diff --git a/codex-rs/Cargo.lock b/codex-rs/Cargo.lock
index cda2e81d..d1b9cb39 100644
--- a/codex-rs/Cargo.lock
+++ b/codex-rs/Cargo.lock
@@ -785,6 +785,7 @@ dependencies = [
  "codex-protocol",
  "mcp-types",
  "mcp_test_support",
+ "os_info",
  "pretty_assertions",
  "schemars 0.8.22",
  "serde",
@@ -843,6 +844,7 @@ dependencies = [
  "anyhow",
  "clap",
  "codex-protocol",
+ "mcp-types",
  "ts-rs",
 ]
 
@@ -2680,9 +2682,11 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "assert_cmd",
+ "codex-core",
  "codex-mcp-server",
  "codex-protocol",
  "mcp-types",
+ "os_info",
  "pretty_assertions",
  "serde",
  "serde_json",
diff --git a/codex-rs/core/src/default_client.rs b/codex-rs/core/src/default_client.rs
index a2192ffa..36212f25 100644
--- a/codex-rs/core/src/default_client.rs
+++ b/codex-rs/core/src/default_client.rs
@@ -1,5 +1,23 @@
 use reqwest::header::HeaderValue;
 use std::sync::LazyLock;
+use std::sync::Mutex;
+
+/// Set this to add a suffix to the User-Agent string.
+///
+/// It is not ideal that we're using a global singleton for this.
+/// This is primarily designed to differentiate MCP clients from each other.
+/// Because there can only be one MCP server per process, it should be safe for this to be a global static.
+/// However, future users of this should use this with caution as a result.
+/// In addition, we want to be confident that this value is used for ALL clients and doing that requires a
+/// lot of wiring and it's easy to miss code paths by doing so.
+/// See https://github.com/openai/codex/pull/3388/files for an example of what that would look like.
+/// Finally, we want to make sure this is set for ALL mcp clients without needing to know a special env var
+/// or having to set data that they already specified in the mcp initialize request somewhere else.
+///
+/// A space is automatically added between the suffix and the rest of the User-Agent string.
+/// The full user agent string is returned from the mcp initialize response.
+/// Parenthesis will be added by Codex. This should only specify what goes inside of the parenthesis.
+pub static USER_AGENT_SUFFIX: LazyLock<Mutex<Option<String>>> = LazyLock::new(|| Mutex::new(None));
 
 pub const CODEX_INTERNAL_ORIGINATOR_OVERRIDE_ENV_VAR: &str = "CODEX_INTERNAL_ORIGINATOR_OVERRIDE";
 
@@ -32,14 +50,58 @@ pub static ORIGINATOR: LazyLock<Originator> = LazyLock::new(|| {
 pub fn get_codex_user_agent() -> String {
     let build_version = env!("CARGO_PKG_VERSION");
     let os_info = os_info::get();
-    format!(
+    let prefix = format!(
         "{}/{build_version} ({} {}; {}) {}",
         ORIGINATOR.value.as_str(),
         os_info.os_type(),
         os_info.version(),
         os_info.architecture().unwrap_or("unknown"),
         crate::terminal::user_agent()
-    )
+    );
+    let suffix = USER_AGENT_SUFFIX
+        .lock()
+        .ok()
+        .and_then(|guard| guard.clone());
+    let suffix = suffix
+        .as_deref()
+        .map(str::trim)
+        .filter(|value| !value.is_empty())
+        .map_or_else(String::new, |value| format!(" ({value})"));
+
+    let candidate = format!("{prefix}{suffix}");
+    sanitize_user_agent(candidate, &prefix)
+}
+
+/// Sanitize the user agent string.
+///
+/// Invalid characters are replaced with an underscore.
+///
+/// If the user agent fails to parse, it falls back to fallback and then to ORIGINATOR.
+fn sanitize_user_agent(candidate: String, fallback: &str) -> String {
+    if HeaderValue::from_str(candidate.as_str()).is_ok() {
+        return candidate;
+    }
+
+    let sanitized: String = candidate
+        .chars()
+        .map(|ch| if matches!(ch, ' '..='~') { ch } else { '_' })
+        .collect();
+    if !sanitized.is_empty() && HeaderValue::from_str(sanitized.as_str()).is_ok() {
+        tracing::warn!(
+            "Sanitized Codex user agent because provided suffix contained invalid header characters"
+        );
+        sanitized
+    } else if HeaderValue::from_str(fallback).is_ok() {
+        tracing::warn!(
+            "Falling back to base Codex user agent because provided suffix could not be sanitized"
+        );
+        fallback.to_string()
+    } else {
+        tracing::warn!(
+            "Falling back to default Codex originator because base user agent string is invalid"
+        );
+        ORIGINATOR.value.clone()
+    }
 }
 
