diff --git a/docs/testing.md b/docs/testing.md
index 18b4244..69fe4b6 100644
--- a/docs/testing.md
+++ b/docs/testing.md
@@ -67,6 +67,26 @@ Python, simulator, and container inputs. The current recipe targets
 `aarch64-linux-gnu`, matching the development host used for its recorded
 receipt.
 
+## When offline suites cannot reach a failure
+
+Some failure modes are structurally unreachable in the fixture, and a green
+suite says nothing about them. The clearest example: the pinned simulator's
+repositories are ~4 KiB, so concurrent workers can never exhaust an in-flight
+budget that a single repository fits in. The batch submit/consume deadlock that
+ended experiment 4 survived 20 passing suites for exactly that reason, and four
+attempts to force it out of the simulator failed.
+
+A real-network reproduction costs about €0.03. Provision a small cloud box,
+stream the image to it, and run against `bsky.network` with the pressure turned
+up — for that bug, 50 workers and `--backfill-max-inflight-bytes=268435456`
+showed it in 90 seconds, and the before/after was unambiguous. Reach for this
+whenever a hypothesis involves memory pressure, worker contention, or repository
+sizes the simulator cannot produce.
+
+`just backfill-batch-contract` pins the weaker property that a dispatch batch
+far larger than the job queue converges. Its docstring is explicit that it
+passes on the broken code; read it before treating it as coverage.
+
 ## Dashboard and metrics
 
 Fully offline:
diff --git a/justfile b/justfile
index a81c78b..1e74e7e 100644
--- a/justfile
+++ b/justfile
@@ -414,6 +414,11 @@ powerloss-oracle:
 
 # Deterministic, fully offline semantic comparison using the exact pinned
 # upstream simulator, JSS reader, oracle model, and public Go client.
+# batch larger than the job queue must still converge (see the docstring:
+# this does NOT reproduce the production deadlock)
+backfill-batch-contract:
+    uv run tests/backfill_batch_contract.py
+
 differential-oracle:
     zig build -Doptimize=ReleaseSafe
     python3 tests/differential_oracle.py