Subagent AI_APICallError is silently swallowed, causing session to hang

[unthingable]

Description

When a subagent encounters an AI_APICallError (e.g., due to an invalid model ID), the error is logged at ERROR level but never surfaced to the user or returned to the parent agent as a tool result. The subagent session silently dies, and the parent session appears stuck/loading indefinitely.

By contrast, when a primary agent hits the same error, it is surfaced clearly in the TUI:

Error: model: claude-sonnet-4-5-latest

Steps to reproduce

1. Configure a custom model alias with an invalid model ID in opencode.jsonc:

"provider": {
  "anthropic": {
    "models": {
      "sonnet": { "id": "claude-sonnet-4-5-INVALID" }
    }
  }
}

2. Define any subagent that uses this model (JSON or markdown):

"agent": {
  "my-agent": {
    "description": "Test agent",
    "mode": "subagent",
    "model": "anthropic/sonnet"
  }
}

3. In a session, have the primary agent invoke the subagent via the task tool.

4. The subagent's LLM call fails with AI_APICallError. The error appears in --print-logs output:

ERROR service=session.processor error=model: claude-sonnet-4-5-INVALID stack="AI_APICallError: model: claude-sonnet-4-5-INVALID\n    at <anonymous> (../../node_modules/.bun/@ai-sdk+provider-utils@3.0.20+.../dist/index.mjs:936:18)..."

5. But the parent session never receives the error — it just hangs waiting for the subagent.

Expected behavior

• The error should be returned to the parent agent as a failed tool result, so it can report the issue or attempt recovery.
• The TUI should surface the subagent error to the user (e.g., in the subagent panel or as a notification).

Actual behavior

• The AI_APICallError is logged but swallowed.
• The subagent session silently dies.
• The parent session hangs indefinitely waiting for a response that will never come.
• The user has no indication of what went wrong without checking --print-logs or log files.

Related issues

• Tasks/Subagents with Codex / OpenAI are frequently getting stuck with no timeout/retry, which then hangs the session forever #11865 — subagents getting stuck with no timeout/retry (different provider, similar symptom)
• TUI: Parent session appears stuck "loading" when subagent is blocked (waiting user input / hanging tool call); lack of visibility and recovery UX #10802 — parent session appears stuck when subagent is blocked (TUI visibility)

OpenCode version

1.1.53

Operating System

macOS (Darwin 24.6.0)