diff --git a/esphome/components/api/proto.h b/esphome/components/api/proto.h
index 8cb05f630f..26868e3fd5 100644
--- a/esphome/components/api/proto.h
+++ b/esphome/components/api/proto.h
@@ -350,6 +350,9 @@ class ProtoEncode {
   /// Tag must be a single-byte varint (< 128). Always encodes (no zero check).
   static inline void encode_short_string_force(uint8_t *__restrict__ &pos PROTO_ENCODE_DEBUG_PARAM, uint8_t tag,
                                                const StringRef &ref) {
+#ifdef ESPHOME_DEBUG_API
+    assert(ref.size() < 128 && "encode_short_string_force: string exceeds max_data_length < 128");
+#endif
     PROTO_ENCODE_CHECK_BOUNDS(pos, 2 + ref.size());
     pos[0] = tag;
     pos[1] = static_cast<uint8_t>(ref.size());