diff --git a/src/claude_agent_sdk/_internal/message_parser.py b/src/claude_agent_sdk/_internal/message_parser.py
index 574816c6..5ca00be9 100644
--- a/src/claude_agent_sdk/_internal/message_parser.py
+++ b/src/claude_agent_sdk/_internal/message_parser.py
@@ -1,6 +1,7 @@
 """Message parser for Claude Code SDK responses."""
 
 import logging
+import os
 from typing import Any
 
 from .._errors import MessageParseError
@@ -131,6 +132,34 @@ def parse_message(data: dict[str, Any]) -> Message | None:
                         case "text":
                             content_blocks.append(TextBlock(text=block["text"]))
                         case "thinking":
+                            if "signature" not in block:
+                                # Thinking blocks emitted by Anthropic models
+                                # always carry an encrypted ``signature`` used
+                                # for multi-turn round-tripping. A missing
+                                # ``signature`` almost always means the
+                                # upstream is an Anthropic-compatible (but
+                                # non-Anthropic) backend that doesn't generate
+                                # one. See issues #339, #949.
+                                if os.environ.get(
+                                    "CLAUDE_AGENT_SDK_SKIP_UNSIGNED_THINKING"
+                                ):
+                                    logger.warning(
+                                        "Dropping thinking block without "
+                                        "'signature' field "
+                                        "(CLAUDE_AGENT_SDK_SKIP_UNSIGNED_THINKING set)."
+                                    )
+                                    continue
+                                raise MessageParseError(
+                                    "Assistant message contains a 'thinking' "
+                                    "block without 'signature'. This usually "
+                                    "means the upstream is not an Anthropic "
+                                    "model — extended thinking from "
+                                    "non-Anthropic backends is unsupported. "
+                                    "Disable thinking on the upstream, or set "
+                                    "CLAUDE_AGENT_SDK_SKIP_UNSIGNED_THINKING=1 "
+                                    "to drop such blocks.",
+                                    data,
+                                )
                             content_blocks.append(
                                 ThinkingBlock(
                                     thinking=block["thinking"],
diff --git a/tests/test_message_parser.py b/tests/test_message_parser.py
index 7ce2990c..ac9c733c 100644
--- a/tests/test_message_parser.py
+++ b/tests/test_message_parser.py
@@ -278,6 +278,56 @@ def test_parse_assistant_message_with_thinking(self):
         assert isinstance(message.content[1], TextBlock)
         assert message.content[1].text == "Here's my response"
 
+    def test_parse_thinking_missing_signature_raises_clearer_error(self, monkeypatch):
+        """A thinking block without 'signature' raises an explanatory error.
+
+        Bare ``KeyError: 'signature'`` gives users no clue why this happened.
+        The most common cause in the wild (see #339, #949) is an
+        Anthropic-compatible upstream emitting thinking blocks without the
+        encrypted signature, so the message points at that.
+        """
+        monkeypatch.delenv("CLAUDE_AGENT_SDK_SKIP_UNSIGNED_THINKING", raising=False)
+        data = {
+            "type": "assistant",
+            "message": {
+                "content": [
+                    {"type": "thinking", "thinking": "no signature here"},
+                ],
+                "model": "claude-opus-4-1-20250805",
+            },
+        }
+        with pytest.raises(MessageParseError) as exc_info:
+            parse_message(data)
+        msg = str(exc_info.value)
+        assert "signature" in msg
+        assert (
+            "non-Anthropic" in msg or "CLAUDE_AGENT_SDK_SKIP_UNSIGNED_THINKING" in msg
+        )
+
+    def test_parse_thinking_missing_signature_skipped_with_env_var(self, monkeypatch):
+        """With the opt-out env var set, unsigned thinking blocks are dropped.
+
+        This lets users on non-Anthropic upstreams keep using the SDK without
+        crashing on every assistant message. Other blocks in the message are
+        preserved.
+        """
+        monkeypatch.setenv("CLAUDE_AGENT_SDK_SKIP_UNSIGNED_THINKING", "1")
+        data = {
+            "type": "assistant",
+            "message": {
+                "content": [
+                    {"type": "thinking", "thinking": "no signature here"},
+                    {"type": "text", "text": "Here's my response"},
+                ],
+                "model": "claude-opus-4-1-20250805",
+            },
+        }
+        message = parse_message(data)
+        assert isinstance(message, AssistantMessage)
+        assert len(message.content) == 1
+        assert isinstance(message.content[0], TextBlock)
+        assert message.content[0].text == "Here's my response"
+
     def test_parse_assistant_message_with_server_tool_use(self):
         """server_tool_use blocks (e.g. advisor, web_search) are preserved.