OpenAI provider fails: gpt-5-nano missing from models.dev registry causes ProviderModelNotFoundError #6493

Description

When using the OpenCode SDK with OpenAI provider, sessions fail because the internal title agent tries to use gpt-5-nano, which is not in OpenCode's model registry (models.dev), causing ProviderModelNotFoundError.

Irony: gpt-5-nano actually exists and works when called directly via OpenAI API, but OpenCode's internal model lookup fails.

Steps to Reproduce

import { createOpencode } from '@opencode-ai/sdk';

const { client, server } = await createOpencode({ port: 4096 });

// Set up OpenAI auth
await client.auth.set({
  path: { id: 'openai' },
  body: { type: 'api', key: process.env.OPENAI_API_KEY },
});

// Create session and send prompt
const session = await client.session.create({ query: { directory: '/tmp/test' } });
await client.session.prompt({
  path: { id: session.data.id },
  query: { directory: '/tmp/test' },
  body: {
    model: { providerID: 'openai', modelID: 'gpt-5.2-2025-12-11' },
    parts: [{ type: 'text', text: 'Say hello' }],
  },
});
// Session is cancelled immediately

Logs

INFO  service=session.prompt sessionID=ses_xxx cancel
INFO  service=llm providerID=openai modelID=gpt-5-nano sessionID=ses_xxx small=true agent=title stream
ERROR service=acp-command reason=ProviderModelNotFoundError Unhandled rejection
ERROR service=default e=ProviderModelNotFoundError rejection

Verification that gpt-5-nano exists

// Direct OpenAI API call works fine:
const response = await fetch('https://api.openai.com/v1/chat/completions', {
  method: 'POST',
  headers: { 'Authorization': `Bearer ${apiKey}`, 'Content-Type': 'application/json' },
  body: JSON.stringify({
    model: 'gpt-5-nano',
    messages: [{ role: 'user', content: 'Hi' }],
    max_completion_tokens: 5
  })
});
// ✅ Works - returns response from gpt-5-nano-2025-08-07

Root Cause

1. OpenCode's internal title agent hardcodes gpt-5-nano as the small model for OpenAI
2. OpenCode's model registry (from models.dev/api.json) doesn't include gpt-5-nano
3. This causes ProviderModelNotFoundError even though the model exists at OpenAI

Workaround

Using Anthropic provider works correctly - no issues with internal agents.

Suggested Fix

Either:

1. Add gpt-5-nano to the models.dev registry
2. Make the title/small model configurable in opencode.json
3. Skip model validation for internal agents if the provider supports the model

Environment

• OpenCode SDK: @opencode-ai/sdk (latest)
• OpenCode version: 1.0.218
• Node.js: v22.21.1
• OS: Linux (WSL2)

Related Issues

• ProviderModelNotFoundError #916 - ProviderModelNotFoundError (models.dev sync)
• @ai-sdk/openai-compatible max_tokens error for GPT 5.x #5421 - max_tokens vs max_completion_tokens for GPT 5.x