diff --git a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
index 353507a47..1170b4110 100644
--- a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
+++ b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
@@ -2171,11 +2171,21 @@
     "InitializeResponse": {
       "$schema": "http://json-schema.org/draft-07/schema#",
       "properties": {
+        "platformFamily": {
+          "description": "Platform family for the running app-server target, for example `\"unix\"` or `\"windows\"`.",
+          "type": "string"
+        },
+        "platformOs": {
+          "description": "Operating system for the running app-server target, for example `\"macos\"`, `\"linux\"`, or `\"windows\"`.",
+          "type": "string"
+        },
         "userAgent": {
           "type": "string"
         }
       },
       "required": [
+        "platformFamily",
+        "platformOs",
         "userAgent"
       ],
       "title": "InitializeResponse",
diff --git a/codex-rs/app-server-protocol/schema/json/v1/InitializeResponse.json b/codex-rs/app-server-protocol/schema/json/v1/InitializeResponse.json
index 6ace3177b..ada38a658 100644
--- a/codex-rs/app-server-protocol/schema/json/v1/InitializeResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v1/InitializeResponse.json
@@ -1,11 +1,21 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "properties": {
+    "platformFamily": {
+      "description": "Platform family for the running app-server target, for example `\"unix\"` or `\"windows\"`.",
+      "type": "string"
+    },
+    "platformOs": {
+      "description": "Operating system for the running app-server target, for example `\"macos\"`, `\"linux\"`, or `\"windows\"`.",
+      "type": "string"
+    },
     "userAgent": {
       "type": "string"
     }
   },
   "required": [
+    "platformFamily",
+    "platformOs",
     "userAgent"
   ],
   "title": "InitializeResponse",
diff --git a/codex-rs/app-server-protocol/schema/typescript/InitializeResponse.ts b/codex-rs/app-server-protocol/schema/typescript/InitializeResponse.ts
index 8a6bec66e..47978fc8d 100644
--- a/codex-rs/app-server-protocol/schema/typescript/InitializeResponse.ts
+++ b/codex-rs/app-server-protocol/schema/typescript/InitializeResponse.ts
@@ -2,4 +2,14 @@
 
 // This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
 
-export type InitializeResponse = { userAgent: string, };
+export type InitializeResponse = { userAgent: string, 
+/**
+ * Platform family for the running app-server target, for example
+ * `"unix"` or `"windows"`.
+ */
+platformFamily: string, 
+/**
+ * Operating system for the running app-server target, for example
+ * `"macos"`, `"linux"`, or `"windows"`.
+ */
+platformOs: string, };
diff --git a/codex-rs/app-server-protocol/src/protocol/v1.rs b/codex-rs/app-server-protocol/src/protocol/v1.rs
index 3ae995323..81f3cc58a 100644
--- a/codex-rs/app-server-protocol/src/protocol/v1.rs
+++ b/codex-rs/app-server-protocol/src/protocol/v1.rs
@@ -56,6 +56,12 @@ pub struct InitializeCapabilities {
 #[serde(rename_all = "camelCase")]
 pub struct InitializeResponse {
     pub user_agent: String,
+    /// Platform family for the running app-server target, for example
+    /// `"unix"` or `"windows"`.
+    pub platform_family: String,
+    /// Operating system for the running app-server target, for example
+    /// `"macos"`, `"linux"`, or `"windows"`.
+    pub platform_os: String,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
diff --git a/codex-rs/app-server/README.md b/codex-rs/app-server/README.md
index 4a6cd58cc..fca87b692 100644
--- a/codex-rs/app-server/README.md
+++ b/codex-rs/app-server/README.md
@@ -74,7 +74,7 @@ Use the thread APIs to create, list, or archive conversations. Drive a conversat
 
 ## Initialization
 
-Clients must send a single `initialize` request per transport connection before invoking any other method on that connection, then acknowledge with an `initialized` notification. The server returns the user agent string it will present to upstream services; subsequent requests issued before initialization receive a `"Not initialized"` error, and repeated `initialize` calls on the same connection receive an `"Already initialized"` error.
+Clients must send a single `initialize` request per transport connection before invoking any other method on that connection, then acknowledge with an `initialized` notification. The server returns the user agent string it will present to upstream services plus `platformFamily` and `platformOs` strings describing the app-server runtime target; subsequent requests issued before initialization receive a `"Not initialized"` error, and repeated `initialize` calls on the same connection receive an `"Already initialized"` error.
 
