Bug: REST API session model ignored when loading session via ACP #27645

Description

When creating a session via the REST API (POST /session) with a specific model, the model is correctly stored in the database. However, when that session is subsequently loaded (e.g., via POST /session/:id/message or POST /session/:id/prompt_async), the stored model is ignored and replaced with defaultModel().

Root Cause

Two files contribute to this bug:

1. packages/opencode/src/acp/session.ts — load() ignores session data

// Line 46-62
async load(sessionId, cwd, mcpServers, model?) {
  const session = await this.sdk.session.get(...)  // session.model exists here!
  
  const resolvedModel = model  // BUG: ignores session.model, uses only parameter
  // ...
}

The load() method fetches the full session from DB (which includes session.model), but then ignores it and uses only the model parameter.

2. packages/opencode/src/acp/agent.ts — passes defaultModel() to load()

// Around line 599, 684, 739 in loadSession()
const model = await defaultModel(this.config, directory)
await this.sessionManager.load(sessionId, params.cwd, params.mcpServers, model)

loadSession() always resolves the model via defaultModel() and passes it to load(), overriding any model stored in the session.

Impact

This affects any workflow that creates sessions via the REST API with a specific model:

• Third-party MCP servers that create sessions with specific models
• External tools that use the REST API to manage sessions
• Any programmatic session creation where the model should differ from the default

Suggested Fix

In ACPSessionManager.load(), prefer the model stored in the session data over the parameter:

async load(sessionId, cwd, mcpServers, model?) {
  const session = await this.sdk.session.get(...)
  
  // Prefer session-stored model, fall back to parameter
  const sessionModel = session.model
    ? { providerID: session.model.providerID, modelID: session.model.id, variant: session.model.variant }
    : undefined
  const resolvedModel = sessionModel ?? model
  // ...
}

Additionally, in Agent.loadSession(), pass undefined instead of defaultModel() to load(), so the session's own model is used.

Environment

• opencode version: v1.14.51 (latest dev branch as of 2026-05-15, commit f179dcb)
• OS: Linux

Related

• Feature Request: Retain current model and agent selection when switching sessions #4930 (model persistence across sessions)
• Plugins using noReply seem to cause model to switch to agent default  #4475 (model switching with plugins)