diff --git a/CHANGELOG.md b/CHANGELOG.md
index fa520afd..9f24c464 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,7 +23,11 @@ changes accumulate. Track in-flight protocol changes via PRs touching
 `NOTIFICATION_INTRODUCED_IN` maps in
 [`types/version/registry.ts`](types/version/registry.ts).
 
-## [0.3.0] — Unreleased
+## [0.4.0] — Unreleased
+
+Spec version: `0.4.0`
+
+## [0.3.0] — 2026-06-05
 
 Spec version: `0.3.0`
 
diff --git a/clients/go/CHANGELOG.md b/clients/go/CHANGELOG.md
index 2e8a28ed..fa1bafc9 100644
--- a/clients/go/CHANGELOG.md
+++ b/clients/go/CHANGELOG.md
@@ -14,6 +14,10 @@ tag whose matching `## [X.Y.Z]` heading is missing from this file.
 
 ## [Unreleased]
 
+## [0.3.0] — 2026-06-05
+
+Implements AHP 0.3.0.
+
 ### Added
 
 - `McpServerCustomization` now exposes the full MCP lifecycle: `Enabled`,
diff --git a/clients/go/VERSION b/clients/go/VERSION
index 6e8bf73a..0d91a54c 100644
--- a/clients/go/VERSION
+++ b/clients/go/VERSION
@@ -1 +1 @@
-0.1.0
+0.3.0
diff --git a/clients/go/ahptypes/version.generated.go b/clients/go/ahptypes/version.generated.go
index f0c9697b..4cb29ded 100644
--- a/clients/go/ahptypes/version.generated.go
+++ b/clients/go/ahptypes/version.generated.go
@@ -6,12 +6,13 @@ package ahptypes
 
 // ProtocolVersion is the current protocol version (SemVer
 // MAJOR.MINOR.PATCH) that this generated source speaks.
-const ProtocolVersion = "0.3.0"
+const ProtocolVersion = "0.4.0"
 
 // supportedProtocolVersions backs [SupportedProtocolVersions] — held
 // in an unexported slice so callers cannot accidentally mutate the
 // shared backing array.
 var supportedProtocolVersions = []string{
+	"0.4.0",
 	"0.3.0",
 }
 
diff --git a/clients/go/release-metadata.json b/clients/go/release-metadata.json
index e0a29541..a3b35687 100644
--- a/clients/go/release-metadata.json
+++ b/clients/go/release-metadata.json
@@ -1,7 +1,8 @@
 {
   "client": "go",
-  "packageVersion": "0.1.0",
+  "packageVersion": "0.3.0",
   "supportedProtocolVersions": [
+    "0.4.0",
     "0.3.0"
   ]
 }
diff --git a/clients/kotlin/CHANGELOG.md b/clients/kotlin/CHANGELOG.md
index 80d2b093..5e87fbc8 100644
--- a/clients/kotlin/CHANGELOG.md
+++ b/clients/kotlin/CHANGELOG.md
@@ -15,6 +15,10 @@ versions (`*-SNAPSHOT`) are explicitly rejected by the publish pipeline; bump
 
 ## [Unreleased]
 
+## [0.3.0] — 2026-06-05
+
+Implements AHP 0.3.0.
+
 ### Added
 
 - `McpServerCustomization` now exposes the full MCP lifecycle: `enabled`,
diff --git a/clients/kotlin/gradle.properties b/clients/kotlin/gradle.properties
index ad247d70..8213419c 100644
--- a/clients/kotlin/gradle.properties
+++ b/clients/kotlin/gradle.properties
@@ -8,7 +8,7 @@ kotlin.code.style=official
 # Maven coordinates — consumed by build.gradle.kts and validated by the
 # publish workflow against the `kotlin/v*` git tag.
 GROUP=com.microsoft.agenthostprotocol
-VERSION_NAME=0.2.0
+VERSION_NAME=0.3.0
 POM_NAME=agent-host-protocol
 POM_DESCRIPTION=Kotlin/JVM client for the Agent Host Protocol (AHP)
 POM_INCEPTION_YEAR=2026
diff --git a/clients/kotlin/release-metadata.json b/clients/kotlin/release-metadata.json
index 3c8f6295..4c356436 100644
--- a/clients/kotlin/release-metadata.json
+++ b/clients/kotlin/release-metadata.json
@@ -1,7 +1,8 @@
 {
   "client": "kotlin",
-  "packageVersion": "0.2.0",
+  "packageVersion": "0.3.0",
   "supportedProtocolVersions": [
+    "0.4.0",
     "0.3.0"
   ]
 }
diff --git a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Version.generated.kt b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Version.generated.kt
index 46dffdde..fe1116b5 100644
--- a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Version.generated.kt
+++ b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Version.generated.kt
@@ -5,7 +5,7 @@ package com.microsoft.agenthostprotocol.generated
 /**
  * Current protocol version (SemVer `MAJOR.MINOR.PATCH`).
  */
-public const val PROTOCOL_VERSION: String = "0.3.0"
+public const val PROTOCOL_VERSION: String = "0.4.0"
 
