diff --git a/examples/simple_repeater/MyMesh.cpp b/examples/simple_repeater/MyMesh.cpp
index b3789657..be201fb4 100644
--- a/examples/simple_repeater/MyMesh.cpp
+++ b/examples/simple_repeater/MyMesh.cpp
@@ -1741,7 +1741,7 @@ void MyMesh::loop() {
       Serial.println("OTA: aborted, MQTT stop did not complete cleanly - resuming bridge");
       otaAlert("OTA aborted: MQTT stop unclean, bridge resumed");
       setBridgeState(true);
-    } else if (!_cli.getBoard()->otaFromManifest(ota_resolve_base(_prefs.ota_channel), getFirmwareVer(), false, ota_reply)) {
+    } else if (!_cli.getBoard()->otaFromManifest(ota_resolve_base(_ota_update_channel), getFirmwareVer(), false, ota_reply)) {
       Serial.print("OTA: aborted, resuming bridge - "); Serial.println(ota_reply);
       char ota_alert_msg[160];
       snprintf(ota_alert_msg, sizeof(ota_alert_msg), "OTA aborted: %s", ota_reply);
diff --git a/examples/simple_repeater/MyMesh.h b/examples/simple_repeater/MyMesh.h
index a232ef52..c1e03a8e 100644
--- a/examples/simple_repeater/MyMesh.h
+++ b/examples/simple_repeater/MyMesh.h
@@ -123,6 +123,7 @@ class MyMesh : public mesh::Mesh, public CommonCLICallbacks
   CayenneLPP telemetry;
   unsigned long set_radio_at, revert_radio_at;
   unsigned long _ota_update_at = 0;  // deferred `ota update` fire time (0 = none scheduled)
+  uint8_t _ota_update_channel = 0;   // channel `ota update` checked; a later `ota branch` cannot retarget it
   float pending_freq;
   float pending_bw;
   uint8_t pending_sf;
@@ -441,6 +442,7 @@ public:
   bool beginDeferredOtaUpdate() override {
     _ota_update_at = millis() + 2500;
     if (_ota_update_at == 0) _ota_update_at = 1;  // 0 means "none"
+    _ota_update_channel = _prefs.ota_channel;
 #if defined(WITH_MQTT_BRIDGE)
     // Broadcast START now, while the loop still runs (the 2.5 s reply window):
     // the deferred flash blocks the loop and, on success, reboots — so a start
diff --git a/examples/simple_room_server/MyMesh.cpp b/examples/simple_room_server/MyMesh.cpp
index 2e97d491..079e43fa 100644
--- a/examples/simple_room_server/MyMesh.cpp
+++ b/examples/simple_room_server/MyMesh.cpp
@@ -1617,7 +1617,7 @@ void MyMesh::loop() {
     }
 
     char ota_reply[160];
-    if (may_flash && !_cli.getBoard()->otaFromManifest(ota_resolve_base(_prefs.ota_channel), getFirmwareVer(), false, ota_reply)) {
+    if (may_flash && !_cli.getBoard()->otaFromManifest(ota_resolve_base(_ota_update_channel), getFirmwareVer(), false, ota_reply)) {
       Serial.print("OTA: aborted - "); Serial.println(ota_reply);
       may_flash = false;
     }
diff --git a/examples/simple_room_server/MyMesh.h b/examples/simple_room_server/MyMesh.h
index b0eb4e8f..ef7fb515 100644
--- a/examples/simple_room_server/MyMesh.h
+++ b/examples/simple_room_server/MyMesh.h
@@ -136,6 +136,7 @@ class MyMesh : public mesh::Mesh, public CommonCLICallbacks
   TransportKey default_scope;
   unsigned long set_radio_at, revert_radio_at;
   unsigned long _ota_update_at = 0;  // deferred `ota update` fire time (0 = none scheduled)
+  uint8_t _ota_update_channel = 0;   // channel `ota update` checked; a later `ota branch` cannot retarget it
   float pending_freq;
   float pending_bw;
   uint8_t pending_sf;
@@ -433,6 +434,7 @@ public:
   bool beginDeferredOtaUpdate() override {
     _ota_update_at = millis() + 2500;
     if (_ota_update_at == 0) _ota_update_at = 1;  // 0 means "none"
+    _ota_update_channel = _prefs.ota_channel;
     return true;
   }
 
diff --git a/scripts/verify_ota_channel.py b/scripts/verify_ota_channel.py
index 15d91c26..6b74030b 100644
--- a/scripts/verify_ota_channel.py
+++ b/scripts/verify_ota_channel.py
@@ -2,7 +2,8 @@
 """Verify every built observer binary is baked for the expected OTA channel.
 
