diff --git a/codex-rs/protocol/src/models.rs b/codex-rs/protocol/src/models.rs
index 95ced2fb5d..cc73736b27 100644
--- a/codex-rs/protocol/src/models.rs
+++ b/codex-rs/protocol/src/models.rs
@@ -10,6 +10,7 @@ use serde::Deserialize;
 use serde::Deserializer;
 use serde::Serialize;
 use serde::ser::Serializer;
+use serde_with::serde_as;
 use ts_rs::TS;
 
 use crate::local_media::audio_mime_for_path;
@@ -32,6 +33,7 @@ use crate::mcp::CallToolResult;
 use codex_utils_path_uri::PathUri;
 
 mod executed_tool_calls;
+mod item_metadata;
 
 pub use crate::local_media::MAX_PROMPT_AUDIO_INPUT_BYTES;
 pub use crate::local_media::snapshot_local_user_input;
@@ -42,6 +44,7 @@ pub use executed_tool_calls::ExecutedToolCallTruncation;
 pub use executed_tool_calls::bound_executed_tool_calls_for_prompt;
 pub use executed_tool_calls::bound_executed_tool_calls_for_prompt_prioritizing_recent;
 pub use executed_tool_calls::executed_tool_call_metadata_bytes;
+pub use item_metadata::ContentItemKind;
 
 /// Controls the per-command sandbox override requested by a shell-like tool call.
 #[derive(
@@ -904,6 +907,7 @@ pub enum MessagePhase {
 ///
 /// Responses API strongly types this payload. Do not modify it without first getting API
 /// approval and making the corresponding Responses API change.
+#[serde_as]
 #[derive(Debug, Clone, Default, Serialize, Deserialize, PartialEq, Eq, JsonSchema, TS)]
 pub struct InternalChatMessageMetadataPassthrough {
     #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -914,6 +918,12 @@ pub struct InternalChatMessageMetadataPassthrough {
     #[schemars(skip)]
     #[ts(skip)]
     pub create_time: Option<serde_json::Number>,
+    /// Harness-owned classifications aligned with the item's content entries.
+    #[serde_as(deserialize_as = "serde_with::DefaultOnError")]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[schemars(skip)]
+    #[ts(skip)]
+    pub content_item_kinds: Option<Vec<ContentItemKind>>,
     /// Warehouse-only Responses metadata, not part of the public app-server protocol.
     #[serde(default, skip_deserializing, skip_serializing_if = "Option::is_none")]
     #[schemars(skip)]
diff --git a/codex-rs/protocol/src/models/item_metadata.rs b/codex-rs/protocol/src/models/item_metadata.rs
new file mode 100644
index 0000000000..eddd29ee58
--- /dev/null
+++ b/codex-rs/protocol/src/models/item_metadata.rs
@@ -0,0 +1,11 @@
+use serde::Deserialize;
+use serde::Serialize;
+
+/// Harness-owned classification for one position in an item's content array.
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(transparent)]
+pub struct ContentItemKind(pub String);
+
+#[cfg(test)]
+#[path = "item_metadata_tests.rs"]
+mod tests;
diff --git a/codex-rs/protocol/src/models/item_metadata_tests.rs b/codex-rs/protocol/src/models/item_metadata_tests.rs
new file mode 100644
index 0000000000..df8cfadfbd
--- /dev/null
+++ b/codex-rs/protocol/src/models/item_metadata_tests.rs
@@ -0,0 +1,79 @@
+use super::ContentItemKind;
+use crate::models::ContentItem;
+use crate::models::InternalChatMessageMetadataPassthrough;
+use crate::models::ResponseItem;
+use anyhow::Result;
+use pretty_assertions::assert_eq;
+use serde_json::Value;
+use serde_json::json;
+
+#[test]
+fn content_kinds_round_trip_without_restricting_future_values() -> Result<()> {
+    let item = response_item(InternalChatMessageMetadataPassthrough {
+        turn_id: Some("turn-1".to_string()),
+        content_item_kinds: Some(vec![ContentItemKind("future_content_kind".to_string())]),
+        ..Default::default()
+    });
+
+    let serialized = serde_json::to_value(&item)?;
+    assert_eq!(
+        serialized,
+        json!({
+            "type": "message",
+            "role": "user",
+            "content": [{"type": "input_text", "text": "hello"}],
+            "internal_chat_message_metadata_passthrough": {
+                "turn_id": "turn-1",
+                "content_item_kinds": ["future_content_kind"],
+            },
+        })
+    );
+    assert_eq!(serde_json::from_value::<ResponseItem>(serialized)?, item);
+
+    Ok(())
+}
+
+#[test]
+fn malformed_content_kinds_do_not_prevent_loading_response_item() -> Result<()> {
+    for malformed_kinds in [
+        json!("user_text"),
+        json!({"kind": "user_text"}),
+        json!(["user_text", 123]),
+    ] {
+        let item = serde_json::from_value::<ResponseItem>(response_item_json(json!({
+            "turn_id": "turn-1",
+            "content_item_kinds": malformed_kinds,
+        })))?;
+
+        assert_eq!(
+            item,
+            response_item(InternalChatMessageMetadataPassthrough {
+                turn_id: Some("turn-1".to_string()),
+                ..Default::default()
+            })
+        );
+    }
+
+    Ok(())
+}
+
+fn response_item(metadata: InternalChatMessageMetadataPassthrough) -> ResponseItem {
+    ResponseItem::Message {
+        id: None,
+        role: "user".to_string(),
+        content: vec![ContentItem::InputText {
+            text: "hello".to_string(),
+        }],
+        phase: None,
+        internal_chat_message_metadata_passthrough: Some(metadata),
+    }
+}
+
+fn response_item_json(metadata: Value) -> Value {
+    json!({
+        "type": "message",
+        "role": "user",
+        "content": [{"type": "input_text", "text": "hello"}],
+        "internal_chat_message_metadata_passthrough": metadata,
+    })
+}