From 7d5bce3aec69440a375b58b18ffb1c627428b8db Mon Sep 17 00:00:00 2001
From: mateo-berri <277851410+mateo-berri@users.noreply.github.com>
Date: Sat, 31 Oct 2026 03:03:54 -0700
Subject: [PATCH] fix(anthropic): only decode text attachments in standard text
 encodings

---
 .../prompt_templates/factory.py               | 36 ++++++++++++++++++-
 ...llm_core_utils_prompt_templates_factory.py | 24 +++++++++++++
 2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/litellm/litellm_core_utils/prompt_templates/factory.py b/litellm/litellm_core_utils/prompt_templates/factory.py
index 9621373913d..ffb512f63bd 100644
--- a/litellm/litellm_core_utils/prompt_templates/factory.py
+++ b/litellm/litellm_core_utils/prompt_templates/factory.py
@@ -1,4 +1,5 @@
 import base64
+import codecs
 import copy
 import hashlib
 import itertools
@@ -1842,6 +1843,39 @@ def anthropic_document_media_type_for_data_uri(url: str, format_override: str |
     return declared if declared in _ANTHROPIC_DOCUMENT_BASE64_MEDIA_TYPES else None
 
 
+_TEXT_ATTACHMENT_ENCODINGS: Final = frozenset(
+    codecs.lookup(label).name
+    for label in (
+        "utf-8",
+        "utf-16",
+        "utf-16-le",
+        "utf-16-be",
+        "ascii",
+        "latin-1",
+        *(f"iso8859-{part}" for part in (2, 3, 4, 5, 6, 7, 8, 10, 13, 14, 15, 16)),
+        *(f"cp{code_page}" for code_page in (866, 874, *range(1250, 1259))),
+        "koi8-r",
+        "koi8-u",
+        "mac-roman",
+        "mac-cyrillic",
+        "gbk",
+        "gb18030",
+        "big5",
+        "euc-jp",
+        "iso2022-jp",
+        "shift-jis",
+        "euc-kr",
+    )
+)
+
+
+def _text_attachment_encoding(charset: str) -> str:
+    encoding: Final = codecs.lookup(charset).name
+    if encoding not in _TEXT_ATTACHMENT_ENCODINGS:
+        raise LookupError(f"unsupported encoding: {charset}")
+    return encoding
+
+
 def _anthropic_text_document_source(
     media_type_with_parameters: str, data: str
 ) -> AnthropicContentParamSourceText | None:
@@ -1852,7 +1886,7 @@ def _anthropic_text_document_source(
     content_type_header["content-type"] = media_type_with_parameters
     charset: Final = content_type_header.get_content_charset("utf-8")
     try:
-        text: Final = base64.b64decode(data).decode(charset)
+        text: Final = base64.b64decode(data).decode(_text_attachment_encoding(charset))
     except (LookupError, UnicodeDecodeError) as decode_error:
         raise litellm.BadRequestError(
             message=f"Text attachment could not be read as {charset!r}: {decode_error}. "
diff --git a/tests/unit/litellm_core_utils/prompt_templates/test_litellm_core_utils_prompt_templates_factory.py b/tests/unit/litellm_core_utils/prompt_templates/test_litellm_core_utils_prompt_templates_factory.py
index 32e90965e8e..0ba858e9f3f 100644
--- a/tests/unit/litellm_core_utils/prompt_templates/test_litellm_core_utils_prompt_templates_factory.py
+++ b/tests/unit/litellm_core_utils/prompt_templates/test_litellm_core_utils_prompt_templates_factory.py
@@ -9218,6 +9218,30 @@ def test_anthropic_messages_pt_text_document_keeps_charset_named_by_the_data_uri
     ]
 
 
+@pytest.mark.parametrize(
+    ("charset", "text"),
+    [("shift_jis", "日本語"), ("windows-1252", "café"), ("KOI8-R", "привет"), ("utf-16", "naïve")],
+)
+def test_anthropic_messages_pt_reads_a_text_attachment_in_a_legacy_encoding_its_data_uri_names(charset, text):
+    data_uri: Final = f"data:text/plain;charset={charset};base64," + base64.b64encode(text.encode(charset)).decode()
+    messages = [{"role": "user", "content": [{"type": "file", "file": {"file_data": data_uri}}]}]
+
+    result = anthropic_messages_pt(messages=messages, model="claude-sonnet-5", llm_provider="anthropic")
+
+    assert result[0]["content"] == [
+        {"type": "document", "source": {"type": "text", "media_type": "text/plain", "data": text}}
+    ]
+
+
+@pytest.mark.parametrize("charset", ["punycode", "utf-7"])
+def test_anthropic_messages_pt_rejects_a_text_attachment_charset_that_is_not_a_supported_text_encoding(charset):
+    data_uri: Final = f"data:text/plain;charset={charset};base64," + base64.b64encode(b"abc").decode()
+    messages = [{"role": "user", "content": [{"type": "file", "file": {"file_data": data_uri}}]}]
+
+    with pytest.raises(litellm.BadRequestError, match=f"unsupported encoding: {charset}"):
+        anthropic_messages_pt(messages=messages, model="claude-sonnet-5", llm_provider="anthropic")
+
+
 def test_anthropic_messages_pt_native_tool_use_input_survives_guardrail_redaction():
     messages = [
         {"role": "user", "content": "hi"},