diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5fcc8ac..2638b9b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,8 @@ All notable changes to `cpp-sdk` are recorded here. Format follows [Keep a Chang
 
 ## [Unreleased]
 
+## [0.4.0] - 2026-10-09
+
 ### Added
 
 - `ebus_homie`, a CMake target (`homie/`, [doc/core.md](doc/core.md)): the Homie model, `/set` dispatch, the controller, id and payload validation, the JSON helpers, `HomieTransport`, and the log and clock hooks. It depends on `ebus_mqtt` and ArduinoJson, not on `ebus_discovery`. A CI step compiles it with only its own, `ebus_mqtt`'s and ArduinoJson's include directories. ([#3](https://github.com/electrification-bus/cpp-sdk/issues/3))
@@ -52,7 +54,8 @@ All notable changes to `cpp-sdk` are recorded here. Format follows [Keep a Chang
 - The core's Unity suites, built and run with CMake and CTest.
 - `library.json`, so a PlatformIO project can add the core with a `lib_deps` git URL.
 
-[Unreleased]: https://github.com/electrification-bus/cpp-sdk/compare/v0.3.0...HEAD
+[Unreleased]: https://github.com/electrification-bus/cpp-sdk/compare/v0.4.0...HEAD
+[0.4.0]: https://github.com/electrification-bus/cpp-sdk/compare/v0.3.0...v0.4.0
 [0.3.0]: https://github.com/electrification-bus/cpp-sdk/compare/v0.2.0...v0.3.0
 [0.2.0]: https://github.com/electrification-bus/cpp-sdk/compare/v0.1.0...v0.2.0
 [0.1.0]: https://github.com/electrification-bus/cpp-sdk/releases/tag/v0.1.0
diff --git a/README.md b/README.md
index 8ea8aca..8cbab53 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@ A portable C++17 core for devices and controllers on the [Electrification Bus](h
 
 ## Status
 
-Version 0.3.0, alpha. Known limits:
+Version 0.4.0, alpha. Known limits:
 
 - The POSIX demo build has no TLS; it connects to plain-TCP brokers only.
 - After a broker loses its retained messages, esp32-sdk's reconnect does not restore `$description`; it does not yet call `Device::forgetDescriptionHash()` ([After a reconnect](doc/core.md#after-a-reconnect)).
diff --git a/library.json b/library.json
index 0a81c0c..f32d94c 100644
--- a/library.json
+++ b/library.json
@@ -1,6 +1,6 @@
 {
   "name": "ebus_core",
-  "version": "0.3.0",
+  "version": "0.4.0",
   "description": "Portable eBus / Homie 5 core: the Device, Node and Property model, /set dispatch, the controller, id and payload validation, broker discovery. Depends on the C/C++ standard library and ArduinoJson; a port supplies the MQTT transport, log and clock. See doc/core.md.",
   "keywords": "homie, mqtt, ebus, iot",
   "repository": {