PromptPex vs Vellum

PromptPex is an open-source tool for developers who keep AI prompts in their codebase and want to examine how models follow prompt requirements. It treats prompts as functions, extracts output rules expressed in natural language, and generates unit tests to check whether a model follows those rules. It can also evaluate prompt performance across models, using an LLM to judge whether outputs meet stated requirements. Test generation can be shaped with scenarios, expansions, custom instructions, and effort levels. The CLI accepts Markdown or text templates, Prompty files, and GitHub Models prompt YAML files. Setup lists OpenAI, Azure OpenAI, GitHub Models, and Ollama as supported providers. PromptPex is packaged as an npm CLI and can run in Docker; its setup guide also describes a local web interface. Documented integrations include OpenAI Evals, Azure OpenAI Stored Completions, GitHub Models Evals, and GitHub Actions. Microsoft is identified as the maker. The project is intended for research, and its maker advises against downstream use without additional evaluation of robustness, safety, harm, and bias. Vague prompts can lead to suboptimal results, and rule extraction may be less accurate with complex or idiomatic language.

Vellum is a personal AI assistant that can read files, manage calendars, send email, run code, and control a computer. It saves preferences and context in its workspace for later conversations, and users can create, install, and manage skills. Listed OAuth services include Discord, GitHub, Google services, HubSpot, Linear, Twitter (X), Asana, Notion, Microsoft Outlook, and Todoist. Vellum says OAuth tokens stay in a credential vault rather than being exposed to the language model. Sensitive actions prompt for approval, and users can set trust rules for standing permissions. By default, conversation context is sent to Anthropic to generate responses; OpenAI, Google, OpenRouter, Fireworks, and local Ollama are listed alternatives. An internet connection is required. Vellum Cloud runs assistants in isolated environments with private storage and 24/7 availability. The Free plan includes first tasks and pay-as-you-go credits. Mighty costs $30 a month, Super $100 a month, and Ultra $200 a month. There is no shipped Linux desktop client, but Linux users can use the web app, Chrome extension, or self-host the runtime.