diff --git a/new_changelogs/20261029T083420Z-security-sandbox-policy-intersection.md b/new_changelogs/20261029T083420Z-security-sandbox-policy-intersection.md
new file mode 100644
index 000000000..c1d6e0c91
--- /dev/null
+++ b/new_changelogs/20261029T083420Z-security-sandbox-policy-intersection.md
@@ -0,0 +1,3 @@
+- **Embedded and preload sandbox policies now intersect.** An allowing
+  in-process checker no longer bypasses a stricter shared-memory or file grant
+  list; an operation must satisfy every active containment layer.
diff --git a/runtime/libaether_sandbox_preload.c b/runtime/libaether_sandbox_preload.c
index 5c646380f..029b6634b 100644
--- a/runtime/libaether_sandbox_preload.c
+++ b/runtime/libaether_sandbox_preload.c
@@ -104,15 +104,16 @@ static int check_grant(const char* category, const char* resource) {
         match_target = resolved;
     }
 
-    // In-process levels take priority (embedded Python mode)
+    // An embedded policy and preload grants are independent boundaries.
+    // When both are active, the operation must satisfy both policies.
     if (aether_sandbox_check_resolved) {
         int result = aether_sandbox_check_resolved(category, match_target);
-        if (result >= 0) {
-            if (!result) log_deny(category, match_target);
-            return result;
+        if (result == 0) {
+            log_deny(category, match_target);
+            return 0;
         }
     }
-    // Fall back to file-based grants (LD_PRELOAD mode)
+    // Apply file/shared-memory grants as the second policy when active.
     if (!sandbox_active) return 1;
     for (int i = 0; i < grant_count; i++) {
         if (grants[i].cat[0] == '*' && grants[i].pat[0] == '*') return 1;
diff --git a/tests/integration/sandbox_policy_intersection/harness.c b/tests/integration/sandbox_policy_intersection/harness.c
new file mode 100644
index 000000000..d7548c2bd
--- /dev/null
+++ b/tests/integration/sandbox_policy_intersection/harness.c
@@ -0,0 +1,47 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+typedef int (*aether_sandbox_check_fn)(const char*, const char*);
+
+static int allow_check(const char* category, const char* resource) {
+    (void)category;
+    (void)resource;
+    return 1;
+}
+
+static int deny_check(const char* category, const char* resource) {
+    (void)category;
+    (void)resource;
+    return 0;
+}
+
+static aether_sandbox_check_fn active_check = allow_check;
+
+int aether_sandbox_check_resolved(const char* category, const char* resource) {
+    return active_check(category, resource);
+}
+
+int aether_sandbox_levels_here(void) {
+    return 1;
+}
+
+int main(int argc, char** argv) {
+    if (argc != 3) {
+        fprintf(stderr, "usage: %s <allow|deny> <file>\n", argv[0]);
+        return 2;
+    }
+    active_check = strcmp(argv[1], "deny") == 0
+        ? deny_check : allow_check;
+
+    int fd = open(argv[2], O_RDONLY);
+    if (fd < 0) return errno == EACCES ? 3 : 4;
+    char buf[64];
+    ssize_t n = read(fd, buf, sizeof buf);
+    close(fd);
+    if (n < 0) return 5;
+    if (write(STDOUT_FILENO, buf, (size_t)n) != n) return 6;
+    return 0;
+}
diff --git a/tests/integration/sandbox_policy_intersection/test_sandbox_policy_intersection.sh b/tests/integration/sandbox_policy_intersection/test_sandbox_policy_intersection.sh
new file mode 100644
index 000000000..10c3e16a2
--- /dev/null
+++ b/tests/integration/sandbox_policy_intersection/test_sandbox_policy_intersection.sh
@@ -0,0 +1,62 @@
+#!/bin/sh
+# When both the embedded checker and preload grants are active, an operation
+# must satisfy both policies. Neither policy may replace the other.
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+ROOT="$(cd "$SCRIPT_DIR/../../.." && pwd)"
+LIB="$ROOT/build/libaether_sandbox.so"
+
+case "$(uname -s 2>/dev/null)" in
+    Linux|FreeBSD) ;;
+    *)
+        echo "  [SKIP] sandbox_policy_intersection: preload is Linux and FreeBSD only"
+        exit 0 ;;
+esac
+if [ ! -f "$LIB" ]; then
+    echo "  [SKIP] sandbox_policy_intersection: preload library not built"
+    exit 0
+fi
+
+TMP="$(cd "$(mktemp -d)" && pwd -P)"
+trap 'rm -rf "$TMP"' EXIT
+if ! ${CC:-cc} -std=c11 -Wall -Wextra -Werror -rdynamic \
+        "$SCRIPT_DIR/harness.c" -o "$TMP/harness"; then
+    echo "  [FAIL] sandbox_policy_intersection: harness did not compile"
+    exit 1
+fi
+
+printf 'intersection sentinel\n' > "$TMP/data.txt"
+printf 'exec:/not-used\n' > "$TMP/deny-read.grants"
+printf 'fs_read:%s\n' "$TMP/data.txt" > "$TMP/allow-read.grants"
+fail=0
+
+run_case() {
+    label="$1"
+    checker="$2"
+    grants="$3"
+    expected="$4"
+    if LD_PRELOAD="$LIB" AETHER_SANDBOX_GRANTS="$grants" \
+            AETHER_SANDBOX_LOG=none "$TMP/harness" "$checker" "$TMP/data.txt" \
+            >"$TMP/out" 2>"$TMP/err"; then
+        got=allow
+    else
+        got=deny
+    fi
+    if [ "$got" = "$expected" ]; then
+        echo "  [PASS] $label"
+    else
+        echo "  [FAIL] $label: got $got, expected $expected"
+        fail=1
+    fi
+}
+
+run_case "preload denial survives an allowing embedded checker" \
+    allow "$TMP/deny-read.grants" deny
+run_case "embedded denial survives an allowing preload policy" \
+    deny "$TMP/allow-read.grants" deny
+run_case "both policies allowing permits the read" \
+    allow "$TMP/allow-read.grants" allow
+
+echo ""
+echo "sandbox_policy_intersection: $([ "$fail" -eq 0 ] && echo PASS || echo FAIL)"
+[ "$fail" -eq 0 ]