diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 84a3e5735..265ee7aea 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -172,21 +172,11 @@ jobs:
       # (shared effect application) is complete.
       # See: .progress/012_20260217_dst_architecture_fix.md
 
+      - name: Dependency isolation regression
+        run: bash scripts/test-dependency-isolation.sh
+
       - name: Dependency isolation
-        run: |
-          # Check temper-jit has no production dep on temper-verify
-          if cargo tree --no-dev -p temper-jit 2>/dev/null | grep -q temper-verify; then
-            echo "FAIL: temper-jit has production dependency on temper-verify"
-            exit 1
-          fi
-          # Check no stateright/proptest in production crates
-          for crate in temper-jit temper-server temper-runtime; do
-            if cargo tree --no-dev -p "$crate" 2>/dev/null | grep -qE 'stateright|proptest'; then
-              echo "FAIL: $crate production binary includes stateright or proptest"
-              exit 1
-            fi
-          done
-          echo "Dependency isolation: OK"
+        run: bash scripts/check-dependency-isolation.sh
 
   # ─────────────────────────────────────────────────────
   # Gate 3: Full test suite (minutes)
diff --git a/scripts/check-dependency-isolation.sh b/scripts/check-dependency-isolation.sh
new file mode 100755
index 000000000..fa0017bd7
--- /dev/null
+++ b/scripts/check-dependency-isolation.sh
@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+# Check default-feature, non-dev dependency graphs used by the CI isolation gate.
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$ROOT"
+
+for crate in temper-jit temper-server temper-runtime; do
+    # Buffer the complete graph and check Cargo's exit before inspecting it.
+    # A grep -q pipeline can hide query errors or cause a producer SIGPIPE.
+    if graph="$(cargo tree --edges no-dev -p "$crate" --color never)"; then
+        if [[ ! "$graph" =~ [^[:space:]] ]]; then
+            echo "FAIL: cargo tree returned an empty dependency graph for $crate" >&2
+            exit 1
+        fi
+    else
+        status=$?
+        echo "FAIL: cargo tree --edges no-dev -p $crate failed (exit $status)" >&2
+        exit "$status"
+    fi
+
+    if [[ "$crate" == temper-jit && "$graph" =~ (^|[[:space:]])temper-verify[[:space:]] ]]; then
+        echo "FAIL: temper-jit has production dependency on temper-verify" >&2
+        exit 1
+    fi
+    if [[ "$graph" =~ (^|[[:space:]])(stateright|proptest)[[:space:]] ]]; then
+        echo "FAIL: $crate production binary includes stateright or proptest" >&2
+        exit 1
+    fi
+done
+
+echo "Dependency isolation: OK"
diff --git a/scripts/test-dependency-isolation.sh b/scripts/test-dependency-isolation.sh
new file mode 100755
index 000000000..5d083288a
--- /dev/null
+++ b/scripts/test-dependency-isolation.sh
@@ -0,0 +1,110 @@
+#!/usr/bin/env bash
+# Exercise the same guard CI invokes, with controlled Cargo output and exits.
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+GUARD="${1:-$ROOT/scripts/check-dependency-isolation.sh}"
+TASK_TMP="$(mktemp -d "${TMPDIR:-/tmp}/temper-dependency-test.XXXXXX")"
+trap 'rm -rf "$TASK_TMP"' EXIT
+mkdir "$TASK_TMP/bin"
+
+cat > "$TASK_TMP/bin/cargo" <<'MOCK'
+#!/usr/bin/env bash
+set -euo pipefail
+printf '%s\n' "$*" >> "$CALLS"
+# The old CI query is accepted here so the RED detects its swallowed failure,
+# rather than merely testing an argument mismatch.
