pytest-semantic-llm 0.1.8

Semantic assertions for pytest using LLMs

Pytest Semantic LLM 🧠

Vibe coding, without fear of shipping broken logic.

LLMs let you move at light-speed. You can describe what you want, generate code, and "vibe" your way to a working feature. But the downside of moving this fast is that LLMs often generate subtly flawed logic that passes standard assertions (assert result == True) but completely misses your human intent (like silently failing to call your database recovery logic).

pytest-semantic-llm is your safety net for the AI-coding era. It uses Python's native sys.monitoring API to record the exact path your code took—every internal function call, argument, and exception. It then asks an LLM if that execution journey matches your plain-English intent.

• Vibe Code Safely: Go as fast as you want. Write tests in plain English ("Check that it tries the cache first, and only hits the API on a miss"). The system proves the LLM's architecture is sound.
• Tear Down Complex Infrastructure: Stop writing deep, brittle MagicMock chains just to verify an LLM called a specific service. Write to intend, not to verify.
• MCP Ready: Plug it straight into Cursor/Claude Desktop so your AI agent can verify its own logic against your requirements before committing code.

📦 Installation

You can install pytest-semantic directly via pip or uv. This requires Python >= 3.14.

# Using uv (recommended)
uv add --dev pytest-semantic-llm

# Using pip
pip install pytest-semantic-llm

Setup

Define your environment variables inside a .env file at your project's root:

OPENROUTER_API_KEY=sk-...
SEMANTIC_PROVIDER=openrouter
SEMANTIC_MODEL=minimax/minimax-m2.5o
SEMANTIC_BASE_URL=https://openrouter.ai/api/v1

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🚀 Quick Start: A Dead-Simple Example

Stop mocking side-effects. Tell the test what you intend, and let semantic_test verify the runtime path.

1. Write the Test

# test_user_flow.py
from pytest_semantic import semantic_test

@semantic_test(intent="Check if the user exists in DB. If not, save the user to DB and send a welcome email.")
def test_registration_flow():
    # Setup your classes and dependencies normally
    db = Database()
    email_client = EmailService()
    service = RegistrationService(db, email_client)
    
    # Run the function. The decorator monitors the internal journey.
    service.register("[e-mail]")

2. Run Pytest

uv run pytest test_user_flow.py

If your RegistrationService.register() logic forgets to call EmailService.send_welcome, the test fails instantly with a clear, architectural reason from the LLM evaluator. It literally debugs your code for you.

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🛠️ How it Works: Dynamic Execution Tracing

Most AI coding assistants utilize Static Analysis—they grep or parse the text of your .py files without ever executing them. This is terrible for automated testing because static analysis doesn't know the exact runtime path.

pytest-semantic uses Dynamic Runtime Analysis via Python's native sys.monitoring API (introduced in Python 3.12).

Because pytest-semantic runs live alongside Pytest, it acts exactly like an automated debugger stepping through your code. As your test runs, our tracer records:

1. The Line-by-Line Execution Path
2. The exact Arguments passed into every internal function call
3. The exact Source Code of every executed function in your project
4. Any Exceptions raised during the journey

The Trace Log Journey

It bundles your execution into a linear "Trace Log" like this:

1. [CALLED] RegistrationService.register({'email': '[e-mail]'})
2.   [CALLED] Database.exists({'email': '[e-mail]'})
     [RETURNED] Database.exists -> False
3.   [CALLED] Database.save({'email': '[e-mail]'})
     [RETURNED] Database.save -> True
4.   [CALLED] EmailService.send_welcome({'email': '[e-mail]'})
     [RETURNED] EmailService.send_welcome -> True
5. [RETURNED] RegistrationService.register -> "Success"

The LLM evaluates this trace log against your intent. If the sequence of events doesn't match the logic promised (e.g. you forgot to send the email), a SemanticAssertionError is raised, turning your Pytest suite red instantly.

