For each surviving mutant, parse the mutmut sandbox trampoline file and
render the mutated function as it appears in the source — with the
differing lines wrapped in `# MUTANT START` / `# MUTANT END` comments,
matching the format from Meta's ACH paper (arXiv 2501.12862, Table 1).
Renames the function header back to its original name so the agent sees
the function as it would appear in the file. Falls back to the unified
diff if the trampoline lookup fails.
Handles replace, insert, and delete diff ops; uses difflib's
SequenceMatcher to find the differing line ranges.
The unified diff is preserved in a collapsible <details> block as
secondary context.
mutmut's mutant-name prefix is x_ (single underscore), so a function
named _foo produces mutants x__foo__mutmut_N. The previous regex
\.x__(.+)__mutmut_ ate the function's leading underscore as part of
the prefix. Changed to \.x_(.+)__mutmut_ so leading underscores are
preserved in the captured function name; verified for normal, leading-
underscore, and dunder-method names.
Replaces the inline bash markdown generation with a Python script that:
- Groups survivors by function (one section per function, function body
shown once per section, surviving mutants nested as subsections)
- Embeds each enclosing function's source via Python AST (so the agent
has full context, not just a 3-line `mutmut show` diff)
- Inlines the existing test file(s) listed in [tool.mutmut].tests_dir
- Writes an ACH-style task description at the bottom following the
prompt template from arXiv 2501.12862
Output goes to mutation-report.md (artifact) and the head of the file
is appended to $GITHUB_STEP_SUMMARY for at-a-glance visibility.
