diff --git a/codex-rs/core/src/mcp_connection_manager.rs b/codex-rs/core/src/mcp_connection_manager.rs
index 387ea1730c..eecda78887 100644
--- a/codex-rs/core/src/mcp_connection_manager.rs
+++ b/codex-rs/core/src/mcp_connection_manager.rs
@@ -163,6 +163,10 @@ impl McpConnectionManager {
                                 name: "codex-mcp-client".to_owned(),
                                 version: env!("CARGO_PKG_VERSION").to_owned(),
                                 title: Some("Codex".into()),
+                                // This field is used by Codex when it is an MCP
+                                // server: it should not be used when Codex is
+                                // an MCP client.
+                                user_agent: None,
                             },
                             protocol_version: mcp_types::MCP_SCHEMA_VERSION.to_owned(),
                         };
diff --git a/codex-rs/mcp-client/src/main.rs b/codex-rs/mcp-client/src/main.rs
index 10cfe389bf..d25bca4ba3 100644
--- a/codex-rs/mcp-client/src/main.rs
+++ b/codex-rs/mcp-client/src/main.rs
@@ -64,6 +64,9 @@ async fn main() -> Result<()> {
             name: "codex-mcp-client".to_owned(),
             version: env!("CARGO_PKG_VERSION").to_owned(),
             title: Some("Codex".to_string()),
+            // This field is used by Codex when it is an MCP server: it should
+            // not be used when Codex is an MCP client.
+            user_agent: None,
         },
         protocol_version: MCP_SCHEMA_VERSION.to_owned(),
     };
diff --git a/codex-rs/mcp-server/tests/common/mcp_process.rs b/codex-rs/mcp-server/tests/common/mcp_process.rs
index 93e8e10d55..ec7953ded6 100644
--- a/codex-rs/mcp-server/tests/common/mcp_process.rs
+++ b/codex-rs/mcp-server/tests/common/mcp_process.rs
@@ -138,6 +138,7 @@ impl McpProcess {
                 name: "elicitation test".into(),
                 title: Some("Elicitation Test".into()),
                 version: "0.0.0".into(),
+                user_agent: None,
             },
             protocol_version: mcp_types::MCP_SCHEMA_VERSION.into(),
         };