Langchain Py

LangChain Python (v1.3-focused)

Practical workflow for building reliable LangChain systems with correct 1.x APIs and strong multi-agent design.

When to activate

• Building or refactoring Python agents with create_agent
• Designing multi-agent systems (subagents, handoffs, router, skills)
• Implementing middleware/context engineering/stateful tools
• Integrating Tavily or provider-specific tools
• Migrating snippets from stale pre-1.0 docs to current semantics

Version baseline (important)

Assume LangChain 1.3.x semantics (langchain-core on the 1.4.x line) with LangGraph 1.2.x unless the user explicitly requests another version.

• Prefer from langchain.agents import create_agent
• Prefer middleware-based dynamics (dynamic_prompt, wrap_model_call, wrap_tool_call)
• Use Command(update=...) for state updates in tools/handoffs
• Treat old langgraph.prebuilt.create_react_agent snippets as migration candidates — the langgraph.prebuilt module is deprecated; canonical home is langchain.agents
• Legacy chain abstractions (LLMChain, initialize_agent, AgentExecutor, old memory classes) live in langchain-classic; AgentExecutor reaches end of maintenance December 2026 — migrate to create_agent or a raw LangGraph StateGraph
• LangGraph 1.2 adds per-node execution controls (timeouts, error-recovery policies, graceful shutdown hooks), the streaming API v3 (typed per-channel projections), and beta DeltaChannel for incremental checkpoint writes on large state

If uncertain, load references/version-scope.md first.

Recommended workflow

1. Scope: identify version, providers, and required capabilities (single agent vs multi-agent).
2. Choose architecture:
   • Single agent for simple tasks and small toolsets
   • Subagents for domain separation and centralized coordination
   • Handoffs for sequential/user-facing state transitions
   • LangGraph custom workflow for deterministic control gates
3. Define tools with strong names, typed signatures, and clear docstrings.
4. Engineer context via middleware (prompt/tool/model/response format selection).
5. Add memory/persistence with checkpointer/store only where needed.
6. Harden with error middleware, interrupts (HITL), and traceability.
7. Verify version correctness (no legacy imports/patterns).

Multi-agent design rules

• Subagents: supervisor calls specialists as tools; great for domain boundaries.
• Handoffs: tool-driven state transitions; best for staged conversational workflows.
• Router: classification + dispatch for fast parallel domain fan-out.
• Skills: on-demand context loading while one main agent remains in control.

When implementing handoffs:

• Keep message history valid (tool-call and ToolMessage pairing).
• Use Command.PARENT only when explicit parent-graph routing is required.
• Pass minimal context between agents; summarize rather than dumping internal traces.

Tavily integration guidance

Use Tavily as a focused retrieval/search tool, not as a blanket dependency for every agent.

• Add Tavily mainly to research subagents.
• Keep max_results and depth bounded to control token/latency costs.
• Use domain filters (include_domains, exclude_domains) for precision.

Load references/tavily-integration.md for concrete patterns.

Anti-patterns

• Mixing pre-1.0 and 1.x APIs in same implementation
• Giving every agent every tool (routing degrades quickly)
• Handoffs without valid message/tool pairing
• Stateful workflows without checkpointer persistence
• Copy-pasting old blog snippets without migration audit

Outcome expectations

• Architecture choice is explicit and justified.
• Generated code is valid for LangChain 1.3.x / LangGraph 1.2.x.
• Multi-agent context boundaries are intentional and testable.
• Tooling and middleware are minimal but sufficient for reliability.

Resources

Load on demand:

• references/version-scope.md — 1.3.x / LangGraph 1.2.x scope, migration-critical rules, changelog highlights
• references/multi-agent-patterns.md — pattern selection and tradeoffs
• references/handoffs-and-command.md — state-machine handoffs, Command, message validity
• references/subagents-supervisor.md — supervisor/subagent layering and information flow
• references/tavily-integration.md — LangChain + Tavily setup and practical constraints
• references/api-cheatsheet.md — fast API checklist for create_agent, ToolRuntime, Command