diff --git a/src/anthropic/_compat.py b/src/anthropic/_compat.py
index eee0bc4c7..6a2579902 100644
--- a/src/anthropic/_compat.py
+++ b/src/anthropic/_compat.py
@@ -120,6 +120,12 @@ def model_copy(model: _ModelT, *, deep: bool = False) -> _ModelT:
     return model.model_copy(deep=deep)
 
 
+def model_fields_set(model: pydantic.BaseModel) -> set[str]:
+    if PYDANTIC_V1:
+        return model.__fields_set__  # type: ignore
+    return model.model_fields_set
+
+
 def model_json(model: pydantic.BaseModel, *, indent: int | None = None) -> str:
     if PYDANTIC_V1:
         return model.json(indent=indent)  # type: ignore
diff --git a/src/anthropic/lib/streaming/_beta_messages.py b/src/anthropic/lib/streaming/_beta_messages.py
index c7d3d6c2c..a199c52e3 100644
--- a/src/anthropic/lib/streaming/_beta_messages.py
+++ b/src/anthropic/lib/streaming/_beta_messages.py
@@ -14,6 +14,7 @@
 
 from ..._types import NotGiven, not_given
 from ..._utils import consume_sync_iterator, consume_async_iterator
+from ..._compat import model_fields_set
 from ..._models import build, construct_type, construct_type_unchecked
 from ._beta_types import (
     BetaCitationEvent,
@@ -536,7 +537,8 @@ def accumulate_event(
         elif event.delta.type == "compaction_delta":
             if content.type == "compaction":
                 content.content = event.delta.content
-                content.encrypted_content = event.delta.encrypted_content
+                if "encrypted_content" in model_fields_set(event.delta):
+                    content.encrypted_content = event.delta.encrypted_content
         else:
             # we only want exhaustive checking for linters, not at runtime
             if TYPE_CHECKING:  # type: ignore[unreachable]
diff --git a/tests/lib/streaming/test_beta_messages.py b/tests/lib/streaming/test_beta_messages.py
index fc2af491d..b49823ad9 100644
--- a/tests/lib/streaming/test_beta_messages.py
+++ b/tests/lib/streaming/test_beta_messages.py
@@ -569,6 +569,43 @@ def test_compaction(self, respx_mock: MockRouter) -> None:
 
             assert_compaction_response([event for event in stream], stream.get_final_message())
 
+    @pytest.mark.respx(base_url=base_url)
+    def test_compaction_delta_without_encrypted_content_retains_metadata(self, respx_mock: MockRouter) -> None:
+        sse_lines = [
+            "event: message_start\n",
+            'data: {"type":"message_start","message":{"id":"msg_01","type":"message","role":"assistant","content":[],"model":"claude-opus-4-7","stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":30,"output_tokens":1}}}\n\n',
+            "event: content_block_start\n",
+            'data: {"type":"content_block_start","index":0,"content_block":{"type":"compaction","content":null,"encrypted_content":null}}\n\n',
+            "event: content_block_delta\n",
+            'data: {"type":"content_block_delta","index":0,"delta":{"type":"compaction_delta","content":"Initial summary.","encrypted_content":"opaque_meta"}}\n\n',
+            "event: content_block_delta\n",
+            'data: {"type":"content_block_delta","index":0,"delta":{"type":"compaction_delta","content":"Updated summary."}}\n\n',
+            "event: content_block_stop\n",
+            'data: {"type":"content_block_stop","index":0}\n\n',
+            "event: message_delta\n",
+            'data: {"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"output_tokens":10}}\n\n',
+            "event: message_stop\n",
+            'data: {"type":"message_stop"}\n\n',
+        ]
+        respx_mock.post("/v1/messages").mock(
+            return_value=httpx2.Response(200, content="".join(sse_lines).encode("utf-8"))
+        )
+
+        with sync_client.beta.messages.stream(
+            max_tokens=1024,
+            messages=[{"role": "user", "content": "Say hello there!"}],
+            model="claude-opus-4-7",
+        ) as stream:
+            events = list(stream)
+            final_message = stream.get_final_message()
+
+        compaction_events = [e for e in events if isinstance(e, BetaCompactionEvent)]
+        assert len(compaction_events) == 2
+        assert compaction_events[0].encrypted_content == "opaque_meta"
+        assert compaction_events[1].encrypted_content == "opaque_meta"
+        assert final_message.content[0].type == "compaction"
+        assert final_message.content[0].encrypted_content == "opaque_meta"
+
     @pytest.mark.respx(base_url=base_url)
     def test_fallback_relabels_model(self, respx_mock: MockRouter) -> None:
         respx_mock.post("/v1/messages").mock(