Subagent tool calls silently abort with MessageAbortedError (zero response tokens) on custom Anthropic provider — 1.18.4+ #39456

Description

Environment

• macOS: 15.7.7
• Node.js: v24.13.1
• opencode: 1.18.4, 1.18.8, 1.18.9 (occasional success on 1.18.3; 100% reproducible from 1.18.4)
• Custom provider: Anthropic protocol, model deepseek-v4-pro

Problem

When the main agent spawns a subagent via the task tool, the subagent receives its prompt normally and decides to call a tool (e.g. read, glob, grep). At that point the tool call never completes — it hangs with "Tool execution aborted" (MessageAbortedError) until the subagent times out. The subagent produces no output and the main agent receives nothing back, silently degrading the task.

Main-agent tool calls in the same session work fine. Only subagent tool execution is affected.

Evidence (from opencode.db)

• message table: All aborted messages show {input: 0, output: 0} tokens — opencode dispatched the API request but never received any response tokens.
• part table tool stats: Main agent — 6957 completed. Subagent — 2073 completed / 65 errors, of which 47 are "Tool execution aborted" (5–53 minute hangs).
• Intermittent: Failures and successes interleave within the same parent session. Retrying the same subagent task often succeeds immediately.
• Skewed distribution: 79% of failures came from one parent session (768 messages, 631 tool calls, ~165 hours alive). However, another parent with 5000+ messages and 4000+ tool calls had zero subagent failures — session size alone is not sufficient.
• Concurrent correlation: Multiple failure timestamps coincide with parent task tool errors, suggesting parent session state propagates to child subagents.

Hypothesis

Client-side race condition or connection issue in the subagent subsystem. The request is dispatched but opencode never receives any response tokens. Long-lived parent sessions and heavy tool usage increase the probability, but the intermittent nature suggests a timing issue rather than a deterministic threshold.

Plugins

No response

OpenCode version

1.18.9

Steps to reproduce

No response

Screenshot and/or share link

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Operating System

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Terminal

No response