Modular Prompt Library vs Prompt Assay

Modular Prompt Library is a web platform for building, managing, sharing, testing, and comparing AI prompts. Users can assemble prompts from reusable components and use them across projects. Version history records changes and supports returning to an earlier version, while shared libraries let teams review and collaborate on prompt work. For testing, users connect their own OpenAI, Anthropic, or Google API keys, fill prompt variables, adjust temperature, top-p, and max tokens, and save presets for a prompt version. The comparison environment displays outputs side by side with latency, token counts, and estimated cost. The site says keys are encrypted at rest, usage is billed directly by the model provider, and prompts, variables, and outputs remain in the user's workspace. Personal Account is free for personal use, with up to five workspaces and unlimited prompts and modules. Starter is free for up to five users or three workspaces; Growth costs $2.50 a month and Professional costs $5 a month. Organizational accounts have a 60-day free trial, and no credit card is required to start.

Prompt Assay is a web and API workbench for authoring, refining, versioning, testing, and shipping LLM prompts and Agent Skills. Its critique feature evaluates clarity, completeness, structure, technique use, robustness, and efficiency, with written reasons. An in-editor AI pair can brainstorm, critique, improve, rewrite, compare, and convert content. Connect your own Anthropic, OpenAI, or Google models to run a prompt across 2–5 models in parallel and compare outputs, latency, token use, and cost; optional LLM-as-Judge scoring is available. For Agent Skills, users can create multi-file bundles, critique them, and run Behavioral Evals across Claude, GPT, and Gemini. Imports accept Markdown, Anthropic JSON, OpenAI JSON, or raw text; its bundle preserves versions, tags, fragments, and metadata. The Free plan is free, with 250 calls monthly and one seat; seven initial platform-funded calls are available before BYOK is needed for ongoing use. Solo costs $49 a month and includes API and TypeScript SDK access. The public API and SDK require Solo or higher.