Agent.get() returns undefined without error handling, causing crash #9437

Description

Agent.get() returns undefined when an agent doesn't exist, but callers like createUserMessage() don't handle this case, causing a crash:

TypeError: undefined is not an object (evaluating 'agent.name')
  at createUserMessage (src/session/prompt.ts:831:14)

The issue is in src/agent/agent.ts line 246-248:

export async function get(agent: string) {
  return state().then((x) => x[agent])  // Returns undefined if not found
}

And src/session/prompt.ts line 825, 833:

const agent = await Agent.get(input.agent ?? (await Agent.defaultAgent()))
// ...
agent: agent.name,  // Crashes here when agent is undefined

This can be triggered by any caller passing an invalid agent name - in my case, it was the plannotator plugin defaulting to "build" when I had that agent disabled.

Plugins

@plannotator/[email protected]

OpenCode version

v1.1.25

Steps to reproduce

1. Disable the default build agent in opencode config:

   { "agent": { "build": { "disable": true } } }

2. Install plannotator plugin
3. Use plannotator to create and approve a plan (without changing agent switch setting in plannotator UI)
4. OpenCode crashes with TypeError

Screenshot and/or share link

N/A - crash occurs before UI can render

Operating System

Fedora Linux

Terminal

Ghostty

Suggested Fix

Either:

1. Agent.get() should throw a descriptive error when agent not found
2. Or createUserMessage() should validate and provide a helpful message like: "Agent 'build' not found. Available agents: [list]"

This is a defensive fix - callers shouldn't need to know that Agent.get() can return undefined.

Related

• plannotator issue: Agent switch default of "build" causes OpenCode crash when build agent is disabled backnotprop/plannotator#94 (addresses the root cause on their side)
• Similar TUI-layer crashes: opentui: fatal: undefined is not an object (evaluating 'local.agent.current().name') #9332, opentui: fatal: undefined is not an object (evaluating 'local.agent.current().name') #9393, opentui: fatal: undefined is not an object (evaluating 'local.agent.current().name') #9405 (same error pattern, different code path)