generation block.
• packages/llm/src/protocols/openai-chat.ts fromRequest emits max_tokens: generation?.maxTokens, which is undefined, so the key is dropped.
• packages/core/src/session/runner/model.ts (withDefaults) does put limit.output into route.defaults.limits, but nothing projects it into generation.maxTokens. v1 did this via ProviderTransform.maxOutputTokens (min(limit.output, 32000) || 32000).

The Anthropic route (anthropic-messages.ts) is built from the same request, so it is likely affected too, but I only verified the openai-compatible path.

Related: #46595 (same root cause on Bedrock), #29363 (v1 32K cap of limit.output).

Plugins

none

OpenCode version

@opencode-ai/cli 0.0.0-beta-19086 (opencode2). Not reproducible on opencode 1.18.20.

Steps to reproduce

1. Config:

   "provider": {
     "my-proxy": {
       "npm": "@ai-sdk/openai-compatible",
       "options": { "baseURL": "https://…/v1", "apiKey": "…" },
       "models": {
         "combo-deepseek-v4-flash": { "limit": { "context": 1000000, "output": 131072 } }
       }
     }
   }

2. Point the base URL at anything that logs request bodies (a local proxy is enough).
3. Send a prompt from opencode2: the body has no max_tokens. Send the same prompt from opencode 1.18.20: max_tokens: 32000 is present.
4. With a thinking model, long turns end at the provider's default output cap, reasoning only, no text.

Screenshot and/or share link

n/a (request bodies above)

Operating System

macOS 15.7.9

Terminal

n/a (not terminal-specific)