From e196384a6a116caa947ff86191fa03e559dfe342 Mon Sep 17 00:00:00 2001
From: "claude[bot]" <41898282+claude[bot]@users.noreply.github.com>
Date: Fri, 10 Oct 2025 23:59:57 +0000
Subject: [PATCH] Change middleware return types from list to Sequence

This fixes covariance issues in middleware method signatures by using
Sequence instead of list for return type annotations. Lists are invariant
while Sequences are covariant, allowing middleware subclasses to properly
type their return values.

Fixes #2055

Co-authored-by: William Easton <strawgate@users.noreply.github.com>
---
 src/fastmcp/server/middleware/middleware.py | 26 ++++++++++++---------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/src/fastmcp/server/middleware/middleware.py b/src/fastmcp/server/middleware/middleware.py
index 0b78e4866..ba2d25c23 100644
--- a/src/fastmcp/server/middleware/middleware.py
+++ b/src/fastmcp/server/middleware/middleware.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import logging
-from collections.abc import Awaitable
+from collections.abc import Awaitable, Sequence
 from dataclasses import dataclass, field, replace
 from datetime import datetime, timezone
 from functools import partial
@@ -164,8 +164,10 @@ class Middleware:
     async def on_read_resource(
         self,
         context: MiddlewareContext[mt.ReadResourceRequestParams],
-        call_next: CallNext[mt.ReadResourceRequestParams, list[ReadResourceContents]],
-    ) -> list[ReadResourceContents]:
+        call_next: CallNext[
+            mt.ReadResourceRequestParams, Sequence[ReadResourceContents]
+        ],
+    ) -> Sequence[ReadResourceContents]:
         return await call_next(context)
 
     async def on_get_prompt(
@@ -178,27 +180,29 @@ class Middleware:
     async def on_list_tools(
         self,
         context: MiddlewareContext[mt.ListToolsRequest],
-        call_next: CallNext[mt.ListToolsRequest, list[Tool]],
-    ) -> list[Tool]:
+        call_next: CallNext[mt.ListToolsRequest, Sequence[Tool]],
+    ) -> Sequence[Tool]:
         return await call_next(context)
 
     async def on_list_resources(
         self,
         context: MiddlewareContext[mt.ListResourcesRequest],
-        call_next: CallNext[mt.ListResourcesRequest, list[Resource]],
-    ) -> list[Resource]:
+        call_next: CallNext[mt.ListResourcesRequest, Sequence[Resource]],
+    ) -> Sequence[Resource]:
         return await call_next(context)
 
     async def on_list_resource_templates(
         self,
         context: MiddlewareContext[mt.ListResourceTemplatesRequest],
-        call_next: CallNext[mt.ListResourceTemplatesRequest, list[ResourceTemplate]],
-    ) -> list[ResourceTemplate]:
+        call_next: CallNext[
+            mt.ListResourceTemplatesRequest, Sequence[ResourceTemplate]
+        ],
+    ) -> Sequence[ResourceTemplate]:
         return await call_next(context)
 
     async def on_list_prompts(
         self,
         context: MiddlewareContext[mt.ListPromptsRequest],
-        call_next: CallNext[mt.ListPromptsRequest, list[Prompt]],
-    ) -> list[Prompt]:
+        call_next: CallNext[mt.ListPromptsRequest, Sequence[Prompt]],
+    ) -> Sequence[Prompt]:
         return await call_next(context)