Fern vs Vellum
Side-by-side comparison of AI visibility scores, market position, and capabilities
Automatic multi-language SDK and API documentation generator from OpenAPI specs; $13M Bessemer-led Series A from YC W23 serving Cohere and ElevenLabs competing with Stainless for API developer experience.
About
Fern is a New York-based API tooling company that automatically generates multi-language SDKs and API documentation from OpenAPI specifications — enabling API companies to publish high-quality client libraries in TypeScript, Python, Java, Go, Ruby, and C# from a single source of truth, eliminating the significant engineering effort of manually building and maintaining SDKs for each programming language a developer audience uses. Founded in 2022 by Danny Sheridan and Deep Singhvi and a Y Combinator Winter 2023 graduate, Fern raised $13 million total including a $9 million Series A led by Bessemer Venture Partners with 30 employees.
LLM application development platform with prompt management, evaluation, and RAG workflows; structured AI feature development competing with LangSmith and Weights & Biases Prompts.
About
Vellum is an AI product development platform providing prompt management, model comparison, workflow orchestration, and production monitoring tools for engineering and product teams building LLM-powered applications — enabling teams to iterate on AI features with rigorous evaluation frameworks rather than ad-hoc prompt tweaking. Founded in 2023 by Andrew Kirima and Noa Flaherty in San Francisco, Vellum has raised approximately $12 million and targets AI-forward product teams at growth companies who need structured workflows for LLM feature development, testing, and deployment.\n\nVellum's platform covers the LLM application development lifecycle: Prompt Workshop for managing and versioning prompt templates with variable substitution, Evaluations for testing prompts against datasets to measure output quality before deployment, Document Index for building RAG (retrieval-augmented generation) pipelines with semantic search over enterprise documents, and Workflows for orchestrating multi-step AI pipelines with branching logic and human-in-the-loop review steps. The monitoring dashboard tracks production LLM performance, latency, and cost across models.\n\nIn 2025, Vellum competes in the rapidly growing LLM development tools market against LangSmith (LangChain's commercial platform), Weights & Biases Prompts, Helicone, Braintrust, and Humanloop for AI application observability and evaluation. The market has grown explosively as companies productionize LLM features and need rigorous quality control processes. Vellum's differentiation is its end-to-end workflow — from prompt development through evaluation to production monitoring — in a single platform rather than requiring separate tools for each stage. The 2025 strategy focuses on expanding workflow complexity support (longer multi-agent pipelines), growing enterprise adoption with SSO and access controls, and adding AI-powered evaluation that automatically judges output quality.
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