diff --git a/esphome/components/ota_base/ota_backend_arduino_esp32.cpp b/esphome/components/ota_base/ota_backend_arduino_esp32.cpp
index b89c61472a..f239544cfe 100644
--- a/esphome/components/ota_base/ota_backend_arduino_esp32.cpp
+++ b/esphome/components/ota_base/ota_backend_arduino_esp32.cpp
@@ -34,7 +34,10 @@ OTAResponseTypes ArduinoESP32OTABackend::begin(size_t image_size) {
   return OTA_RESPONSE_ERROR_UNKNOWN;
 }
 
-void ArduinoESP32OTABackend::set_update_md5(const char *md5) { Update.setMD5(md5); }
+void ArduinoESP32OTABackend::set_update_md5(const char *md5) {
+  Update.setMD5(md5);
+  this->md5_set_ = true;
+}
 
 OTAResponseTypes ArduinoESP32OTABackend::write(uint8_t *data, size_t len) {
   size_t written = Update.write(data, len);
@@ -49,7 +52,9 @@ OTAResponseTypes ArduinoESP32OTABackend::write(uint8_t *data, size_t len) {
 }
 
 OTAResponseTypes ArduinoESP32OTABackend::end() {
-  if (Update.end()) {
+  // Use strict validation (false) when MD5 is set, lenient validation (true) when no MD5
+  // This matches the behavior of the old web_server OTA implementation
+  if (Update.end(!this->md5_set_)) {
     return OTA_RESPONSE_OK;
   }
 
diff --git a/esphome/components/ota_base/ota_backend_arduino_esp32.h b/esphome/components/ota_base/ota_backend_arduino_esp32.h
index 6fb9454c64..e3966eb2f6 100644
--- a/esphome/components/ota_base/ota_backend_arduino_esp32.h
+++ b/esphome/components/ota_base/ota_backend_arduino_esp32.h
@@ -16,6 +16,9 @@ class ArduinoESP32OTABackend : public OTABackend {
   OTAResponseTypes end() override;
   void abort() override;
   bool supports_compression() override { return false; }
+
+ private:
+  bool md5_set_{false};
 };
 
 }  // namespace ota_base
diff --git a/esphome/components/ota_base/ota_backend_arduino_esp8266.cpp b/esphome/components/ota_base/ota_backend_arduino_esp8266.cpp
index 1133ce7b05..5df2ed0a58 100644
--- a/esphome/components/ota_base/ota_backend_arduino_esp8266.cpp
+++ b/esphome/components/ota_base/ota_backend_arduino_esp8266.cpp
@@ -43,7 +43,10 @@ OTAResponseTypes ArduinoESP8266OTABackend::begin(size_t image_size) {
   return OTA_RESPONSE_ERROR_UNKNOWN;
 }
 
-void ArduinoESP8266OTABackend::set_update_md5(const char *md5) { Update.setMD5(md5); }
+void ArduinoESP8266OTABackend::set_update_md5(const char *md5) {
+  Update.setMD5(md5);
+  this->md5_set_ = true;
+}
 
 OTAResponseTypes ArduinoESP8266OTABackend::write(uint8_t *data, size_t len) {
   size_t written = Update.write(data, len);
@@ -58,7 +61,9 @@ OTAResponseTypes ArduinoESP8266OTABackend::write(uint8_t *data, size_t len) {
 }
 
 OTAResponseTypes ArduinoESP8266OTABackend::end() {
-  if (Update.end()) {
+  // Use strict validation (false) when MD5 is set, lenient validation (true) when no MD5
+  // This matches the behavior of the old web_server OTA implementation
+  if (Update.end(!this->md5_set_)) {
     return OTA_RESPONSE_OK;
   }
 
diff --git a/esphome/components/ota_base/ota_backend_arduino_esp8266.h b/esphome/components/ota_base/ota_backend_arduino_esp8266.h
index 3f9982a514..f399013121 100644
--- a/esphome/components/ota_base/ota_backend_arduino_esp8266.h
+++ b/esphome/components/ota_base/ota_backend_arduino_esp8266.h
@@ -21,6 +21,9 @@ class ArduinoESP8266OTABackend : public OTABackend {
 #else
   bool supports_compression() override { return false; }
 #endif
+
+ private:
+  bool md5_set_{false};
 };
 
