Best Free AI / LLM Developer Tools (2026)
Everything you need to build and ship with large language models — counting tokens, estimating cost, fitting the context window, wrangling JSON output, and generating tool definitions. All client-side, no upload, no signup.
Tools
LLM Token Counter
Count tokens for GPT, Claude, and Gemini and predict cost before you send. Runs locally.
AI API Cost Calculator
Estimate monthly cost across 100+ models with prompt-caching and batch discounts. Prices refreshed daily.
Context Window Checker
See if your prompt fits GPT-4o, o1, Claude, Gemini or DeepSeek before you hit 'context length exceeded'.
Prompt Token Breakdown
Split a prompt into segments and see which part eats the most tokens and cost.
LLM JSON Extractor
Pull every JSON block out of a ChatGPT / Claude reply — fences stripped, errors auto-repaired.
Schema → Tool Definition
Generate OpenAI, Anthropic, and Gemini function-calling definitions from one sample, side by side.
Chat Messages Validator
Validate a Chat Completions messages array — roles, content, and structure — before you hit a 400.
Prompt Template Tester
Fill {{variable}} placeholders in a prompt template and preview the final prompt, with unfilled vars flagged.
Prompt Diff
Compare two prompt versions line by line and see the token delta between them.
Embedding Cost Calculator
Estimate the cost of embedding a corpus across OpenAI, Voyage, Cohere, and Gemini models.
Vector Storage Calculator
Estimate the RAM / disk your embeddings need by count, dimensions, and precision.
Prompt Cache Calculator
See how much prompt caching saves given your prefix size, volume, and hit rate.
Batch API Cost Calculator
Compare real-time vs Batch API cost and the ~50% bulk discount for offline jobs.
Prompt Injection Detector
Scan untrusted input for injection / jailbreak patterns with a heuristic risk score.
Text Chunker for RAG
Split long text into overlapping chunks for embeddings with live token counts.
Cosine Similarity Calculator
Compare embedding vectors with cosine, dot product, and Euclidean distance.
Reference
LLM API Pricing Comparison
Sortable table of input/output/cached prices and context windows across 100+ models.
LLM API Errors — Meaning & Fix
429, context_length_exceeded, 401 and more — what each means and how to fix it.
LLM API Parameters
What temperature, top_p, max_tokens and the rest do, and how they differ across providers.
Why these are the best: Every tool runs entirely in your browser — your prompts, keys, and data never leave your device. Token counts match official tokenizers within ±3%, prices are refreshed daily from the public LiteLLM catalog, and the JSON tools handle the messy real-world output models actually produce.