From 29a2b4f3b774654cea09a50839c97c1ec0c43c19 Mon Sep 17 00:00:00 2001
From: MUHAMMAD SALMAN HUSSAIN <160324527+mshsheikh@users.noreply.github.com>
Date: Mon, 1 Jun 2026 18:18:40 +0500
Subject: [PATCH] Detect and reject duplicate tool names within individual MCP
 servers

## Problem
The existing duplicate tool name validation only checked for conflicts across multiple MCP servers. It collected names from a single server using a set comprehension, which inherently discards duplicates. When a misconfigured or buggy MCP server returned multiple tools with identical names, the later entry would silently overwrite the earlier one. This caused non-deterministic tool registration, unpredictable agent behavior, and made it extremely difficult to trace the root cause of tool routing failures.

## Solution
Introduced an explicit intra-server duplicate detection step before the existing cross-server validation. The updated logic uses collections.Counter to track the frequency of each tool name returned by a server. If any name appears more than once, the system raises a UserError with a precise message identifying the offending server and the duplicated names. The original cross-server duplicate check remains fully intact and operates on the deduplicated set of names.

## Technical Implementation
- Replaced the initial set comprehension with a Counter to capture name frequencies per server response
- Added a filtered iteration to identify names with a count greater than one
- Introduced a targeted UserError for intra-server duplicates that includes the server name for immediate troubleshooting
- Converted the Counter keys back to a set to preserve the existing cross-server intersection logic
- Leveraged the already-imported collections.Counter, requiring zero new dependencies
- Maintains full backward compatibility with no changes to public method signatures, error types, or external APIs

## Code Changes
Location: MCPUtil.get_all_function_tools() inside the for server in servers: loop

### Before:
```python
            server_tool_names = {tool.name for tool in server_tools}
            duplicate_tool_names = sorted(server_tool_names & tool_names)
            if duplicate_tool_names:
                raise UserError(
                    "Duplicate tool names found across MCP servers: "
                    f"{', '.join(duplicate_tool_names)}"
                )
```

### After:
```python
            server_tool_name_counts = Counter(tool.name for tool in server_tools)
            intra_server_duplicate_tool_names = sorted(
                name for name, count in server_tool_name_counts.items() if count > 1
            )
            if intra_server_duplicate_tool_names:
                raise UserError(
                    "Duplicate tool names found within MCP server "
                    f"'{server.name}': {', '.join(intra_server_duplicate_tool_names)}"
                )

            server_tool_names = set(server_tool_name_counts)
            duplicate_tool_names = sorted(server_tool_names & tool_names)
            if duplicate_tool_names:
                raise UserError(
                    "Duplicate tool names found across MCP servers: "
                    f"{', '.join(duplicate_tool_names)}"
                )
```

## Impact and Verification
- Prevents silent tool overwrites when an MCP server returns duplicate tool definitions
- Provides clear, actionable error messages that immediately identify the source of naming conflicts
- Preserves all existing cross-server validation behavior and error formatting
- Verified against the existing test suite, which passes without modification
- Introduces no new dependencies or runtime overhead
- Safe for immediate merge as a backward-compatible stability improvement
---
 src/agents/mcp/util.py | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/agents/mcp/util.py b/src/agents/mcp/util.py
index bf00cb2b79..22250f027b 100644
--- a/src/agents/mcp/util.py
+++ b/src/agents/mcp/util.py
@@ -277,7 +277,17 @@ async def get_all_function_tools(
                 agent,
                 failure_error_function=failure_error_function,
             )
-            server_tool_names = {tool.name for tool in server_tools}
+            server_tool_name_counts = Counter(tool.name for tool in server_tools)
+            intra_server_duplicate_tool_names = sorted(
+                name for name, count in server_tool_name_counts.items() if count > 1
+            )
+            if intra_server_duplicate_tool_names:
+                raise UserError(
+                    "Duplicate tool names found within MCP server "
+                    f"'{server.name}': {', '.join(intra_server_duplicate_tool_names)}"
+                )
+
+            server_tool_names = set(server_tool_name_counts)
             duplicate_tool_names = sorted(server_tool_names & tool_names)
             if duplicate_tool_names:
                 raise UserError(