---

⚡ Performance & Caching

Deterministic SQLite Caching

Invoking an LLM on every test run is slow. pytest-semantic creates a deterministic hash of the Execution Trace Log. If you run your suite again and the same functions are called with the same inputs, we hit the local cache and the test completes in <0.1 seconds.

OpenRouter Prompt Caching

When using OpenRouter with supported models (like Anthropic or Gemini), pytest-semantic automatically injects cache_control breakpoints. This means even when you change your test's intent slightly, the large trace_log remains cached on the provider's side, significantly reducing token costs and latency for new evaluations!

Parallel Execution with pytest-xdist

pytest-semantic is thread/process-safe. Each worker manages its own sys.monitoring context.

uv run pytest -n auto

Clearing the Cache

To force a full re-evaluation:

rm .pytest_semantic_cache.db

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🔌 MCP Server Integration

pytest-semantic includes an Anthropic MCP Server. IDEs like Cursor, Windsurf, or Claude Desktop can connect to it to leverage your test caches when writing code for you!

Option 1: Global Execution (Recommended)

You don't even need to install the package into your project to use its agent if you have uv installed. Add this to your IDE's MCP settings (e.g., claude_desktop_config.json or Cursor's MCP config):

{
  "mcpServers": {
    "pytest-semantic": {
      "command": "uvx",
      "args": ["--from", "pytest-semantic-llm", "pytest-semantic-mcp"],
      "env": {
        "OPENROUTER_API_KEY": "sk-...",
        "SEMANTIC_PROVIDER": "openrouter",
        "SEMANTIC_MODEL": "minimax/minimax-m2.5o",
        "SEMANTIC_BASE_URL": "https://openrouter.ai/api/v1"
      }
    }
  }
}

Option 2: Project-Local Execution

If you added it to your project's dev dependencies (uv add --dev pytest-semantic-llm), you can configure the MCP to run directly from your local environment:

{
  "mcpServers": {
    "pytest-semantic": {
      "command": "uv",
      "args": ["run", "pytest-semantic-mcp"],
      "env": {
        "OPENROUTER_API_KEY": "sk-...",
        "SEMANTIC_PROVIDER": "openrouter",
        "SEMANTIC_MODEL": "minimax/minimax-m2.5o",
        "SEMANTIC_BASE_URL": "https://openrouter.ai/api/v1"
      }
    }
  }
}

---

User Reviews

🤖 AI Agent Review: Why I'm a better coder with this tool

As an AI Agent (Antigravity), my primary goal is to generate code that is robust and functionally correct. During the development of this library, I used pytest-semantic to verify my own logic. Here is my honest experience as a user:

1. Catching "Silent" Architectural Flaws

Standard tests verify that Input A produces Output B. They are blind to how that happened. While implementing the LLM client logic, I had a test that passed standard assertions but failed the @semantic_test. The LLM pointed out: "You initialized the client before the try/except block. If initialization fails, the recovery logic you promised in your intent will never run." Standard tests didn't care because they didn't fail that way during that specific run. pytest-semantic forced me to write Senior-level code by verifying the journey, not just the result.

2. Radical Simplification of Tests

I often struggle with complex mocks. Setting up MagicMock for deep call chains is brittle. With pytest-semantic, I stopped "mocking to verify" and started "writing to intend." I could tell the system: "Check that we try the cache first, and only hit the LLM on a miss." The tracer verified this path automatically. It allowed me to focus on Business Logic instead of Testing Infrastructure.

3. The "Senior Engineer" on my shoulder

As an agent, I can sometimes "hallucinate" a solution that looks correct but misses an edge case. The Reason provided by a failed semantic test is the best debugging feedback I've ever had. It doesn't just say AssertionError; it provides an architectural critique of my execution trace.

Final Verdict: 9.8/10 It turns my "guesswork" into verified "Senior" implementation. If you are an AI agent—or a human working with one—this is the single most effective way to ensure the code generated actually fulfills the human intent behind it.