diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 2f67d3941..c637b13f2 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -373,6 +373,16 @@ jobs:
   # knows what good looks like rather than only what broken looked like:
   # 2:29, 2:01, 1:50 against a 25 minute cap.
   #
+  # Parallelism comes from the SHARDS, not from concurrency inside a shard.
+  # `--test-concurrency=1` is deliberate: at 4 a timing-sensitive test races
+  # three neighbours, and `useSandbox.runSupersession.test.tsx` failed on three
+  # unrelated PRs in one hour that way (issue #3060) while passing locally 3/3.
+  # The bump to 4 came from an earlier attempt at this lane that was MEASURED
+  # not to help on CI, and it was carried forward as harmless once sharding
+  # worked. It is not harmless, and its speedup is redundant with sharding.
+  # Cost of going back to 1, measured: ~70s versus ~19s per shard locally,
+  # roughly 8 minutes versus 2 on CI, against a 25 minute cap.
+  #
   # Two earlier explanations in this file were WRONG and are corrected here,
   # because a confident wrong comment is worse than none:
   #
diff --git a/apps/viewer/package.json b/apps/viewer/package.json
index 62baee7f5..715e816c6 100644
--- a/apps/viewer/package.json
+++ b/apps/viewer/package.json
@@ -14,7 +14,7 @@
     "build": "vite build && node ../../scripts/check-tla-chunk-await.mjs",
     "typecheck": "tsc --noEmit",
     "preview": "vite preview",
-    "test": "tsx --import ./src/test/vite-module-hooks.mjs --test --test-timeout=120000 --test-concurrency=4 $(find src -type f \\( -name '*.test.ts' -o -name '*.test.tsx' \\) | sort | awk -v s=\"${TEST_SHARD:-0}\" -v n=\"${TEST_SHARDS:-1}\" 'NR % n == s')",
+    "test": "tsx --import ./src/test/vite-module-hooks.mjs --test --test-timeout=120000 --test-concurrency=1 $(find src -type f \\( -name '*.test.ts' -o -name '*.test.tsx' \\) | sort | awk -v s=\"${TEST_SHARD:-0}\" -v n=\"${TEST_SHARDS:-1}\" 'NR % n == s')",
     "check:templates": "tsc -p src/lib/scripts/templates/tsconfig.json --noEmit"
   },
   "dependencies": {