 Observer builds carry three manifest bases (src/helpers/OtaChannel.h), each stored
-behind a tag so it can be read back from the binary:
+behind a tag so it can be read back from the binary (plus an ota-compat tag, which a
+channel switch reads from the downloaded image before booting it):
 
   ota-base-native:<url>   the channel the build OTAs from by default
   ota-base-stable:<url>   production, the target of `ota branch prod`
@@ -26,6 +27,7 @@ import re
 import sys
 
 TAG_RE = re.compile(rb"ota-base-(native|stable|dev):([\x21-\x7e]*)\x00")
+COMPAT_RE = re.compile(rb"ota-compat:([0-9]+(?:\+[a-z]+)*)\x00")
 
 
 def read_tags(data):
@@ -49,6 +51,10 @@ def check(data, expect, native_url, stable_url, dev_url):
         found = sorted(tags[name])
         if found != [want]:
             problems.append("ota-base-%s: expected [%s], found %s" % (name, want, found or "nothing"))
+    # Without its compat tag a build cannot be the target of a channel switch.
+    compat = sorted({m.group(1).decode() for m in COMPAT_RE.finditer(data)})
+    if len(compat) != 1:
+        problems.append("ota-compat: expected one value, found %s" % (compat or "nothing"))
     return problems
 
 
@@ -62,7 +68,7 @@ def self_test():
     P, B = "https://h/v", "https://h/beta/v"
 
     def blob(native, stable=P, dev=B):
-        return b"\x00junk\x00ota-base-native:%s\x00ota-base-stable:%s\x00ota-base-dev:%s\x00" % (
+        return b"\x00junk\x00ota-base-native:%s\x00ota-base-stable:%s\x00ota-base-dev:%s\x00ota-compat:1\x00" % (
             native.encode(), stable.encode(), dev.encode())
 
     cases = [
@@ -74,6 +80,8 @@ def self_test():
         ("wrong dev URL fails", blob(P, dev="https://other/v"), "prod", P, False),
         ("conflicting native tags fail", blob(P) + blob(B), "prod", P, False),
         ("workflow URL off-channel fails", blob(P), "prod", B, False),
+        ("missing compat tag fails", blob(P).replace(b"ota-compat:1", b"ota-compat:"), "prod", P, False),
+        ("conflicting compat tags fail", blob(P) + b"ota-compat:2+eth\x00", "prod", P, False),
     ]
     failed = 0
     for name, data, expect, native, ok in cases:
diff --git a/src/helpers/ESP32Board.cpp b/src/helpers/ESP32Board.cpp
index 206a3a7c..7c013c23 100644
--- a/src/helpers/ESP32Board.cpp
+++ b/src/helpers/ESP32Board.cpp
@@ -78,6 +78,9 @@ bool ESP32Board::startOTAUpdate(const char* id, char reply[], bool force_ap) {
 #include <freertos/FreeRTOS.h>
 #include <freertos/task.h>
 #include <esp_partition.h>
+#include <esp_ota_ops.h>
+#include <esp_image_format.h>
+#include "OtaChannel.h"
 
 // Embedded CA bundle (produced by board_build.embed_files). Weak so non-bundle
 // builds still link; we check for presence at runtime.
@@ -123,6 +126,31 @@ static void ota_parseVersion(const char* ver, char* base_out, size_t base_sz, in
   }
 }
 
+// Read the compat tag of the image just written to `part`, bounded by the image's own
+// length: bytes past it can be a stale, tagged image from an earlier flash.
+static bool ota_readImageCompat(const esp_partition_t* part, OtaCompat* out) {
+  const esp_partition_pos_t pos = { part->address, part->size };
+  esp_image_metadata_t meta;
+  if (esp_image_verify(ESP_IMAGE_VERIFY_SILENT, &pos, &meta) != ESP_OK) return false;
+  static const size_t kChunk = 2048, kTail = 64;  // tail > tag + value, so a split tag is seen whole
+  static uint8_t buf[kTail + kChunk];
+  size_t have = 0;
+  for (uint32_t off = 0; off < meta.image_len;) {
+    size_t n = meta.image_len - off;
+    if (n > kChunk) n = kChunk;
+    if (esp_partition_read(part, off, buf + have, n) != ESP_OK) return false;
+    have += n;
+    off += n;
+    const char* v = ota_compat_find(buf, have);
+    if (v) return ota_compat_parse(v, out);
+    if (have > kTail) {
+      memmove(buf, buf + have - kTail, kTail);
+      have = kTail;
+    }
+  }
+  return false;
+}
+
 // Canonical signature of the FLASHED partition table — MUST match
 // scripts/partition_signature.py: each entry "type:subtype:offset:size" in
 // lowercase hex, sorted by offset, joined by ','. Lets `ota update` compare the
@@ -334,13 +362,14 @@ bool ESP32Board::otaFromManifestImpl(const char* manifest_base, const char* curr
   bool same_base = (own_base[0] && avail_base[0] && strcmp(own_base, avail_base) == 0);
   bool have_builds = (own_build >= 0 && avail_build >= 0);
   bool diff_base = (own_base[0] && avail_base[0] && !same_base);
-  // Build counters are per channel, so build numbers only compare within the native channel.
+  // Another channel's image always differs (its native base does), even from the same
+  // commit; build counters are per channel, so build numbers only compare natively.
   bool cross_channel = (strcmp(manifest_base, OTA_MANIFEST_BASE) != 0);
 