+crate=''
+while [[ $# -gt 0 ]]; do
+    if [[ "$1" == '-p' ]]; then
+        crate="$2"
+        break
+    fi
+    shift
+done
+if [[ "$crate" == "$TARGET_CRATE" ]]; then
+    case "$MODE" in
+        cargo-error)
+            printf '%s v0.1.0\n' "$crate"
+            echo 'cargo fixture: dependency query failed' >&2
+            exit 42
+            ;;
+        sigpipe)
+            printf '%s v0.1.0\n' "$crate"
+            echo 'cargo fixture: interrupted output' >&2
+            exit 141
+            ;;
+        empty) exit 0 ;;
+        whitespace) printf ' \t\n'; exit 0 ;;
+    esac
+fi
+printf '%s v0.1.0\n' "$crate"
+if [[ "$crate" == "$TARGET_CRATE" ]]; then
+    case "$MODE" in
+        forbidden|large-forbidden)
+            printf '├── %s v1.0.0\n' "$FORBIDDEN"
+            ;;
+        allowed)
+            printf '├── proptest-helper v1.0.0\n└── stateright-helper v1.0.0\n'
+            ;;
+    esac
+    if [[ "$MODE" == 'large-forbidden' ]]; then
+        for ((i = 0; i < 10000; i++)); do
+            printf '├── allowed-dependency-%s v1.0.0\n' "$i"
+        done
+    fi
+fi
+MOCK
+chmod +x "$TASK_TMP/bin/cargo"
+
+check_case() {
+    local name="$1" mode="$2" crate="$3" forbidden="$4" expected="$5" diagnostic="$6"
+    local actual=0
+    local output="$TASK_TMP/$name.stdout" errors="$TASK_TMP/$name.stderr"
+    local calls="$TASK_TMP/$name.calls"
+    if PATH="$TASK_TMP/bin:$PATH" MODE="$mode" TARGET_CRATE="$crate" \
+        FORBIDDEN="$forbidden" CALLS="$calls" bash "$GUARD" > "$output" 2> "$errors"; then
+        actual=0
+    else
+        actual=$?
+    fi
+    if [[ "$actual" -ne "$expected" ]]; then
+        cat "$output" "$errors" >&2
+        printf 'FAIL %s: expected exit %s, got %s\n' "$name" "$expected" "$actual" >&2
+        exit 1
+    fi
+    if [[ "$expected" -eq 0 ]]; then
+        grep -Fx 'Dependency isolation: OK' "$output"
+        printf 'tree --edges no-dev -p %s --color never\n' \
+            temper-jit temper-server temper-runtime > "$TASK_TMP/expected.calls"
+        diff -u "$TASK_TMP/expected.calls" "$calls"
+    else
+        if grep -F 'Dependency isolation: OK' "$output" "$errors"; then
+            printf 'FAIL %s: failure reported success\n' "$name" >&2
+            exit 1
+        fi
+        grep -F "$diagnostic" "$errors"
+    fi
+    printf 'PASS %s (exit %s)\n' "$name" "$actual"
+}
+
+# First case is the discriminating regression for the original if-pipeline.
+check_case cargo-error-jit cargo-error temper-jit '' 42 'cargo fixture: dependency query failed'
+check_case allowed allowed temper-jit '' 0 ''
+check_case jit-verifier forbidden temper-jit temper-verify 1 'temper-jit has production dependency on temper-verify'
+# temper-verify itself is only forbidden in temper-jit, not the other roots.
+for crate in temper-server temper-runtime; do
+    check_case "$crate-verifier-allowed" forbidden "$crate" temper-verify 0 ''
+done
+for crate in temper-jit temper-server temper-runtime; do
+    for forbidden in stateright proptest; do
+        check_case "$crate-$forbidden" forbidden "$crate" "$forbidden" 1 "$crate production binary includes stateright or proptest"
+    done
+    check_case "$crate-empty" empty "$crate" '' 1 "cargo tree returned an empty dependency graph for $crate"
+done
+for crate in temper-server temper-runtime; do
+    check_case "$crate-cargo-error" cargo-error "$crate" '' 42 'cargo fixture: dependency query failed'
+done
+check_case whitespace whitespace temper-jit '' 1 'cargo tree returned an empty dependency graph for temper-jit'
+check_case sigpipe sigpipe temper-jit '' 141 'cargo fixture: interrupted output'
+check_case large-forbidden large-forbidden temper-jit proptest 1 'temper-jit production binary includes stateright or proptest'