Tools for building AI agents, with engineering notes

31 tools across MCP, evals, vector databases, observability and agent frameworks. Every fact is checked against the official source.

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MCP servers & tooling

Servers, SDKs and registries for the Model Context Protocol.

5 tools

Evaluation

Test and evaluate prompts, agents and RAG systems.

5 tools

Vector databases

Storage and retrieval for embeddings.

6 tools

Observability

Tracing, monitoring and evals for LLM apps and agents.

6 tools

Agent frameworks

Libraries for building agents and multi-agent workflows.

9 tools

All tools

MCP reference servers

Model Context Protocol Servers.

Pricing: not verified yet, check the tool's site

Engineering note: Collection of MCP servers maintained under the official modelcontextprotocol GitHub org. Start here to see how reference servers are structured before writing your own.

MCP Python SDK

The official Python SDK for Model Context Protocol servers and clients.

Pricing: not verified yet, check the tool's site

Engineering note: Official SDK, so it tracks the protocol directly. Covers both the server and client side in Python.

FastMCP

The fast, Pythonic way to build MCP servers and clients.

Pricing: not verified yet, check the tool's site

Engineering note: Python library for building MCP servers and clients. Compare its API against the official SDK before choosing.

MCP Inspector

Visual testing tool for MCP servers.

Pricing: not verified yet, check the tool's site

Engineering note: Use it to poke at a server you are building without wiring up a full agent first.

Smithery

Registry and connection layer that connects agents to thousands of tools and services, with auth, credentials and sessions handled for you.

Pricing: not verified yet, check the tool's site

Engineering note: Homepage lists hosted MCP servers with usage counts and a CLI (npx smithery) for adding and calling tools. Its connection auth is powered by the open-source agent.pw. Check how credentials are stored before sending real accounts through any hosted gateway.

promptfoo

Test your prompts, agents, and RAGs. Red teaming, pentesting and vulnerability scanning for AI. Compare GPT, Claude, Gemini, DeepSeek and more.

Pricing: not verified yet, check the tool's site

Engineering note: Declarative configs with CLI and CI/CD integration, so evals can run as a pipeline step. Repo says it is used by OpenAI and Anthropic.

DeepEval

The LLM Evaluation Framework.

Pricing: not verified yet, check the tool's site

Engineering note: Repo lives under the confident-ai org. Check the repo docs for the metric list before assuming it covers agent-specific cases.

Ragas

Supercharge Your LLM Application Evaluations.

Pricing: not verified yet, check the tool's site

Engineering note: The repo has moved to the vibrantlabsai org (GitHub redirects the old explodinggradients path). Pin to the current org URL in your docs.

Inspect AI

A framework for large language model evaluations.

Pricing: not verified yet, check the tool's site

Engineering note: Published by the UK Government BEIS org on GitHub. A framework to write evals as code.

OpenAI Evals

A framework for evaluating LLMs and LLM systems, and an open-source registry of benchmarks.

Pricing: not verified yet, check the tool's site

Engineering note: Combines a framework with a registry of benchmarks. Useful as a reference for eval structure.

Qdrant

Vector database (repo: qdrant/qdrant) with a managed Qdrant Cloud and a free OSS option.

Pricing: Cloud: free tier forever (single node, 0.5 vCPU / 1GB RAM / 4GB disk). Standard tier is usage-based (vCPU, memory, storage, backups, paid inference tokens, billed hourly). Premium: minimum spend. Source: qdrant.tech/pricing

Engineering note: Free tier is capped at 1GB RAM and 4GB disk and has no high availability. Paid Standard tier lists a 99.5% uptime SLA. Migration tool from OSS to Cloud is offered.

Chroma

Open-source search infrastructure for AI; Chroma Cloud offers serverless vector, full-text and metadata search.

Pricing: Cloud Starter $0/mo + usage (with $5 free credits); Team $250/mo + usage; Enterprise custom. Usage: $2.50/GiB written, $0.33/GiB-month stored, $0.0075/TiB queried, $0.09/GiB returned. Source: trychroma.com/pricing

Engineering note: Pricing is purely usage-based on writes, storage, queries and egress, so estimate with their calculator. Chroma says data export is easy since it is open source.

Weaviate

Open-source vector database that stores objects and vectors, combining vector search with structured filtering.

Pricing: Cloud: Free plan (100k objects, 1GB memory, 10GB disk, 1 collection); Flex from $45/mo pay-as-you-go; Premium from $400/mo (contact sales). Source: weaviate.io/pricing

Engineering note: Pricing is by total vector dimensions plus storage, with compression affecting price. Hybrid search is available on all plans including free. Site notes a September 2026 price book update.

