
OpenAPI Experts
for reliable API contracts, matched in minutes with vetted freelance specialistsHire experts who design clear API contracts, generate documentation and connect OpenAPI with tools such as Swagger UI, Postman and API gateways. FRATCH finds precise matches quickly among vetted, available freelancers.
Meet FRATCH Experts who have recently used OpenAPI
Timo H.
Last position:
Software Developer at Smiths Detection
- Extension of a Swing-based UI for recording Security Restricted Articles during baggage screening.
- Introduction of a new workflow for recording Deboarding Events.
- Extension of the reporting system for Security Restricted Articles and Deboarding Events.
- Implementation using JBoss Wildfly, PostgreSQL, spring-boot 3.x, Java 8/21, Swing, JSF, JAX-RS/Resteasy, Docker, Maven, Kiro.
Max R.
Last position:
Senior AI Engineer and Technical Advisor (Freelancer) at dialpad
Serving as AI technical advisor to Dialpad's Telephony Engineering division while building AI products hands-on. Currently developing MCP-based tooling for AI text and voice agents: agent authoring, integrations and adoption of existing agents, shared authentication and API contracts, and durable workflow state. This includes text and voice tests that evaluate agent behavior, latency and task completion.
In parallel, helping Dialpad's engineers adopt QualityLayer and use Claude Code and Codex effectively. This ongoing work covers context management for large codebases, reusable plan-implement-test-review workflows connected to CI/CD, playbooks, hands-on workshops, labs and pair programming.
Technologies: AI Agent Products (Text & Voice Agents, MCP, LLMs, Python, TypeScript), Agent Tooling (Authoring, Integrations, OAuth, Durable State), Quality & Evaluation (Text & Voice Tests, Evals, Regression, Traces), Agentic Engineering (Claude Code, Codex, QualityLayer, Hooks, Prompts), Context Engineering (RAG, Vector Stores, Codebase Embeddings), Software Delivery (CI/CD Integration, Agentic Loops, PR Automation), Enablement & Coaching (Workshops, Hands-On Labs, Pair Programming)
Reinhold M.
Last position:
Business Requirements Engineer, Proxy Product Owner, Tester at AXA Konzern AG
Working Environment
- Insurance / Claims Management (Property, Liability, Automotive)
- REST-based ClaimAPI with Guidewire ClaimCenter
Scope
- Further development of the internal REST-based ClaimAPI with Guidewire ClaimCenter as the backend and various portals/front-end offices as clients, and with the following objectives: + increased AI-supported claims processing outside of Guidewire ClaimCenter + and support for all lines (property, liability, automotive).
- Further development of automated claims processing
Activities
- Interfacing IT & Business
- Design activities with documentation in Confluence, Jira: + Requirements engineering (functional and non-functional) + Design of REST endpoints and coordination with interface partners (esp. AI-colleagues) + Identification, design and evaluation of alternative solutions (moving away from a monolithic approach toward fine-grained endpoints, with code reuse across all lines if possible) + Creation / refinement of product backlog items in Jira, i.e. epics, user stories and tasks, incl. alignment with developers & POs during (pre-)refinement sessions + Using AXA Secure GPT when creating / refining user stories (description, acceptance criteria) and designing REST endpoints + Estimation and prioritization of user stories and tasks during Sprint Planning sessions + Coordination and support during the implementation of Product Backlog items + Driving decision making with preparation and presentation of decision-making documents
- Quality assurance: + Test management, including test case creation using Xray in Jira + Test execution using Bruno as REST client and in Guidewire ClaimCenter + Submission of test artifacts for PO approval + Analysis of production bugs
Methods Big Room Planning (aka PI-Planning in SAFe), Scrum, BPMN, EPK, UML, ERM, DDD
Tools / Tech Stack Teams, Confluence, Jira with Xray, OpenAPI/Swagger, REST, JSON, XML, MS Office, Bruno, GitHub repository (reading), Guidewire ClaimCenter, AXA Secure GPT (AXA's in-house GenAI solution based on Azure OpenAI)
Onur K.
Last position:
Project Manager & Outsourcing Manager at SENEC GmbH (EnBW Group)
- Built a scalable nearshore IT developer hub (Croatia, Czech Republic, Poland) as an independent company through a BOT model (Build – Operate – Transfer)
- Identified, selected, and managed full-service agencies; introduced management and control mechanisms, including KPIs, SLAs, and regular service reviews
- Prepared and reviewed data processing agreements and framework contracts in coordination with Legal & Compliance; integrated regulatory requirements (including KRITIS) into process design
- Advised on cloud vs. on-premise strategies, data storage, and authorization concepts; supported Procurement with tendering and service provider evaluations
- Managed change and process harmonization between internal teams and nearshore partners; reported to executive management, CFO, and CIO
Result: Scalable IT developer hub with an audit-ready governance model, reduced operating costs, and accelerated product development.
