diff --git a/esphome/components/api/api_server.cpp b/esphome/components/api/api_server.cpp
index f25a9bc0e2..67a117e68f 100644
--- a/esphome/components/api/api_server.cpp
+++ b/esphome/components/api/api_server.cpp
@@ -28,11 +28,7 @@ static const char *const TAG = "api";
 // APIServer
 APIServer *global_api_server = nullptr;  // NOLINT(cppcoreguidelines-avoid-non-const-global-variables)
 
-APIServer::APIServer() {
-  global_api_server = this;
-  // Pre-allocate shared write buffer
-  shared_write_buffer_.reserve(64);
-}
+APIServer::APIServer() { global_api_server = this; }
 
 void APIServer::setup() {
   ControllerRegistry::register_controller(this);
diff --git a/esphome/components/api/api_server.h b/esphome/components/api/api_server.h
index 28f60343e0..323acc2efb 100644
--- a/esphome/components/api/api_server.h
+++ b/esphome/components/api/api_server.h
@@ -268,7 +268,11 @@ class APIServer : public Component,
 
   // Vectors and strings (12 bytes each on 32-bit)
   std::vector<std::unique_ptr<APIConnection>> clients_;
-  std::vector<uint8_t> shared_write_buffer_;  // Shared proto write buffer for all connections
+  // Shared proto write buffer for all connections.
+  // Not pre-allocated: all send paths call prepare_first_message_buffer() which
+  // reserves the exact needed size. Pre-allocating here would cause heap fragmentation
+  // since the buffer would almost always reallocate on first use.
+  std::vector<uint8_t> shared_write_buffer_;
 #ifdef USE_API_HOMEASSISTANT_STATES
   std::vector<HomeAssistantStateSubscription> state_subs_;
 #endif