OpenAPI Experts in Berlin
for reliable API delivery, matched in minutes with vetted and available freelancersHire experts who design OpenAPI specifications, generate documentation and client libraries, and integrate REST services into modern products. FRATCH finds a precise match quickly from vetted, available freelancers.
Meet FRATCH Experts in Berlin, who have recently used OpenAPI
Toralf C.
Last position:
IT Consultant in PKI Software Development at BWI
- Implementation of a full-stack application with Angular as frontend framework and Java as backend language
- Use of the Swagger OpenAPI specification for REST communication between frontend and backend applications
- Control of a card printer via the Nexus Card SDK framework
- Design and implementation of card management software with web interface and JavaFX module for printer monitoring
- Use of Cucumber tests for test automation and creation of test artifacts for testing outside the development environment
- Use of Wireshark for network investigations to analyze the API of a third-party product and to check the REST API of the self-developed software
Caleb C.
Last position:
Senior Python Backend Engineer at Simon-Kucher
Transmission system operator – network provisioning platform (team lead)
- Designed the saga orchestrator in Python that provisions network fabrics with rollback.
- Split IP, identity, and resource allocation into bounded contexts with their own databases and typed clients.
- Implemented data access using DDD repositories and units of work, with no SQL in the domain code.
- Required integration tests using real PostgreSQL and RabbitMQ containers, and API fuzzing for every service.
- Introduced an agent workflow where a spec starts each change and a human approves every merge.
- Rolled out OpenTelemetry auto-instrumentation through GitOps across eighteen Kubernetes clusters.
- Led incident response for stuck provisioning and recorded every outage as a post-mortem.
Languages and software used: Python, FastAPI, Pydantic v2, SQLAlchemy async, PostgreSQL, RabbitMQ, Celery, pytest, testcontainers, schemathesis, Helm, Flux CD, GitLab CI, OpenTelemetry, pi coding agent.
Software product company – datacenter simulation SaaS (backend development)
- Built the asynchronous simulation pipeline as the second-largest contributor among more than twenty team members.
- Tagged every image by commit SHA and semver in the GitHub Actions deployment workflows.
- Diagnosed the RabbitMQ memory alarm that froze every simulation and fixed the limits in the Helm chart.
- Converted the test suite to function-style pytest, with coverage reported in every pipeline.
Languages and software used: Python, FastAPI, SQLAlchemy, Alembic, Celery, RabbitMQ, Redis, PostgreSQL, S3, EKS, Helm, Keycloak, GitHub Actions, pytest, mypy, ruff, SonarQube.
Internal AI platform – model serving and telemetry
- Built the FastAPI service that deploys open-weight models on GPU tiers as OpenAI-compatible endpoints.
- Upgraded production EKS by two Kubernetes minor versions and fixed zone-bound storage faults.
- Traced every model call, including its token count, cost, and latency, through the OpenTelemetry operator.
Languages and software used: Python, FastAPI, vLLM, LangGraph, pydantic-ai, OpenAI and Anthropic APIs, EKS, Helm, Flux CD, Terraform, GitLab CI, OpenTelemetry, Grafana.
Tim D.
Last position:
Product Owner at mkk meine krankenkasse
Product ownership for the new bonus component: a fully digital rewards program in the customer app, going live on 1.1.2027.
- Translated requirements from the business team, app team, and development into epics and stories, and managed delivery
- Designed interfaces between the app, Python backend, and health insurance systems (mkk API, 21c, d.3)
Mukund B.
Last position:
Voice AI Chatbot - Real-Time Audio Assistant
- ▶ Built real-time voice assistant (STT → LLM → TTS pipeline) benchmarking and evaluating multiple STT providers including faster-whisper and Azure Speech. achieved sub-3s latency, Groq API (Llama 3) with multi-turn memory - directly handling edge cases in dictation, names and passcode recognition.
Rüdiger S.
Last position:
Full-Stack Software Engineer / Consultant for Digitalization at ARTEVENT
Designed, built, and launched an internal event planning web application used by over 100 department leads for a large event, despite having no dedicated testing phase.
Ensured smooth, failure-free operation during first production use, leading to the tool being adopted for future events.
Automated catering calculations and related workflows, significantly reducing email communication and manual computation effort for meal planning.
Managed deployment and hosting on a Linux server using Coolify, including application setup and runtime operations.