Sercan T.
Last position:
Co-Founder & Lead Software Architect at Pflege-Pfad
- Focus: System architecture, cloud-native platforms, microservices, API design
- Product: Pflege-Pfad is a digital placement platform that directly connects relatives of people in need of care with verified care services and caregivers – without an agency and without ongoing fees.
- Business analysis & process design:
- Analysis of the German care market and identification of the key pain points of both target groups.
- Modeling of core business processes: registration, verification, care request, application, placement, and rating.
- Definition of the business model as a freemium/premium model with optional contact unlocking.
- Creation of user stories and requirements documentation for relatives, care services, and administrators.
- Design of trust and quality assurance mechanisms with document upload, admin review process, and rating system.
- Coordination with stakeholders and validation of product decisions with potential users.
- Technical implementation:
- Design and implementation of the entire platform architecture as the sole developer.
- Design and implementation of a REST API with Spring Boot and Kotlin, including JWT-based authentication.
- Development of the frontend as a single-page application with Angular 17.
- Implementation of the AWS infrastructure with EC2, RDS PostgreSQL, S3, CloudFront, and IAM.
- Document upload with AWS S3 using presigned URLs to verify care services.
- Email notifications via Resend API.
- AI-supported care service search via OpenAI API.
- Implementation of complete user flows such as registration, login, password reset, and the placement process.
- Development of an admin panel for user and care service management as well as analytics.
- CI/CD with GitHub Actions and containerized deployments with Docker.
- End-to-end tests with Playwright.
Technologies: Kotlin, Spring Boot 3, Spring Security, JWT, JPA/Hibernate, PostgreSQL, Angular 17, TypeScript, RxJS, AWS (EC2, ECS, S3, CloudFront CDN, RDS PostgreSQL, IAM), nginx, GitHub Actions, Playwright, Maven, Git, OpenAI API, Resend API, Docker, Scrum, i18n (DE/EN/TR), Kiro, feature flag architecture.
Kiriakos K.
Last position:
Tech Lead / Architect : OTTO API Platform at OTTO
Maturing their API practices on both a business and technology level. My role covers strategy, architecture, developer advocacy as well as hands-on software engineering, enabling both technical teams and business leadership to adopt and act on API-centric principles effectively. Coincidentally, we also establish GitOps, DX and platform best practices with this project.
Highlights:
- Aligning executives with the initiative by clarifying strategy, replacing misconceptions and myths with facts, clarifying the value of existing assets and enabling informed decision-making
- Formulating a way forward for API Lifecycle Management at OTTO
- Driving platform progress and fostering developer engagement by hands-on engineering work towards strategic goals
API Lifecycle Management, Team Topologies, Organizational Evolution, Regulatory, Platform Advocate, Developer Platform, Communities of Practice, Terraform, Kotlin, Kafka, Kong, WSO2, Apigee, Gravitee, Backstage, AsyncAPI, OpenAPI, API Design, AWS, React, Node.js, TypeScript, Redocly, reactive programming, CDC, Golang, Gin, GitOps, DX (developer experience), stakeholder management, roadmaps, workshops, discovery.
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
Boian V.
Last position:
Solution Architect at DB InfraGO AG
The Base Services of DB InfraGO form a central data hub between the company’s IT systems. Common Data Services are developed that distribute data from source systems to a variety of downstream systems (via JMS) and make them available (via REST API). The existing service landscape based on TIBCO is being migrated to DB InfraGO’s cloud-native platform.
- Architecture design and implementation for performance-optimized bulk data processing of several million datasets at specific times during the day
- Technical specification and documentation of business requirements
- Integration of various subsystems (including SAP and Salesforce) through the reimplementation of more than 40 microservices based on Spring Boot
- Migration of TIBCO Based microservices from the Enterprise Integration Platform to the Cloud Native Platform using Spring-Boot
- Establishment of a deployment pipeline using GitLab CI/CD, Artifactory, and automated deployment to a Kubernetes environment with the help of ArgoCD
- Definition and implementation of automated unit, integration, and regression tests
Team Size: 9
Technologies/Tools: Java, Spring Boot, ActiveMQ(JMS), JUnit, Tibco Business Works, Gitlab (CI/CD), Kubernetes (Amazon AWS), ArgoCD, postgreSQL, Oracle, Postman, Hoppscotch, openAPI, Sonarqube
Will O.
