diff --git a/flake.nix b/flake.nix
index 4a5b5039eb3..35cce191e95 100644
--- a/flake.nix
+++ b/flake.nix
@@ -673,6 +673,14 @@
             if [ -x "$shim" ]; then
               export PATH="$(dirname "$shim"):$PATH"
             fi
+
+            # Each nested shell (direnv, then `nix develop`, then another
+            # inside a gate) prepends this shell's ~160 store paths again,
+            # about 10 KB. mbx copies PATH into two variables at ~3.6 times
+            # its size, and Linux rejects any one variable over 128 KiB, so
+            # four nested shells failed every Cargo launch with E2BIG. Keep
+            # each directory's first entry only: lookups resolve as before.
+            PATH=$(printf '%s' "$PATH" | awk -v RS=: '!seen[$0]++ { printf "%s%s", sep, $0; sep = ":" }')
           '';
 
           env = {
diff --git a/quest/m0/ci/README.md b/quest/m0/ci/README.md
index d7a6897dee1..eac73276287 100644
--- a/quest/m0/ci/README.md
+++ b/quest/m0/ci/README.md
@@ -168,4 +168,3 @@ The historical samples locate candidates; they are not controlled benchmarks.
 - [The next test-CPU cuts](/quest/m0/ci/test-cost-followups.md) - the slow-test leftovers from #6987, cut at their source
 - [Biome checks every file it can parse](/quest/m0/ci/biome-whole-repo.md) - Biome runs over the whole repository and main is clean under it
 - [dispatch.sh's tests pass without KVM](/quest/m0/ci/dispatch-test-kvm.md) - `sh/dispatch-query.test.py` fakes the KVM probe instead of failing 20 tests on hosts without `/dev/kvm`
-- [mbx launches Cargo with any argument length](/quest/m0/ci/mbx-arg-max.md) - no "Argument list too long" in `sh/rs/select.test.py`'s recipe test
diff --git a/quest/m0/ci/mbx-arg-max.md b/quest/m0/ci/mbx-arg-max.md
deleted file mode 100644
index 294ea2d6cf9..00000000000
--- a/quest/m0/ci/mbx-arg-max.md
+++ /dev/null
@@ -1,21 +0,0 @@
-# [S] mbx launches Cargo with any argument length
-
-## Goal
-
-mbx launches Cargo however long its argument list or environment is, and
-`RecipeTest.test_unexecuted_bench_is_not_compiled` (`sh/rs/select.test.py`)
-passes in `just ci`.
-
-## Plan
-
-Planned 2026-11-01 (maintainer, follow-up planning). A `just ci` run during
-[moq-dev/moq#7567](https://github.com/moq-dev/moq/pull/7567) failed in that
-test, because mbx couldn't launch Cargo: "Argument list too long" (`E2BIG`).
-
-- First find which part goes over the limit: the argv, or the environment
-  the test passes (`os.environ` plus its overrides).
-- Then fix it at the cause, for example by passing long lists through a file
-  or by trimming what mbx forwards, rather than skipping the test.
-- If the limit is hit inside mbx itself rather than in this repository's
-  wrapper, the fix belongs upstream, and this quest pins the release that
-  carries it.