Feature parity with cosmic-ray and mutmut

[sakost]

Summary

fest already implements many core mutation testing features, but cosmic-ray and mutmut offer several capabilities that fest currently lacks. This issue tracks the gap analysis and implementation of missing features.

fest Current State (v0.1.1)

Mutation Operators (17 built-in)

arithmetic_op, comparison_op, boolean_op, return_value, negate_condition, remove_decorator, constant_replace, exception_swallow, break_continue, unary_op, zero_iteration_loop, augmented_assign, statement_deletion, bitwise_op, remove_super_call, variable_replace, variable_insert

Infrastructure

• Coverage-guided mutation (skip uncovered lines)
• Persistent pytest worker pool (Unix sockets)
• Session management (SQLite, stop/resume, incremental)
• Per-file configuration with glob-based overrides
• Operator and path filtering
• Pragma support (`# pragma: no mutate`)
• Seed-based deterministic mutations
• Text/JSON/HTML reports
• Custom text-pattern mutators (TOML config)

---

Missing Features from cosmic-ray

1. Distributed execution

• cosmic-ray supports "distributors" — plugins that control where/how tests run (local parallel, message bus, cloud)
• Celery-based distribution for running mutations across multiple machines
• HTTP-based worker distribution
• fest status: Only local parallel execution (rayon thread pool)

2. Operator provider plugin system

• cosmic-ray uses stevedore-based plugin discovery for operator providers
• Third-party packages can register new operators via entry points
• fest status: Config types exist for python and dylib plugins but not yet wired into the registry

3. Interceptors

• cosmic-ray has "interceptors" — hooks that run before/after each mutation test
• Can be used to skip mutations, modify environments, etc.
• fest status: No equivalent

4. `init` / `exec` split workflow

• cosmic-ray separates mutation generation (`init`) from execution (`exec`)
• Allows inspecting/filtering mutants before running tests
• fest status: Single `run` command does everything; session DB partially enables this

5. Operator listing/inspection

• cosmic-ray has `cosmic-ray operators` command to list available operators
• fest status: No equivalent CLI command

---

Missing Features from mutmut

1. `apply` command

• mutmut allows writing a specific mutant to disk (`mutmut apply `)
• Useful for debugging why a mutant survived
• fest status: Not implemented

2. Interactive browse TUI

• mutmut has `mutmut browse` — interactive terminal UI for reviewing results
• Press `f` to retest a function, `m` to retest a module
• fest status: No post-run interactive mode

3. Type checker integration

• mutmut can filter invalid mutants using mypy/pyrefly (e.g., `x: str = 'foo'` → `x: str = None` is caught by type checker)
• Reduces false positive survived mutants
• fest status: Not implemented

4. Stack depth limiting

• mutmut can limit test-to-function stack depth
• Lower values = faster runs + more localized tests, but more false survivors
• fest status: Not implemented

5. Test selection per mutant

• mutmut identifies which tests are relevant per mutant and only runs those
• Major speed improvement for large test suites
• fest status: Uses coverage to skip uncovered mutants, but runs full test suite for covered ones

6. `show` command

• mutmut can display surviving mutants grouped by function/module
• fest status: Reports show flat lists

---

Suggested Priority

Feature	Priority	Complexity
Test selection per mutant	High	High
`apply` command	High	Low
Operator listing CLI	High	Low
Type checker integration	Medium	Medium
Interactive browse mode	Medium	High
Distributed execution	Low	Very High
Interceptors/hooks	Low	Medium
`init`/`exec` split	Low	Medium
Stack depth limiting	Low	Medium

References

• cosmic-ray documentation
• cosmic-ray GitHub
• mutmut GitHub
• mutmut documentation
• Existing design doc: `docs/plans/2026-03-05-cosmic-ray-parity-design.md`