 /// Create a reqwest client with default `originator` and `User-Agent` headers set.
@@ -114,6 +176,28 @@ mod tests {
         assert_eq!(ua_header.to_str().unwrap(), expected_ua);
     }
 
+    #[test]
+    fn test_invalid_suffix_is_sanitized() {
+        let prefix = "codex_cli_rs/0.0.0";
+        let suffix = "bad\rsuffix";
+
+        assert_eq!(
+            sanitize_user_agent(format!("{prefix} ({suffix})"), prefix),
+            "codex_cli_rs/0.0.0 (bad_suffix)"
+        );
+    }
+
+    #[test]
+    fn test_invalid_suffix_is_sanitized2() {
+        let prefix = "codex_cli_rs/0.0.0";
+        let suffix = "bad\0suffix";
+
+        assert_eq!(
+            sanitize_user_agent(format!("{prefix} ({suffix})"), prefix),
+            "codex_cli_rs/0.0.0 (bad_suffix)"
+        );
+    }
+
     #[test]
     #[cfg(target_os = "macos")]
     fn test_macos() {
diff --git a/codex-rs/core/src/mcp_connection_manager.rs b/codex-rs/core/src/mcp_connection_manager.rs
index 387ea173..9fc72755 100644
--- a/codex-rs/core/src/mcp_connection_manager.rs
+++ b/codex-rs/core/src/mcp_connection_manager.rs
@@ -17,7 +17,7 @@ use anyhow::Result;
 use anyhow::anyhow;
 use codex_mcp_client::McpClient;
 use mcp_types::ClientCapabilities;
-use mcp_types::Implementation;
+use mcp_types::McpClientInfo;
 use mcp_types::Tool;
 
 use serde_json::json;
@@ -159,7 +159,7 @@ impl McpConnectionManager {
                                 // indicates this should be an empty object.
                                 elicitation: Some(json!({})),
                             },
-                            client_info: Implementation {
+                            client_info: McpClientInfo {
                                 name: "codex-mcp-client".to_owned(),
                                 version: env!("CARGO_PKG_VERSION").to_owned(),
                                 title: Some("Codex".into()),
diff --git a/codex-rs/mcp-client/src/main.rs b/codex-rs/mcp-client/src/main.rs
index 10cfe389..a9f4e335 100644
--- a/codex-rs/mcp-client/src/main.rs
+++ b/codex-rs/mcp-client/src/main.rs
@@ -17,10 +17,10 @@ use anyhow::Context;
 use anyhow::Result;
 use codex_mcp_client::McpClient;
 use mcp_types::ClientCapabilities;
-use mcp_types::Implementation;
 use mcp_types::InitializeRequestParams;
 use mcp_types::ListToolsRequestParams;
 use mcp_types::MCP_SCHEMA_VERSION;
+use mcp_types::McpClientInfo;
 use tracing_subscriber::EnvFilter;
 
 #[tokio::main]
@@ -60,7 +60,7 @@ async fn main() -> Result<()> {
             sampling: None,
             elicitation: None,
         },
-        client_info: Implementation {
+        client_info: McpClientInfo {
             name: "codex-mcp-client".to_owned(),
             version: env!("CARGO_PKG_VERSION").to_owned(),
             title: Some("Codex".to_string()),
diff --git a/codex-rs/mcp-server/Cargo.toml b/codex-rs/mcp-server/Cargo.toml
index 335e4ce6..c219c729 100644
--- a/codex-rs/mcp-server/Cargo.toml
+++ b/codex-rs/mcp-server/Cargo.toml
@@ -41,6 +41,7 @@ uuid = { version = "1", features = ["serde", "v4"] }
 [dev-dependencies]
 assert_cmd = "2"
 mcp_test_support = { path = "tests/common" }
+os_info = "3.12.0"
 pretty_assertions = "1.4.1"
 tempfile = "3"
 wiremock = "0.6"
diff --git a/codex-rs/mcp-server/src/message_processor.rs b/codex-rs/mcp-server/src/message_processor.rs
index dccd86e9..8018095d 100644
--- a/codex-rs/mcp-server/src/message_processor.rs
+++ b/codex-rs/mcp-server/src/message_processor.rs
@@ -14,6 +14,8 @@ use codex_protocol::mcp_protocol::ConversationId;
 use codex_core::AuthManager;
 use codex_core::ConversationManager;
 use codex_core::config::Config;
+use codex_core::default_client::USER_AGENT_SUFFIX;
+use codex_core::default_client::get_codex_user_agent;
 use codex_core::protocol::Submission;
 use mcp_types::CallToolRequestParams;
 use mcp_types::CallToolResult;
@@ -208,6 +210,14 @@ impl MessageProcessor {
             return;
         }
 
