Benchmark models by building one feature in your repo

Same contract, isolated envs: 47 hidden tests, 4 contract mutations and a binary check. Opus killed 4/4 mutants at 100× the cost.

In short

What it does: Same contract, isolated envs: 47 hidden tests, 4 contract mutations and a binary check. Opus killed 4/4 mutants at 100× the cost.

Agents: Claude, Codex · Category: Coding

Steps in brief:

  1. Pick a real mid-size slice of your project and write the contract.
  2. Have each model build it in isolated environments (Claude Code and Codex).
  3. Before looking at any build, write hidden tests from the contract alone (47 here).

+3 more steps in the copyable prompt.

Who made it: Damian Pala (X post, 10 Sep 2026) · source

Last updated · based on 336 use cases on agentsetups, each credited to its author.

by Damian Pala10 Sep 2026

Prompt for your agent

Goal: Pick the right model for my real work, not public benchmarks.
Setup: agents used in source: Claude, Codex; tools/MCPs stated: mutation testing.
Steps:
1. Pick a real mid-size slice of your project and write the contract.
2. Have each model build it in isolated environments (Claude Code and Codex).
3. Before looking at any build, write hidden tests from the contract alone (47 here).
4. Mutate the contract (4 mutations) and run each model’s own tests against them.
5. Check behaviour on the real binary by hand; compare cost and wall time.
6. Review the contract before dispatching: that’s where the expensive model earned its keep.
Rules: ask me before anything irreversible (sending, posting, paying, deleting); don’t invent data, results or tools I haven’t given you; tell me what you couldn’t verify.
Based on: Damian Pala — https://x.com/iamdamianpala/status/2098086060081221977

Steps are from the source. Includes a “Based on” credit line.

I tried this

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Agents, models and tools only what the source states

Agents
Claude Codex
Models
GPT-5.6 Luna DeepSeek V4.1 Flash Claude Opus 5
Tools
  • mutation testing

Sources & credit