Free Developer Tools

Free online developer tools, grouped by the job

Search usually starts from the task, not from a brand name. These groups are the same categories as the filters above, with the context for when each tool is the right one.

Compare code, JSON and environment files

Use the code diff checker when the input is source, a config file or any other text. It highlights the lines and words that changed and can export a patch. Use the JSON diff when both sides are JSON: object key order and insignificant whitespace stop looking like edits, and nested fields stay readable. The .env file diff is the narrower case — two environment files, missing keys called out, and a .env.example you can commit without copying secret values.

None of these tools talk to git. Paste the two sides, read the result, and keep the history in your repository.

Check syntax, decode tokens and turn curl into code

The syntax checker covers 60 language modes in a private editor. It is a fast way to see that a snippet is structurally broken. It is not a substitute for the language’s compiler, typechecker or test suite, and a clean result does not mean the project builds. The JWT decoder splits a token into header and payload and can verify a signature when you supply the key. Verification answers “was this signed with this secret?” It does not answer “will the API accept this token right now?” The cURL to code converter turns a copied curl command into a starting point in common languages, including headers and the request body.

Preview Markdown and test regular expressions

The Markdown previewer renders tables, task lists, math and Mermaid diagrams next to the source, and it can export Markdown or HTML. Remote images stay blocked unless you turn them on, because loading them would tell that server your address. The regex tester runs a pattern against sample text in the page, which is the right place to try an expression before you drop it into application code. Flavor differences still exist: confirm the result in the language that will run it in production.

Read cron expressions and Unix timestamps

A cron line is easy to misread, especially around day-of-month and day-of-week. The cron expression parser explains the fields and lists upcoming run times so you can see whether “every weekday at 9” is what you actually wrote. The Unix timestamp converter turns epoch seconds or milliseconds into a calendar time, and the other way around, with the timezone made explicit. Log lines, JWT exp claims and database columns all show up here.

Convert CSV, YAML, JSON samples and phone numbers

The CSV to JSON converter and the YAML to JSON converter are format translators. YAML converts in both directions. They do not validate a business schema; they produce structured text you can paste into an editor or a test fixture. The JSON to types converter reads one sample payload and drafts TypeScript, Zod, Python or Go types. Treat the draft as a start: a single example cannot show every optional field or every enum. The E.164 phone number formatter normalizes a number to the international form used by telephony APIs. Formatting is not the same as checking that the number can receive a call or a WhatsApp message — that check is a live lookup in the API Playground.

Calculate permissions, subnets and Compose services

The chmod calculator maps octal modes, symbolic modes and the read, write and execute bits, which is useful when a file landed on disk as 644 and a process expected something else. The CIDR calculator expands a prefix into network address, broadcast address and usable host range so a security-group rule or a VPC subnet can be checked before it is applied. Docker run to Compose rewrites a docker run command as a Compose service, including ports, environment and volumes that the flags expressed. Review the result: some runtime flags do not have a one-line Compose equivalent.

Sign messages and inspect email authentication records

The HMAC generator computes or checks a keyed hash (SHA-256 and the other common digests) without sending the secret anywhere. Use it when a webhook signature does not match and you want to recompute the expected value from the raw body. The SPF, DKIM and DMARC inspector parses those TXT records, counts SPF DNS lookups against the ten-lookup limit, and can build a new record from common mail-provider presets. It inspects the text you paste. Publishing the record, and seeing what the live DNS zone returns, still happens at your DNS host or through a DNS lookup tool.

What these pages will not do

They will not store an account history, run your test suite, or query a third-party API on your behalf. Formatters and decoders answer questions about text you already have. If the question is “is this phone valid?”, “does this mailbox exist?” or “where is this IP?”, use the API Playground. For encoders, hash generators, beautifiers and DNS or SSL lookups, the free tools directory is the rest of the catalog, and the list under “More developer utilities” links the ones developers open most often.