fix(mcp_semantic_filter): keep tool names whole in filter response header (#32282)

The x-litellm-semantic-filter-tools response header was sliced mid-name at
MAX_MCP_SEMANTIC_FILTER_TOOLS_HEADER_LENGTH with a trailing "...", so the
admin UI test panel rendered the last selected tool name chopped. Truncate
the CSV at a tool name boundary instead so the header only ever carries
complete names, and note in the test panel how many selected tools did not
fit in the header

litellm/proxy/hooks/mcp_semantic_filter/hook.py

@@ -24,6 +24,18 @@ if TYPE_CHECKING:
     from litellm.router import Router
 
 
+def _truncate_csv_at_tool_name_boundary(tool_names_csv: str, max_length: int) -> str:
+    """Cap a CSV of tool names to max_length, dropping any name that does not fit whole."""
+    if len(tool_names_csv) <= max_length:
+        return tool_names_csv
+
+    head = tool_names_csv[: max_length + 1]
+    if "," not in head:
+        return ""
+
+    return head.rsplit(",", 1)[0]
+
+
 class SemanticToolFilterHook(CustomLogger):
     """
     Pre-call hook that filters MCP tools semantically.
@@ -327,11 +339,12 @@ class SemanticToolFilterHook(CustomLogger):
 
         # Add CSV of filtered tool names (nginx-safe length)
         tool_names_csv = metadata.get("litellm_semantic_filter_tools", "")
-        if tool_names_csv:
-            if len(tool_names_csv) > MAX_MCP_SEMANTIC_FILTER_TOOLS_HEADER_LENGTH:
-                tool_names_csv = tool_names_csv[: MAX_MCP_SEMANTIC_FILTER_TOOLS_HEADER_LENGTH - 3] + "..."
-
-            headers["x-litellm-semantic-filter-tools"] = tool_names_csv
+        header_safe_csv = _truncate_csv_at_tool_name_boundary(
+            tool_names_csv=tool_names_csv,
+            max_length=MAX_MCP_SEMANTIC_FILTER_TOOLS_HEADER_LENGTH,
+        )
+        if header_safe_csv:
+            headers["x-litellm-semantic-filter-tools"] = header_safe_csv
 
         return headers
 

tests/test_litellm/proxy/_experimental/mcp_server/test_semantic_tool_filter.py

@@ -1050,3 +1050,64 @@ class TestGetToolsByNames:
         )
 
         assert matched == []
+
+
+@pytest.mark.asyncio
+async def test_semantic_filter_headers_hook_emits_only_complete_tool_names():
+    """
+    Regression test for LIT-4215.
+
+    The x-litellm-semantic-filter-tools header used to be sliced mid-name at
+    MAX_MCP_SEMANTIC_FILTER_TOOLS_HEADER_LENGTH with a "..." suffix, so the UI
+    rendered a chopped tool name as the last entry. The header must only ever
+    contain complete tool names, in their original order, within the cap.
+    """
+    from litellm.constants import MAX_MCP_SEMANTIC_FILTER_TOOLS_HEADER_LENGTH
+    from litellm.proxy._experimental.mcp_server.semantic_tool_filter import (
+        SemanticMCPToolFilter,
+    )
+    from litellm.proxy.hooks.mcp_semantic_filter import SemanticToolFilterHook
+
+    filter_instance = SemanticMCPToolFilter(
+        embedding_model="text-embedding-3-small",
+        litellm_router_instance=Mock(),
+        top_k=10,
+        similarity_threshold=0.3,
+        enabled=True,
+    )
+    hook = SemanticToolFilterHook(filter_instance)
+
+    tool_names = [f"metrics_mcp-very_long_tool_name_for_header_{i:02d}" for i in range(8)]
+    data = {
+        "metadata": {
+            "litellm_semantic_filter_stats": "40->8",
+            "litellm_semantic_filter_tools": ",".join(tool_names),
+        }
+    }
+
+    headers = await hook.async_post_call_response_headers_hook(
+        data=data,
+        user_api_key_dict=Mock(),
+        response=None,
+    )
+
+    assert headers is not None
+    assert headers["x-litellm-semantic-filter"] == "40->8"
+
+    tools_header = headers["x-litellm-semantic-filter-tools"]
+    assert len(tools_header) <= MAX_MCP_SEMANTIC_FILTER_TOOLS_HEADER_LENGTH
+    emitted_names = tools_header.split(",")
+    assert emitted_names == tool_names[: len(emitted_names)]
+    assert 0 < len(emitted_names) < len(tool_names)
+
+
+def test_truncate_csv_at_tool_name_boundary_edges():
+    from litellm.proxy.hooks.mcp_semantic_filter.hook import (
+        _truncate_csv_at_tool_name_boundary,
+    )
+
+    assert _truncate_csv_at_tool_name_boundary(tool_names_csv="a,b,c", max_length=150) == "a,b,c"
+    assert _truncate_csv_at_tool_name_boundary(tool_names_csv="abc,def", max_length=3) == "abc"
+    assert _truncate_csv_at_tool_name_boundary(tool_names_csv="ab,cd,ef", max_length=5) == "ab,cd"
+    assert _truncate_csv_at_tool_name_boundary(tool_names_csv="ab,cd,ef", max_length=4) == "ab"
+    assert _truncate_csv_at_tool_name_boundary(tool_names_csv="single_name_longer_than_cap", max_length=10) == ""

ui/litellm-dashboard/src/components/Settings/AdminSettings/MCPSemanticFilterSettings/MCPSemanticFilterTestPanel.test.tsx

@@ -103,6 +103,18 @@ describe("MCPSemanticFilterTestPanel", () => {
     expect(screen.getByText("wiki-fetch")).toBeInTheDocument();
     expect(screen.getByText("github-search")).toBeInTheDocument();
     expect(screen.getByText("slack-post")).toBeInTheDocument();
+    expect(screen.queryByText(/more selected tools not shown/i)).not.toBeInTheDocument();
+  });
+
+  it("should note how many selected tools are missing when the header list is incomplete", () => {
+    const testResult: TestResult = {
+      totalTools: 40,
+      selectedTools: 8,
+      tools: ["metrics_mcp-node_query_by_id", "metrics_mcp-latency_query_api", "inventory_mcp-site_lookup"],
+    };
+    render(<MCPSemanticFilterTestPanel {...buildProps({ testResult })} />);
+
+    expect(screen.getByText("+5 more selected tools not shown")).toBeInTheDocument();
   });
 
   it("should not render the results section when testResult is null", () => {

ui/litellm-dashboard/src/components/Settings/AdminSettings/MCPSemanticFilterSettings/MCPSemanticFilterTestPanel.tsx

@@ -103,6 +103,11 @@ export default function MCPSemanticFilterTestPanel({
                           </li>
                         ))}
                       </ul>
+                      {testResult.selectedTools > testResult.tools.length && (
+                        <Typography.Text type="secondary" style={{ display: "block", marginTop: 8 }}>
+                          +{testResult.selectedTools - testResult.tools.length} more selected tools not shown
+                        </Typography.Text>
+                      )}
                     </div>
                   </div>
                 )}