Skip to main content

The Four Ways to Build

The four paths onto the Claude Developer Platform, separated by two questions: who supplies the harness, and who supplies the deployment.

Hand-drawn ink-line illustration on terracotta: a loopy hand places the last of four cream squares into a two-by-two arrangement, a one-line profile face observing.

The 2x2​

Anthropic's platform offers four distinct ways to build an agent. They stop being confusing the moment you sort them by the harness (the agent loop) and the deployment (the infrastructure it runs on). Details below are drawn from platform.claude.com/docs and code.claude.com/docs; verify against the live docs, beta surfaces drift.

PathYou writeHarnessDeployment
Messages API, manual loopthe tool-use loop itselfyouyou
Messages API + Tool Runnerjust your tool functionsthe SDKyou
Claude Agent SDKa prompt plus optionsthe SDK (the Claude Code harness as a library)you
Managed Agents (beta)an agent configAnthropicAnthropic

Three of the four leave deployment to the builder. Managed Agents is the only path where Anthropic supplies both halves: hosted sandbox, durable sessions, Vaults for secrets, Scheduled Deployments, capacity.

One confusion worth killing early: Tool Runner is not the Agent SDK. Tool Runner is a helper inside the regular Anthropic SDK that loops over tools you define. The Agent SDK is Claude Code packaged as a library, with built-in file, shell, and web tools. Both are harness-only.

Choosing between them​

  • One well-scoped call (classify, extract, summarize): just the Messages API. Do not build an agent. This is Anthropic's own standing advice; Building effective agents (December 2024) urges "finding the simplest solution possible, and only increasing complexity when needed."
  • Custom tools, API-first, you want the loop: Messages API with Tool Runner.
  • Fast local iteration, or production on infrastructure you want to control: Agent SDK, plus your own sandbox, session store, and isolation.
  • Production without the infrastructure lift: Managed Agents.

What switching paths has done in practice​

  • OffDeal moved its M&A agent to the Agent SDK and reports eval accuracy rising from 25% to 85% overall, with the switch itself doing the first leg: "Just switching to the SDK moved our score on that eval from 25% to 60%." (source)
  • Freedom Forever benchmarked the Agent SDK on web automation at a "97.5% success rate on complex web automation benchmarks where alternative frameworks hit 20–60%." (source)
  • Pendo tried building its own agent infrastructure for about three months, then reached production on Managed Agents in "About three days." (source)
  • Sentry shipped its Managed Agents integration "in weeks instead of months" with one engineer. (source)

The honest caveats​

Managed Agents is beta and, at time of writing, not available through Bedrock or Vertex; confirm current status in the platform docs before committing an architecture to it. And sometimes the right tier is the lowest one: if a script or a single call solves the problem, every row above it is overhead. The wiki's Build on Claude tool walks a specific product through this exact decision.

Further Reading​