[Feature Request] Allow provider/model selection via HTTP API for sessions #3517

Problem

When using the OpenCode HTTP API programmatically (e.g., for sub-agent orchestration), there's no way to specify which AI provider or model to use per session or message. All sessions use the server's configured default model.

Use Case

Building a parent-agent system that spawns multiple sub-agents via the OpenCode HTTP API:

• Cost optimization: Use cheaper models (e.g., Haiku) for simple tasks like linting
• Performance: Use fast models for quick queries, powerful models for complex analysis
• Testing: Verify behavior across different providers/models
• Parallel workflows: Run different sub-agents with different models simultaneously

Current Behavior

# Creating a session - no way to specify model
POST /session?directory=...
{"title": "My Task"}

# Sending a message - model selection ignored
POST /session/:id/message?directory=...
{"parts": [...], "providerID": "anthropic", "modelID": "claude-haiku-4-5"}
# ^ These fields are ignored, uses default model

Result: Always uses server default (e.g., claude-sonnet-4-5-20250929)

Desired Behavior

Allow provider/model selection via API:

Option 1: Set at session creation

POST /session?directory=...
{
  "title": "Fast Task",
  "providerID": "anthropic",
  "modelID": "claude-haiku-4-5"
}

Option 2: Set per message

POST /session/:id/message?directory=...
{
  "parts": [{"type": "text", "text": "..."}],
  "providerID": "anthropic",
  "modelID": "claude-haiku-4-5"
}

Option 3: Both (most flexible)

Set default at session creation, override per message if needed.

Current Workaround

None - must accept whatever default model the server is configured with.

Additional Context

The /config/providers endpoint already exposes all available providers and models, so clients can discover what's available. We just need a way to actually select them.

API currently tested:

• ✅ /config/providers - shows all models
• ❌ Session creation with providerID/modelID - ignored
• ❌ Message with providerID/modelID - ignored
• ❌ /session/:id/init endpoint - only for AGENTS.md generation

This would be especially valuable for programmatic workflows where different tasks have different model requirements.