From af4390a5298614525441b6dfb169a1028198ee2b Mon Sep 17 00:00:00 2001
From: Zawwar <105767627+Zawwarsami16@users.noreply.github.com>
Date: Tue, 12 May 2026 20:08:25 -0400
Subject: [PATCH] fix(tools): raise clear error for duplicate tool names in
 tool_runner

---
 src/anthropic/lib/tools/_beta_runner.py | 13 +++-
 tests/lib/tools/test_runners.py         | 82 +++++++++++++++++++++++++
 2 files changed, 94 insertions(+), 1 deletion(-)

diff --git a/src/anthropic/lib/tools/_beta_runner.py b/src/anthropic/lib/tools/_beta_runner.py
index 52e486988..a781fcf80 100644
--- a/src/anthropic/lib/tools/_beta_runner.py
+++ b/src/anthropic/lib/tools/_beta_runner.py
@@ -72,7 +72,18 @@ def __init__(
         max_iterations: int | None = None,
         compaction_control: CompactionControl | None = None,
     ) -> None:
-        self._tools_by_name = {tool.name: tool for tool in tools}
+        tools_list = list(tools)
+        seen: dict[str, int] = {}
+        for tool in tools_list:
+            seen[tool.name] = seen.get(tool.name, 0) + 1
+        duplicates = sorted(name for name, count in seen.items() if count > 1)
+        if duplicates:
+            raise ValueError(
+                "Duplicate tool name(s) passed to tool_runner: "
+                + ", ".join(repr(name) for name in duplicates)
+                + ". Tool names must be unique within a single tool_runner call."
+            )
+        self._tools_by_name = {tool.name: tool for tool in tools_list}
         self._params: ParseMessageCreateParamsBase[ResponseFormatT] = {
             **params,
             "messages": [message for message in params["messages"]],
diff --git a/tests/lib/tools/test_runners.py b/tests/lib/tools/test_runners.py
index 6b92e7a9b..ef02f2361 100644
--- a/tests/lib/tools/test_runners.py
+++ b/tests/lib/tools/test_runners.py
@@ -695,3 +695,85 @@ def test_tool_runner_method_in_sync(sync: bool, client: Anthropic, async_client:
             "stream",
         },
     )
+
+
+def test_tool_runner_rejects_duplicate_tool_names_sync(client: Anthropic) -> None:
+    @beta_tool
+    def get_weather(location: str) -> BetaFunctionToolResultType:
+        """Look up the weather for a location."""
+        return location
+
+    @beta_tool(name="get_weather")
+    def other_weather(city: str) -> BetaFunctionToolResultType:
+        """A second tool that happens to use the same name."""
+        return city
+
+    with pytest.raises(ValueError) as excinfo:
+        client.beta.messages.tool_runner(
+            max_tokens=1024,
+            model="claude-haiku-4-5",
+            tools=[get_weather, other_weather],
+            messages=[{"role": "user", "content": "hi"}],
+        )
+
+    msg = str(excinfo.value)
+    assert "Duplicate tool name" in msg
+    assert "'get_weather'" in msg
+
+
+async def test_tool_runner_rejects_duplicate_tool_names_async(async_client: AsyncAnthropic) -> None:
+    @beta_async_tool
+    async def get_weather(location: str) -> BetaFunctionToolResultType:
+        """Look up the weather for a location."""
+        return location
+
+    @beta_async_tool(name="get_weather")
+    async def other_weather(city: str) -> BetaFunctionToolResultType:
+        """A second tool that happens to use the same name."""
+        return city
+
+    with pytest.raises(ValueError) as excinfo:
+        async_client.beta.messages.tool_runner(
+            max_tokens=1024,
+            model="claude-haiku-4-5",
+            tools=[get_weather, other_weather],
+            messages=[{"role": "user", "content": "hi"}],
+        )
+
+    msg = str(excinfo.value)
+    assert "Duplicate tool name" in msg
+    assert "'get_weather'" in msg
+
+
+def test_tool_runner_lists_all_duplicate_names(client: Anthropic) -> None:
+    @beta_tool
+    def get_weather(location: str) -> BetaFunctionToolResultType:
+        """get weather"""
+        return location
+
+    @beta_tool(name="get_weather")
+    def get_weather_dup(city: str) -> BetaFunctionToolResultType:
+        """duplicate of get_weather"""
+        return city
+
+    @beta_tool
+    def lookup(query: str) -> BetaFunctionToolResultType:
+        """lookup a thing"""
+        return query
+
+    @beta_tool(name="lookup")
+    def lookup_dup(q: str) -> BetaFunctionToolResultType:
+        """duplicate of lookup"""
+        return q
+
+    with pytest.raises(ValueError) as excinfo:
+        client.beta.messages.tool_runner(
+            max_tokens=1024,
+            model="claude-haiku-4-5",
+            tools=[get_weather, get_weather_dup, lookup, lookup_dup],
+            messages=[{"role": "user", "content": "hi"}],
+        )
+
+    msg = str(excinfo.value)
+    assert "'get_weather'" in msg
+    assert "'lookup'" in msg