 /**
  * Every protocol version this library is willing to negotiate, ordered
@@ -16,5 +16,6 @@ public const val PROTOCOL_VERSION: String = "0.3.0"
  * protocol versions if the host doesn't accept the newest one.
  */
 public val SUPPORTED_PROTOCOL_VERSIONS: List<String> = listOf(
+    "0.4.0",
     "0.3.0",
 )
diff --git a/clients/rust/CHANGELOG.md b/clients/rust/CHANGELOG.md
index 7ce5cef3..f40505ea 100644
--- a/clients/rust/CHANGELOG.md
+++ b/clients/rust/CHANGELOG.md
@@ -15,6 +15,10 @@ matching `## [X.Y.Z]` heading is missing from this file.
 
 ## [Unreleased]
 
+## [0.3.0] — 2026-06-05
+
+Implements AHP 0.3.0.
+
 ### Added
 
 - `McpServerCustomization` now exposes the full MCP lifecycle: `enabled`,
diff --git a/clients/rust/Cargo.lock b/clients/rust/Cargo.lock
index 9987bfbc..b4ce821e 100644
--- a/clients/rust/Cargo.lock
+++ b/clients/rust/Cargo.lock
@@ -4,7 +4,7 @@ version = 3
 
 [[package]]
 name = "ahp"
-version = "0.2.0"
+version = "0.3.0"
 dependencies = [
  "ahp-types",
  "ahp-ws",
@@ -18,7 +18,7 @@ dependencies = [
 
 [[package]]
 name = "ahp-types"
-version = "0.2.0"
+version = "0.3.0"
 dependencies = [
  "serde",
  "serde_json",
@@ -27,7 +27,7 @@ dependencies = [
 
 [[package]]
 name = "ahp-ws"
-version = "0.2.0"
+version = "0.3.0"
 dependencies = [
  "ahp",
  "futures-util",
diff --git a/clients/rust/Cargo.toml b/clients/rust/Cargo.toml
index 6c89c169..a9517f0c 100644
--- a/clients/rust/Cargo.toml
+++ b/clients/rust/Cargo.toml
@@ -7,7 +7,7 @@ members = [
 ]
 
 [workspace.package]
-version = "0.2.0"
+version = "0.3.0"
 edition = "2021"
 rust-version = "1.75"
 license = "MIT"
@@ -22,5 +22,5 @@ tokio = { version = "1", features = ["sync", "rt", "macros", "time"] }
 tokio-util = "0.7"
 futures-util = "0.3"
 tracing = "0.1"
-ahp-types = { path = "crates/ahp-types", version = "0.2.0" }
-ahp = { path = "crates/ahp", version = "0.2.0" }
+ahp-types = { path = "crates/ahp-types", version = "0.3.0" }
+ahp = { path = "crates/ahp", version = "0.3.0" }
diff --git a/clients/rust/crates/ahp-types/src/version.rs b/clients/rust/crates/ahp-types/src/version.rs
index f827dc25..cdd4b359 100644
--- a/clients/rust/crates/ahp-types/src/version.rs
+++ b/clients/rust/crates/ahp-types/src/version.rs
@@ -5,7 +5,7 @@
 #![allow(missing_docs)]
 
 /// Current protocol version (SemVer `MAJOR.MINOR.PATCH`).
-pub const PROTOCOL_VERSION: &str = "0.3.0";
+pub const PROTOCOL_VERSION: &str = "0.4.0";
 
 /// Every protocol version this crate is willing to negotiate, ordered
 /// most-preferred-first. The first entry equals [`PROTOCOL_VERSION`].
@@ -13,4 +13,4 @@ pub const PROTOCOL_VERSION: &str = "0.3.0";
 /// Consumers building `InitializeParams` should pass this slice (or a
 /// derived `Vec<String>`) so the same client binary can fall back to
 /// older protocol versions if the host doesn't accept the newest one.
-pub const SUPPORTED_PROTOCOL_VERSIONS: &[&str] = &["0.3.0"];
+pub const SUPPORTED_PROTOCOL_VERSIONS: &[&str] = &["0.4.0", "0.3.0"];
diff --git a/clients/rust/release-metadata.json b/clients/rust/release-metadata.json
index 0a17eb6f..26cc27cd 100644
--- a/clients/rust/release-metadata.json
+++ b/clients/rust/release-metadata.json
@@ -1,7 +1,8 @@
 {
   "client": "rust",
-  "packageVersion": "0.2.0",
+  "packageVersion": "0.3.0",
   "supportedProtocolVersions": [
+    "0.4.0",
     "0.3.0"
   ]
 }
diff --git a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Version.generated.swift b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Version.generated.swift
index b6d50116..1179b0c9 100644
--- a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Version.generated.swift
+++ b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Version.generated.swift
@@ -3,7 +3,7 @@
 import Foundation
 
