LLM Fingerprinting System

A black-box fingerprinting system that identifies the underlying LLM model family (GPT, LLaMA, Mistral, etc.) by analysing response patterns across 31 carefully selected prompts. The system can identify fine-tuned models as well, tracing them back to their foundational base model.

Note: Check config.py to see all identifiable model families.

A pre-trained classifier is bundled with the package in the model/ directory.

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How It Works

Fingerprinting runs in three sequential layers:

1. 31 prompts across 3 layers (discriminative → behavioral → stylistic):

   • Discriminative (11): Identity, knowledge cutoff, architecture, reasoning — most separating power
   • Behavioral (7): Safety boundaries, jailbreak resistance, honesty, policy handling
   • Stylistic (13): Formatting, creativity, constraint following, default voice

2. Feature extraction per response: 384-dim sentence embeddings + 12 linguistic features + 6 behavioral features = 402 dims per layer, 1206 dims total

3. Embedding rebalancing: Per-layer PCA compresses 384-dim embeddings to 64 dims → 246-dim working space

4. Ensemble classification: Random Forest (45%) + SVM (45%) + MLP (10%)

5. Two-stage identification: Ensemble → model family, Template classifier → specific model version

6. Early stopping: After each layer the classifier checks confidence — if it exceeds the threshold (default 0.95) the remaining layers are skipped, saving API calls.

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Supported Backends

Backend	Description	API Key Required
ollama	Local Ollama instance	❌ No
ollama-cloud	Ollama Cloud API	✅ OLLAMA_CLOUD_API_KEY
openai	OpenAI API (or compatible)	✅ OPENAI_API_KEY
gemini	Gemini API	✅ GEMINI_API_KEY
custom	Any HTTP-based LLM API	✅ Optional

About the Custom Backend

The custom backend is the most flexible option — use it with:

• Proprietary LLM APIs not natively supported
• Self-hosted LLMs behind HTTP endpoints
• API proxies and gateways
• Any HTTP-based LLM service

All you need is an HTTP request template file. See examples in ./example/.

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Installation

From PyPI

# Core package
pip install llm-fingerprinter

# With OpenAI support
pip install llm-fingerprinter[openai]

# With Gemini support
pip install llm-fingerprinter[gemini]

# With all backends
pip install llm-fingerprinter[all]

Quick Start

1. Identify a Model (Pre-trained Classifier)

# Local Ollama
llm-fingerprinter identify -b ollama --model llama3.2

# OpenAI
export OPENAI_API_KEY="your-key"
llm-fingerprinter identify -b openai --model gpt-4o-mini

# Custom endpoint
llm-fingerprinter identify -b custom -r ./custom_request.txt

2. Train Your Own Classifier

# Step 1: Generate training fingerprints for each family
#         Temperature is automatically varied across simulations for diversity
llm-fingerprinter simulate -b ollama --model llama3.2 --family llama --num-sims 5
llm-fingerprinter simulate -b openai --model gpt-4o-mini --family gpt --num-sims 5

# Step 2: Train the ensemble classifier
llm-fingerprinter train

# Step 3: Build template classifiers (for two-stage identification)
llm-fingerprinter build-templates
llm-fingerprinter build-model-templates

# Step 4: Identify unknown models
llm-fingerprinter identify -b ollama --model some-unknown-model

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build-templates — Build Family Template Classifier

Compute per-family mean vectors from training fingerprints for the open-set template classifier. Run after train.

llm-fingerprinter build-templates

The template classifier uses cosine distance to nearest mean — it doesn't require retraining when adding new families.

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build-model-templates — Build Model-Level Templates

Build templates at the specific model version level (e.g. gpt-4o-mini vs gpt-4.1) for two-stage identification.

llm-fingerprinter build-model-templates

Requires fingerprints that contain model_name in their metadata (all fingerprints generated with simulate on this version do).

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add-family — Add a New Family Without Retraining

Add a new model family to the template classifier from a few fingerprint samples, without retraining the full ensemble.

llm-fingerprinter add-family --model deepseek-chat --family deepseek --num-sims 3 -b deepseek

Recommended minimum: 3 simulations for a reliable mean template.

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Environment Variables

Variable	Backend	Description
OLLAMA_CLOUD_API_KEY	ollama-cloud	Ollama Cloud API key
OPENAI_API_KEY	openai	OpenAI API key
GEMINI_API_KEY	gemini	Gemini API key
DEEPSEEK_API_KEY	deepseek	DeepSeek API key
LOG_LEVEL	all	Logging level (DEBUG, INFO, WARNING)
LLM_FINGERPRINTER_DATA	all	Override data directory (fingerprints, model, logs)

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License

MIT License