LangChain's guide to building a custom agent harness: context is everything
iamrobotbear · x · 2026-09-13
LangChain engineer Sydney Runkle published a guide on building domain-specific agent harnesses, amplified by founder Harrison Chase with Garry Tan's line: "Either you die a system of record or you live long enough to become a domain-specific harness."
Key points:
- Agent = model + harness. The harness is the scaffolding connecting the model to the real world; an agent is only as good as the context delivered at every step.
- createagent is the base primitive—pass a model, tools, and system prompt to get a working agent, then add custom prompting, business logic, and guardrails.
- Deep Agents and the Claude Agent SDK ship opinionated middleware stacks (memory, context management, sandboxing) for fast production setups, but finer-grained customization is where custom harnesses earn their keep.
Related event: LangChain Publishes Guide to Building Domain-Specific Agent Harnesses(2 posts)→
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