How Resume Matching Algorithms Actually Work (Open Source Breakdown)

ATS systems aren’t magic. They’re algorithms. Here’s exactly how they decide whether your resume gets seen or trashed.

You submit a resume. Somewhere between you clicking “Apply” and a recruiter opening your file, an algorithm makes a decision about you. You’ve probably heard that 75% of resumes are rejected before a human ever reads them. That number traces back to a 2012 vendor claim with no published data, but the algorithm is real, and it decides how your resume is ranked before a recruiter opens it.

Most advice about beating ATS systems amounts to “use the right keywords.” That’s true, but it’s like saying “score more goals” is a soccer strategy. It’s technically correct and completely useless without understanding the mechanism.

We’ll use our own open-source implementation, Resume Matcher, as a reference.

How Systems Actually Score You: The Three Layers

Every modern resume matching system operates on three layers, in order of increasing sophistication.

Layer 1: Keyword Matching

The simplest and oldest approach. The algorithm extracts keywords from the job description, extracts keywords from your resume, and counts the overlap.

This is where most job seekers stop thinking. “If the job says Python, put Python on my resume.” And they’re right, but keyword matching alone is crude. It treats “Python” the programming language and “python” the snake identically. It doesn’t understand that “React.js” and “ReactJS” and “React” are the same thing. It can’t tell that “built scalable microservices” is relevant to a job requiring “distributed systems architecture.”

Pure keyword matching is a filter, not an evaluator. It catches the obvious mismatches (you don’t know Java, the job requires Java) but misses nuanced fits. Our resume keywords guide covers how to find and place the terms this layer looks for.

Layer 2: Semantic Similarity

This is where modern ATS systems get more sophisticated.

Instead of matching exact words, semantic matching converts both the resume and the job description into mathematical representations called embeddings. These are long lists of numbers that capture the meaning of text, not just the words.

In this vector space, “machine learning engineer” and “ML engineer” are close together. “Team leadership” and “managed a cross-functional team of 12” are close together. “REST API development” and “built RESTful web services” are close together, even though they share almost no words.

Resume Matcher takes a different route to the same goal. It doesn’t compute embeddings itself. Instead, it uses a large language model to read the job description the way a recruiter would, pulling out the skills, tools, and requirements that matter, including ones phrased differently from how you’d describe them. It then works those keywords into your resume, rewording your real experience so it lines up with whatever matching the employer’s ATS uses, whether that’s exact keywords or semantic similarity. It doesn’t invent experience you don’t have.

Layer 3: Structured Extraction and Scoring

The most sophisticated layer extracts structured data from both documents and compares them field by field.

From the job description, the algorithm extracts required skills (explicit and implicit), years of experience requirements, education requirements, certifications needed, and seniority level.

From your resume, it extracts listed skills, inferred skills from work experience descriptions, years of experience per skill (calculated from dates), education credentials, and certifications.

Then it scores each dimension and produces an overall match rating. A candidate who matches 8 out of 10 required skills, has the right education, and meets the experience threshold scores higher than someone who matches 5 out of 10 skills, even if both resumes contain the word “Python.”

This is why Resume Matcher asks you to upload your full resume and paste the complete job description, not just a list of keywords. The language model needs the full context to pick out the right keywords and tailor your resume intelligently.

How to Optimize for All Three Layers

Understanding these three layers changes how you optimize.

For Layer 1 (keyword matching): Use the exact terms from the job description. If they say “project management,” don’t write “managed projects.” If they say “Kubernetes,” don’t write “container orchestration” and hope the ATS figures it out. Include both the specific term and the broader concept.

For Layer 2 (semantic matching): Write descriptive bullet points that explain what you did and how, not just one-word skill lists. “Designed and implemented a real-time data pipeline processing 2M events per day using Apache Kafka and Spark” carries far more semantic information than a skills section that just lists “Kafka, Spark.”

For Layer 3 (structured scoring): Make your experience easy to parse. Use standard section headings (Experience, Education, Skills, Certifications). Include dates with your positions. List your actual title. Don’t make the algorithm guess. Structured data that’s easy to extract scores better than creative formatting that looks impressive to humans but confuses parsers.

How Resume Matcher Implements This

Resume Matcher is open source, so you can inspect exactly how the matching works. The architecture is a FastAPI backend with a Next.js frontend, using LiteLLM to support multiple AI providers, including fully local processing with Ollama, so your resume data never leaves your machine.

The matching pipeline works in steps. First, you upload your master resume, a comprehensive document with all your experience, not a tailored version. Then you paste a job description. A large language model extracts the keywords, skills, and requirements from the job description and compares them with your resume to find matches and gaps. Then it generates a tailored version: which bullet points to emphasize, which keywords to work in, and what’s missing.

The key design decision is that Resume Matcher doesn’t just tell you “your match score is 72%.” It tells you why: which specific requirements you’re matching, which you’re missing, and how to close the gap with content you already have in your master resume.

You can run it locally with Ollama (completely private, no data leaves your machine), or connect to cloud providers like OpenAI, Anthropic, Google Gemini, or DeepSeek for more powerful analysis.

The Open Source Advantage

Commercial ATS checkers like JobScan charge $50/month and run your resume through a proprietary black box. You get a score but no insight into how it’s calculated.

Resume Matcher is free, open source (Apache 2.0), and transparent. The matching logic is in the codebase. You can read it, modify it, and verify it. It’s been used by over 200,000 job seekers, has 28,000+ stars on GitHub, and was used as the resume scorer in an ICML 2025 paper by Stanford researchers on fair hiring under LLM resume manipulation.

More importantly, it runs on your hardware. Your resume contains sensitive personal information: your address, your employment history, your salary expectations. Running analysis locally with Ollama means that data never touches a third-party server.

Quick Start

If you want to try Resume Matcher yourself, the fastest path is:

1. Clone the repository from github.com/srbhr/Resume-Matcher.
2. Start the backend with FastAPI and the frontend with Next.js.
3. Configure your preferred AI provider (Ollama for local, or any cloud provider via API key).
4. Upload your resume, paste a job description, and get instant analysis.

Full setup instructions are in the documentation, and there’s a Docker image if you prefer containerized deployment.

For developers who want to contribute, the project has an active community on Discord and welcomes PRs, from bug fixes to entirely new features. The roadmap includes an AI-powered content canvas and multi-job description optimization.

Now You Know How the Algorithm Thinks

ATS systems aren’t trying to trick you. They’re trying to efficiently filter thousands of applications to find the best matches. Understanding how the algorithm works (keyword matching, semantic similarity, and structured extraction) gives you a concrete framework for optimization instead of vague advice about “using the right keywords.”

For the step-by-step workflow, see how to match your resume to a job description.

The job description is your cheat sheet. The algorithm is your opponent. And now you know how it thinks.