fix(core): avoid mutation in google-genai standard content (#40023)

Co-authored-by: Hotragn Pettugani <103170876+Hotragn@users.noreply.github.com>

libs/core/langchain_core/messages/block_translators/google_genai.py

@@ -527,10 +527,15 @@ def _convert_to_v1_from_genai(message: AIMessage) -> list[types.ContentBlock]:
     if grounding_metadata:
         citations = translate_grounding_metadata_to_citations(grounding_metadata)
 
-        for block in converted_blocks:
+        for i, block in enumerate(converted_blocks):
             if block["type"] == "text" and citations:
-                # Add citations to text blocks (only the first text block)
-                block["annotations"] = cast("list[types.Annotation]", citations)
+                # Add citations to text blocks (only the first text block).
+                # Replaced rather than assigned into: `block` is the caller's own
+                # dict from `message.content`.
+                converted_blocks[i] = cast(
+                    "types.TextContentBlock",
+                    {**block, "annotations": citations},
+                )
                 break
 
     # Audio is stored on the message.additional_kwargs

libs/core/tests/unit_tests/messages/block_translators/test_google_genai.py

@@ -1,8 +1,9 @@
 """Tests for Google GenAI block translator."""
 
+from copy import deepcopy
 from typing import Any
 
-from langchain_core.messages import AIMessageChunk
+from langchain_core.messages import AIMessage, AIMessageChunk
 from langchain_core.messages.block_translators.google_genai import (
     translate_grounding_metadata_to_citations,
 )
@@ -331,3 +332,31 @@ def test_content_blocks_index_propagation_is_one_to_one() -> None:
 
     assert indices == [0, 1, 2]
     assert len(indices) == len(set(indices))
+
+
+_GROUNDING_METADATA = {
+    "grounding_chunks": [{"web": {"uri": "https://example.com", "title": "Example"}}],
+    "grounding_supports": [
+        {
+            "segment": {"start_index": 0, "end_index": 5},
+            "grounding_chunk_indices": [0],
+        }
+    ],
+}
+
+
+def test_grounding_citations_do_not_mutate_content() -> None:
+    """Attaching grounding citations must leave `message.content` unchanged."""
+    message = AIMessage(
+        content=[{"type": "text", "text": "hello"}],
+        response_metadata={
+            "model_provider": "google_genai",
+            "grounding_metadata": _GROUNDING_METADATA,
+        },
+    )
+    original_content = deepcopy(message.content)
+
+    blocks = message.content_blocks
+
+    assert message.content == original_content
+    assert "annotations" in blocks[0]