Hired and guided a communication designer on UX while independently owning all technical decisions and implementation.
Marvin M.
Last position:
Co-founder & CTO · Freelance Software Engineer (AI & SaaS) at Self-employed
- Self-employed · Berlin, Germany
- Co-founded the company and own the entire technical side: product architecture, backend, frontend, infrastructure and operations.
- Designed and built the Automated Booking System (ABS) as well as the core platform and payment logic.
- Development of AI-powered SaaS products, from architecture through backend and frontend to production operation.
- Technical consulting on product architecture, data protection and permissible automation.
- Building, running and monetising my own API products for AI agents, each available as a REST interface and as an MCP server.
- Full ownership of architecture, infrastructure, billing, legal texts and go-to-market.
Murad H.
Last position:
Founder & Technical Lead at Hubpoint.Ai
- Founded an AI-powered scheduling and business-management SaaS for SMBs, owning technology strategy, architecture, product development, UX, billing and go-to-market execution.
- Architected and shipped a multi-tenant platform with REST APIs, RBAC, CRM, billing and notifications, powering the manager dashboard, admin console, booking experience and iOS/Android applications.
- Led and mentored 7 software engineers, 1 DevOps engineer, 1 QA engineer and 1 UX/UI designer, while remaining hands-on across backend, frontend and product delivery.
- Built AI voice and chat agents using Python/FastAPI, OpenAI and Anthropic APIs, RAG, pgvector and tool calling; integrated Twilio, Google Calendar/Meet, Stripe and Firebase.
- Owned production infrastructure and automated delivery across separate environments using Docker, Nginx, GitHub Actions and Grafana; represented the company at accelerators and international startup events.
Selected stack: Python, FastAPI, Node.js, Vue 3, React/Next.js, React Native, PostgreSQL, Redis, Docker
Yavuz Ö.
Last position:
Full-Stack Developer – Practical Project Work at Sybarium Limited
Company in the industrial sector
Projects completed during retraining
- Rebuilt the company website from WordPress using a modern stack with Astro and TypeScript, deployed on Vercel (live at sybarium.vercel.app); replaced the legacy CMS with a faster, maintainable system.
- Reduced page load time by ~80% (from 5 s to under 1 s) through optimization, caching, and asset optimization.
- Developed n8n automation workflows that integrate several REST APIs and internal systems, saving more than 3 hours of manual work per week.
Muzamal A.
Last position:
Data Scientist / AI Consultant at HelmX
- Delivered AI and data science solutions, including LLM-based chatbots and data pipelines, improving operational efficiency.
- Collaborated on product features, achieving measurable impact and maintaining strong client relationships.
Marcus B.
Last position:
Alpecin.de - Dr. Wolff Group
- Development of a Component Library based on a Design System for several international brands such as Alpecin, Karex, Linola and Plantur 39
- Integration of Headless CMS, PIM and Commerce solutions
- UI/UX development
- Prototype development
- MVP development
- API design and development
- SaaS development (Whitelabel/Multi-tenent)
- Headless CMS integration
- Design Systems Digital Signage
- Internationalization (i18n)
- Javascript
- Component Library
- Tailwind CSS
- Storybook
- HTML
- Vue 3
- Storyblok
- Github Actions
- SEO
- i18n Theming
- React
Ibrahim H.
Last position:
Senior Full Stack / AI Engineer at Punktum Digital GmbH
- Context: Healthcare and laboratory teams required faster document analysis, treatment-planning support, and reliable AI workflows for MR/VR-assisted operations.
- Contribution: Built the AI healthcare platform, model/agent workflows, VR-glasses deployment platform, REST APIs, Next.js/React interfaces, and CI/CD pipelines.
- Impact: Delivered a production-ready AI product foundation that improved clinical document review, supported laboratory automation, and made VR fleet deployment manageable across environments.
Tech: TypeScript, Next.js, Node.js, React, Java, Spring Boot, Python, PyTorch, TensorFlow, Docker, PostgreSQL, OpenAPI, GitLab, GitHub Actions.
Ursula R.
Last position:
Sabbatical with Training at Own Sabbatical
Qaiser A.
