diff --git a/data-src/index.js b/data-src/index.js
index 0c3b735..c7a3f3c 100644
--- a/data-src/index.js
+++ b/data-src/index.js
@@ -4169,9 +4169,14 @@ class Firmware {
         div.style.display = '';
     }
     updateFirmware() {
+        let chip = document.getElementById('divContainer').getAttribute('data-chipmodel') || '';
+        // Firmware from before the field leaves it out: undefined, not false.
+        if (proto.model.header.otaSlot === false) {
+            ui.infoMessage(`This device has no OTA slot, so its firmware is flashed over USB with SomfyController.onboard.esp32${chip}.bin. That image holds the web UI and an empty configuration too, so take a backup first and restore it afterwards.`);
+            return;
+        }
         let div = this.createFileUploader('/updateFirmware');
         let inst = div.querySelector('div[id=divInstText]');
-        let chip = document.getElementById('divContainer').getAttribute('data-chipmodel') || '';
         let html = `<div style="font-size:14px;margin-bottom:20px;">Select a binary file [SomfyController.ino.esp32${chip}.bin] containing the device firmware then press the Upload File button.</div>`;
         if (this.isMobile()) {
             html += `<div style="width:100%;color:red;text-align:center;font-weight:bold;"><span style="margin-top:7px;width:100%;background:yellow;padding:3px;display:inline-block;border-radius:5px;background:white;">WARNING<span></div>`;
diff --git a/data-src/js/model.js b/data-src/js/model.js
index 2411996..df80ea1 100644
--- a/data-src/js/model.js
+++ b/data-src/js/model.js
@@ -140,7 +140,7 @@ ProtoModel.ITEMS = [
 // have it.
 ProtoModel.FIELDS = {
     header: ['maxRooms', 'maxShades', 'maxGroups', 'maxGroupedShades', 'maxLinkedRemotes', 'startingAddress', 'bitLength',
-        'proto', 'serverId', 'hostname', 'model', 'chipModel', 'fw', 'connType'],
+        'proto', 'serverId', 'hostname', 'model', 'chipModel', 'otaSlot', 'fw', 'connType'],
     room: ['roomId', 'name', 'sortOrder'],
     shadeState: ['shadeId', 'position', 'target', 'direction', 'myPos', 'tiltPosition', 'tiltTarget', 'tiltDirection',
         'myTiltPos', 'flags', 'lastRollingCode'],
diff --git a/docs/PROTOCOL.md b/docs/PROTOCOL.md
index 1937f23..3e5d18d 100644
--- a/docs/PROTOCOL.md
+++ b/docs/PROTOCOL.md
@@ -53,7 +53,7 @@ A number is never reused. A retired type stays reserved.
    - `F`: fields, as `[key, name, kind, default]`, kind being `b` bool, `i` integer, `n` number, `s` string, `l` list, `o` object
    - `E`: error codes, as `[number, name]`
    - `Q`: feeds, as `[number, name]`
-3. Then the device: `header` (limits, starting address, what a new shade gets, server id, hostname, model, chip, firmware, connection type), `rooms`, `shades`, `groups`, `repeaters` and `status` (memory, and WiFi or Ethernet). A frame is at most 4 KB, an item is never split across frames, and several sections can share one.
+3. Then the device: `header` (limits, starting address, what a new shade gets, server id, hostname, model, chip, whether it has an OTA slot (`otaSlot`, left out when it has), firmware, connection type), `rooms`, `shades`, `groups`, `repeaters` and `status` (memory, and WiFi or Ethernet). A frame is at most 4 KB, an item is never split across frames, and several sections can share one.
 4. The client sends `next` after each `sync` frame, and the device answers with the following one. With no `next` for 30 s the device closes the socket.
 5. `syncEnd` follows the last `sync` frame.
 
diff --git a/src/ConfigSettings.cpp b/src/ConfigSettings.cpp
index 42def2d..3424e66 100644
--- a/src/ConfigSettings.cpp
+++ b/src/ConfigSettings.cpp
@@ -7,6 +7,7 @@
 #include "Utils.h"
 #include "Secrets.h"
 #include "esp_chip_info.h"
+#include "esp_ota_ops.h"
 #include "esp_log.h"
 