   int behind = 0;
   bool up_to_date;
   if (cross_channel) {
-    up_to_date = hash_equal;
+    up_to_date = false;
   } else if (same_base && have_builds) {
     behind = avail_build - own_build;
     up_to_date = (behind <= 0);
@@ -411,9 +440,35 @@ bool ESP32Board::otaFromManifestImpl(const char* manifest_base, const char* curr
     int d = (int)((int64_t)cur * 10 / total);
     if (d != ota_progress_decile) { ota_progress_decile = d; Serial.printf("OTA: %d%%\n", d * 10); }
   });
-  httpUpdate.onEnd([]() { Serial.println("OTA: write complete, rebooting..."); });
-  httpUpdate.rebootOnUpdate(true);  // reboots into the new image on success
+  httpUpdate.onEnd([]() { Serial.println("OTA: write complete"); });
+  // A channel switch is checked before it may boot (below); native updates reboot here.
+  httpUpdate.rebootOnUpdate(!cross_channel);
   t_httpUpdate_return ret = httpUpdate.update(uclient, file_url);
+  if (ret == HTTP_UPDATE_OK && cross_channel) {
+    const esp_partition_t* running = esp_ota_get_running_partition();
+    const esp_partition_t* target = esp_ota_get_boot_partition();
+    OtaCompat own = {0, 0}, img = {0, 0};
+    const char* volatile own_tag = ota_compat_tag;  // volatile: the tag must stay in the image
+    ota_compat_parse(own_tag + sizeof(OTA_COMPAT_TAG) - 1, &own);
+    bool tagged = target && target != running && ota_readImageCompat(target, &img);
+    if (tagged && ota_compat_ok(own, img)) {
+      Serial.println("OTA: channel switch compatible, rebooting...");
+      delay(100);
+      ESP.restart();
+    }
+    // Point boot back at the running image so the refused build never starts.
+    bool reverted = (esp_ota_set_boot_partition(running) == ESP_OK);
+    inhibit_sleep = false;
+    if (!tagged) {
+      snprintf(reply, 160, "ERR: channel switch refused: %s has no compat tag; cable flash%s",
+               avail_disp, reverted ? "" : " [boot revert FAILED]");
+    } else {
+      snprintf(reply, 160, "ERR: channel switch refused: target compat %d/%x < this node %d/%x; cable flash%s",
+               img.gen, img.caps, own.gen, own.caps, reverted ? "" : " [boot revert FAILED]");
+    }
+    Serial.print("OTA: "); Serial.println(reply);
+    return false;
+  }
 
