Custom primary agents fail silently in web UI #29468

Description

Custom primary agents defined in opencode.json (or .opencode/agents/*.md) appear in the web UI dropdown but cannot process chat messages. Submitting a message produces an error sound with no visible error or console output. Built-in agents (build, plan) work normally.

Tested and confirmed on v1.15.7 through v1.15.10. Even a minimal custom agent with no prompt, no model, and no permissions fails the same way.

OpenCode version

v1.15.10 (also reproduced on v1.15.7, v1.15.8, v1.15.9)

Operating System

Windows 11

Terminal

Windows Terminal (for CLI tests). Issue is in the web UI (opencode web).

Steps to reproduce

1. Create an opencode.json with a custom primary agent:
   {
   "agent": {
   "my-agent": {
   "mode": "primary",
   "description": "Test custom agent"
   }
   }
   }
2. Run opencode web and open the browser
3. Select the custom agent from the dropdown
4. Type a message and submit
5. Expected: agent processes the message and responds
6. Actual: error sound plays, no response, no console errors

Additional bugs found during investigation

• GET /agent?workspace=X crashes with a 500 error
• GET /agent?directory=X filters response to only native:true (built-in) agents, excluding all custom agents
• The SPA calls GET /agent?directory=&workspace=, triggering both issues
• A workaround for the /agent endpoint issues is to strip directory and workspace params at the proxy layer

Notes

• The affected API endpoint (/agent) correctly returns the custom agent with mode:"primary" and native:false
• The client-side filter (mode !== "subagent" && !hidden) correctly includes the agent in the dropdown
• The failure happens server-side during session creation
• Overriding the built-in plan agent with a custom prompt also fails (the built-in prompt is used regardless)
• Custom primary agents work correctly in CLI/TUI mode

Plugins

No response

OpenCode version

No response

Steps to reproduce

No response

Screenshot and/or share link

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Operating System

No response

Terminal

No response