diff --git a/CLAUDE.md b/CLAUDE.md
index 62ae0a2c..bb4c8f6f 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -141,8 +141,19 @@ src/
 
   `env -u RUSTFLAGS` is load-bearing: a RUSTFLAGS env var REPLACES every
   cargo-config rustflags entry, so it silently drops `-Ctarget-cpu=x86-64-v4`
-  and the arm measures v3 while claiming v4 (the trap `scripts/masking-parity.sh`
-  documents).
+  and the arm does NOT measure v4 while claiming to (the trap
+  `scripts/masking-parity.sh` documents).
+
+  > ⊘ **CORRECTED 2026-09-16 (coderabbit, #313).** This read "*and the arm
+  > measures v3*". Wrong, and this session's own measurement is what disproves
+  > it: RUSTFLAGS replaces **every** config rustflags entry, so it drops the
+  > DEFAULT config's target-cpu too, not just the overlay's. Measured on one
+  > unit — `RUSTFLAGS="-D warnings"` produced **zero** `-Ctarget-cpu` flags,
+  > against 65 with the env unset. What you actually get is rustc's own default
+  > for the target, i.e. the `x86-64` baseline (SSE2), which is LOWER than v3
+  > and is the tier `simd_avx2.rs`'s intrinsics SIGILL on. The sentence was
+  > wrong before the native flip as well; the flip only changed which config
+  > gets discarded.
 
   **That same mechanism had silently disabled the whole config in CI, and it is
   the more serious half (found 2026-09-16).** `.github/workflows/ci.yaml` sets a
diff --git a/Dockerfile b/Dockerfile
index 08e979aa..b8bd2bd4 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -72,6 +72,15 @@ COPY ndarray-rand/benches/ ndarray-rand/benches/
 # detects AVX-512 at runtime via LazyLock<Tier> even when compiled for v3;
 # compile-time v3 just means the scalar/AVX2 fallback paths are used when the
 # runtime check fails. Both paths produce identical results.
+# The cargo CONFIG DIRECTORY, required by the `--config` flags below and easy to
+# forget: this Dockerfile COPYs selectively by design (see the note above), so a
+# file that is not named here does not exist in the image. Adding `--config
+# .cargo/config-v3.toml` without this line makes cargo fail on a missing
+# configuration file BEFORE it compiles anything — which is exactly what
+# happened on #313 and was caught in review after merge, not by a build (there
+# is no Docker daemon in the dev container, so neither image is built here).
+COPY .cargo/ .cargo/
+
 # The tier is passed as a CONFIG, not as `ENV RUSTFLAGS` (changed 2026-09-16).
 # A RUSTFLAGS env REPLACES every cargo-config `rustflags` entry rather than
 # joining it, so `ENV RUSTFLAGS="-C target-cpu=x86-64-v3"` did set the tier —
diff --git a/Dockerfile.avx512 b/Dockerfile.avx512
index 21d2e1e9..f53f1802 100644
--- a/Dockerfile.avx512
+++ b/Dockerfile.avx512
@@ -50,6 +50,11 @@ COPY examples/ examples/
 COPY benches/ benches/
 COPY ndarray-rand/benches/ ndarray-rand/benches/
 
+# The cargo CONFIG DIRECTORY, required by the `--config` flags below. This
+# Dockerfile COPYs selectively, so a file not named here is absent from the
+# image and cargo fails on a missing configuration file before compiling.
+COPY .cargo/ .cargo/
+
 # AVX-512 pinned: compile-time dispatch, everything inlined.
 #
 # Passed as a CONFIG, not `ENV RUSTFLAGS` (changed 2026-09-16): a RUSTFLAGS env
diff --git a/scripts/masking-parity.sh b/scripts/masking-parity.sh
index 4dff97cb..787c652b 100755
--- a/scripts/masking-parity.sh
+++ b/scripts/masking-parity.sh
@@ -26,8 +26,21 @@ case "$ARM" in
     # `env -u RUSTFLAGS`: a workflow-global RUSTFLAGS (CI sets "-D warnings")
     # REPLACES every cargo-config rustflags entry, so a `--config
     # .cargo/config-v4.toml` passed through CARGO_ARGS would silently lose its
-    # `-Ctarget-cpu=x86-64-v4` and this arm would measure v3 while claiming
-    # v4 — the exact trap the tier4 CI job hit. Clearing it lets the config win.
+    # `-Ctarget-cpu=x86-64-v4` — the exact trap the tier4 CI job hit. Clearing
+    # it lets the config win.
+    #
+    # What you get INSTEAD is not v3 (corrected 2026-09-16, coderabbit on #314;
+    # this comment said "would measure v3"). RUSTFLAGS replaces EVERY entry,
+    # including the DEFAULT `.cargo/config.toml`'s own `-Ctarget-cpu`, so no
+    # target-cpu reaches rustc at all and the build lands on rustc's generic
+    # `x86-64` baseline — SSE2, BELOW v3, and the tier `simd_avx2.rs`'s
+    # intrinsics SIGILL on. Measured on one unit: `RUSTFLAGS="-D warnings"`
+    # emitted ZERO `-Ctarget-cpu` flags against 65 with the env unset.
+    #
+    # This arm NAMES NO TIER by design: it builds with whatever config wins,
+    # which by default is `target-cpu=native` (the host). Read the program's
+    # own `avx512f=` header line for the tier; pin `.cargo/config-v3.toml`
+    # through CARGO_ARGS when you specifically want AVX2.
     env -u RUSTFLAGS cargo ${CARGO_ARGS:-} build --release --manifest-path "$MANIFEST" --bin simd-masking-parity
     "$TD/release/simd-masking-parity"
     ;;