langchain-oss-primer

It answers three questions every project must resolve upfront:

1. Which framework? — LangChain, LangGraph, or Deep Agents
2. Which agent archetype? — maps your use case to the right API and patterns
3. What to install and which skills to load next

Load this skill first. Once you've decided on a framework and agent type, follow the "Next Skills" section at the bottom — it tells you exactly which skills to invoke next based on your choices.

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Step 1 — Pick Your Framework

The three frameworks are layered, not competing. Each builds on the one below:

┌─────────────────────────────────────────┐
│              Deep Agents                │  ← batteries included
│   (planning, memory, skills, files)     │
├─────────────────────────────────────────┤
│               LangGraph                 │  ← custom orchestration
│    (nodes, edges, state, persistence)   │
├─────────────────────────────────────────┤
│               LangChain                 │  ← foundation
│      (models, tools, prompts, RAG)      │
└─────────────────────────────────────────┘

Answer these questions in order:

Question	Yes →	No →
User needs or wants planning, persistent memory, complex task management, long-running tasks, out-of-the-box file management, on-demand skills, or built-in middleware, subagents, easy expansion capabilities?	Deep Agents	↓
Needs custom control flow — specified loops, branching, deterministic parallel workers, or manually instrumented human-in-the-loop?	LangGraph	↓
Single-purpose agent with a fixed set of tools?	LangChain (create_agent)	↓
Simple prompt pipeline or retrieval chain with no agent loop?	LangChain (direct model / chain)	—

Higher layers depend on lower ones only when necessary — you can mix them. A LangGraph graph can be a subagent inside Deep Agents; LangChain tools work inside both.

	LangChain	LangGraph	Deep Agents
Control flow	Fixed (tool loop)	Custom (graph)	Managed (middleware)
Middleware	Callbacks only	✗ None	✓ Explicit, configurable
Planning	✗	Manual	✓ TodoListMiddleware
File management	✗	Manual	✓ FilesystemMiddleware
Persistent memory	✗	With checkpointer	✓ MemoryMiddleware
Subagent delegation	✗	Manual	✓ SubAgentMiddleware
On-demand skills	✗	✗	✓ SkillsMiddleware
Human-in-the-loop	✗	Manual interrupt	✓ HumanInTheLoopMiddleware
Custom graph edges	✗	✓ Full control	Limited
Setup complexity	Low	Medium	Low

Middleware is a concept specific to Deep Agents (explicit middleware layer). LangGraph has no middleware — behavior is wired directly into nodes and edges. If a user asks for built-in hooks or automatic middleware, route to Deep Agents.

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