Last position:
Freelance Lead DevOps Engineer at Schwarz Gruppe Produktion
Bootstrapping a CloudOps team and building a multi-cloud provider backend for a low-code Internal Developer Platform (IDP) with env zero
Introducing user story mapping, ADRs, milestones, and backlog management
Designing and developing core APIs, setting up CI/CD pipelines, OpenTofu/Terraform scripts
Representing and communicating the team with third-party stakeholders (e.g. env zero)
(Cross-)team coaching on DevOps, software design, Terraform, Golang, and agile practices
Karthikeyan R.
Last position:
Full-Stack Developer — Own Product at Self-employed
Java 21 · Spring Boot 3 · Keycloak · PostgreSQL · Docker · Nginx · GitHub Actions · DigitalOcean · React 18 · TypeScript · Plasmo
- Architected and shipped a production-ready Job Application Tracker end-to-end: REST API with 5-stage workflow, pagination, sorting, and dynamic filtering — full ownership from design to live cloud deployment on DigitalOcean.
- Implemented production-grade identity management: OAuth 2.0 / OpenID Connect / JWT / RBAC via Keycloak, applying Hexagonal Architecture and DDD principles.
- Built automated CI/CD pipeline (GitHub Actions); containerised with Docker; Nginx reverse proxy with path-based routing and SSL termination.
- Developed a Chrome Extension (Plasmo framework, Manifest V3) that auto-fills job applications directly from LinkedIn into the tracker — demonstrates full product thinking across backend API and browser client.
Maurizio F.
Last position:
Python Software Developer at Schönhofer Sales and Engineering GmbH
- Implemented a command line interface (CLI) for integration of REST APIs of various microservices for end users
- Centralized and simplified interaction with services through the CLI
- Implemented a REST microservice for custom data schemas based on an API-first approach
- Developed event-driven control with RabbitMQ to connect to other services
- Deployed services using Docker and Kubernetes and extended the CLI
- Managed complexity and data volume handling through the microservice
Discover over 15,000 top freelancers
Statistics of experts using OpenAPI
Aggregated from the professional profiles of matched freelancers.
Experience
19 years
Position duration
2.2 years (Germany: 1.8 years)
Positions per freelancer
10 (Germany: 14)
Top business areas
Information Technology, Product Development, Quality Assurance
Top industries
Information Technology, Banking and Finance, Education
Certification focus areas
Information Technology, Project Management, Product Development
Bachelor's degree or higher
90% (Germany: 89%)
Master's degree or higher
35% (Germany: 51%)
Certifications per freelancer
2
Most common languages
German, English, Spanish
Speak two or more languages
83% (Germany: 94%)
Based on our profile pool as of 29 Oct 2026.
Daily rate distribution
The chart shows how the daily rates of experts in this technology in Berlin are distributed, based on recent contracts on our platform. Each bar covers a rate range — its height shows the share of experts charging within that range.
Average rates of experts in Berlin using OpenAPI
Rates are based on recent contracts and do not include FRATCH margin.
The average daily rate is the mean of all daily rates from recent contracts of comparable freelancers on our platform.
The median daily rate is the middle value of all daily rates — half of comparable freelancers charge less, half charge more. Unlike the average, it is barely affected by outliers.
Calculated based on our freelancers’ daily rates as of 29 Oct 2026. Actual rates may vary depending on seniority level, experience, skill specialization, project complexity, and engagement length.
OpenAPI experts industry focus
See which industries our matched freelancers work in most often — every figure is calculated live from the freelancers on FRATCH.
- Information Technology (97%)
- Banking and Finance (48%)
- Education (38%)
- Energy (34%)
- Professional Services (34%)
- Retail (34%)
- Automotive (31%)
- Healthcare (31%)
Please note that freelancers can work across multiple industries, so percentages overlap.
About the technology
API Contracts
OpenAPI is a machine-readable standard for describing HTTP APIs. Teams use OpenAPI documents to define endpoints, parameters, request bodies, authentication, responses, and error formats before implementation. The specification creates a shared contract between product teams, backend services, client applications, and external partners.
Design and Delivery
OpenAPI supports the full API lifecycle, from planning and review to testing and release. Strong specialists turn business requirements into clear resource models and reusable schemas, then keep the contract aligned with the running service.
- Define REST endpoints and response models
- Document authentication, pagination, and errors
- Validate requests and responses against the contract
- Manage compatible changes across API versions
Tooling Ecosystem
The ecosystem includes Swagger UI and Swagger Editor for interactive documentation and design, along with generators for client libraries, server stubs, and typed models. Professionals also work with tools such as Redoc, Postman, Stoplight, Spectral, and CI pipelines that lint and validate specifications.
