OpenCode export drops the results of parallel tool calls written one call per message #98

What happened?

When parallel tool calls are recorded one call per assistant message, with each result in its own later user message, converting to opencode drops the results: every call is written with an empty output.

The OpenCode writer folds a ToolResult into its call's state.output only when the result sits in the message right after the call (src/harness/opencode.rs, assistant_part, messages.get(*idx + 1)). With calls A and B written as separate assistant messages followed by result A and result B, call A's next message is call B, and call B's next message holds only result A, so neither call finds its result.

Expected: each call keeps its own result, wherever it sits in the following user messages of the same turn.

How to reproduce

run.json (Simple document):

{
  "messages": [
    { "role": "user", "content": "read both" },
    { "role": "assistant", "content": [{ "type": "tool_use", "id": "call_a", "name": "Read", "input": { "file_path": "/a" } }] },
    { "role": "assistant", "content": [{ "type": "tool_use", "id": "call_b", "name": "Read", "input": { "file_path": "/b" } }] },
    { "role": "user", "content": [{ "type": "tool_result", "tool_use_id": "call_a", "content": "content a" }] },
    { "role": "user", "content": [{ "type": "tool_result", "tool_use_id": "call_b", "content": "content b" }] },
    { "role": "assistant", "content": "done" }
  ]
}

convert(run, "simple", "opencode") (or txcript continue ./run.json --with opencode) writes both tool parts with "output": "".

The same conversation as a library test, on main at c60e686:

assertion `left == right` failed
  left: [("call_a", ""), ("call_b", "")]
 right: [("call_a", "content a"), ("call_b", "content b")]

Versions

• txcript: main at c60e686 (also 0.14.4).
• Agents and their versions: target OpenCode (any version; the writer is shared by the 2.0 export and the 2.1 document of fix(opencode): read and write OpenCode 2.1 sessions #97).
• Operating system: Windows 11.

I have a fix with this test ready: look past the next message, through the following user messages of the turn, for the call's result.