From c0fd8f6012f9ff2a8fe6681770439d54ec915bf8 Mon Sep 17 00:00:00 2001
From: mateo-berri <277851410+mateo-berri@users.noreply.github.com>
Date: Tue, 15 Sep 2026 11:03:10 -0700
Subject: [PATCH] fix(anthropic): merge a case-variant Anthropic-Beta client
 header instead of clobbering it

---
 .../messages/transformation.py                | 23 +++++++++++--------
 ...est_anthropic_messages_per_turn_control.py | 12 ++++++++++
 2 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py b/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
index 67cda1984cb..27cdac34116 100644
--- a/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
+++ b/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
@@ -689,16 +689,20 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         """
         beta_values: Final[set] = set()
 
-        # Get existing beta headers if any
-        existing_beta: Final = headers.get("anthropic-beta")
-        if existing_beta:
-            beta_values.update(b.strip() for b in existing_beta.split(","))
+        existing_beta: Final = tuple(
+            piece.strip()
+            for key, value in headers.items()
+            if key.lower() == "anthropic-beta"
+            for piece in value.split(",")
+            if piece.strip()
+        )
+        beta_values.update(existing_beta)
 
         # Check for context management
         context_management_param: Final = optional_params.get("context_management")
         if context_management_param is not None:
             # Check edits array for compact_20260112 type
-            edits: Final = context_management_param.get("edits", [])
+            edits: Final = context_management_param.get("edits", ())
             has_compact = False
             has_other = False
 
@@ -740,7 +744,8 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         if AnthropicModelInfo().is_tool_search_used(tools):
             beta_values.add(get_tool_search_beta_header(custom_llm_provider))
 
-        if beta_values:
-            headers["anthropic-beta"] = ",".join(sorted(beta_values))
-
-        return headers
+        if not beta_values:
+            return headers
+        merged: Final = {key: value for key, value in headers.items() if key.lower() != "anthropic-beta"}
+        merged["anthropic-beta"] = ",".join(sorted(beta_values))
+        return merged
diff --git a/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_messages_per_turn_control.py b/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_messages_per_turn_control.py
index 7ed3526c4b6..d444258d02f 100644
--- a/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_messages_per_turn_control.py
+++ b/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_messages_per_turn_control.py
@@ -83,6 +83,18 @@ def test_forwarded_client_betas_survive_alongside_the_added_one():
     assert PER_TURN_CONTROL in _betas(headers)
 
 
+def test_case_variant_client_beta_header_is_merged():
+    """A client or config can spell the header ``Anthropic-Beta``; the proxy forwards it as
+    is, so the merge must read it whatever the casing and write one canonical header instead
+    of a lowercase one that clobbers it."""
+    headers = _validate(
+        _claude_code_turn({"effort": "low"}), headers={"Anthropic-Beta": "interleaved-thinking-2025-05-14"}
+    )
+
+    assert [key for key in headers if key.lower() == "anthropic-beta"] == ["anthropic-beta"]
+    assert _betas(headers) == {"interleaved-thinking-2025-05-14", PER_TURN_CONTROL}
+
+
 def test_added_per_turn_control_beta_survives_the_anthropic_allowlist():
     """The proxy filters ``anthropic-beta`` against the bundled allowlist right after
     the headers are built. A name missing from it is dropped silently, which would turn