Prompt Assay

Prompt Assay is a focused workbench for building, evaluating, and maintaining prompts and Agent Skills. It is best for prompt specialists, model researchers, shared-library teams, and developers who need repeatable assets rather than occasional chat experiments. Its strongest advantage is cross-provider evaluation in one workflow; the trade-offs are BYOK dependence after the starter calls, one-seat entry plans, and a $49 monthly Solo commitment.

Overview

The product follows a prompt from drafting and refinement through versioning, testing, and delivery, then applies a similar process to multi-file Agent Skill bundles. BYOK connections cover Anthropic, OpenAI, and Google models, so teams can assess provider differences without rebuilding the workflow in separate tools. That makes Prompt Assay a better fit for structured prompt engineering and model assessment than for casual, one-off use.

Provider keys are encrypted at rest, excluded from logs, and used only for calls a user initiates. Provider responses are generally not retained on the server; evaluation test outputs and Skills Behavioral Eval results are exceptions. Prompt Assay says customer content is not used for training. Organizations requiring formal compliance certifications should account for the absence of current SOC 2 and ISO 27001 attestations.

Key features

Authoring, critique, and version control

An in-editor AI partner can brainstorm, critique, improve, rewrite, compare, and convert prompts. Its critique scores clarity, completeness, structure, technique use, robustness, and efficiency, adding written rationale rather than leaving users with a bare grade. Version history, templates, tags, fragments, and metadata support a reusable library; the import and export bundle preserves those elements. Markdown, Anthropic JSON, OpenAI JSON, and raw-text imports make migration practical for common prompt formats.

Multi-model comparison

A single prompt can run against two to five BYOK models in parallel. Side-by-side output, latency, token use, and cost expose the operational differences that a single response hides, while optional LLM-as-Judge scoring adds a comparative signal. This is particularly useful for researchers and teams choosing a provider or tuning a prompt for more than one model.

Agent Skills and delivery

Users can author multi-file Agent Skill bundles, critique them, and run Behavioral Evals across Claude, GPT, and Gemini. Developers can retrieve assets through the public REST API or TypeScript SDK, but those interfaces require Solo or a higher tier and use API-key authentication. The free plan therefore works for exploration, not for production retrieval.

Pricing

Free: billed at $0 per month, with seven platform-funded calls to start, 250 AI calls per month, one seat, a seven-day version view, and BYOK required for ongoing use. It is a useful low-risk introduction, but the call ceiling, short history window, and single-seat design limit sustained team work.

Solo: $49 a month for one seat. It adds unlimited AI calls on your keys, full prompt and Skill version history, and the REST API and TypeScript SDK. This is the clearest fit for an individual professional or developer who needs automation and durable history; it remains expensive for light personal use.

Team: $99 per seat per month, with a minimum of three seats and annual billing available, supporting three to 15 seats. Shared organization libraries, evaluation suites, and owner, admin, and member roles justify the jump when collaboration is central, but the minimum commitment makes it unsuitable for a small two-person effort.

Enterprise: custom contract pricing, with unlimited seats, SAML SSO, custom tier controls, a data-processing agreement, and priority support. It targets organizations with identity, procurement, and support requirements rather than buyers seeking a published budget.

Platforms

Prompt Assay is offered through a web application and an API. The TypeScript SDK is part of the API offering on Solo and higher plans.

Who it's for

Prompt engineers gain structured critique and version history; researchers gain parallel runs and measurable latency, token, cost, and judge comparisons; teams gain shared libraries and role controls on Team; developers gain programmatic retrieval on Solo or above. It is less suitable for users who want a generous collaborative free tier, built-in provider access without their own keys, or formal SOC 2/ISO 27001 coverage.

Pros and cons

• Pros: Cross-provider runs cover two to five models with operational metrics, making prompt decisions more evidence-based.
• Pros: Agent Skill authoring and Behavioral Evals extend the workflow beyond single prompt files.
• Pros: Encrypted key handling, limited retention, and no training on customer content provide clear data boundaries.
• Cons: Free usage drops to BYOK after seven funded calls and is capped at 250 calls monthly.
• Cons: Solo is single-seat at $49 a month, while Team requires at least three seats.
• Cons: API and SDK access are paywalled, and the product lacks SOC 2 and ISO 27001 attestations.

Alternatives

Vellum is worth choosing when broad platform coverage, including mobile, desktop, extension, web, and self-hosting, matters more than Prompt Assay's focused comparison workflow.

PromptHelm suits buyers wanting a free gateway allowance of 10,000 calls per month, though it limits prompts, versions, seats, organizations, and log retention.

Agenta is a stronger alternative for teams prioritizing agent runs, evaluations, trace retention, or self-hosting on its free options.

Impromptо is the alternative to consider for a lower-priced early-bird team plan with playground runs and panel-based workspaces.

Langfuse fits observability-focused teams that need usage units, retained data access, and self-hosting options.

Modular Prompt Library is appropriate for personal prompt and module organization with testing through your own API keys.

PromptDocker offers another freemium option for prompt workflow management.

PromptFluent is better suited to browser-based prompt storage, search, favorites, and on-demand scans.

Verdict

Choose Prompt Assay if you need disciplined prompt and Agent Skill development with measurable comparisons across Anthropic, OpenAI, and Google models, plus an API for shipping approved assets. Look elsewhere if collaboration must start free, if you cannot supply provider keys, or if certified compliance is a purchase requirement.