The Claude Code SDK and the Birth of HaaS (Harness as a Service) (www.vtrivedy.com)

Anthropic’s Claude Code SDK effectively turns the model wrapper that used to live in apps into a reusable, extendable “harness” — a shift the author dubs Harness as a Service (HaaS). Rather than thinking in chat API calls (client.chat.completions.create → client.responses.create → agent.query), builders get a batteries‑included agent runtime ready for fast iteration. That harness bundles conversation and context management, a tool/MCP layer, permissions, session/filesystem state, loop control/error handling, observability, and optimized Claude integrations (automatic prompt caching and context compaction). The immediate payoff: dramatically reduced Time to First Feedback (TTFF) so teams can ship v0.1 agents quickly and iterate on real behavior instead of rebuilding infra. The post lays out practical customization guidance — obsess over the system prompt, design/compose tools or MCPs for clear, atomic actions, surface key context as markdown or memory files, and use subagents (YAML-defined) when you need specialization or parallelization. Technical implications are substantial: harnesses commodify agent infra, letting teams concentrate on prompts, tools, and domain context while leveraging built-in production essentials (error handling, monitoring, permissions). The author predicts an open-harness ecosystem — an “Open App Store for Agents” — where many user-facing AI products plug into shared harnesses, accelerating adoption and specialization across AI/ML applications.