Top 5 Best API Testing Tools for Developers: Postman vs Insomnia vs Thunder Client

API testing is no longer an afterthought in the software development lifecycle. According to the 2023 State of API Report by Postman, nearly 90% of developers now use APIs daily, and the average organization manages over 1,000 internal and external APIs. As the number of endpoints grows, so does the complexity of validating them. Whether you are debugging a RESTful service, mocking a GraphQL schema, or automating a CI/CD pipeline, the tool you choose can save you hours—or cost you a full afternoon of frustration.

I have spent the last six years building and testing microservices across fintech and e-commerce platforms. In that time, I have cycled through nearly a dozen API clients. Below, I break down the top five tools that dominate the landscape in 2025, with a deep dive into the three most popular: Postman, Insomnia, and Thunder Client. I will give you the facts, the trade-offs, and the specific use cases where each shines.

Why the Right API Testing Tool Matters

Before we jump into the list, let’s set the baseline. A good API testing tool should do more than just send a GET request. It needs to handle:

• Environment management (dev, staging, production variables)
• Authentication flows (OAuth 2.0, API keys, JWT)
• Automation and scripting (pre-request scripts, test assertions)
• Collaboration (sharing collections with teammates)
• Performance (response time, memory footprint)

If a tool fails on any of these for your specific workflow, it becomes a bottleneck. The best tool is not the one with the most features—it is the one that fits your team’s context.

1. Postman: The Industry Standard (But Is It Too Heavy?)

Postman is the default choice for a reason. With over 25 million developers using it, it has become the de facto standard for API development. The tool supports everything from REST and GraphQL to gRPC and WebSockets. Its collection runner allows you to chain requests, set data-driven tests, and integrate with CI/CD via Newman, its command-line companion.

Strengths:

• Feature depth: Postman offers a full testing suite, including test scripts in JavaScript, assertion libraries, and mock servers. You can build a complete API contract test suite without leaving the app.
• Collaboration: Workspaces and shared collections make it easy for teams to maintain a single source of truth. The free tier supports up to three collaborators, which is sufficient for small projects.
• Ecosystem: The Postman API and integrations with tools like Swagger, OpenAPI, and Jenkins are mature. You can generate client code in multiple languages directly from a collection.

Weaknesses:

• Performance: Postman is a resource hog. The Electron-based desktop app regularly consumes 500MB+ of RAM, which is painful on a 8GB MacBook with Docker running.
• Learning curve: The UI is cluttered. New developers often struggle to find the difference between a folder, a collection, and an environment. The sheer number of buttons can be overwhelming.
• Cost: While the free tier is generous, advanced features like API governance, reporting, and priority support require a paid plan starting at $14/user/month.

Best for: Large teams with complex integration testing needs, and organizations that require deep collaboration and audit trails.

2. Insomnia: The Lightweight Powerhouse

Insomnia, originally a Kickstarter-backed project, has carved out a niche for developers who want a robust tool without the bloat. After being acquired by Kong in 2019, it pivoted to focus heavily on GraphQL and OpenAPI support. The current version (9.x) offers a clean, tab-based interface that feels like a native app, even though it is built on Electron.

Strengths:

• GraphQL first-class support: Insomnia is the best tool for GraphQL testing. You can write GraphQL queries with autocomplete, define variables, and even generate documentation from your schema.
• Performance: It is noticeably faster than Postman. Startup time is under two seconds, and memory usage stays around 200-300MB. This matters when you are switching between multiple projects.
• Simplicity: The UI is uncluttered. Environments and variables are easy to set up, and the pre-request scripts are straightforward for basic automation.

Weaknesses:

• Collaboration is limited: The free tier does not support real-time team collaboration. You can share collections via Git sync, but it is not as seamless as Postman’s workspace model.
• Test assertions are basic: While Insomnia supports scripting via JavaScript, the testing framework is less mature than Postman’s. Chai assertions are built-in, but you will miss the rich reporting and data-driven testing options.
• Plugin ecosystem is smaller: There are fewer community plugins compared to Postman, so you may need to write custom scripts for niche integrations.

Best for: Individual developers and small teams, especially those working heavily with GraphQL or looking for a fast, lightweight client.

3. Thunder Client: The VS Code Native

Thunder Client takes a different approach—it lives entirely inside Visual Studio Code. If you are already using VS Code daily (which over 70% of developers do, according to Stack Overflow’s 2024 survey), Thunder Client eliminates the need to switch contexts. It is a lightweight extension that supports REST, GraphQL, and WebSockets.

Strengths:

• Zero context switching: You stay in your editor. No separate app, no Electron overhead, no background processes. This is a huge productivity boost for developers who live in VS Code.
• Speed: Requests execute quickly, and the UI is minimal. The “code snippet” feature generates curl, Python, and JavaScript code instantly, which is handy for documentation.
• Pricing: The core features are free. A Pro version ($19 one-time payment) adds team collaboration, cloud sync, and environment variables. This is significantly cheaper than Postman’s recurring subscription.

