Context Drift Killed Our Longest-Running Agent Sessions

Fazm Team··3 min read

Context Drift Killed Our Longest-Running Agent Sessions

Our longest agent sessions kept producing bizarre results. Not errors - the agent would complete its work successfully, but the output would be subtly wrong. Off-topic. Solving a problem nobody asked it to solve.

How Context Drift Works

In a long-running session, the agent's understanding of its objective gradually shifts. Each step introduces small interpretation changes. The agent reads a file and adjusts its mental model slightly. It encounters an unexpected result and compensates by reframing the goal. Ten steps in, the agent is confidently executing a plan that has drifted meaningfully from the original intent.

This is not hallucination in the traditional sense. The agent is not making things up. It is genuinely working toward a goal - just not the one you gave it. Each individual step seems reasonable. The drift is invisible until you compare the final output to the original request.

Why Long Sessions Are Vulnerable

Short sessions survive because there is not enough time to drift. The agent gets a task, executes it, and finishes before any meaningful shift can occur. But sessions that run for 30 minutes, an hour, or longer accumulate micro-drifts that compound.

The context window itself contributes. As the window fills, earlier instructions get compressed or pushed out. The agent's most recent actions start to define its objective more than the original prompt does.

The Fix - Explicit Checkpoints

We introduced explicit checkpoints where the agent pauses, summarizes its current understanding of the objective, and waits for human confirmation before continuing. Not after every step - that would be too slow. At natural transition points between phases of work.

The summary forces the agent to articulate what it thinks it is doing. When the summary does not match the original intent, you catch the drift before it compounds further. The human confirms, the agent continues with a corrected understanding, and the session stays on track.

The Pattern

Every long-running agent session should have checkpoints. The longer the session, the more checkpoints it needs. Think of them as course corrections - small adjustments that prevent large deviations.

Context drift is not a bug you can fix with better prompts. It is a property of how language models process sequential information. The only reliable solution is periodic re-grounding.

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Fazm is an open source macOS AI agent. Open source on GitHub.

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