diff --git a/docs/dqn-wire-up-audit.md b/docs/dqn-wire-up-audit.md
index a941bbc32..54375ecf2 100644
--- a/docs/dqn-wire-up-audit.md
+++ b/docs/dqn-wire-up-audit.md
@@ -8723,3 +8723,26 @@ Per `pearl_canary_input_freshness_launch_order.md`: this launch order is the smo
 2. **Hold-cost consumer site found at exactly the canonical site.** `experience_kernels.cu:3089` (segment_complete branch) was the source of the `cost=0.006251` diagnostic value emitted at HEALTH_DIAG[2] in pfh9n. The 2 sibling sites (3553, 3617) cover the per-bar positioned-Hold and per-bar flat-Hold branches. All three are migrated atomically with the same fallback semantics so the per-bar Hold cost subtraction is consistent across all 3 reward composition paths.
 
 3. **Producer launch ordering.** The SP16-P1 producer fires AFTER the per-step `hold_rate_observer` chain via the `if (collector.executed) {…}` block in training_loop.rs. The new SP16-P2 producer fires immediately after SP16-P1 inside the same block, so the launch ordering is correct by construction. The `pearl_canary_input_freshness_launch_order.md` invariant is preserved.
+
+## Build infra: ensure-binary cache key bug (2026-05-08)
+
+Discovered during SP16 validation (4gb88 → 8mzkj forced switch from L40S to H100).
+
+The build.rs fix landed in commit `98a81981a` made Cargo's incremental
+build properly arch-aware (added `cargo:rerun-if-env-changed=CUDA_COMPUTE_CAP`
+to 6 build.rs files). However, the Argo `ensure-binary` step caches the
+compiled binary at `/data/bin/$SHORT_SHA/` keyed by **commit SHA only**.
+
+When a workflow on L40S (sm=89) populates the cache for a given SHA, then
+a subsequent workflow on H100 (sm=90) for the SAME SHA cache-hits and
+reuses the SM 89 binary. Training fails at runtime with:
+
+```
+rmsnorm cubin load: DriverError(CUDA_ERROR_NO_BINARY_FOR_GPU)
+```
+
+**Workaround used**: bump SHA with no-op commit to force fresh build.
+
+**Proper fix**: extend cache key in `scripts/argo-train.sh` (or workflow
+template) to include `cuda-compute-cap`, e.g., `/data/bin/$SHORT_SHA-sm$CC/`.
+Filed as future infrastructure cleanup.