Best Prompt Optimizer Tools in 2026: 8 Options Compared (Free and Paid)

Quick answer: the best prompt optimizer depends on where your prompts run. If you write prompts by hand for ChatGPT, Claude, or Gemini and want them rewritten and saved, use a hosted optimizer like Prompt Builder (free tier, paid plans from $9 a month). If you want something free and self-hosted with your own API key, use the open-source Prompt Optimizer project. If your prompts live inside an app and you can score outputs, use DSPy plus an eval tool like Promptfoo. OpenAI's dataset-backed prompt optimizer is no longer a long-term option: its Evals platform goes read-only on October 31, 2026 and shuts down on November 30, 2026.

This guide compares 8 prompt optimizer tools by price, what they actually change in a prompt, and who they fit. Prices and features were checked on October 25, 2026.

Why this list changed in the last two months

Two of the best-known prompt optimizers left the market in 2026.

• PromptPerfect closed on September 1, 2026 after Elastic acquired Jina AI. We covered the PromptPerfect shutdown and alternatives in March.
• OpenAI's prompt optimizer in the platform dashboard is tied to the Evals platform. OpenAI's deprecation notice says existing evals become read-only on October 31, 2026 and the Evals dashboard and API shut down on November 30, 2026. OpenAI's own migration guide points developers to Promptfoo.

If you relied on either one, you need a replacement that does two jobs: rewrite the prompt, and give you some way to tell whether the rewrite is better.

What a prompt optimizer actually changes

A prompt optimizer takes the prompt you already have and rebuilds it so the model has less to guess. The good ones change the same four things every time:

1. Goal: one clear sentence about the outcome, not a topic.
2. Context: who the output is for, what the model should know, what already exists.
3. Constraints: length, tone, what to avoid, what must be included.
4. Output format: headings, a table, JSON, a word count, a number of options.

This is not cosmetic. In our prompt structure study, 256 prompts across GPT-5.6, Claude Sonnet 5, Gemini 3.5 Flash, and Grok 4.5 were blind-scored. The structured 4-part version averaged 18.97 out of 20, against 9.56 for a bare one-line ask, a 98% gap. That gap is what you are paying an optimizer to close.

Here is a typical before and after:

	Prompt
Before	Write a cold email for my SaaS.
After	You are writing a first-touch cold email for a B2B SaaS that schedules shifts for restaurants. Reader: an operations manager at a 5 to 20 location restaurant group. Goal: get a 15-minute call. Constraints: under 120 words, no buzzwords, one concrete number about time saved, no attachments or links except one calendar link. Output: 3 subject lines, then the email body, then one follow-up line to send 4 days later.

The second prompt is longer, but every added line removes a decision the model would otherwise make badly.

The 8 best prompt optimizer tools in 2026

Tool	Price	Best for	Needs your API key	Built-in testing
Prompt Builder Optimizer	Free tier, then $9, $19, or $49 a month	People who write prompts for ChatGPT, Claude, Gemini, Grok	No	Run prompts in the app
Prompt Optimizer (open source)	Free (AGPL-3.0), you pay model usage	Self-hosting, privacy, power users	Yes	Yes, compare evaluation
OpenAI prompt optimizer	Included with the API platform until Nov 30, 2026	Teams with datasets and graders on OpenAI	Yes (platform account)	Yes, via Evals
Aiven Prompt Optimizer	Free, no signup	Cutting tokens in long system prompts	No	No
PromptOptimizer.tools	Free	Quick one-off rewrites of text and image prompts	No	No
DSPy	Free, open source	Developers optimizing prompts inside a pipeline	Yes	Yes, against your metric
Promptfoo	Free, open source	Measuring whether a rewrite is actually better	Yes	Yes, that is its job
DIY meta-prompt	Free	Anyone with a chat subscription	No	Manual

1. Prompt Builder Optimizer

Prompt Builder is a prompt generator and optimizer for ChatGPT, Claude, Gemini, Grok, DeepSeek, Llama, Mistral, Perplexity, and Cohere. The Optimizer works on a prompt you already use: paste it in, it analyzes what is missing, asks up to three short clarifying questions, and rebuilds the prompt for the model you picked. Every refine is saved on a version timeline, so you can compare against where you started and roll back.

What makes it different from a one-shot rewriter is what happens after the rewrite. You can save the prompt to a Library, attach reusable Instructions (your voice, your product context, your constraints) that apply to every future prompt, and run the prompt inside the app with several models.

Pricing: free plan with 5 prompts a month, no card. Starter is $9 a month (1,000 credits), Pro is $19 a month (3,000 credits), and Unlimited is $49 a month. Yearly billing saves 20%, and paid plans have a 14-day money-back guarantee. See pricing.

Best for: marketers, founders, writers, and support teams who write prompts by hand every day and want them consistent. Start with the AI Prompt Generator, or go straight to the ChatGPT or Claude generator.

Skip it if: your prompts live in code and you need automatic optimization against a test set. Use DSPy for that.

2. Prompt Optimizer (open source, by linshenkx)

The most popular open-source option, with about 39,000 GitHub stars. It runs as a web app, a desktop app, a Chrome extension, or a Docker container, and it also exposes an MCP server so tools like Claude Desktop can call it. It optimizes both system prompts and user prompts, supports multi-round iteration, and has analysis and side-by-side compare evaluation, which most free tools lack.

You bring your own API key (OpenAI, Gemini, DeepSeek, Grok, and any OpenAI-compatible endpoint, including local Ollama). Processing happens in the browser, with requests going straight to the model provider.