When Expertise Helps
Companies bring in freelance OpenAPI specialists when an API is difficult to understand, documentation has drifted, or several teams need a dependable contract. Berlin companies can also benefit from professionals who combine remote delivery with clear on-site collaboration when workshops, architecture reviews, or partner integrations require it.
- Standardize inconsistent API documentation
- Prepare a public or partner-facing API
- Introduce contract testing into delivery workflows
- Migrate Swagger 2.0 descriptions to OpenAPI 3
Skills Beyond the Specification
OpenAPI work is closely tied to REST design, HTTP semantics, JSON Schema, OAuth 2.0, OpenID Connect, and API gateways. Useful specialists understand the implementation language and framework behind the service, as well as version control, automated testing, security reviews, and documentation workflows.
Quality Signals
A strong professional separates design decisions from generated output and explains trade-offs around compatibility, naming, schemas, and security. They inspect real API behavior rather than trusting documentation alone, test generated clients, and establish ownership so the specification remains useful after the engagement. For Berlin-based collaboration, language fit and disciplined async communication matter alongside technical depth.
Frequently asked questions
Curious about OpenAPI? Here are the answers that come up again and again.
OpenAPI is used to describe HTTP APIs in a structured, machine-readable format. It helps teams design endpoints, generate documentation and client libraries, validate traffic, and align service implementations with the contract.
OpenAPI usually describes REST-style HTTP APIs, where clients call defined resources and receive specified responses. GraphQL uses a query language and a flexible schema, while gRPC commonly uses Protocol Buffers and strongly defined remote procedure calls; the right choice depends on client needs, transport, tooling, and system boundaries.
A capable OpenAPI specialist should understand REST, HTTP, JSON Schema, OAuth 2.0, OpenID Connect, and API gateway concepts. Experience with Swagger UI, Redoc, Postman, Spectral, contract testing, version control, and the service’s implementation stack is also valuable.
A small documentation update may suit a specialist with focused OpenAPI experience, while a company-wide API standard needs broader design and governance skills. Assess the scope, number of services, security requirements, generation workflow, and migration risk before choosing the level of expertise.
Yes. OpenAPI projects are well suited to remote collaboration because specifications, reviews, validation rules, and generated artifacts can be managed in version control. On-site workshops in Berlin can still help when teams need to agree on domain models, ownership, or partner integration requirements.
A high-quality OpenAPI specification is complete, consistent, readable, and accurate against the running API. Check reusable schemas, explicit error responses, security definitions, compatibility rules, linting results, and whether another team can generate or use documentation without extra explanation.
OpenAPI is the current name of the specification that evolved from the Swagger specification. Swagger remains the name of a related tooling ecosystem, including products such as Swagger UI and Swagger Editor, so the terms are connected but not interchangeable in every context.
An OpenAPI freelancer may deliver a new specification, an API design review, generated client or server artifacts, interactive documentation, validation rules, and contract tests. They can also define contribution standards and migration steps so internal teams can maintain the contract after the engagement.
The average hourly rate of freelancers in Berlin, Germany who have used OpenAPI in their recent projects is 92 €, which corresponds to a daily rate of about 740 € based on an 8-hour working day.
Of the freelancers in Berlin, Germany who have used OpenAPI in their recent projects, 90% hold at least a Bachelor's degree and 35% hold at least a Master's degree.
On average, freelancers in Berlin, Germany who have used OpenAPI in their recent projects have 19 years of professional experience, with a single engagement typically lasting around 2.2 years.
The most common languages among freelancers in Berlin, Germany who have used OpenAPI in their recent projects are German (93%), English (90%), and Spanish (10%).
The most common industries among freelancers in Berlin, Germany who have used OpenAPI in their recent projects are Information Technology (97%), Banking and Finance (48%), and Education (38%).
The most common business areas among freelancers in Berlin, Germany who have used OpenAPI in their recent projects are Information Technology (97%), Product Development (93%), and Quality Assurance (55%).
Main locations of FRATCH Experts, who have recently used OpenAPI
Our freelancers and interim experts are at home across the DACH region — available on-site in the major business hubs or fully remote. Choose a location to discover matched specialists, local market insights and up-to-date availability.
Hamburg 
Munich 
Cologne 
Frankfurt 
Stuttgart 
Dusseldorf Countries:
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