diff --git a/libs/partners/fireworks/langchain_fireworks/chat_models.py b/libs/partners/fireworks/langchain_fireworks/chat_models.py
index 22a34bec14..78306dbb7a 100644
--- a/libs/partners/fireworks/langchain_fireworks/chat_models.py
+++ b/libs/partners/fireworks/langchain_fireworks/chat_models.py
@@ -222,6 +222,25 @@ def _allowed_content_part_keys() -> frozenset[str]:
 _ALLOWED_CONTENT_PART_KEYS: frozenset[str] = _allowed_content_part_keys()
 
 
+_DROPPED_CONTENT_BLOCK_TYPES: frozenset[str] = frozenset(
+    {
+        "tool_use",
+        "thinking",
+        "reasoning",
+        "reasoning_content",
+        "function_call",
+        "code_interpreter_call",
+    }
+)
+"""Content block types the chat completions wire format does not carry.
+
+These arise from provider-specific or canonical v1 content (e.g. Anthropic
+`tool_use`/`thinking` blocks, or a `reasoning` block on an AIMessage) that
+reaches `_convert_message_to_dict` as conversation history. Fireworks rejects
+them, so they are dropped rather than forwarded.
+"""
+
+
 def _sanitize_chat_completions_content(content: Any) -> Any:
     """Strip non-wire keys from content blocks before serializing to Fireworks.
 
@@ -292,13 +311,7 @@ def _format_message_content(content: Any) -> Any:
     for block in content:
         if isinstance(block, dict) and "type" in block:
             btype = block["type"]
-            if btype in (
-                "tool_use",
-                "thinking",
-                "reasoning_content",
-                "function_call",
-                "code_interpreter_call",
-            ):
+            if btype in _DROPPED_CONTENT_BLOCK_TYPES:
                 continue
             if is_data_content_block(block):
                 formatted.append(
diff --git a/libs/partners/fireworks/tests/unit_tests/test_chat_models.py b/libs/partners/fireworks/tests/unit_tests/test_chat_models.py
index 448d2b6e0b..b665ef0e40 100644
--- a/libs/partners/fireworks/tests/unit_tests/test_chat_models.py
+++ b/libs/partners/fireworks/tests/unit_tests/test_chat_models.py
@@ -44,6 +44,7 @@ from langchain_core.messages import (
 from langchain_fireworks import ChatFireworks
 from langchain_fireworks.chat_models import (
     _ALLOWED_CONTENT_PART_KEYS,
+    _DROPPED_CONTENT_BLOCK_TYPES,
     FireworksContextOverflowError,
     _acompletion_with_retry,
     _completion_with_retry,
@@ -402,16 +403,23 @@ def test_format_message_content_passes_through_existing_image_url() -> None:
     assert formatted == blocks
 
 
-@pytest.mark.parametrize(
-    "btype",
-    [
+def test_dropped_content_block_types_membership() -> None:
+    """Pin the drop-list so a removal is a deliberate, visible change.
+
+    The parametrized test below derives its cases from the constant, so it
+    tracks additions for free but cannot catch a deletion.
+    """
+    assert {
         "tool_use",
         "thinking",
+        "reasoning",
         "reasoning_content",
         "function_call",
         "code_interpreter_call",
-    ],
-)
+    } == _DROPPED_CONTENT_BLOCK_TYPES
+
+
+@pytest.mark.parametrize("btype", sorted(_DROPPED_CONTENT_BLOCK_TYPES))
 def test_format_message_content_drops_unsupported_block_types(btype: str) -> None:
     """Block types not part of the OpenAI chat completions wire format are stripped."""
     blocks = [