diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 00000000..b82b93d4
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,20 @@
+# Changelog
+
+## 0.4.0 (2025-09-17)
+
+### Protocol
+
+No changes.
+
+### Rust Library
+
+- Make `Agent` and `Client` dyn compatible (you'll need to annotate them with `#[async_trait]`) [#97](https://github.com/zed-industries/agent-client-protocol/pull/97)
+- `ext_method` and `ext_notification` methods are now more consistent with the other trait methods [#95](https://github.com/zed-industries/agent-client-protocol/pull/95)
+  - There are also distinct types for `ExtRequest`, `ExtResponse`, and `ExtNotification`
+- Rexport `serde_json::RawValue` for easier use [#95](https://github.com/zed-industries/agent-client-protocol/pull/95)
+
+### Typescript Library
+
+- Use Stream abstraction instead of raw byte streams [#93](https://github.com/zed-industries/agent-client-protocol/pull/93)
+  - Makes it easier to use with websockets instead of stdio
+- Improve type safety for method map helpers [#94](https://github.com/zed-industries/agent-client-protocol/pull/94)
diff --git a/Cargo.lock b/Cargo.lock
index 30dbad11..10ab9375 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -19,7 +19,7 @@ checksum = "320119579fcad9c21884f5c4861d16174d0e06250625266f50fe6898340abefa"
 
 [[package]]
 name = "agent-client-protocol"
-version = "0.3.1-alpha.1"
+version = "0.4.0"
 dependencies = [
  "anyhow",
  "async-broadcast",
diff --git a/Cargo.toml b/Cargo.toml
index 60d004ce..15fc8f91 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "agent-client-protocol"
 authors = ["Zed <hi@zed.dev>"]
-version = "0.3.1-alpha.1"
+version = "0.4.0"
 edition = "2024"
 license = "Apache-2.0"
 description = "A protocol for standardizing communication between code editors and AI coding agents"
diff --git a/package-lock.json b/package-lock.json
index 589f1f7a..10c5f6be 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "@zed-industries/agent-client-protocol",
-  "version": "0.3.1-alpha.1",
+  "version": "0.4.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@zed-industries/agent-client-protocol",
-      "version": "0.3.1-alpha.1",
+      "version": "0.4.0",
       "license": "Apache-2.0",
       "dependencies": {
         "zod": "^3.0.0"
diff --git a/package.json b/package.json
index dd0145b2..b3d6b368 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@zed-industries/agent-client-protocol",
-  "version": "0.3.1-alpha.1",
+  "version": "0.4.0",
   "publishConfig": {
     "access": "public"
   },