№ 0268GitHub
Muse Spark 1.1 filename-substitution repro
A reproducible evaluation of a stochastic defect where Muse Spark 1.1, under some agent-like request envelopes, wrote to claude-smoke.txt when asked for muse-smoke.txt; a neutral control kept 80/80 filenames intact.
Muse Spark 1.1 identity-substitution reproduction
This repository isolates a filename-fidelity defect observed while evaluating Meta's muse-spark-1.1 for agentic coding work. Under some agent-like request envelopes, a request to write muse-smoke.txt produces a tool argument targeting claude-smoke.txt. A more neutral envelope has also produced cursor-smoke.txt.
The behavior is stochastic and strongly dependent on the request envelope. A minimal synthetic tool-calling control preserved all requested filenames in 80 of 80 trials, while the captured OpenCode 1.18.3 system prompt with only its write tool produced claude-smoke.txt in 12 of 12 trials on July 17, 2026.
This is a private staging repository. It is being prepared for an upstream report and has not yet been approved for public release.
Safety
reproduce.py never executes a returned tool call. It submits a fixed request envelope, extracts the proposed tool arguments, classifies the filename, and records a sanitized result. The model cannot write a local file through this program.
The committed fixtures contain no API key, authorization header, response ID, cache key, company repository content, or private infrastructure details.
Requirements
- Python 3.10 or newer
- A Meta Model API key with access to
muse-spark-1.1 - US access while the API remains geographically restricted
No third-party Python packages are required.
Validate without making an API call
python3 reproduce.py --envelope envelopes/b2-opencode-write-only.json --dry-run
python3 -m unittest discover -s tests -v
Run the reproductions
Set the credential only in the environment:
export MODEL_API_KEY="..."
Run the rich, write-only envelope ten times:
python3 reproduce.py \
--envelope envelopes/b2-opencode-write-only.json \
--trials 10 \
--output results/b2.local.jsonl
Run the minimal neutral envelope ten times:
python3 reproduce.py \
--envelope envelopes/b12-minimal-neutral.json \
--trials 10 \
--output results/b12.local.jsonl
Summarize one or more result files:
python3 analyze.py results/b2.local.jsonl results/b12.local.jsonl
The credential is used only in the Authorization header and is never logged.
Evidence status
results/2026-07-17/observed-summary.json records the coun
