Context Engineering Inside the Harness: 4 Mechanisms That Beat Context Overflow and Goal Loss on Long-Horizon Tasks - MarkTechPost Discord Linkedin Reddit X Home Open Source/Weights AI Agents Tutorials Voice AI Robotics Newsletter → Partner with Us Search News Hub News Hub Premium Content Read our exclusive articles Facebook Instagram X Home Open Source/Weights AI Agents Tutorials Voice AI Robotics Newsletter → Partner with Us News Hub Search Home Open Source/Weights AI Agents Tutorials Voice AI Robotics Newsletter → Partner with Us Home Editors Pick Agentic AI Context Engineering Inside the Harness: 4 Mechanisms That Beat Context Overflow and... Editors Pick Agentic AI AI Agents Artificial Intelligence AI Infrastructure Technology AI Shorts Applications Harness Language Model Machine Learning Staff Context Engineering Inside the Harness: 4 Mechanisms That Beat Context Overflow and Goal Loss on Long-Horizon Tasks By Asif Razzaq - September 12, 2026 An agent, in its simplest form, is an LLM calling tools in a loop. That loop works for short jobs. Give it a task that runs for an hour and 200 tool calls, and it breaks in 2 predictable ways. The AWS Samples design guide for autonomous cloud coding agents names them directly: shallow agents suffer from context overflow, get distracted (goal loss), and do not maintain state over long periods. The layer that fixes this is not the model. It is the harness, which AWS describes as managing everything but the model. This article opens up that layer. Compaction, memory strategy, context budgeting, and todo-state are the machinery that turns a shallow loop into a deep agent. We look at how LangChain Deep Agents , Claude Code , Manus , OpenAI Codex , and Amazon Bedrock AgentCore implement each one, with the actual thresholds they ship. Why a bigger window does not fix it The obvious fix is a larger context window. The evidence says it helps less than expected. Chroma’s Context Rot report evaluated 18 LLMs, including GPT-4.1, Claude 4, Gemini 2.5, and Qwen3, and found that performance grows increasingly unreliable as input length grows, even on simple retrieval tasks. Anthropic’s context engineering guide explains the mechanism: attention creates n² pairwise relationships for n tokens, so every added token depletes a finite “attention budget.” Context is a resource with diminishing returns, not a bucket. For an agent loop, this is worse than it sounds. Manus reports that a typical task needs around 50 tool calls, and that the input-to-output token ratio runs near 100:1. Each observation lands in context and stays there. The original instruction drifts toward the middle of the window, which is exactly where recall degrades. Goal loss is not only a model bug. It is the expected outcome of an unmanaged context on a long enough task. Mechanism 1: Context budgeting and offloading The first job of a harness is deciding what never enters the window at all. Deep Agents ships 2 offloading rules with hard numbers. When a tool response exceeds 20,000 tokens, it is written to the filesystem and replaced with a file path plus a preview of the first 10 lines. When session context crosses 85% of the model’s window, older write and edit tool calls, whose full file contents already live on disk, are truncated to a pointer. Only after offloading runs out of room does the harness fall back to summarization. Claude Code applies the same budgeting to what loads before the first prompt. Auto memory is capped at the first 200 lines or 25KB. MCP tool schemas stay deferred by default, with only tool names listed, and full schemas load on demand via tool search. After compaction, any re-read file over 5,000 tokens comes back as a path reference rather than content. The context window simulation in the Claude Code docs makes the payoff concrete: a research subagent reads 6,100 tokens of files and returns a 420-token result to the parent. That subagent pattern is budgeting at the architecture level. Anthropic’s guide notes that each subagent may burn tens of thousands of tokens exploring, but returns a distilled summary, often 1,000 to 2,000 tokens. The AWS AgentCore walkthrough builds exactly this: a coordinator spawns 3 browser subagents in parallel, each in its own MicroVM, and an analyst subagent receives only their structured findings. AWS reports a 4 to 6 minute expected runtime, and notes that sequential processing would take up to 3x longer. Mechanism 2: Compaction When offloading is not enough, the harness summarizes. Compaction is the practice of taking a conversation nearing the window limit, summarizing it, and reinitiating a new context with the summary. It is also where goal loss most often happens, because a lossy summary can drop the one constraint that mattered. The implementations differ in what they promise to keep. Claude Code’s compaction prompt preserves architectural decisions, unresolved bugs, and implementation details while discarding redundant tool outputs. Right after compaction it re-reads up to 5 of the files modified most recently, reloads the rules matching those files, and re-injects invoked skill bodies, capped at 5,000 tokens per skill and 25,000 total. The docs are explicit that detailed instructions from early in the conversation may be lost, which is why persistent rules belong in the project-root CLAUDE.md, which is re-injected from disk. Users can steer the pass with /compact focus on the auth bug fix or move the trigger point with /autocompact . Deep Agents made goal preservation a structural feature. Its summary is a structured document with dedicated fields for session intent, artifacts created, and next steps. The LangChain team added those fields after forced-summarization experiments showed the change improved performance. The full original transcript is also written to the filesystem, so a fact that was summarized away can be recovered by read_file later. Compaction has moved into the API lay