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Sprint projectSep 14, 2026Saudi Arabia

Closing Content Does Not Close the Channel: Measured Residual Capacity in Co-Tenant Agent Evaluation Infrastructure

Fatimah Emad Eldin · Team Permitted Operations

Submitted to AI Incident Response Sprint. Sprint projects are early-stage work by participants, not Apart Research publications.

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Report: Closing Content Does Not Close the Channel: Measured Residual Capacity in Co-Tenant Agent Evaluation Infrastructure

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Presentation: Closing Content Does Not Close the Channel: Measured Residual Capacity in Co-Tenant Agent Evaluation Infrastructure

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Shared mutable infrastructure in an evaluation harness permits information flow between co-tenant agents that authorization controls cannot detect: every operation involved is permitted. We measure both what such a channel can carry and whether agents use it. On a synthetic model of such a cache, closing file content — the control payload-inspecting monitors implement — still leaves 3075 bits, carried across names, existence, size and timestamp; a path-inspecting monitor still leaves 512. Closing directory names leaves 13 bits in an object’s size; only a fixed content-addressed manifest reaches zero, at the same task cost and checkable without the lab’s network. We report the residual, not the fraction removed, which moves with a ceiling we chose. Across 40 and 35 episodes agents read the store almost always and deposited in none, even under the unsolvable tasks that motivated the incident — so a lab contains capacity, not current propensity

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  1. Shows two AI agents can secretly send messages just by changing a file's name or timing, not its content - something most safety tools miss. Careful, honest work. They even admitted when one of their own predictions was wrong.

  2. This is a unique measurement study, I particularly like the detailed breakdown.

    Please consider running the ladder against the real OCI registry.

  3. The central question here is interesting and worth investigating: can agents still exchange information through a shared cache after its file contents are hidden? The project builds encoders that use remaining cache properties to transmit messages and makes a useful point about containment: deleting content may not remove every communication channel. I also appreciated the honesty of the negative results such as the agent failing to deposit messages.

    My main reservation is how the paper’s claims go further than the evidence supports. The experiments show how much information the tested methods could transmit, not the maximum that any method could transmit. This would be important if someone uses these results to decide whether a cache is safe to share. The claim about what monitoring can detect also needs to account for false positives. I would separate the demonstrated results from these broader claims, then test the proposed restriction on a realistic workload. Does it reduce communication between agents while preserving useful cache access? This would be useful information that an evaluation team could use.

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Cite this project

@misc{eldin2026closing,
  title = {{Closing Content Does Not Close the Channel: Measured Residual Capacity in Co-Tenant Agent Evaluation Infrastructure}},
  author = {Fatimah Emad Eldin},
  year = {2026},
  month = sep,
  note = {Submitted to AI Incident Response Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/closing-content-does-not-close-the-channel-measured-residual-capacity-in-cotenant-agent-evaluation-infrastructure-zwbt}},
  url = {https://apartresearch.com/sprints/projects/closing-content-does-not-close-the-channel-measured-residual-capacity-in-cotenant-agent-evaluation-infrastructure-zwbt}
}

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