+        let client_info = params.client_info;
+        let name = client_info.name;
+        let version = client_info.version;
+        let user_agent_suffix = format!("{name}; {version}");
+        if let Ok(mut suffix) = USER_AGENT_SUFFIX.lock() {
+            *suffix = Some(user_agent_suffix);
+        }
+
         self.initialized = true;
 
         // Build a minimal InitializeResult. Fill with placeholders.
@@ -224,10 +234,11 @@ impl MessageProcessor {
             },
             instructions: None,
             protocol_version: params.protocol_version.clone(),
-            server_info: mcp_types::Implementation {
+            server_info: mcp_types::McpServerInfo {
                 name: "codex-mcp-server".to_string(),
                 version: env!("CARGO_PKG_VERSION").to_string(),
                 title: Some("Codex".to_string()),
+                user_agent: get_codex_user_agent(),
             },
         };
 
diff --git a/codex-rs/mcp-server/tests/common/Cargo.toml b/codex-rs/mcp-server/tests/common/Cargo.toml
index 88ad93f5..6bdef423 100644
--- a/codex-rs/mcp-server/tests/common/Cargo.toml
+++ b/codex-rs/mcp-server/tests/common/Cargo.toml
@@ -9,9 +9,11 @@ path = "lib.rs"
 [dependencies]
 anyhow = "1"
 assert_cmd = "2"
+codex-core = { path = "../../../core" }
 codex-mcp-server = { path = "../.." }
 codex-protocol = { path = "../../../protocol" }
 mcp-types = { path = "../../../mcp-types" }
+os_info = "3.12.0"
 pretty_assertions = "1.4.1"
 serde = { version = "1" }
 serde_json = "1"
diff --git a/codex-rs/mcp-server/tests/common/mcp_process.rs b/codex-rs/mcp-server/tests/common/mcp_process.rs
index 64f2cc38..66d546e2 100644
--- a/codex-rs/mcp-server/tests/common/mcp_process.rs
+++ b/codex-rs/mcp-server/tests/common/mcp_process.rs
@@ -26,13 +26,13 @@ use codex_protocol::mcp_protocol::SendUserTurnParams;
 
 use mcp_types::CallToolRequestParams;
 use mcp_types::ClientCapabilities;
-use mcp_types::Implementation;
 use mcp_types::InitializeRequestParams;
 use mcp_types::JSONRPC_VERSION;
 use mcp_types::JSONRPCMessage;
 use mcp_types::JSONRPCNotification;
 use mcp_types::JSONRPCRequest;
 use mcp_types::JSONRPCResponse;
+use mcp_types::McpClientInfo;
 use mcp_types::ModelContextProtocolNotification;
 use mcp_types::ModelContextProtocolRequest;
 use mcp_types::RequestId;
@@ -111,7 +111,7 @@ impl McpProcess {
                 roots: None,
                 sampling: None,
             },
-            client_info: Implementation {
+            client_info: McpClientInfo {
                 name: "elicitation test".into(),
                 title: Some("Elicitation Test".into()),
                 version: "0.0.0".into(),
@@ -129,6 +129,14 @@ impl McpProcess {
         .await?;
 
         let initialized = self.read_jsonrpc_message().await?;
+        let os_info = os_info::get();
+        let user_agent = format!(
+            "codex_cli_rs/0.0.0 ({} {}; {}) {} (elicitation test; 0.0.0)",
+            os_info.os_type(),
+            os_info.version(),
+            os_info.architecture().unwrap_or("unknown"),
+            codex_core::terminal::user_agent()
+        );
         assert_eq!(
             JSONRPCMessage::Response(JSONRPCResponse {
                 jsonrpc: JSONRPC_VERSION.into(),
@@ -142,7 +150,8 @@ impl McpProcess {
                     "serverInfo": {
                         "name": "codex-mcp-server",
                         "title": "Codex",
-                        "version": "0.0.0"
+                        "version": "0.0.0",
+                        "user_agent": user_agent
                     },
                     "protocolVersion": mcp_types::MCP_SCHEMA_VERSION
                 })
diff --git a/codex-rs/mcp-server/tests/suite/user_agent.rs b/codex-rs/mcp-server/tests/suite/user_agent.rs
index 19de87a5..718e1452 100644
--- a/codex-rs/mcp-server/tests/suite/user_agent.rs
+++ b/codex-rs/mcp-server/tests/suite/user_agent.rs
@@ -1,4 +1,3 @@
-use codex_core::default_client::get_codex_user_agent;
 use codex_protocol::mcp_protocol::GetUserAgentResponse;
 use mcp_test_support::McpProcess;
 use mcp_test_support::to_response;
@@ -34,11 +33,18 @@ async fn get_user_agent_returns_current_codex_user_agent() {
     .expect("getUserAgent timeout")
     .expect("getUserAgent response");
 