 }  // namespace ota_base
diff --git a/esphome/components/ota_base/ota_backend_arduino_libretiny.cpp b/esphome/components/ota_base/ota_backend_arduino_libretiny.cpp
index 052a9faed9..2596e3c2a3 100644
--- a/esphome/components/ota_base/ota_backend_arduino_libretiny.cpp
+++ b/esphome/components/ota_base/ota_backend_arduino_libretiny.cpp
@@ -34,7 +34,10 @@ OTAResponseTypes ArduinoLibreTinyOTABackend::begin(size_t image_size) {
   return OTA_RESPONSE_ERROR_UNKNOWN;
 }
 
-void ArduinoLibreTinyOTABackend::set_update_md5(const char *md5) { Update.setMD5(md5); }
+void ArduinoLibreTinyOTABackend::set_update_md5(const char *md5) {
+  Update.setMD5(md5);
+  this->md5_set_ = true;
+}
 
 OTAResponseTypes ArduinoLibreTinyOTABackend::write(uint8_t *data, size_t len) {
   size_t written = Update.write(data, len);
@@ -49,7 +52,9 @@ OTAResponseTypes ArduinoLibreTinyOTABackend::write(uint8_t *data, size_t len) {
 }
 
 OTAResponseTypes ArduinoLibreTinyOTABackend::end() {
-  if (Update.end()) {
+  // Use strict validation (false) when MD5 is set, lenient validation (true) when no MD5
+  // This matches the behavior of the old web_server OTA implementation
+  if (Update.end(!this->md5_set_)) {
     return OTA_RESPONSE_OK;
   }
 
diff --git a/esphome/components/ota_base/ota_backend_arduino_libretiny.h b/esphome/components/ota_base/ota_backend_arduino_libretiny.h
index b1cf1df738..33eebeb95a 100644
--- a/esphome/components/ota_base/ota_backend_arduino_libretiny.h
+++ b/esphome/components/ota_base/ota_backend_arduino_libretiny.h
@@ -15,6 +15,9 @@ class ArduinoLibreTinyOTABackend : public OTABackend {
   OTAResponseTypes end() override;
   void abort() override;
   bool supports_compression() override { return false; }
+
+ private:
+  bool md5_set_{false};
 };
 
 }  // namespace ota_base
diff --git a/esphome/components/ota_base/ota_backend_arduino_rp2040.cpp b/esphome/components/ota_base/ota_backend_arduino_rp2040.cpp
index bcb87f3547..589187f615 100644
--- a/esphome/components/ota_base/ota_backend_arduino_rp2040.cpp
+++ b/esphome/components/ota_base/ota_backend_arduino_rp2040.cpp
@@ -43,7 +43,10 @@ OTAResponseTypes ArduinoRP2040OTABackend::begin(size_t image_size) {
   return OTA_RESPONSE_ERROR_UNKNOWN;
 }
 
-void ArduinoRP2040OTABackend::set_update_md5(const char *md5) { Update.setMD5(md5); }
+void ArduinoRP2040OTABackend::set_update_md5(const char *md5) {
+  Update.setMD5(md5);
+  this->md5_set_ = true;
+}
 
 OTAResponseTypes ArduinoRP2040OTABackend::write(uint8_t *data, size_t len) {
   size_t written = Update.write(data, len);
@@ -58,7 +61,9 @@ OTAResponseTypes ArduinoRP2040OTABackend::write(uint8_t *data, size_t len) {
 }
 