Last position:
Solution Architect at Polizei Behörde Stuttgart
Designed a migration strategy for WLS applications to BKA Cloud and Stack IT
Created a hybrid cloud for internal testing and development with Kubernetes
Transformed the IT landscape from an outdated monolithic architecture to a cloud-based architecture
Integrated new CI/CD processes using Azure Pipelines
Quadrupled the processing throughput for personnel security checks
Migrated personnel security screening applications to Docker-based applications
Implemented CI/deployment pipelines using Azure DevOps
Environment: Java Springboot, Kubernetes, Oracle Weblogic, OpenID, Camunda, JUnit, TOGAF
Karen M.
Last position:
Personal AI Engineering Project — Croky AI at Crocky AI
Product:
- Built a production-ready AI platform for generating brand-aware marketing images and videos from product data, user requirements, and uploaded media.
- Own the platform architecture, technical roadmap, API design, security, deployment workflow, operational reliability, and model-provider strategy.
- Developed the core platform in .NET and built supporting AI and workflow prototypes in Python, applying language-independent API contracts and structured interfaces between services and model providers.
- Implemented reliable background processing with RabbitMQ, persisted workflow state, idempotent handling, retries, failure recovery, logging, secure storage, authorization, and credit accounting.
- Made pragmatic build-versus-buy and model-routing decisions based on reliability, latency, cost, and maintainability rather than novelty.
Agent Orchestration & RAG Systems
- Built and compared agent workflows using Microsoft Agent Framework, LangGraph, and LangChain, including tool use, conditional routing, clarification steps, state management, and hand-offs between agents.
- Implemented reusable .NET components for agents, prompts, tools, model providers, structured responses, and retrieval with pyvector, making it easier to change AI providers without rewriting the core workflow.
Waseem S.
Last position:
Solution Architect / Subject Matter Expert at FRMCS Device Manufacturer
Participation in the EU-funded MORANE-2 project to validate the Future Railway Mobile Communication System (FRMCS) in real-world use cases: preparation of the requirements specifications for TOBA and the associated test specifications.
Technologies: 5GS (Radio & Core), FRMCS, MCx (MCPTT, MCData, MCVideo), MC Gateway UE, MCx Priority/Security/QoS Management, ETCS, SIP, IMS
Fred H.
Last position:
Software Architect and Developer at Personal project
Recurring problem in my own AI-assisted projects: requirements analysis, use cases, and architecture decisions can be created quickly with AI support, but remain difficult to follow and scattered across Markdown files – knowledge is lost as soon as it is no longer in the context window. arknet turns requirements engineering and architecture knowledge into structured, verifiable data instead of plain text: requirements, use cases, and architecture decisions form a consistently linked knowledge graph, traceable from requirement to architecture decision – queryable by both people and AI agents. Technically based on RDF/OWL and a custom MCP server.
Result: Working MCP daemon, Docker image published automatically to GHCR, nine hexagonal modules, eleven ADRs (including an Open-Core licensing model). Requirements engineering and Ubiquitous Language hexagons are active. Public as a Community Edition under Apache-2.0 since 07/2026 (github.com/kogn-io/arknet), together with the Claude Code plugin and GHCR image; Open-Core model.
Label: Java, Maven, RDF, RDF4J, OWL, SPARQL, Model Context Protocol, Spring AI, Docker, GitHub, Git, Claude Code, Obsidian, DDD, Hexagonal Architecture, ArchUnit, JUnit, AssertJ, Interface Development, Software Architecture, Continuous Integration, Knowledge Management
Ramazan C.
Last position:
Fullstack-/DevOps Engineer at BKA (Federal Criminal Police Office)
Development and further development of an internal platform for managing and providing technical resources, virtual machines, and infrastructure services. The platform supports self-service processes and covers functions that are conceptually comparable to cloud management solutions like Azure or AWS.
