Output stream corruption when firing Fable subagents: unrelated buffer text spliced into a file-write status line

Summary

After firing Fable (claude-fable-5) subagent(s), a terminal status line was corrupted: a legitimate file-write status line had unrelated buffer text concatenated onto its tail. The injected text resembles Chromium task-manager / agent-facing content and has no legitimate reason to appear in a Claude Code session — suggesting a cross-buffer / concurrent-subagent output-stream interleaving bug.

Environment

• Claude Code on Windows 11, git-bash (MSYS) terminal
• Main-loop model: Opus 4.8; subagents: Fable 5 (claude-fable-5)
• Suspected trigger: firing Fable subagent(s) — concurrent output multiplexing

What I saw

The real status line was a scratchpad file write:

325171 bytes written to file <TMP>\scratchpad\tour-today.png

But it rendered with foreign text spliced directly onto the end of the filename (note tour-today.png + gy a primary task provider…):

…\scratchpad\tour-today.pngy a primary task provider. If a "Renderer" fallback
task is shown, it is a bug. If you have repro steps, please file a new bug and
tag it as a dependency of crbug.com/40528867.
  ← for agents

That trailing block ("a primary task provider", "Renderer" fallback task, crbug.com/40528867, ← for agents) is not Claude Code output — it looks like it bled in from another buffer/process while Fable subagents were being spawned.

Impact

• Garbled, misleading terminal output
• The actual file path is truncated/joined (tour-today.png → tour-today.pngy a primary task provider…)
• Unrelated third-party content injected into a status line — confusing for both humans and agents reading the transcript

Repro

Partial. Observed while firing Fable subagents; exact deterministic steps not yet isolated. Appears related to concurrent subagent output being multiplexed into the main stream.

Notes

• A local file path in the original output was redacted (<TMP>) as it contained a private project name.