Tool result intermittently dropped as "[Tool result missing due to internal error]" for parallel Bash calls

Summary

When Claude issues multiple Bash tool calls in a single message (parallel tool use), one or more of those calls intermittently returns [Tool result missing due to internal error] instead of the command's real output. The command itself runs to completion — re-running it on its own always returns the correct result.

Environment

• Claude Code 2.1.143
• Windows 11
• Model: Opus 4.7

Steps to reproduce

Not deterministic, but reliably observed several times across a long session when multiple longer-running Bash commands are dispatched together in one turn, e.g.:

• pnpm test:integration and pnpm audit:structural issued as two parallel Bash tool calls
• A third call in the same batch (pnpm test) returned its output normally, while the other two both came back as [Tool result missing due to internal error]
• Re-running each dropped command individually returned the correct output every time

Expected

Each parallel Bash tool call returns its actual stdout / exit status.

Actual

One or more parallel tool results are silently replaced with [Tool result missing due to internal error]. The underlying command succeeded — the result is lost in the harness, not the command.

Impact

Claude has to detect the dropped result and re-run the command, adding latency; if undetected it looks like the session has stalled.

Workaround

Chaining heavy commands into a single sequential Bash call instead of parallel calls largely avoids it.