diff --git a/codex-rs/app-server-protocol/src/protocol/v2.rs b/codex-rs/app-server-protocol/src/protocol/v2.rs
index 4d20f024a9..d7462cddb2 100644
--- a/codex-rs/app-server-protocol/src/protocol/v2.rs
+++ b/codex-rs/app-server-protocol/src/protocol/v2.rs
@@ -2671,16 +2671,23 @@ pub struct DynamicToolCallParams {
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct DynamicToolCallResponse {
+    #[serde(flatten)]
+    pub result: DynamicToolCallResult,
+    pub success: bool,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(untagged, rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub enum DynamicToolCallResult {
     /// Preferred structured tool output (for example text + images) that is
     /// forwarded directly to the model as content items.
-    ///
-    /// At least one of `content_items` or `output` must be set.
-    pub content_items: Option<Vec<DynamicToolCallOutputContentItem>>,
+    ContentItems {
+        #[serde(rename = "contentItems")]
+        content_items: Vec<DynamicToolCallOutputContentItem>,
+    },
     /// Legacy plain-text tool output.
-    ///
-    /// At least one of `content_items` or `output` must be set.
-    pub output: Option<String>,
-    pub success: bool,
+    Output { output: String },
 }
 
 /// App-server-facing dynamic tool output items.
@@ -3089,10 +3096,11 @@ mod tests {
     #[test]
     fn dynamic_tool_response_serializes_content_items() {
         let value = serde_json::to_value(DynamicToolCallResponse {
-            content_items: Some(vec![DynamicToolCallOutputContentItem::InputText {
-                text: "dynamic-ok".to_string(),
-            }]),
-            output: None,
+            result: DynamicToolCallResult::ContentItems {
+                content_items: vec![DynamicToolCallOutputContentItem::InputText {
+                    text: "dynamic-ok".to_string(),
+                }],
+            },
             success: true,
         })
         .unwrap();
@@ -3100,7 +3108,6 @@ mod tests {
         assert_eq!(
             value,
             json!({
-                "output": null,
                 "success": true,
                 "contentItems": [
                     {
diff --git a/codex-rs/app-server/src/dynamic_tools.rs b/codex-rs/app-server/src/dynamic_tools.rs
index 91a76018b6..c63ec45d34 100644
--- a/codex-rs/app-server/src/dynamic_tools.rs
+++ b/codex-rs/app-server/src/dynamic_tools.rs
@@ -1,7 +1,7 @@
 use codex_app_server_protocol::DynamicToolCallResponse;
+use codex_app_server_protocol::DynamicToolCallResult;
 use codex_core::CodexThread;
 use codex_protocol::dynamic_tools::DynamicToolResponse as CoreDynamicToolResponse;
-use codex_protocol::models::FunctionCallOutputContentItem;
 use codex_protocol::protocol::Op;
 use std::sync::Arc;
 use tokio::sync::oneshot;
@@ -36,31 +36,26 @@ pub(crate) async fn on_call_response(
         }
     };
 
-    let mut response =
-        serde_json::from_value::<DynamicToolCallResponse>(value).unwrap_or_else(|err| {
-            error!("failed to deserialize DynamicToolCallResponse: {err}");
-            DynamicToolCallResponse {
-                content_items: None,
-                output: Some("dynamic tool response was invalid".to_string()),
-                success: false,
-            }
-        });
-
-    if response.content_items.is_none() && response.output.is_none() {
-        error!("dynamic tool response must include output or contentItems");
-        response.output = Some("dynamic tool response must include output or contentItems".into());
-        response.success = false;
-    }
-
-    let content_items = response.content_items.map(|items| {
-        items
-            .into_iter()
-            .map(Into::into)
-            .collect::<Vec<FunctionCallOutputContentItem>>()
+    let response = serde_json::from_value::<DynamicToolCallResponse>(value).unwrap_or_else(|err| {
+        error!("failed to deserialize DynamicToolCallResponse: {err}");
+        DynamicToolCallResponse {
+            result: DynamicToolCallResult::Output {
+                output: "dynamic tool response was invalid".to_string(),
+            },
+            success: false,
+        }
     });
+
+    let (output, content_items) = match response.result {
+        DynamicToolCallResult::ContentItems { content_items } => (
+            None,
+            Some(content_items.into_iter().map(Into::into).collect()),
+        ),
+        DynamicToolCallResult::Output { output } => (Some(output), None),
+    };
     let response = CoreDynamicToolResponse {
         call_id: call_id.clone(),
-        output: response.output,
+        output,
         success: response.success,
         content_items,
     };
diff --git a/codex-rs/app-server/tests/suite/v2/dynamic_tools.rs b/codex-rs/app-server/tests/suite/v2/dynamic_tools.rs
index 0fa854f87a..d670c096cc 100644
--- a/codex-rs/app-server/tests/suite/v2/dynamic_tools.rs
+++ b/codex-rs/app-server/tests/suite/v2/dynamic_tools.rs
@@ -7,6 +7,7 @@ use app_test_support::to_response;
 use codex_app_server_protocol::DynamicToolCallOutputContentItem;
 use codex_app_server_protocol::DynamicToolCallParams;
 use codex_app_server_protocol::DynamicToolCallResponse;
+use codex_app_server_protocol::DynamicToolCallResult;
 use codex_app_server_protocol::DynamicToolSpec;
 use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::RequestId;
@@ -203,9 +204,10 @@ async fn dynamic_tool_call_round_trip_sends_output_to_model() -> Result<()> {
 
     // Respond to the tool call so the model receives a function_call_output.
     let response = DynamicToolCallResponse {
-        output: Some("dynamic-ok".to_string()),
+        result: DynamicToolCallResult::Output {
+            output: "dynamic-ok".to_string(),
+        },
         success: true,
-        content_items: None,
     };
     mcp.send_response(request_id, serde_json::to_value(response)?)
         .await?;
@@ -331,9 +333,10 @@ async fn dynamic_tool_call_round_trip_sends_content_items_to_model() -> Result<(
         .map(Into::into)
         .collect::<Vec<FunctionCallOutputContentItem>>();
     let response = DynamicToolCallResponse {
-        output: None,
+        result: DynamicToolCallResult::ContentItems {
+            content_items: response_content_items,
+        },
         success: true,
-        content_items: Some(response_content_items),
     };
     mcp.send_response(request_id, serde_json::to_value(response)?)
         .await?;