Panic: 'byte index 80 is not a char boundary; it is inside ''' when
tool output contained multi-byte UTF-8 characters (smart quotes from
web search results).
Fixed 4 unsafe byte-index slices:
- thread.rs:281: message preview &content[..80] → chars().take(80)
- loop_engine.rs:556: tool output &str[..4000] → chars().take(4000)
- loop_engine.rs:579: output tail &str[len-8000..] → chars().skip()
- scripting.rs:82: stdout tail &str[len-N..] → chars().skip()
All now use .chars().take() or .chars().skip() which respect character
boundaries. Follows CLAUDE.md rule: "Never use byte-index slicing on
user-supplied or external strings."
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>
Monty creates a fresh runtime per code step, so variables are lost
between steps. This caused the model to re-paste tool results from
system messages, wasting tokens.
Fix: maintain a `persisted_state` JSON dict in the ExecutionLoop that
accumulates across steps:
- Tool results stored by tool name: state["web_search"] = {results...}
- Return values stored: state["last_return"], state["step_0_return"]
- Injected as a `state` Python variable in each new MontyRun
Now the model can do:
Step 1: results = web_search(query="...") # tool result saved in state
Step 2: data = state["web_search"] # access previous result
summary = llm_query("summarize", str(data))
FINAL(summary)
System prompt updated to document the `state` variable.
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>
Cross-referenced our implementation against the official RLM (alexzhang13/rlm),
fast-rlm (avbiswas/fast-rlm), and Prime Intellect's verifiers implementation.
Key enhancements:
- FINAL(answer) / FINAL_VAR(name): explicit termination pattern matching
all three reference implementations. Code can signal completion at any
point, not just via return value.
- llm_query_batched(prompts): parallel recursive sub-calls via tokio::spawn,
matching fast-rlm's asyncio.gather pattern and Prime Intellect's llm_batch.
- Output truncation increased to 8000 chars (from 120), matching Prime
Intellect's 8192 default. Shows [TRUNCATED: last N chars] or [FULL OUTPUT].
- Step 0 orientation preamble: auto-injects context metadata (message count,
total chars, goal, last user message preview) before first code step,
matching fast-rlm's auto-print pattern.
- Error-to-LLM flow: Python parse errors, runtime errors, NameErrors,
OS errors, and async errors now flow back as stdout content instead of
terminating the step, enabling LLM self-correction on next iteration.
Only VM panics (catch_unwind) terminate as EngineError.
74 tests passing, zero clippy warnings.
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>