- Responsible involvement in the design, development, and implementation of new backend and frontend features
- Hands-on development with Java, Spring Boot, Python, and Angular
- Implementation of REST interfaces, business logic, validations, and integrations into existing system landscapes
- Further development of modern web interfaces with Angular, including connection to backend services
- Participation in architecture and design decisions within the team, especially with regard to scalability, maintainability, and clean interfaces
- Containerization and deployment of applications with Docker, Kubernetes, and Helm
- Support with CI/CD processes and deployment to Kubernetes-based environments
- Work in the environment of vSphere, Broadcom, GitLab CI/CD, ArgoCD, Maven, npm, and NuGet
- Close collaboration with developers, business teams, DevOps, and other technical stakeholders
- Analysis of technical requirements, deriving suitable solutions, and independent implementation in an agile team
- Use of GitHub Copilot to support code generation, refactoring, test case creation, and technical documentation
Methods/ tools/ technologies: Languages & frameworks: Java (21), Spring Boot (4.x), Python, Angular, Robot Framework, Kubernetes, Helm Persistence: PostgreSQL, MongoDB, Hibernate, Liquibase Architecture & communication: REST, gRPC, GraphQL, Apache Kafka, OpenAPI, Microservices, Event Driven, Domain Driven Design Cloud & infrastructure: Terraform, Docker, Rancher, Helm, Ansible Security: OAuth2, MS (Entra ID), web security, Keycloak (extensions for detailed group rights) DevOps: GitLab CI/CD, Ansible, Maven, Gradle, Grafana, Prometheus, Git, GitHub Copilot Testing & QM: JUnit, Robot Framework, automated component and integration tests, E2E tests with Playwright, Testcontainers, EasyMock Methodology & approach: Kanban, JIRA, Confluence, Clean Code
Aurelia H.
Last position:
Software Full Stack Developer at INNIO Jenbacher GmbH & Co OG
- Implementing and extending an Electron app based on TypeScript, React and Node.js to display stream content and technical data
- Extending a Kotlin backend for message processing via Kafka
- Extending various React modules
- Programming custom solutions for customers, including testing
- Creating documentation
- Development in accordance with Scrum processes
Used skills: React, React Hooks, Jest, Vitest, Typescript, Electron, IntelliJ, Vite, Github, Jira, Node.js, yarn, npm, pnpm, Swagger, OpenAPI, Storybook, Docker, Prettier, Eslint, tailwindcss, i18n, GraphQL, CoPilot, Postgres, JSON, Sqlite, trpc, influxdb, Java, Spring, Kotlin, Kafka, Git, Sonarcube
Marijn S.
Last position:
Senior Software Engineer at Puls Security GmbH
Optimizing and acceleration of our Gitlab CI pipeline
Conceptual work for the PoC of the Zero Trust system
Extension of the policy-engine backend in Go
Extension of the policy-testing mechanism in Python
Architectural design of the PEP component of Zero Trust
Documentation of the product
Technologies: Zero Trust, Go, Python, Gitlab CI, Docker, JWT, Domain-Driven Design
Discover over 15,000 top freelancers
Statistics of experts using OpenAPI
Aggregated from the professional profiles of matched freelancers.
Experience
19 years

Position duration
1.8 years

Positions per freelancer
14

Top business areas
Information Technology, Product Development, Quality Assurance

Top industries
Information Technology, Banking and Finance, Automotive

Certification focus areas
Information Technology, Product Development, Project Management
Bachelor's degree or higher
89%
Master's degree or higher
51%
Doctorate
10%

Certifications per freelancer
2

Most common languages
German, English, French

Speak two or more languages
94%
Based on our profile pool as of 16 Oct 2026.
Daily rate distribution
The chart shows how the daily rates of experts in this technology 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 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 16 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 (96%)
- Banking and Finance (56%)
- Automotive (47%)
- Government and Administration (42%)
- Retail (42%)
- Manufacturing (39%)
- Transportation (37%)
- Telecommunication (33%)
Please note that freelancers can work across multiple industries, so percentages overlap.
About the technology
API Contracts
OpenAPI is a machine-readable specification for describing HTTP APIs. It defines endpoints, parameters, request bodies, responses, authentication and data models in a shared contract. Teams use YAML or JSON documents to align frontend, backend, QA and external consumers before and during implementation.
What It Builds
OpenAPI supports public, partner and internal APIs for web applications, mobile services, integrations and microservices. A well-maintained contract can drive documentation, validation, testing and client generation across the delivery lifecycle.
- Define resources, operations and response models
- Document authentication, errors and pagination
- Generate client libraries and server stubs
- Validate requests and responses against the contract
Ecosystem And Tooling
The ecosystem includes Swagger UI for interactive documentation, Swagger Editor for authoring and tools such as Redoc for reference portals. Professionals also connect OpenAPI with Postman, Stoplight, API gateways, contract-testing tools and CI pipelines. They work with JSON Schema, OAuth 2.0, OpenID Connect and webhooks where the API requires them.