 /// Current protocol version (SemVer `MAJOR.MINOR.PATCH`).
-public let PROTOCOL_VERSION: String = "0.3.0"
+public let PROTOCOL_VERSION: String = "0.4.0"
 
 /// Every protocol version this package is willing to negotiate,
 /// ordered most-preferred-first. The first entry equals
@@ -13,5 +13,6 @@ public let PROTOCOL_VERSION: String = "0.3.0"
 /// `InitializeParams` so the same client binary can fall back to older
 /// protocol versions if the host doesn't accept the newest one.
 public let SUPPORTED_PROTOCOL_VERSIONS: [String] = [
+    "0.4.0",
     "0.3.0",
 ]
diff --git a/clients/swift/CHANGELOG.md b/clients/swift/CHANGELOG.md
index 1f6e5383..cdea34ae 100644
--- a/clients/swift/CHANGELOG.md
+++ b/clients/swift/CHANGELOG.md
@@ -17,6 +17,10 @@ the tag matches the version pinned in [`VERSION`](VERSION).
 
 ## [Unreleased]
 
+## [0.3.0] — 2026-06-05
+
+Implements AHP 0.3.0.
+
 ### Added
 
 - `McpServerCustomization` now exposes the full MCP lifecycle: `enabled`,
diff --git a/clients/swift/VERSION b/clients/swift/VERSION
index 0ea3a944..0d91a54c 100644
--- a/clients/swift/VERSION
+++ b/clients/swift/VERSION
@@ -1 +1 @@
-0.2.0
+0.3.0
diff --git a/clients/swift/release-metadata.json b/clients/swift/release-metadata.json
index de75c746..07f8c330 100644
--- a/clients/swift/release-metadata.json
+++ b/clients/swift/release-metadata.json
@@ -1,7 +1,8 @@
 {
   "client": "swift",
-  "packageVersion": "0.2.0",
+  "packageVersion": "0.3.0",
   "supportedProtocolVersions": [
+    "0.4.0",
     "0.3.0"
   ]
 }
diff --git a/clients/typescript/CHANGELOG.md b/clients/typescript/CHANGELOG.md
index eda16335..2210359b 100644
--- a/clients/typescript/CHANGELOG.md
+++ b/clients/typescript/CHANGELOG.md
@@ -20,6 +20,10 @@ hotfix escape hatch.
 
 ## [Unreleased]
 
+## [0.3.0] — 2026-06-05
+
+Implements AHP 0.3.0.
+
 ### Added
 
 - `McpServerCustomization` now exposes the full MCP lifecycle: `enabled`,
diff --git a/clients/typescript/package-lock.json b/clients/typescript/package-lock.json
index 51c3e519..f3900bf2 100644
--- a/clients/typescript/package-lock.json
+++ b/clients/typescript/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "@microsoft/agent-host-protocol",
-  "version": "0.2.0",
+  "version": "0.3.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@microsoft/agent-host-protocol",
-      "version": "0.2.0",
+      "version": "0.3.0",
       "license": "MIT",
       "devDependencies": {
         "@types/node": "^22.10.0",
diff --git a/clients/typescript/package.json b/clients/typescript/package.json
index c63e10b6..cf80469a 100644
--- a/clients/typescript/package.json
+++ b/clients/typescript/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@microsoft/agent-host-protocol",
-  "version": "0.2.0",
+  "version": "0.3.0",
   "description": "TypeScript client for the Agent Host Protocol (AHP).",
   "type": "module",
   "license": "MIT",
diff --git a/clients/typescript/release-metadata.json b/clients/typescript/release-metadata.json
index a66bf694..0220cb4e 100644
--- a/clients/typescript/release-metadata.json
+++ b/clients/typescript/release-metadata.json
@@ -1,7 +1,8 @@
 {
   "client": "typescript",
-  "packageVersion": "0.2.0",
+  "packageVersion": "0.3.0",
   "supportedProtocolVersions": [
+    "0.4.0",
     "0.3.0"
   ]
 }
diff --git a/types/version/registry.ts b/types/version/registry.ts
index 4adc151b..c4c98cfb 100644
--- a/types/version/registry.ts
+++ b/types/version/registry.ts
@@ -15,7 +15,7 @@ import type { ServerNotificationMap } from '../messages.js';
  *
  * Formatted as a [SemVer](https://semver.org) `MAJOR.MINOR.PATCH` string.
  */
-export const PROTOCOL_VERSION = '0.3.0';
+export const PROTOCOL_VERSION = '0.4.0';
 
 /**
  * Every protocol version a client built from this source tree is willing
@@ -34,6 +34,7 @@ export const PROTOCOL_VERSION = '0.3.0';
  * `scripts/verify-release-metadata.ts`.
  */
 export const SUPPORTED_PROTOCOL_VERSIONS: readonly string[] = Object.freeze([
+  '0.4.0',
   '0.3.0',
 ]);