Compaction Laundering: Provenance Destruction in Context-Compacting Coding Agents
Coding agents that run long enough must compact their context, summarizing the conversation, discarding the original, and carrying the summary forward as memory. We show that in popular open-source coding agents this step is an exploitable trust boundary. Harnesses that flatten the typed conversation into a single plain string, marking each message with a text label such as [User]:, destroy message provenance in code, before the summarizer runs. An attacker who controls anything the agent reads can forge a user turn that survives compaction as a genuine instruction. Across five flattening harnesses (pi, opencode, Cline, Aider, OpenHands) the forged approval launders into the agent's memory 87-100% of the time, up to a ~908,000-token context window; across four role-typed harnesses (Codex, Gemini CLI, Qwen Code, Continue) the identical attack succeeds 0% of the time. We follow the attack to executed consequences. A laundered standing approval drives real tool-running agents to execute attacker-controlled curl | sh in a sandbox, and end to end in the live pi TUI at ~50% against a 0% paired clean arm. The same channel reliably steers an agent's plan away from a file an attacker designates off-limits, which can blind a code review to a planted backdoor. Because the provenance is destroyed deterministically in harness code, no prompt-level defense can restore it; we measure a code-level provenance check that catches 77% of laundering while demoting 2% of legitimate summary content. The defect also has a straightforward fix at the serializer, escaping the role labels before flattening; the role-typed harnesses avoid it entirely by keeping messages role-typed.