 static const char *TAG = "Config";
@@ -144,6 +145,7 @@ bool ConfigSettings::begin() {
       break;
   }
   ESP_LOGD(TAG, "Chip Model ESP32-%s", this->chipModel);
+  this->otaSlot = esp_ota_get_next_update_partition(nullptr) != nullptr;
   this->fwVersion.parse(FW_VERSION);
   uint64_t mac = ESP.getEfuseMac();
   for(int i=0; i<17; i=i+8) {
diff --git a/src/ConfigSettings.h b/src/ConfigSettings.h
index 043ebe4..1aa5946 100644
--- a/src/ConfigSettings.h
+++ b/src/ConfigSettings.h
@@ -163,6 +163,9 @@ class ConfigSettings: BaseSettings {
     char serverId[10] = "";
     char hostname[32] = "ESPSomfyRTS";
     char chipModel[10] = "ESP32";
+    // Whether the partition table has a second app slot for /updateFirmware
+    // to write.  Builds with one 3 MB slot (C5, C6, debug) are flashed over USB.
+    bool otaSlot = true;
     conn_types_t connType = conn_types_t::unset;
     appver_t fwVersion;
     appver_t appVersion;
diff --git a/src/ProtoSchema.h b/src/ProtoSchema.h
index dc5a853..a46644e 100644
--- a/src/ProtoSchema.h
+++ b/src/ProtoSchema.h
@@ -169,6 +169,7 @@
   FIELD(hostname, "hn", 's', 0) \
   FIELD(model, "md", 's', 0) \
   FIELD(chipModel, "cp", 's', 0) \
+  FIELD(otaSlot, "os", 'b', 1) \
   FIELD(connType, "ct", 's', 0) \
   /* memory, wifi and ethernet */ \
   FIELD(freeHeap, "fr", 'i', 0) \
diff --git a/src/entryPoints/HttpRoutes.cpp b/src/entryPoints/HttpRoutes.cpp
index 14f1bac..5379de4 100644
--- a/src/entryPoints/HttpRoutes.cpp
+++ b/src/entryPoints/HttpRoutes.cpp
@@ -164,6 +164,11 @@ void registerHttpRoutes(AsyncWebServer &server) {
   // updateFirmware - file upload
   server.on(AsyncURIMatcher::exact("/updateFirmware"), HTTP_POST,
     [](AsyncWebServerRequest *request) {
+      // Nothing was written or stopped, so there is nothing to reboot for.
+      if(!settings.otaSlot) {
+        request->send(400, _encoding_json, F("{\"status\":\"ERROR\",\"desc\":\"This device has no OTA slot, so its firmware is flashed over USB.\"}"));
+        return;
+      }
       if(Update.hasError())
         request->send(500, _encoding_json, "{\"status\":\"ERROR\",\"desc\":\"Error updating firmware: \"}");
       else
@@ -172,7 +177,7 @@ void registerHttpRoutes(AsyncWebServer &server) {
       rebootDelay.rebootTime = millis() + 500;
     },
     [](AsyncWebServerRequest *request, String filename, size_t index, uint8_t *data, size_t len, bool final) {
-      if(!uploadAllowed(request, index)) return;
+      if(!uploadAllowed(request, index) || !settings.otaSlot) return;
       if(index == 0) {
         webServer.uploadSuccess = false;
         ESP_LOGI(TAG, "Update: %s", filename.c_str());
diff --git a/src/outputs/ProtoSocket.cpp b/src/outputs/ProtoSocket.cpp
index 81c5533..9c381e8 100644
--- a/src/outputs/ProtoSocket.cpp
+++ b/src/outputs/ProtoSocket.cpp
@@ -77,6 +77,7 @@ static void writeHeader(ProtoWriter &w) {
   w.add(proto_field_t::hostname, settings.hostname);
   w.add(proto_field_t::model, "ESPSomfyRTS");
   w.add(proto_field_t::chipModel, settings.chipModel);
+  w.add(proto_field_t::otaSlot, settings.otaSlot);
   w.add(proto_field_t::fw, settings.fwVersion.name);
   w.add(proto_field_t::connType, net.connTypeName());
 }
diff --git a/test/e2e/test_routes.py b/test/e2e/test_routes.py
index 8bf4859..d6df206 100644
--- a/test/e2e/test_routes.py
+++ b/test/e2e/test_routes.py
@@ -11,7 +11,8 @@ import urllib.request
 
 import pytest
 
-from proto_client import ProtoClient
+from proto_client import ProtoClient, run_sync
+from test_settings import uptime
 
 # Every route that needs the token under Secured, sent without a body or
 # parameters, so a route that let one through would still change nothing.
@@ -169,3 +170,22 @@ def test_a_socket_from_the_page_the_device_served_is_accepted(host, token):
     c.close()
     # The device frees the slot when it sees the disconnect.
     time.sleep(0.5)
+
+
+def test_a_firmware_upload_without_an_ota_slot_is_refused(host, token, connect, schema):
+    """Builds with one app slot (C5, C6, debug) say so in the header, and
+    refuse the upload without stopping the radio or rebooting."""
+    c = connect(schema=schema.hash)
+    _, frames, _ = run_sync(c)
+    key = schema.keys["header"]
+    header = next(schema.expand(f["d"][key]) for f in frames if key in f["d"])
+    if header.get("otaSlot", schema.default("otaSlot")):
+        pytest.skip("the device has an OTA slot: an upload would start an update")
+    uptime_before = uptime(schema, frames)
+    status, _, body = upload(host, "/updateFirmware", auth(token))
+    assert status == 400, body
+    assert json.loads(body)["desc"] == "This device has no OTA slot, so its firmware is flashed over USB."
+    # The old reply rebooted half a second later.
+    time.sleep(2)
+    _, frames, _ = run_sync(connect(schema=schema.hash))
+    assert uptime(schema, frames) > uptime_before, "the device restarted"
diff --git a/test/js/model.test.js b/test/js/model.test.js
index 3d8cd4e..d48fadb 100644
--- a/test/js/model.test.js
+++ b/test/js/model.test.js
@@ -37,6 +37,19 @@ test('a field the sync left out is its default', () => {
     for (const g of m.groupList()) assert.ok(Array.isArray(g.linkedShades));
 });
 
+test('otaSlot is false only when the device says so', () => {
+    // The recording is from firmware before the field.
+    assert.equal(synced().header.otaSlot, undefined);
+    const schema = recordedSchema();
+    schema.add({ F: [['os', 'otaSlot', 'b', 1]] });
+    const header = deviceSections(schema).find((d) => d.header).header;
+    const m = new ProtoModel(schema);
+    m.applySync({ header: header });
+    assert.equal(m.header.otaSlot, true);
+    m.applySync({ header: schema.expand(Object.assign({ os: 0 }, schema.pack(header))) });
+    assert.equal(m.header.otaSlot, false);
+});
+
 test('the lists come in their sort order', () => {
     const m = synced();
     const orders = m.shadeList().map((s) => s.sortOrder);