Milvus

High-performance, cloud-native vector database built for scalable vector ANN search.

Pricing: not verified yet, check the tool's site

Engineering note: Described as cloud-native and built for scalable approximate nearest neighbor search.

pgvector

Open-source vector similarity search for Postgres.

Pricing: not verified yet, check the tool's site

Engineering note: Runs inside Postgres, so embeddings can sit next to your relational data and use your existing database ops.

LanceDB

Developer-friendly OSS embedded retrieval library for multimodal AI.

Pricing: not verified yet, check the tool's site

Engineering note: Embedded library rather than a separate server, which changes the deployment model compared with Qdrant or Milvus.

Langfuse

Open source agent evals and observability: trace, evaluate and improve LLM applications.

Pricing: Cloud: Hobby free (50k units/mo, 30 days data, 2 users); Core $29/mo; Pro $199/mo; Enterprise $2,499/mo. Extra usage from $8 per 100k units. Self-hosting is free. Source: langfuse.com/pricing

Engineering note: Billing counts traces, observations and scores as units, so a chatty multi-step agent consumes units fast. Supports Python/JS SDKs and OpenTelemetry. Includes prompt management and datasets.

Arize Phoenix

AI observability and evaluation.

Pricing: not verified yet, check the tool's site

Engineering note: Repo lives under the Arize-ai org. Verify the self-host and hosted options in their docs.

Helicone

Open source LLM observability platform. One line of code to monitor, evaluate and experiment.

Pricing: Hobby: 10,000 free requests, 7-day retention, 1 seat. Pro $79/mo; Team $799/mo; both usage-based on top. Source: helicone.ai/pricing

Engineering note: Integration is described as one line of code. Pricing page also lists gateway features such as caching, rate limits and fallbacks.

OpenLLMetry

Open-source observability for your GenAI or LLM application, based on OpenTelemetry.

Pricing: not verified yet, check the tool's site

Engineering note: Built on OpenTelemetry, which helps if you already ship traces to an existing backend.

Opik

Debug, evaluate and monitor LLM applications, RAG systems and agentic workflows with tracing, automated evaluations and production dashboards.

Pricing: not verified yet, check the tool's site

Engineering note: Repo lives under the comet-ml org. Combines tracing, evals and dashboards in one tool.

LangSmith

LangChain's platform for tracing, evaluation, deployment and agent tooling.

Pricing: Developer $0/seat (5k base traces/mo); Plus $39/seat/mo (10k base traces/mo); Enterprise custom. Usage in LSUs at $1.00 each. Source: langchain.com/pricing

Engineering note: Base traces are retained 14 days; extended traces 180 days for an extra fee. Pricing page says LangSmith does not train on your data.

LangChain

The agent engineering platform.

Pricing: not verified yet, check the tool's site

Engineering note: Same org as LangGraph and LangSmith (listed here), so the three are designed to be used together.

LangGraph

Build resilient agents.

Pricing: not verified yet, check the tool's site

Engineering note: Same org as LangChain. LangSmith's pricing page mentions deployment for LangGraph APIs.

CrewAI

Framework for orchestrating role-playing, autonomous AI agents.

Pricing: not verified yet, check the tool's site

Engineering note: Role-based multi-agent design: you define agents with roles that collaborate on tasks.

AutoGen

A programming framework for agentic AI.

Pricing: not verified yet, check the tool's site

Engineering note: Microsoft project. Check the repo README for current status and recommended successor before starting new work.

LlamaIndex

The document processing platform for AI.

Pricing: not verified yet, check the tool's site

Engineering note: Positions itself as a document processing platform; check docs for its agent features.

Pydantic AI

How Python does AI. Agents, realtime voice, image generation, embeddings. Every model, every interface, typed end to end.

Pricing: not verified yet, check the tool's site

Engineering note: From the Pydantic team. Selling point is end-to-end typing in Python.

smolagents

A barebones library for agents that think in code.

Pricing: not verified yet, check the tool's site

Engineering note: From Hugging Face. Agents write actions as code, so sandbox execution is something to plan for.

OpenAI Agents SDK (Python)

A lightweight, powerful framework for multi-agent workflows.

Pricing: not verified yet, check the tool's site

Engineering note: Official OpenAI library for multi-agent workflows in Python.

Mastra

The modern TypeScript framework for AI-powered applications and agents.

Pricing: not verified yet, check the tool's site

Engineering note: One of the TypeScript-first options in this list; the rest are mostly Python.