Pricing: free, licensed AGPL-3.0. You pay your model provider per token.

Best for: developers and privacy-minded users who are fine managing API keys. The web version hits CORS errors with some providers; the desktop app avoids that.

3. OpenAI prompt optimizer (shutting down)

OpenAI's optimizer lives in the platform dashboard. You build a dataset with at least three rows of responses, add Good/Bad annotations, written critiques, or grader results, then click Optimize. It rewrites the prompt against that feedback, and OpenAI's own docs warn that an optimized prompt can still do worse than the original on specific inputs.

It is a good tool for teams already on Evals, but it is on a deadline: read-only on October 31, 2026, shut down on November 30, 2026. Export your datasets and prompts now. If you used it as a casual "ChatGPT prompt optimizer," a hosted optimizer is the closer replacement. If you used it with graders, move to Promptfoo for testing and DSPy for the automatic rewriting.

4. Aiven Prompt Optimizer

A free web tool from the database company Aiven. No signup. It offers modes rather than one generic rewrite: reduce tokens, improve structure, add examples, optimize for Managed Agents, or follow your own instructions. Aiven says prompts are only sent when you click Optimize and are not used to train models.

Best for: trimming a long system prompt that has grown over months. The token-reduction mode is the main reason to use it.

5. PromptOptimizer.tools

A free, single-box rewriter for text and image prompts with a 5,000-character limit. The site says it has optimized more than 384,000 prompts. There are no accounts, no history, and no testing, which is the point: paste, rewrite, copy.

Best for: occasional users who need one better prompt right now and nothing else.

6. DSPy

DSPy is an open-source Python framework from Stanford NLP. Instead of rewriting a prompt once, you define the task as a program, give it a metric and some examples, and an optimizer such as MIPROv2 searches for better instructions and few-shot examples automatically.

Best for: engineers with a pipeline in production and a way to score outputs. It is the most powerful option here and the least useful for someone writing prompts in a chat window.

7. Promptfoo

Promptfoo does not rewrite prompts. It tests them: you define test cases and assertions, run every prompt version across models, and get a pass rate. It is the tool OpenAI recommends for teams leaving Evals.

Best for: pairing with any optimizer above. An optimizer without a test is a guess.

8. The DIY meta-prompt

If you only need an occasional rewrite and already pay for ChatGPT, Claude, or Gemini, ask the model to optimize your prompt. This works better than most people expect if the instruction is specific:

Rewrite the prompt below so a model can follow it without guessing.
Keep my intent. Add: a one-sentence goal, the audience, constraints
(length, tone, must-include, must-avoid), and an exact output format.
If something important is missing, ask me up to 3 questions first.

Prompt: [paste here]

The cost is your time. You will rewrite this meta-prompt per model and lose good versions in old chats, which is the problem dedicated tools solve.

How to choose in 30 seconds

• You write prompts by hand for several models and want them saved: Prompt Builder.
• You want free and private, and you have API keys: open-source Prompt Optimizer.
• You were on OpenAI's optimizer with graders: Promptfoo for testing, DSPy for automatic rewriting.
• You need fewer tokens in a long system prompt: Aiven.
• You need one better prompt, once: PromptOptimizer.tools or the DIY meta-prompt.

How to test whether the optimized prompt is better

Do not trust the rewrite because it looks more professional. Run this 10-minute check:

1. Pick 5 real inputs, including one awkward edge case.
2. Run the original and the optimized prompt on each, same model, same settings.
3. Score each output 0 to 5 on two things that matter for the task (for example, accuracy and format).
4. Keep the optimized version only if it wins on at least 4 of the 5 inputs and does not lose badly on the edge case.

If you run the same prompt across models, test per model. In our study, adding a worked example helped Gemini and Grok but slightly hurt Claude, so the "best" prompt is not the same everywhere. Our Claude prompt engineering guide covers the Claude-specific differences.

FAQ

What is a prompt optimizer? A prompt optimizer is a tool that rewrites an existing prompt so an AI model can follow it with less guessing. It typically adds a clear goal, context, constraints, and an output format, and the better tools let you test the result against the original.

Is OpenAI's prompt optimizer going away? Yes. The dataset-backed prompt optimizer is part of OpenAI's Evals platform, which becomes read-only on October 31, 2026 and is scheduled to shut down on November 30, 2026. OpenAI points developers to Promptfoo as the migration path.

What is the best free prompt optimizer? For a hosted tool with no setup, Prompt Builder's free plan (5 prompts a month) and Aiven's free optimizer are the easiest. For unlimited free use with your own API key, the open-source Prompt Optimizer project is the strongest option.

Is there a prompt optimizer for Claude? Yes. Prompt Builder lets you pick Claude as the target model and rebuilds the prompt for how Claude follows instructions, and the open-source Prompt Optimizer works with Claude through any compatible endpoint. You can also start from the free Claude Prompt Generator.

Do prompt optimizers work for image prompts? Some do. PromptOptimizer.tools and the open-source Prompt Optimizer both handle image prompts. For Midjourney and Stable Diffusion specifics, see our image prompt tips.

What replaced PromptPerfect? PromptPerfect closed on September 1, 2026. Our PromptPerfect alternative page compares the replacements and covers how to move your saved prompts.

Bottom line

The prompt optimizer market got smaller in 2026, which makes the choice easier. Pick a hosted optimizer if you write prompts by hand, an open-source one if you want control and your own keys, and DSPy with Promptfoo if your prompts run in code. Whichever you pick, test the rewrite on five real inputs before you trust it. For more options beyond optimization, see our roundup of the best prompt builder tools in 2026.