LangChain / LangGraph engineering pitfalls and verified fixes. Covers DeepAgents, structured output, OpenAI-compatible model integration (including Chinese provider adapters: DeepSeek, Qwen, GLM, etc.), middleware, streaming, multi-agent orchestration, and other common development issues. Use when h

LangChain Dev Guide A systematic summary of typical issues, non obvious behaviors, and verified solutions encountered in real engineering with the LangChain / LangGraph ecosystem. Every entry comes from a real development scenario and is organized by category. [!IMPORTANT] This skill is an engineering practice reference , not an introductory tutorial. Each entry assumes the developer is already familiar with basic LangChain concepts (agent, tool, message, graph). How to Use 1. First use the "Scenario Index" below to locate the category file your problem belongs to. 2. When unsure which category applies, search keywords directly in the "Common Issues Quick Reference". 3. For ContextSeek / semantic memory: start with [contextseek middleware.md](reference/contextseek middleware.md) to identify your scenario, then go to [contextseek params.md](reference/contextseek params.md) for specific parameter configuration issues. 4. Once you find the relevant section, read it in depth — every entry follows the structure Symptom → Cause → Solution → Lessons learned . Scenario Index Category File Trigger Scenarios Deep Agents [reference/deepagents.md](reference/deepagents.md) Model selection, filesystem backend, disabling the general purpose sub agent, file permissions, long term memory, long SKILL.md truncated by read file 100 line default Structured Output [reference/structured output.md](reference/structured output.md) Model level method selection, create agent strategies, missing fields, unsupported tool choice , provider side 400 errors on forced schema tool selection OpenAI compatible Model Integration [reference/model integration.md](reference/model integration.md) Pitfalls when using ChatOpenAI against OpenAI compatible providers, integrating Reasoning models (chain of thought / reasoning content ) CN Model Integration [reference/cn models/README.md](reference/cn models/README.md) Generating LangChain integration classes for Chinese providers (DeepSeek, Qwen, GLM, Moonshot) Middleware [reference/middleware.md](reference/middleware.md) Middleware execution order, state schema merging, HITL resume values, modifying state from wrap model call Streaming Output [reference/streaming.md](reference/streaming.md) Choosing between stream events and stream , distinguishing tokens from multiple LLMs, disabling streaming, custom progress events Multi Agent Orchestration [reference/multi agent.md](reference/multi agent.md) subagents vs handoffs, tool per agent vs dispatch, retrieving subagent state, trimming subagent boilerplate, quickly building handoff setups Other Common Issues [reference/common issues.md](reference/common issues.md) High frequency standalone issues that don't fit the categories above. Currently includes: tools returning data to both the model and the application layer, MCP tools unable to access runtime context, invalid tool calls , and dynamic system prompt placeholders ContextSeek — Use Case Scenarios [reference/contextseek middleware.md](reference/contextseek middleware.md) Agent loses context across sessions, tool call auditing, cross topic knowledge discovery (dream), SRE provenance / confidence tracing, enterprise knowledge cold start (DataPlug) ContextSeek — Parameter & Config Issues [reference/contextseek params.md](reference/contextseek params.md) scope isolation, auto store / record tool calls write volume, auto compact throttling and shutdown, retrieval tags / min score filtering, tool arg overrides, dream trigger conditions, dream item decay, evidence chain vs chain confidence, DataPlug vs ctx.add(), plug() scope priority, auto dream dual gate triggering Common Issues Quick Reference Keyword / Error Where to Look Which model to choose / Deep Agent performing poorly deepagents issue 1 Filesystem backend / local files / file permissions deepagents issues 2 / 4 Disabling the default sub agent / general purpose deepagents issue 3 Long term memory / store deepagents issue 5 SKILL.md truncated / only first 100 lines read / read file limit / progressive disclosure deepagents issue 6 with structured output returning None / missing fields structured output issue 1 create agent / response format / ProviderStrategy / ToolStrategy structured output issue 1 with structured output / function calling / tool choice unsupported / deepseek reasoner does not support this tool choice structured output issue 2 OpenAI compatible model / ChatOpenAI not working model integration issue 1 Reasoning model / reasoning content / chain of thought lost model integration issue 2 Chinese model / CN provider / DeepSeek / Qwen / GLM / Moonshot cn models README langchain cn models (embedded) / generate integration class cn models README Middleware order messed up / before/after counterintuitive middleware issue 1 state schema fields not merged / input/output control middleware issue 2 interrupt resume value missing / HITL middleware issue 3 Modifying state inside wrap model call has no effect middleware issue 4 Choosing between astream events and astream for streaming streaming issue 1 Distinguishing token sources across multiple LLMs streaming issue 2 Disabling streaming for a specific model streaming issue 3 Custom events from inside a tool not being emitted streaming issue 4 Multi agent: subagents vs handoffs multi agent issue 1 Single dispatch tool vs one tool per agent multi agent issue 2 interrupt can't see subagent state multi agent issue 3 Too much subagent wrapper boilerplate multi agent issue 4 Quickly building a handoff based multi agent setup multi agent issue 5 Tool returning data to both the model and the app layer / artifact / Command(update=...) common issues issue 1 MCP tool can't access user id / store / state / API key common issues issue 2 invalid tool calls / tool never executes / malformed tool call JSON common issues issue 3 Dynamic system prompt placeholders / format prompt / Jinja2 prompt variables common issues issue 4 Agent loses context across sessions — personal assistant or support bot contextseek middleware issue 1 Multi tool data pipeline agent — auditing tool call decisions contextseek middleware issue 2 Research agent accumulates raw notes — cross topic pattern discovery contextseek middleware issue 3 SRE incident postmortem agent — tracing knowledge confidence and conflicts contextseek middleware issue 4 Enterprise knowledge migration — agent is retrieval ready on day one contextseek middleware issue 5

langchain-dev-guide