+    let os_info = os_info::get();
+    let user_agent = format!(
+        "codex_cli_rs/0.0.0 ({} {}; {}) {} (elicitation test; 0.0.0)",
+        os_info.os_type(),
+        os_info.version(),
+        os_info.architecture().unwrap_or("unknown"),
+        codex_core::terminal::user_agent()
+    );
+
     let received: GetUserAgentResponse =
         to_response(response).expect("deserialize getUserAgent response");
-    let expected = GetUserAgentResponse {
-        user_agent: get_codex_user_agent(),
-    };
+    let expected = GetUserAgentResponse { user_agent };
 
     assert_eq!(received, expected);
 }
diff --git a/codex-rs/mcp-types/src/lib.rs b/codex-rs/mcp-types/src/lib.rs
index 988bec2a..2f862be8 100644
--- a/codex-rs/mcp-types/src/lib.rs
+++ b/codex-rs/mcp-types/src/lib.rs
@@ -482,13 +482,23 @@ pub struct ImageContent {
 
 /// Describes the name and version of an MCP implementation, with an optional title for UI representation.
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, TS)]
-pub struct Implementation {
+pub struct McpClientInfo {
     pub name: String,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub title: Option<String>,
     pub version: String,
 }
 
+/// Describes the name and version of an MCP implementation, with an optional title for UI representation.
+#[derive(Debug, Clone, PartialEq, Deserialize, Serialize, TS)]
+pub struct McpServerInfo {
+    pub name: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub title: Option<String>,
+    pub version: String,
+    pub user_agent: String,
+}
+
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, TS)]
 pub enum InitializeRequest {}
 
@@ -502,7 +512,7 @@ impl ModelContextProtocolRequest for InitializeRequest {
 pub struct InitializeRequestParams {
     pub capabilities: ClientCapabilities,
     #[serde(rename = "clientInfo")]
-    pub client_info: Implementation,
+    pub client_info: McpClientInfo,
     #[serde(rename = "protocolVersion")]
     pub protocol_version: String,
 }
@@ -516,7 +526,7 @@ pub struct InitializeResult {
     #[serde(rename = "protocolVersion")]
     pub protocol_version: String,
     #[serde(rename = "serverInfo")]
-    pub server_info: Implementation,
+    pub server_info: McpServerInfo,
 }
 
 impl From<InitializeResult> for serde_json::Value {
diff --git a/codex-rs/mcp-types/tests/suite/initialize.rs b/codex-rs/mcp-types/tests/suite/initialize.rs
index 04778f2a..2bd3c789 100644
--- a/codex-rs/mcp-types/tests/suite/initialize.rs
+++ b/codex-rs/mcp-types/tests/suite/initialize.rs
@@ -1,10 +1,10 @@
 use mcp_types::ClientCapabilities;
 use mcp_types::ClientRequest;
-use mcp_types::Implementation;
 use mcp_types::InitializeRequestParams;
 use mcp_types::JSONRPC_VERSION;
 use mcp_types::JSONRPCMessage;
 use mcp_types::JSONRPCRequest;
+use mcp_types::McpClientInfo;
 use mcp_types::RequestId;
 use serde_json::json;
 
@@ -58,7 +58,7 @@ fn deserialize_initialize_request() {
                 sampling: None,
                 elicitation: None,
             },
-            client_info: Implementation {
+            client_info: McpClientInfo {
                 name: "acme-client".into(),
                 title: Some("Acme".to_string()),
                 version: "1.2.3".into(),
diff --git a/codex-rs/protocol-ts/Cargo.toml b/codex-rs/protocol-ts/Cargo.toml
index 9faa9344..1131a621 100644
--- a/codex-rs/protocol-ts/Cargo.toml
+++ b/codex-rs/protocol-ts/Cargo.toml
@@ -16,6 +16,7 @@ path = "src/main.rs"
 
 [dependencies]
 anyhow = "1"
+mcp-types = { path = "../mcp-types" }
 codex-protocol = { path = "../protocol" }
 ts-rs = "11"
 clap = { version = "4", features = ["derive"] }
diff --git a/codex-rs/protocol-ts/src/lib.rs b/codex-rs/protocol-ts/src/lib.rs
index 08cb9407..776c8ba3 100644
--- a/codex-rs/protocol-ts/src/lib.rs
+++ b/codex-rs/protocol-ts/src/lib.rs
@@ -16,6 +16,7 @@ pub fn generate_ts(out_dir: &Path, prettier: Option<&Path>) -> Result<()> {
     ensure_dir(out_dir)?;
 
     // Generate TS bindings
+    mcp_types::InitializeResult::export_all_to(out_dir)?;
     codex_protocol::mcp_protocol::ConversationId::export_all_to(out_dir)?;
     codex_protocol::mcp_protocol::InputItem::export_all_to(out_dir)?;
     codex_protocol::mcp_protocol::ClientRequest::export_all_to(out_dir)?;