diff --git a/codex-rs/exec-server/src/posix/mcp.rs b/codex-rs/exec-server/src/posix/mcp.rs
index 92368a32c2..e4e7b25d92 100644
--- a/codex-rs/exec-server/src/posix/mcp.rs
+++ b/codex-rs/exec-server/src/posix/mcp.rs
@@ -143,3 +143,50 @@ pub(crate) async fn serve(
     let tool = ExecTool::new(bash_path, execve_wrapper, policy);
     tool.serve(stdio()).await
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+    use serde_json::json;
+
+    /// Verify that the way we use serde does not compromise the desired JSON
+    /// schema via schemars. In particular, ensure that the `login` and
+    /// `timeout_ms` fields are optional.
+    #[test]
+    fn exec_params_json_schema_matches_expected() {
+        let schema = schemars::schema_for!(ExecParams);
+        let actual = serde_json::to_value(schema).expect("schema should serialize");
+
+        assert_eq!(
+            actual,
+            json!({
+                "$schema": "https://json-schema.org/draft/2020-12/schema",
+                "title": "ExecParams",
+                "type": "object",
+                "properties": {
+                    "command": {
+                        "description": "The bash string to execute.",
+                        "type": "string"
+                    },
+                    "login": {
+                        "description": "Launch Bash with -lc instead of -c: defaults to true.",
+                        "type": ["boolean", "null"]
+                    },
+                    "timeout_ms": {
+                        "description": "The timeout for the command in milliseconds.",
+                        "format": "uint64",
+                        "minimum": 0,
+                        "type": ["integer", "null"]
+                    },
+                    "workdir": {
+                        "description":
+                            "The working directory to execute the command in. Must be an absolute path.",
+                        "type": "string"
+                    }
+                },
+                "required": ["command", "workdir"]
+            })
+        );
+    }
+}