Fix order-dependent collision in _build_anthropic_tool_name_maps

Use a two-pass approach: first pre-register all already-valid tool names
in the 'used' set, then sanitize/disambiguate names that need rewriting.
This ensures valid names always have priority regardless of input order,
preventing duplicate tool names on the wire when e.g. 'foo/bar' appears
before 'foo_bar' in the tool list.

Add regression test for the reversed ordering case.

litellm/llms/anthropic/chat/transformation.py

@@ -175,14 +175,22 @@ def _build_anthropic_tool_name_maps(
     """
     forward: Dict[str, str] = {}
     used: set = set()
+
+    # First pass: reserve slots for names that are already valid so they
+    # always have priority regardless of input order.
     for original in original_names:
         if not isinstance(original, str) or not original:
             continue
         candidate = _basic_sanitize_anthropic_tool_name(original)
         if candidate == original:
-            # Already valid. Reserve the slot but don't put it in `forward`
-            # -- we want untouched names to skip translation entirely.
             used.add(candidate)
+
+    # Second pass: sanitize/disambiguate names that need rewriting.
+    for original in original_names:
+        if not isinstance(original, str) or not original:
+            continue
+        candidate = _basic_sanitize_anthropic_tool_name(original)
+        if candidate == original:
             continue
         # Disambiguate against names already chosen this request.
         unique = candidate
@@ -197,6 +205,8 @@ def _build_anthropic_tool_name_maps(
         used.add(unique)
     reverse = {v: k for k, v in forward.items()}
     return forward, reverse
+
+
 REASONING_EFFORT_TO_OUTPUT_CONFIG_EFFORT: Dict[str, str] = {
     "low": "low",
     "minimal": "low",

tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py

@@ -4026,6 +4026,23 @@ def test_build_anthropic_tool_name_maps_three_way_collision():
     assert reverse == {"foo_bar_2": "foo/bar", "foo_bar_3": "foo.bar"}
 
 
+def test_build_anthropic_tool_name_maps_reverse_order_collision():
+    """REGRESSION: when the invalid name appears *before* the valid name that
+    its sanitized form collides with, both must still end up with distinct
+    names on the wire."""
+    from litellm.llms.anthropic.chat.transformation import (
+        _build_anthropic_tool_name_maps,
+    )
+
+    forward, reverse = _build_anthropic_tool_name_maps(["foo/bar", "foo_bar"])
+    # The valid name keeps its slot untouched.
+    assert "foo_bar" not in forward
+    # The rewritten one gets a disambiguating suffix.
+    assert forward["foo/bar"] == "foo_bar_2"
+    assert reverse == {"foo_bar_2": "foo/bar"}
+    assert "foo_bar" not in reverse
+
+
 def test_map_openai_params_does_not_pollute_optional_params_with_internal_keys():
     """REGRESSION: ``optional_params`` is what becomes the JSON body sent to
     Anthropic (``data = {**optional_params}``). It MUST NOT carry LiteLLM-