 OTAResponseTypes ArduinoRP2040OTABackend::end() {
-  if (Update.end()) {
+  // Use strict validation (false) when MD5 is set, lenient validation (true) when no MD5
+  // This matches the behavior of the old web_server OTA implementation
+  if (Update.end(!this->md5_set_)) {
     return OTA_RESPONSE_OK;
   }
 
diff --git a/esphome/components/ota_base/ota_backend_arduino_rp2040.h b/esphome/components/ota_base/ota_backend_arduino_rp2040.h
index fb6e90bb53..6d622d4a5a 100644
--- a/esphome/components/ota_base/ota_backend_arduino_rp2040.h
+++ b/esphome/components/ota_base/ota_backend_arduino_rp2040.h
@@ -17,6 +17,9 @@ class ArduinoRP2040OTABackend : public OTABackend {
   OTAResponseTypes end() override;
   void abort() override;
   bool supports_compression() override { return false; }
+
+ private:
+  bool md5_set_{false};
 };
 
 }  // namespace ota_base
diff --git a/esphome/components/web_server_base/web_server_base.cpp b/esphome/components/web_server_base/web_server_base.cpp
index 1503ee3bdd..a683ee85eb 100644
--- a/esphome/components/web_server_base/web_server_base.cpp
+++ b/esphome/components/web_server_base/web_server_base.cpp
@@ -67,6 +67,7 @@ void OTARequestHandler::schedule_ota_reboot_() {
 void OTARequestHandler::ota_init_(const char *filename) {
   ESP_LOGI(TAG, "OTA Update Start: %s", filename);
   this->ota_read_length_ = 0;
+  this->ota_success_ = false;
 }
 
 void OTARequestHandler::handleUpload(AsyncWebServerRequest *request, const String &filename, size_t index,
@@ -140,8 +141,15 @@ void OTARequestHandler::handleUpload(AsyncWebServerRequest *request, const Strin
 
   // Finalize
   if (final) {
+    ESP_LOGD(TAG, "OTA final chunk: index=%u, len=%u, total_read=%u, contentLength=%u", index, len,
+             this->ota_read_length_, request->contentLength());
+
+    // For Arduino framework, the Update library tracks expected size from firmware header
+    // If we haven't received enough data, calling end() will fail
+    // This can happen if the upload is interrupted or the client disconnects
     error_code = this->ota_backend_->end();
     if (error_code == ota_base::OTA_RESPONSE_OK) {
+      this->ota_success_ = true;
 #ifdef USE_OTA_STATE_CALLBACK
       // Report completion before reboot - use call_deferred since we're in web server task
       this->parent_->state_callback_.call_deferred(ota_base::OTA_COMPLETED, 100.0f, 0);
@@ -159,8 +167,9 @@ void OTARequestHandler::handleUpload(AsyncWebServerRequest *request, const Strin
 
 void OTARequestHandler::handleRequest(AsyncWebServerRequest *request) {
   AsyncWebServerResponse *response;
-  // Send response based on whether backend still exists (error) or was reset (success)
-  response = request->beginResponse(200, "text/plain", !this->ota_backend_ ? "Update Successful!" : "Update Failed!");
+  // Use the ota_success_ flag to determine the actual result
+  const char *msg = this->ota_success_ ? "Update Successful!" : "Update Failed!";
+  response = request->beginResponse(200, "text/plain", msg);
   response->addHeader("Connection", "close");
   request->send(response);
 }
diff --git a/esphome/components/web_server_base/web_server_base.h b/esphome/components/web_server_base/web_server_base.h
index 6f3a770e42..99087ddfa8 100644
--- a/esphome/components/web_server_base/web_server_base.h
+++ b/esphome/components/web_server_base/web_server_base.h
@@ -159,6 +159,7 @@ class OTARequestHandler : public AsyncWebHandler {
   uint32_t last_ota_progress_{0};
   uint32_t ota_read_length_{0};
   WebServerBase *parent_;
+  bool ota_success_{false};
 
  private:
   std::unique_ptr<ota_base::OTABackend> ota_backend_{nullptr};