✦ 26 free tools, 100% client-side

Free Developer Tools

Fast, privacy-first utilities for developers. Everything runs in your browser — nothing you enter is ever sent to a server.

Data

{ }

JSON Formatter

Format, validate, and beautify JSON instantly.

#ID

UUID Generator

Generate v4 UUIDs or ULIDs in bulk, one click to copy.

T:00

Timestamp Converter

Convert Unix timestamps to human-readable dates and back, in any timezone, instantly.

⏱

Cron Parser

Paste a cron expression to get a plain-English explanation and the next 5 scheduled run times.

Encoding

B64

Base64 Encode/Decode

Encode text to Base64 or decode Base64 back to plain text, live as you type.

%20

URL Encode/Decode

Encode text or full URLs into percent-encoded form, or decode them back to readable text, instantly.

Security

JWT

JWT Decoder

Decode a JSON Web Token's header and payload instantly and inspect expiry.

#H

Hash Generator

Generate MD5, SHA-1, SHA-256, and SHA-512 hashes from text instantly.

402

x402 Payment Validator

Decode and validate an x402 protocol payment payload (the header AI agents and APIs exchange for HTTP 402 machine payments), including a real EIP-712/EIP-3009 ECDSA signature check against the declared sender.

Text

.*

Regex Tester

Test regular expressions against sample text with live match highlighting and capture group breakdown.

Design

◍

Color Converter

Convert colors between HEX, RGB, and HSL formats live, with an instant preview swatch.

AI

GPT

Token Counter

Count tokens for GPT-5, GPT-4o, and older OpenAI models as you type, using the exact same tokenizer OpenAI's API uses.

[1]

Citations Viewer

Paste a Claude Citations API response to see the answer text with each cited passage highlighted and matched to its source document, location, and quoted text.

TS

Signature Validator

Getting a 400 error or silently broken multi-turn tool use from the Gemini 3 API? Paste your request or response to check function-calling turns for missing thoughtSignature fields, functionResponse ordering mistakes, and missing id echoes.

MCP

MCP Validator

Paste a Model Context Protocol tool definition — a single tool object, an array, or a full tools/list response — to check it against the spec: missing name or inputSchema, a name most clients will reject, a required property that doesn't exist, malformed annotations, and more.

MRL

Embedding Truncator

Truncate a Matryoshka-style embedding vector to fewer dimensions and correctly L2-renormalize it — the step most people skip when truncating OpenAI, Gemini, Cohere, or Nomic embeddings by hand.

SSE

AI SDK Stream Debugger

Paste a raw SSE response from a custom Vercel AI SDK backend and validate it against the UI Message Stream Protocol — catches malformed JSON, missing required fields, orphaned deltas, and unrecognized part types that silently break useChat.

M28

MCP Migration Checker

MCP server or client stopped working after updating to the 2026-07-28 spec's stateless rewrite? Paste a request or response to check for the removed initialize handshake, missing _meta protocol fields, and malformed Multi Round-Trip Request results.

APM

Agent Plugin Validator

Paste a plugin.json and optional mcp.json to check them against the Agent Plugins 1.0.0 specification (OpenAI, AWS, Cursor, Microsoft, and Vercel's Aug 2026 standard for portable agent extensions) — catches an invalid $schema, a malformed plugin name, disallowed fields, and broken MCP server configs.

APP

MCP Apps Validator

Paste a tool's _meta.ui block, a UI resource from resources/read, or a ui/* postMessage envelope to check it against the MCP Apps 1.0 specification — catches a wrong resourceUri scheme, an incorrect mimeType, malformed CSP domain lists, and invalid ui/* message shapes.

ACP

ACP Validator

Paste a JSON-RPC message and check it against the Agent Client Protocol (ACP) — the Zed-originated, now multi-vendor standard for editor-to-coding-agent communication.

SKL

SKILL.md Validator

Validate a SKILL.md file's YAML frontmatter against the open Agent Skills standard.

TSK

MCP Tasks Validator

Validate a Model Context Protocol Task/status object, a tasks/get, tasks/cancel, or tasks/list message, a task-creation request, or a tasks capability negotiation object against the official Tasks extension shape.

SRV

server.json Validator

Validate a server.json manifest against the official Model Context Protocol Registry schema before publishing — required fields, name format, package/remote transports, icons, and repository metadata.

OTL

GenAI Span Validator

Validate an OpenTelemetry span's gen_ai.* attributes against the official GenAI semantic conventions — attribute types, enum values, required fields per operation, and span-name template conformance.

MEM

Memory Tool Validator

Check a Claude memory tool command (view/create/str_replace/insert/delete/rename) or a context_management config against Anthropic's published schemas, and test any path for the exact sandbox-escape bug class behind a real, patched CVE.

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Recommended resources

A few other free resources developers tend to reach for. Not sponsored — just genuinely useful.

  • MDN Web Docs — the reference for HTML, CSS, and JavaScript.
  • DevDocs — fast, searchable API docs for dozens of languages and frameworks in one place.
  • Can I Use — browser support tables for web platform features.