   // Only reached on failure (success reboots inside update()).
   inhibit_sleep = false;
diff --git a/src/helpers/OtaChannel.h b/src/helpers/OtaChannel.h
index 2918d83d..974cc4ba 100644
--- a/src/helpers/OtaChannel.h
+++ b/src/helpers/OtaChannel.h
@@ -76,3 +76,57 @@ static inline bool ota_parse_channel(const char* arg, uint8_t* out) {
   if (strcmp(arg, "default") == 0)                            { *out = OTA_CH_NATIVE; return true; }
   return false;
 }
+
+// Compatibility tag, read from a downloaded image before a channel switch boots it.
+// A switch can land on an older build, which would boot without state it cannot read
+// or without a transport this node depends on. Bump OTA_STATE_GEN when a build starts
+// storing state that older builds cannot read.
+//   1: /prefs.json + /mqtt_prefs
+#ifndef OTA_STATE_GEN
+#define OTA_STATE_GEN 1
+#endif
+#define OTA_CAP_ETH 0x01  // carries MQTT over Ethernet
+#if defined(NETWORK_PREFER_ETHERNET)
+#define OTA_CAPS_STR "+eth"
+#else
+#define OTA_CAPS_STR ""
+#endif
+#define OTA_STR_(x) #x
+#define OTA_STR(x) OTA_STR_(x)
+#define OTA_COMPAT_TAG "ota-compat:"
+static const char ota_compat_tag[] = OTA_COMPAT_TAG OTA_STR(OTA_STATE_GEN) OTA_CAPS_STR;
+
+struct OtaCompat {
+  int gen;
+  uint8_t caps;
+};
+
+// Parse the tag value "<gen>[+cap...]"; unknown caps are ignored.
+static inline bool ota_compat_parse(const char* s, OtaCompat* out) {
+  if (*s < '0' || *s > '9') return false;
+  out->gen = 0;
+  out->caps = 0;
+  while (*s >= '0' && *s <= '9') out->gen = out->gen * 10 + (*s++ - '0');
+  while (*s == '+') {
+    const char* cap = ++s;
+    while (*s && *s != '+') s++;
+    if ((size_t)(s - cap) == 3 && memcmp(cap, "eth", 3) == 0) out->caps |= OTA_CAP_ETH;
+  }
+  return *s == 0;
+}
+
+// Find the tag in an image chunk; returns the NUL-terminated value text, or nullptr when
+// the chunk holds no complete tag.
+static inline const char* ota_compat_find(const uint8_t* buf, size_t len) {
+  const size_t tag_len = sizeof(OTA_COMPAT_TAG) - 1;
+  for (size_t i = 0; i + tag_len < len; i++) {
+    if (memcmp(buf + i, OTA_COMPAT_TAG, tag_len) != 0) continue;
+    if (memchr(buf + i + tag_len, 0, len - i - tag_len)) return (const char*)buf + i + tag_len;
+  }
+  return nullptr;
+}
+
+// A target must read this node's state and keep every transport this node has.
+static inline bool ota_compat_ok(const OtaCompat& own, const OtaCompat& target) {
+  return target.gen >= own.gen && (target.caps & own.caps) == own.caps;
+}
diff --git a/test/test_ota_channel/test_ota_channel.cpp b/test/test_ota_channel/test_ota_channel.cpp
index d2a4e482..75ce673d 100644
--- a/test/test_ota_channel/test_ota_channel.cpp
+++ b/test/test_ota_channel/test_ota_channel.cpp
@@ -40,6 +40,39 @@ TEST(OtaChannel, NativeChannelNameMatchesBase) {
   EXPECT_STREQ(ota_native_channel_name(), "prod");
 }
 
+TEST(OtaCompat, OwnTagParses) {
+  OtaCompat own;
+  ASSERT_TRUE(ota_compat_parse(ota_compat_tag + sizeof(OTA_COMPAT_TAG) - 1, &own));
+  EXPECT_EQ(own.gen, OTA_STATE_GEN);
+  EXPECT_EQ(own.caps, 0);
+}
+TEST(OtaCompat, ParsesCapsAndRejectsJunk) {
+  OtaCompat c;
+  ASSERT_TRUE(ota_compat_parse("12+eth+future", &c));
+  EXPECT_EQ(c.gen, 12);
+  EXPECT_EQ(c.caps, OTA_CAP_ETH);
+  EXPECT_FALSE(ota_compat_parse("", &c));
+  EXPECT_FALSE(ota_compat_parse("x1", &c));
+  EXPECT_FALSE(ota_compat_parse("2eth", &c));
+}
+TEST(OtaCompat, FindsCompleteTagOnly) {
+  const char img[] = "\xe9junk\0ota-compat:2+eth\0tail";
+  const char* v = ota_compat_find((const uint8_t*)img, sizeof(img));
+  ASSERT_NE(v, nullptr);
+  EXPECT_STREQ(v, "2+eth");
+  const char cut[] = "junk ota-compat:2+e";  // value runs past the chunk end
+  EXPECT_EQ(ota_compat_find((const uint8_t*)cut, sizeof(cut) - 1), nullptr);
+  const char none[] = "ota-compat";
+  EXPECT_EQ(ota_compat_find((const uint8_t*)none, sizeof(none)), nullptr);
+}
+TEST(OtaCompat, TargetMustKeepStateAndTransports) {
+  EXPECT_TRUE(ota_compat_ok({2, 0}, {2, 0}));
+  EXPECT_TRUE(ota_compat_ok({1, 0}, {2, OTA_CAP_ETH}));
+  EXPECT_FALSE(ota_compat_ok({2, 0}, {1, 0}));                       // cannot read /mqtt.json
+  EXPECT_FALSE(ota_compat_ok({2, OTA_CAP_ETH}, {2, 0}));             // drops Ethernet
+  EXPECT_TRUE(ota_compat_ok({2, OTA_CAP_ETH}, {3, OTA_CAP_ETH}));
+}
+
 int main(int argc, char** argv) {
   ::testing::InitGoogleTest(&argc, argv);
   return RUN_ALL_TESTS();