diff --git a/src/claude_agent_sdk/_internal/message_parser.py b/src/claude_agent_sdk/_internal/message_parser.py
index 4bfe8145..7f893d54 100644
--- a/src/claude_agent_sdk/_internal/message_parser.py
+++ b/src/claude_agent_sdk/_internal/message_parser.py
@@ -124,6 +124,9 @@ def parse_message(data: dict[str, Any]) -> Message:
                     model=data["message"]["model"],
                     parent_tool_use_id=data.get("parent_tool_use_id"),
                     error=data["message"].get("error"),
+                    # Per-step fields from Anthropic API response
+                    message_id=data["message"].get("id"),
+                    usage=data["message"].get("usage"),
                 )
             except KeyError as e:
                 raise MessageParseError(
diff --git a/src/claude_agent_sdk/types.py b/src/claude_agent_sdk/types.py
index 9c09345f..244115e6 100644
--- a/src/claude_agent_sdk/types.py
+++ b/src/claude_agent_sdk/types.py
@@ -574,6 +574,9 @@ class AssistantMessage:
     model: str
     parent_tool_use_id: str | None = None
     error: AssistantMessageError | None = None
+    # Per-step fields from Anthropic API response
+    message_id: str | None = None  # API message ID (e.g., "msg_...")
+    usage: dict[str, Any] | None = None  # Per-step token usage
 
 
 @dataclass