diff --git a/pyproject.toml b/pyproject.toml
index bfce3066..73e1ad6c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "claude-agent-sdk"
-version = "0.1.19"
+version = "0.1.19.1"
 description = "Python SDK for Claude Code"
 readme = "README.md"
 requires-python = ">=3.10"
diff --git a/src/claude_agent_sdk/_internal/message_parser.py b/src/claude_agent_sdk/_internal/message_parser.py
index 4bfe8145..feb9f401 100644
--- a/src/claude_agent_sdk/_internal/message_parser.py
+++ b/src/claude_agent_sdk/_internal/message_parser.py
@@ -99,7 +99,7 @@ def parse_message(data: dict[str, Any]) -> Message:
                             content_blocks.append(
                                 ThinkingBlock(
                                     thinking=block["thinking"],
-                                    signature=block["signature"],
+                                    signature=block.get("signature"),
                                 )
                             )
                         case "tool_use":
diff --git a/src/claude_agent_sdk/types.py b/src/claude_agent_sdk/types.py
index 9c09345f..5ff9ff44 100644
--- a/src/claude_agent_sdk/types.py
+++ b/src/claude_agent_sdk/types.py
@@ -522,7 +522,7 @@ class ThinkingBlock:
     """Thinking content block."""
 
     thinking: str
-    signature: str
+    signature: str | None = None
 
 
 @dataclass