Free online developer tools

Builds an HTTP request from the browser and shows which response headers and body the page is actually allowed to read.

Adjust flex container and item properties with controls and watch the layout change live.

Looks up a GitHub profile by username or numeric ID via the GitHub REST API.

Turns a GitHub issue or pull request, with its comments and referenced code, into a single Markdown document.

Reports how big a GitHub repository is in KB, MB or GB from its name or URL.

Looks up a public GitHub repository and lists stars, forks, branches, languages, contributors and release info.

Adds ids to every heading in an HTML snippet and builds a linked table of contents.

Write HTML in one pane and see it rendered immediately in another, with a copy button.

Runs HTML, CSS and JS in an iframe with configurable sandbox restrictions to see their effect.

Formats JSON that is cut off or malformed, adding indentation as you type and reporting validation state.

Compares two JSON documents and highlights added, removed and changed values with character-level detail for strings.

Define properties and nested objects in a form and copy the JSON Schema it generates.

Try one JavaScript library against OpenAI, Anthropic or Gemini models by picking a provider and pasting a key.

Sends prompts with different structures to OpenAI and shows how many tokens were served from cache.

Extracts package pins from a Pipfile.lock and outputs them in other formats.

Compares two released versions of a Python package by fetching their archives and showing the file differences.

Writes JSON Schema from a short field-per-line notation with types and descriptions.

Finds and downloads archived Git repositories from the Software Heritage archive.

Formats SQL for SQLite, PostgreSQL or MySQL with options for indentation and keyword case.

Select endpoints from a Swagger JSON file and get a smaller spec containing only those and their dependencies.

Checks that pasted XML is well-formed and reports errors with line and column positions.

Loads YAML from text or a URL and shows it as a collapsible tree, remembering which nodes you opened.

Opens a PyPI package or archive URL and lists its files with syntax-highlighted viewing of each one.