Harness Design Long Running Apps

Harness design for long-running application development \ Anthropic Engineering at Anthropic Harness design for long-running application development

Published Mar 24, 2026 Harness design is key to performance at the frontier of agentic coding. Here's how we pushed Claude further in frontend design and long-running autonomous software engineering.

Written by Prithvi Rajasekaran, a member of our Labs team.

Over the past several months I’ve been working on two interconnected problems: getting Claude to produce high-quality frontend designs, and getting it to build complete applications without human intervention. This work originated with earlier efforts on our frontend design skill and long-running coding agent harness , where my colleagues and I were able to improve Claude’s performance well above baseline through prompt engineering and harness design—but both eventually hit ceilings.

To break through, I sought out novel AI engineering approaches that held across two quite different domains, one defined by subjective taste, the other by verifiable correctness and usability. Taking inspiration from Generative Adversarial Networks (GANs), I designed a multi-agent structure with a generator and evaluator agent. Building an evaluator that graded outputs reliably—and with taste—meant first developing a set of criteria that could turn subjective judgments like “is this design good?” into concrete, gradable terms.

I then applied these techniques to long-running autonomous coding, carrying over two lessons from our earlier harness work: decomposing the build into tractable chunks, and using structured artifacts to hand off context between sessions. The final result was a three-agent architecture—planner, generator, and evaluator—that produced rich full-stack applications over multi-hour autonomous coding sessions. Why naive implementations fall short We've previously shown that harness design has a substantial impact on the effectiveness of long running agentic coding. In an earlier experiment , we used an initializer agent to decompose a product spec into a task list, and a coding agent that implemented the tasks one feature at a time before handing off artifacts to carry context across sessions. The broader developer community has converged on similar insights, with approaches like the " Ralph Wiggum " method using hooks or scripts to keep agents in continuous iteration cycles.

But some problems remained persistent. For more complex tasks, the agent still tends to go off the rails over time. While decomposing this issue, we observed two common failure modes with agents executing these sorts of tasks.

First is that models tend to lose coherence on lengthy tasks as the context window fills (see our post on context engineering ). Some models also exhibit "context anxiety," in which they begin wrapping up work prematurely as they approach what they believe is their context limit. Context resets—clearing the context window entirely and starting a fresh agent, combined with a structured handoff that carries the previous agent's state and the next steps—addresses both these issues.

This differs from compaction, where earlier parts of the conversation are summarized in place so the same agent can keep going on a shortened history. While compaction preserves continuity, it doesn't give the agent a clean slate, which means context anxiety can still persist. A reset provides a clean slate, at the cost of the handoff artifact having enough state for the next agent to pick up the work cleanly. In our earlier testing, we found Claude Sonnet 4.5 exhibited context anxiety strongly enough that compaction alone wasn't sufficient to enable strong long task performance, so context resets became essential to the harness design. This solves the core issue, but adds orchestration complexity, token overhead, and latency to each harness run.

A second issue, which we haven’t previously addressed, is self-evaluation. When asked to evaluate work they've produced, agents tend to respond by confidently praising the work—even when, to a human observer, the quality is obviously mediocre. This problem is particularly pronounced for subjective tasks like design, where there is no binary check equivalent to a verifiable software test. Whether a layout feels polished or generic is a judgment call, and agents reliably skew positive when grading their own work.

However, even on tasks that do have verifiable outcomes, agents still sometimes exhibit poor judgment that impedes their performance while completing the task. Separating the agent doing the work from the agent judging it proves to be a strong lever to address this issue. The separation doesn't immediately eliminate that leniency on its own; the evaluator is still an LLM that is inclined to be generous towards LLM-generated outputs. But tuning a standalone evaluator to be skeptical turns out to be far more tractable than making a generator critical of its own work, and once that external feedback exists, the generator has something concrete to iterate against. Frontend design: making subjective quality gradable I started by experimenting on frontend design, where the self-evaluation issue was most visible. Absent any intervention, Claude normally gravitates toward safe, predictable layouts that are technically functional but visually unremarkable.

Two insights shaped the harness I built for frontend design. First, while aesthetics can’t be fully reduced to a score—and individual tastes will always vary—they can be improved with grading criteria that encode design principles and preferences. "Is this design beautiful?" is hard to answer consistently, but "does this follow our principles for good design?" gives Claude something concrete to grade against. Second, by separating frontend generation from frontend grading, we can create a feedback loop that drives the generator toward stronger outputs.

With this in mind, I wrote four grading criteria that I gave to both the generator and evaluator agents in their prompts: Design quality: Does the design feel like a coherent whole rather than a collection of parts? Strong work here means the colors, typography, layout, imagery, and other details combine to create a distinct mood and identity. Originality: Is there evidence of custom decisions, or is this template layouts, library defaults, and AI-generated patterns? A human designer should recognize deliberate creative…