{"slug": "why-100-line-coverage-is-lying-to-you-in-ai-generated-code-and-how-we-catch-it", "title": "Why 100% Line Coverage is Lying to You in AI-Generated Code (And How We Catch It)", "summary": "A developer has open-sourced DeployProof, a Python pre-push verification tool that uses diff-scoped AST mutation testing to catch hollow test suites, hallucinated dependencies, and security traps in AI-generated code. The tool reduces verification time from minutes to seconds by targeting only changed lines, and it flags issues like unasserted logic and hardcoded secrets before deployment.", "body_md": "If you have spent the last few months building projects with AI coding assistants (Antigravity, Claude Code, Cursor, Copilot), you have likely experienced this specific frustration:\n\nYou prompt an agent to build a feature or fix a bug. The agent writes tests. You run `pytest`\n\n, and all green checkmarks appear with **100% line coverage**. You feel confident and push to production — only to discover after deployment that the tests were completely hollow and missed critical edge-case logic.\n\nLine coverage measures whether a line of code was **executed**, not whether its logic was actually **asserted**.\n\nTo solve this, I built and open-sourced **DeployProof** — a deterministic pre-push verification tool for Python that catches hollow test suites, hallucinated dependencies, and security traps in seconds before code leaves your local machine.\n\nTo illustrate the problem clearly, consider this simple discount calculator with a 50% threshold cap:\n\n``` php\n# calculator.py\ndef calculate_discount(price: float, rate: float) -> float:\n    if rate > 0.5:\n        return price * 0.5\n    return price * (1.0 - rate)\n```\n\nWhen asked to write unit tests, an LLM might generate this:\n\n``` python\n# test_calculator.py\nfrom calculator import calculate_discount\n\ndef test_calculate_discount_standard():\n    assert calculate_discount(100.0, 0.2) == 80.0\n```\n\nThis single test hits every branch of the standard discount and yields **100% line coverage**.\n\nHowever, if you mutate the logic:\n\n`rate > 0.5`\n\nto `rate > 1.5`\n\n`return price * 0.5`\n\nto `return price * 1.5`\n\n`*`\n\nto `/`\n\n**The test suite still passes 100% green.** The test never asserted the threshold cap or boundary conditions.\n\nTraditional mutation testing tools (like `mutmut`\n\nor `cosmic-ray`\n\n) are powerful, but they typically run against the entire codebase. On a project with hundreds of tests, running a full mutation suite can take 5 to 20 minutes — far too slow to run on every `git commit`\n\nor `pre-push`\n\nhook.\n\n**DeployProof solves this with Diff-Scoped AST Mutation:**\n\nInstead of mutating the entire repository, DeployProof inspects your active `git diff`\n\n(or uncommitted session files) and targets AST mutations strictly to the lines you just wrote or modified.\n\nThis drops verification time from minutes down to **2 to 4 seconds**.\n\n``` bash\n$ deployproof check\n\nDeployProof - LOCAL PRE-CHECK\n====================================================================\nTarget Scope (1 file evaluated):\n  * calculator.py\n\nLocal Pre-Check Mutation Verification:\n  Score:  57.1% (4/7 mutants killed)\n  Status: FAILED (score 57.1% below 80.0%) (threshold: 80.0%)\n  Time:   2.27s\n\nSurviving Mutants (3 unverified changes):\n  [1] calculator.py:2\n      Mutation: Replace numeric constant '0.5' with '1.5'\n      Original: if rate > 0.5:\n      Mutated:  if rate > 1.5:\n\n  [2] calculator.py:3\n      Mutation: Replace numeric constant '0.5' with '1.5'\n      Original: return price * 0.5\n      Mutated:  return price * 1.5\n\n  [3] calculator.py:3\n      Mutation: Replace binary operator '*' with '/'\n      Original: return price * 0.5\n      Mutated:  return price / 0.5\n====================================================================\nPre-check FAILED: Score 57.1% is below threshold 80.0% (3 surviving mutants).\n```\n\nOnce you add tests for the threshold cap (`rate = 0.8`\n\n) and exact boundary (`rate = 0.5`\n\n), all mutants are killed and the pre-push gate passes at **100.0%**.\n\nBeyond hollow tests, AI codebases frequently introduce adjacent failure modes. DeployProof runs 5 additional static verification passes against your active diff:\n\n`except Exception: pass`\n\nblocks and dead code generated to silence errors.`@patch`\n\nand `unittest.mock`\n\nusage that masks broken business logic.`.env`\n\nsecrets and hardcoded API keys (OpenAI, Anthropic, AWS, Stripe).DeployProof is free, open source (MIT), and installs via pip:\n\n```\npip install deployproof\n```\n\nInitialize it in your repository (creates `.deployproof.json`\n\nand sets up the `.git/hooks/pre-push`\n\ngate to block pushes when checks fail):\n\n```\ndeployproof init\n```\n\nRun on-demand verification anytime:\n\n```\ndeployproof check\n```\n\nFor CI/CD pipelines (GitHub Actions, GitLab CI), it provides structured JSON output:\n\n```\ndeployproof check --json\n```\n\nI built DeployProof as an independent solo developer after repeatedly hitting subtle AI test regressions across my own projects.\n\nIf you are using AI coding agents in your daily workflow, I would love for you to try it out, file issues, star the repository, or contribute:\n\n*What subtle failure modes or hollow test patterns have you noticed in your AI coding workflows? Let me know in the comments below!*", "url": "https://wpnews.pro/news/why-100-line-coverage-is-lying-to-you-in-ai-generated-code-and-how-we-catch-it", "canonical_source": "https://dev.to/svspraveen/why-100-line-coverage-is-lying-to-you-in-ai-generated-code-and-how-we-catch-it-4dka", "published_at": "2026-08-29 14:15:45+00:00", "updated_at": "2026-08-29 14:50:12.956670+00:00", "lang": "en", "topics": ["developer-tools", "ai-tools", "ai-safety"], "entities": ["DeployProof", "Antigravity", "Claude Code", "Cursor", "Copilot", "OpenAI", "Anthropic", "AWS"], "alternates": {"html": "https://wpnews.pro/news/why-100-line-coverage-is-lying-to-you-in-ai-generated-code-and-how-we-catch-it", "markdown": "https://wpnews.pro/news/why-100-line-coverage-is-lying-to-you-in-ai-generated-code-and-how-we-catch-it.md", "text": "https://wpnews.pro/news/why-100-line-coverage-is-lying-to-you-in-ai-generated-code-and-how-we-catch-it.txt", "jsonld": "https://wpnews.pro/news/why-100-line-coverage-is-lying-to-you-in-ai-generated-code-and-how-we-catch-it.jsonld"}}