 `initialize.params.capabilities` also supports per-connection notification opt-out via `optOutNotificationMethods`, which is a list of exact method names to suppress for that connection. Matching is exact (no wildcards/prefixes). Unknown method names are accepted and ignored.
 
diff --git a/codex-rs/app-server/src/message_processor.rs b/codex-rs/app-server/src/message_processor.rs
index 7571449da..b0b80c7bf 100644
--- a/codex-rs/app-server/src/message_processor.rs
+++ b/codex-rs/app-server/src/message_processor.rs
@@ -560,7 +560,11 @@ impl MessageProcessor {
                 }
 
                 let user_agent = get_codex_user_agent();
-                let response = InitializeResponse { user_agent };
+                let response = InitializeResponse {
+                    user_agent,
+                    platform_family: std::env::consts::FAMILY.to_string(),
+                    platform_os: std::env::consts::OS.to_string(),
+                };
                 self.outgoing
                     .send_response(connection_request_id, response)
                     .await;
diff --git a/codex-rs/app-server/tests/suite/v2/initialize.rs b/codex-rs/app-server/tests/suite/v2/initialize.rs
index 6b5bf1156..9b5f0cacc 100644
--- a/codex-rs/app-server/tests/suite/v2/initialize.rs
+++ b/codex-rs/app-server/tests/suite/v2/initialize.rs
@@ -46,9 +46,15 @@ async fn initialize_uses_client_info_name_as_originator() -> Result<()> {
     let JSONRPCMessage::Response(response) = message else {
         anyhow::bail!("expected initialize response, got {message:?}");
     };
-    let InitializeResponse { user_agent } = to_response::<InitializeResponse>(response)?;
+    let InitializeResponse {
+        user_agent,
+        platform_family,
+        platform_os,
+    } = to_response::<InitializeResponse>(response)?;
 
     assert!(user_agent.starts_with("codex_vscode/"));
+    assert_eq!(platform_family, std::env::consts::FAMILY);
+    assert_eq!(platform_os, std::env::consts::OS);
     Ok(())
 }
 
@@ -80,9 +86,15 @@ async fn initialize_respects_originator_override_env_var() -> Result<()> {
     let JSONRPCMessage::Response(response) = message else {
         anyhow::bail!("expected initialize response, got {message:?}");
     };
-    let InitializeResponse { user_agent } = to_response::<InitializeResponse>(response)?;
+    let InitializeResponse {
+        user_agent,
+        platform_family,
+        platform_os,
+    } = to_response::<InitializeResponse>(response)?;
 
     assert!(user_agent.starts_with("codex_originator_via_env_var/"));
+    assert_eq!(platform_family, std::env::consts::FAMILY);
+    assert_eq!(platform_os, std::env::consts::OS);
     Ok(())
 }
 
diff --git a/sdk/python/src/codex_app_server/models.py b/sdk/python/src/codex_app_server/models.py
index 7c5bb34de..70c61d44c 100644
--- a/sdk/python/src/codex_app_server/models.py
+++ b/sdk/python/src/codex_app_server/models.py
@@ -95,3 +95,5 @@ class ServerInfo(BaseModel):
 class InitializeResponse(BaseModel):
     serverInfo: ServerInfo | None = None
     userAgent: str | None = None
+    platformFamily: str | None = None
+    platformOs: str | None = None