Weaknesses:

• Limited advanced features: There is no built-in test runner for CI/CD. You can write scripts, but they are not as powerful as Postman’s Newman. For automation-heavy workflows, you will need to combine Thunder Client with other tools.
• No native desktop app: If you do not use VS Code, this tool is irrelevant. It also lacks a mobile version, which Postman and Insomnia offer.
• UI constraints: The sidebar-based interface can feel cramped when dealing with large collections. Managing complex folder hierarchies is less intuitive than in Postman.

Best for: Frontend and full-stack developers who want a quick, integrated solution for manual API testing without leaving their editor.

4. SoapUI: The Heavyweight for SOAP and Enterprise

SoapUI (and its open-source version) is the go-to tool for enterprise environments that still rely on SOAP web services. While REST is dominant, many financial and healthcare systems use SOAP for legacy integrations. SoapUI excels at functional testing, load testing, and security testing of these complex XML-based APIs.

Strengths:

• Protocol support: It handles SOAP, REST, JMS, and AMF out of the box. For SOAP, it automatically parses WSDL files and generates test suites.
• Security testing: The Pro version includes security scans for SQL injection, XSS, and malformed XML. This is a unique feature not found in Postman or Insomnia.
• Data-driven testing: You can connect to databases (JDBC) and Excel files to feed test data into your requests.

Weaknesses:

• Clunky UI: The interface looks dated. It is not intuitive, and the learning curve is steep, especially for developers who are used to modern tools.
• Resource intensive: SoapUI is Java-based and can eat up 1GB+ of RAM when running multiple test suites.
• Overkill for simple REST: If you are building modern microservices, SoapUI is unnecessary. Its power is wasted on simple CRUD operations.

Best for: Enterprise QA teams and developers working with legacy SOAP APIs or requiring integrated security testing.

5. Bruno: The Offline-First, Git-Native Contender

Bruno is a newer player (launched in 2022) that has gained traction among developers who value data privacy and version control. Unlike Postman, which stores your collections on its cloud, Bruno stores everything locally in a text-based format (.bru files). This means you can version your API tests with Git, review changes in pull requests, and work completely offline.

Strengths:

• Privacy and security: Your data never leaves your machine. For organizations with strict compliance requirements (HIPAA, SOC2), this is a game-changer.
• Git-friendly: Collections are plain text, so diffs are readable. You can collaborate via standard Git workflows, which is more transparent than Postman’s cloud sync.
• Lightweight: It is built on Electron but is surprisingly fast, with a clean, minimal interface.

Weaknesses:

• Young ecosystem: The plugin marketplace is sparse. You will not find the same depth of integrations as Postman.
• Limited automation: While Bruno supports scripting (JavaScript), the test runner is less mature. There is no official CI/CD CLI tool yet, though the community has built some workarounds.
• Smaller community: Fewer tutorials, fewer Stack Overflow answers, and fewer examples to reference.

Best for: Privacy-conscious developers and teams that already use Git for everything and want to treat API tests as code.

Head-to-Head Comparison Table

Feature	Postman	Insomnia	Thunder Client	SoapUI	Bruno
Best for	Enterprise teams, collaboration	GraphQL devs, lightweight use	VS Code users	SOAP/enterprise	Privacy, Git-centric teams
GraphQL support	Good	Excellent	Good	Poor	Fair
Automation (CI/CD)	Excellent (Newman)	Fair (CLI available)	Poor (manual only)	Excellent (Maven/Gradle)	Fair (community CLI)
Collaboration	Excellent (cloud)	Fair (Git sync)	Poor (Pro only)	Fair (team server)	Good (via Git)
Memory usage	High (500MB+)	Medium (200-300MB)	Low (VS Code process)	High (1GB+)	Medium
Price	Free tier, $14/user/mo	Free, Pro $8/user/mo	Free, Pro $19 one-time	Free (open source), Pro costly	Free (open source)

The Verdict: Which One Should You Choose?

If you are a solo developer building a REST API and want a fast, no-nonsense tool, Insomnia is the sweet spot. It balances power and simplicity without the resource drain.

If you are leading a team of five or more developers who need to share collections, run integration tests in CI/CD, and maintain a single source of truth, Postman is still the most reliable choice—despite its bloat.

If you never leave VS Code and your testing needs are manual and exploratory, Thunder Client will save you time and RAM.

If you are dealing with SOAP or need built-in security testing, SoapUI is the only serious option.

And if your company has strict data residency rules or you want your API tests to live in the same Git repo as your code, Bruno is worth a serious look.

No single tool wins across every dimension. The best approach is to evaluate your team’s workflow, the protocols you use, and your automation requirements—then pick the tool that removes friction rather than adding it. In the fast-moving world of API development, the right tool is the one you do not have to think about.