ADMIT: Support-Gated Memory-Write Admission for Document QA Agents

Document QA agents do not fail only at the final answer. They can also write unsupported claims into public working memory, and those writes can affect later retrieval and synthesis. We introduce ADMIT (Auditable Decoupled Memory-write admission for Intermediate Traces), a low-budget admission controller for public OCR/text evidence packs. ADMIT applies a deterministic substring-support check $S_{\text{str}}(\hat{y}, B) > 0$ to each public commit and allows one bounded RepairRead. If repair still fails, ADMIT uses a memory-fail-closed / output-fail-open rule: it writes a $\bot$ (NO_COMMIT) sentinel to memory while the final-answer surface may still fall back to the baseline answer. On held-out M3DocVQA, a retrained final-row router closes the answer-surface gap; ADMIT's distinguishing evidence is therefore memory-surface contamination, not final-answer accuracy. The lexical invariant guarantees zero-substring accepted intermediate contamination by construction; the contribution is therefore the public memory-write boundary as a low-cost control surface with decoupled fail actions, not the invariant itself. In a 7-arm dynamic stress diagnostic on three agents, ADMIT enforces the invariant and preserves raw EM at this operating point (zero contaminated trajectories on every agent and no detectable raw-EM loss relative to the fail-open ablation), whereas final-only baselines still leave $12$--$32%$ contamination. ADMIT enforces lexical auditability, not semantic truth: relation-level entailment, citation-aware faithfulness, and visual-region grounding are not claimed, and lower-density VisDoMSlide and text-only FRAMES are reported as boundary cases.