diff --git a/host/psxrecomp_codegen_host.c b/host/psxrecomp_codegen_host.c
index 33f3af39..eb178c7e 100644
--- a/host/psxrecomp_codegen_host.c
+++ b/host/psxrecomp_codegen_host.c
@@ -4353,6 +4353,34 @@ static void host_selfcheck_or_return(const PsxrecompCodegenHostConfig* cfg,
     printf(",\n  \"game_dispatch_present\": %s", game_ok ? "true" : "false");
     printf(",\n  \"bios_backends_present\": %s", bios_ok ? "true" : "false");
     printf(",\n  \"sources_missing\": %s", missing ? "true" : "false");
+    /* Wave-5 F4: overlay_cache is the switch that makes the runtime initialise the
+     * overlay loader at all; without it every streamed overlay is interpreted. CI
+     * asserts this field so a title cannot ship silently without it again. */
+    {
+        char toml_abs[1200];
+        int overlay_cache = 0;
+        if (join_path(toml_abs, sizeof(toml_abs), g_project_root, "game.toml")) {
+            FILE* tf = fopen(toml_abs, "rb");
+            if (tf) {
+                char line[512];
+                while (fgets(line, sizeof(line), tf)) {
+                    const char* p = line;
+                    while (*p == ' ' || *p == '\t') ++p;
+                    if (strncmp(p, "overlay_cache", 13) == 0) {
+                        p += 13;
+                        while (*p == ' ' || *p == '\t') ++p;
+                        if (*p == '=') {
+                            ++p;
+                            while (*p == ' ' || *p == '\t') ++p;
+                            overlay_cache = strncmp(p, "true", 4) == 0;
+                        }
+                    }
+                }
+                fclose(tf);
+            }
+        }
+        printf(",\n  \"overlay_cache_configured\": %s", overlay_cache ? "true" : "false");
+    }
     printf("\n}\n");
     fflush(stdout);
     exit(missing ? 2 : 0);
diff --git a/tools/package_setup_host.sh b/tools/package_setup_host.sh
index 7482e9fa..019b9357 100755
--- a/tools/package_setup_host.sh
+++ b/tools/package_setup_host.sh
@@ -64,6 +64,10 @@ DISPLAY_NAME=""
 RECOMPILER_BUILD="build-recompiler"
 VERSION_ENV="RELEASE_VERSION"
 DISC_HINT="your legally owned game disc"
+# Wave-5 F4: a packaged game.toml without `overlay_cache = true` ships a runtime that never
+# initialises the overlay loader, so every streamed overlay runs on the interpreter (78 of
+# 105 releases measured 2026-09-16). Refuse unless the caller states why.
+SHIP_WITHOUT_OVERLAY_CACHE_BECAUSE=""
 PROJECT_FILES=()
 PROJECT_DIRS=()
 RUNTIME_DIRS=()
@@ -115,6 +119,7 @@ while [[ $# -gt 0 ]]; do
     --root) ROOT="${2:?}"; shift 2 ;;
     --embed-toolchain) EMBED_TOOLCHAIN=1; shift ;;
     --no-embed-toolchain) EMBED_TOOLCHAIN=0; shift ;;
+    --ship-without-overlay-cache-because) SHIP_WITHOUT_OVERLAY_CACHE_BECAUSE="${2:?}"; shift 2 ;;
     *)
       echo "error: unknown arg: $1" >&2
       usage 2
@@ -484,6 +489,17 @@ fi
 
 bash "${STAGE_SDK}" "${stage_args[@]}"
 
+if ! grep -qE '^[[:space:]]*overlay_cache[[:space:]]*=[[:space:]]*true' "${STAGE}/game.toml"; then
+  if [[ -z "${SHIP_WITHOUT_OVERLAY_CACHE_BECAUSE}" ]]; then
+    echo "error: REFUSING TO PACKAGE: ${STAGE}/game.toml has no '[runtime] overlay_cache = true'." >&2
+    echo "       The runtime never initialises the overlay loader without it, so every streamed" >&2
+    echo "       overlay runs on the dirty-RAM interpreter for every player. Add the key, or pass" >&2
+    echo "       --ship-without-overlay-cache-because '<reason>' to record why this title ships without it." >&2
+    exit 1
+  fi
+  echo "warning: packaging without overlay_cache = true (reason: ${SHIP_WITHOUT_OVERLAY_CACHE_BECAUSE})" >&2
+fi
+
 cat >"${STAGE}/README-SETUP.txt" <<EOF
 ${DISPLAY_NAME} ${VERSION} — setup package
 Platform: ${ARTIFACT}