When To Bring In Specialists
Companies often need freelance expertise when an API is being designed, migrated or opened to new consumers. Specialists can establish a consistent contract across legacy services, review an existing specification or automate documentation and validation in delivery pipelines.
- Resolve inconsistencies between implementation and documentation
- Plan versioning and backward-compatible changes
- Prepare an API for external partners or a developer portal
- Introduce contract-first practices across several teams
What Strong Experts Know
Strong professionals understand both the specification and the systems behind it. They model practical resources, choose precise schemas, describe failure cases and keep examples aligned with real behavior. They know when to use reusable components, how to handle nullable data and how to avoid contracts that expose accidental implementation details.
They also review generated output instead of trusting it blindly. Useful experience includes API security, gateway policies, automated tests, CI checks, source control workflows and collaboration with product, frontend and backend teams.
How Quality Is Judged
A useful OpenAPI document is accurate, readable and actionable. Check whether its examples work, responses cover meaningful failure paths, schemas express constraints clearly and authentication is described without exposing secrets. The contract should remain consistent with the running service through review, validation or contract tests.
Ask specialists to explain trade-offs around design-first and code-first workflows, versioning and generated clients. A strong professional can improve an existing specification without forcing unnecessary rewrites and can leave maintainable standards, templates and review practices behind.
Frequently asked questions
Key details about OpenAPI, drawn from the questions we get asked most.
OpenAPI is used to describe HTTP APIs in a standard YAML or JSON format. Companies use the specification to align teams, publish documentation, validate traffic, generate clients and support automated testing.
OpenAPI commonly describes REST-style HTTP APIs, where clients call defined resources and operations. GraphQL uses a query schema, while gRPC relies on protocol buffers and generated service contracts, so the right choice depends on clients, transport needs and existing architecture.
OpenAPI work benefits from knowledge of HTTP, REST design, JSON Schema, OAuth 2.0, OpenID Connect and API security. Experience with Swagger UI, Postman, gateways, CI pipelines and contract testing is also useful when the specification must stay aligned with production services.
OpenAPI expertise should match the project risk rather than a fixed career duration. A straightforward internal API may need focused specification and documentation support, while a public or distributed API requires someone who can handle compatibility, security, versioning and governance.
OpenAPI is well suited to remote collaboration because the contract, examples and review history can live in version control. Clear written communication, shared review practices and familiarity with the team's language expectations matter more than physical location.
OpenAPI quality is visible in accurate operations, useful examples, complete error responses and schemas that reflect real constraints. Ask for a review of an existing specification or a sample design, and check whether the specialist explains trade-offs clearly instead of only producing attractive documentation.
OpenAPI is the current name of the specification that originated from the Swagger specification. Swagger UI and Swagger Editor remain widely used tools for viewing and creating OpenAPI documents, but the specification itself should be identified by its OpenAPI version and supported features.
OpenAPI contract-first design is useful when several teams, clients or partners must agree on behavior before implementation. It helps expose unclear requirements early, but the contract still needs automated validation and regular review so it does not drift from the running API.
The average hourly rate of freelancers who have used OpenAPI in their recent projects is 96 €, which corresponds to a daily rate of about 767 € based on an 8-hour working day.
Of the freelancers who have used OpenAPI in their recent projects, 89% hold at least a Bachelor's degree, 51% hold at least a Master's degree, and 10% hold a doctorate.
On average, freelancers who have used OpenAPI in their recent projects have 19 years of professional experience, with a single engagement typically lasting around 1.8 years.
The most common languages among freelancers who have used OpenAPI in their recent projects are German (96%), English (95%), and French (16%).
The most common industries among freelancers who have used OpenAPI in their recent projects are Information Technology (96%), Banking and Finance (56%), and Automotive (47%).
The most common business areas among freelancers who have used OpenAPI in their recent projects are Information Technology (99%), Product Development (98%), and Quality Assurance (65%).
Main locations of FRATCH Experts, who have recently used OpenAPI
Our freelancers and interim experts are at home all over Germany — available on-site in Berlin, Hamburg, Munich and every major business hub, or fully remote. Choose a city to discover matched specialists, local market insights and up-to-date availability.
In Austria our freelancers and interim experts support companies from Vienna to Graz — on-site where your project needs them, or fully remote. Choose a city to discover matched specialists, local market insights and up-to-date availability.
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