From 91c77e4d73af4abddd5a5af8b79cfe2794b8f26b Mon Sep 17 00:00:00 2001
From: mahenoorsalat <salatmahenoor7.8.6@gmail.com>
Date: Fri, 11 Sep 2026 13:18:24 +0530
Subject: [PATCH] fix(anthropic): handle omitted text field in
 content_block_start (#40689)

---
 litellm/llms/anthropic/chat/handler.py        |  2 +-
 litellm/types/llms/anthropic.py               |  2 +-
 ...anthropic_streaming_content_block_start.py | 51 +++++++++++++++++++
 3 files changed, 53 insertions(+), 2 deletions(-)
 create mode 100644 tests/test_anthropic_streaming_content_block_start.py

diff --git a/litellm/llms/anthropic/chat/handler.py b/litellm/llms/anthropic/chat/handler.py
index d1fe4cadf40..c7f7e2273af 100644
--- a/litellm/llms/anthropic/chat/handler.py
+++ b/litellm/llms/anthropic/chat/handler.py
@@ -891,7 +891,7 @@ class ModelResponseIterator:
                 # Track current content block type for filtering deltas
                 self.current_content_block_type = content_block_start["content_block"]["type"]
                 if content_block_start["content_block"]["type"] == "text":
-                    text = content_block_start["content_block"]["text"]
+                    text = content_block_start["content_block"].get("text") or ""
                 elif (
                     content_block_start["content_block"]["type"] == "tool_use"
                     or content_block_start["content_block"]["type"] == "server_tool_use"
diff --git a/litellm/types/llms/anthropic.py b/litellm/types/llms/anthropic.py
index 365d59a179b..6c64285225c 100644
--- a/litellm/types/llms/anthropic.py
+++ b/litellm/types/llms/anthropic.py
@@ -495,7 +495,7 @@ class ToolUseBlock(TypedDict):
     caller: ToolCaller | None
 
 
-class TextBlock(TypedDict):
+class TextBlock(TypedDict, total=False):
     text: str
 
     type: Literal["text"]
diff --git a/tests/test_anthropic_streaming_content_block_start.py b/tests/test_anthropic_streaming_content_block_start.py
new file mode 100644
index 00000000000..49c39a9307d
--- /dev/null
+++ b/tests/test_anthropic_streaming_content_block_start.py
@@ -0,0 +1,51 @@
+import pytest
+from litellm.llms.anthropic.chat.handler import ModelResponseIterator
+
+
+def test_chunk_parser_content_block_start_omits_text():
+    """
+    Test that ModelResponseIterator.chunk_parser handles content_block_start
+    when the 'text' key is omitted by third-party Anthropic-compatible upstreams.
+    Fixes Issue #40689.
+    """
+    iterator = ModelResponseIterator(streaming_response=[], sync_stream=True)
+
+    # 1. Chunk omitting "text" entirely
+    chunk_without_text = {
+        "type": "content_block_start",
+        "index": 0,
+        "content_block": {"type": "text"},
+    }
+    res = iterator.chunk_parser(chunk_without_text)
+    assert res is not None
+    assert res.choices[0].delta.content == ""
+
+    # 2. Chunk with standard official empty string text
+    chunk_with_empty_text = {
+        "type": "content_block_start",
+        "index": 0,
+        "content_block": {"type": "text", "text": ""},
+    }
+    res = iterator.chunk_parser(chunk_with_empty_text)
+    assert res is not None
+    assert res.choices[0].delta.content == ""
+
+    # 3. Chunk with text as None
+    chunk_with_none_text = {
+        "type": "content_block_start",
+        "index": 0,
+        "content_block": {"type": "text", "text": None},
+    }
+    res = iterator.chunk_parser(chunk_with_none_text)
+    assert res is not None
+    assert res.choices[0].delta.content == ""
+
+    # 4. Chunk with actual initial text
+    chunk_with_text = {
+        "type": "content_block_start",
+        "index": 0,
+        "content_block": {"type": "text", "text": "hello"},
+    }
+    res = iterator.chunk_parser(chunk_with_text)
+    assert res is not None
+    assert res.choices[0].delta.content == "hello"