diff --git a/src/mcp/server/lowlevel/server.py b/src/mcp/server/lowlevel/server.py
index 7e79c94326..0e917aefd0 100644
--- a/src/mcp/server/lowlevel/server.py
+++ b/src/mcp/server/lowlevel/server.py
@@ -148,7 +148,7 @@ def __init__(
         | None = None,
         on_call_tool: Callable[
             [ServerRequestContext[LifespanResultT], types.CallToolRequestParams],
-            Awaitable[types.CallToolResult],
+            Awaitable[types.CallToolResult | types.InputRequiredResult],
         ]
         | None = None,
         on_list_resources: Callable[
@@ -163,7 +163,7 @@ def __init__(
         | None = None,
         on_read_resource: Callable[
             [ServerRequestContext[LifespanResultT], types.ReadResourceRequestParams],
-            Awaitable[types.ReadResourceResult],
+            Awaitable[types.ReadResourceResult | types.InputRequiredResult],
         ]
         | None = None,
         on_subscribe_resource: Callable[
@@ -176,6 +176,11 @@ def __init__(
             Awaitable[types.EmptyResult],
         ]
         | None = None,
+        on_subscriptions_listen: Callable[
+            [ServerRequestContext[LifespanResultT], types.SubscriptionsListenRequestParams],
+            Awaitable[types.EmptyResult],
+        ]
+        | None = None,
         on_list_prompts: Callable[
             [ServerRequestContext[LifespanResultT], types.PaginatedRequestParams | None],
             Awaitable[types.ListPromptsResult],
@@ -183,7 +188,7 @@ def __init__(
         | None = None,
         on_get_prompt: Callable[
             [ServerRequestContext[LifespanResultT], types.GetPromptRequestParams],
-            Awaitable[types.GetPromptResult],
+            Awaitable[types.GetPromptResult | types.InputRequiredResult],
         ]
         | None = None,
         on_completion: Callable[
@@ -242,6 +247,7 @@ def __init__(
             ("resources/read", types.ReadResourceRequestParams, on_read_resource),
             ("resources/subscribe", types.SubscribeRequestParams, on_subscribe_resource),
             ("resources/unsubscribe", types.UnsubscribeRequestParams, on_unsubscribe_resource),
+            ("subscriptions/listen", types.SubscriptionsListenRequestParams, on_subscriptions_listen),
             ("tools/list", types.PaginatedRequestParams, on_list_tools),
             ("tools/call", types.CallToolRequestParams, on_call_tool),
             ("logging/setLevel", types.SetLevelRequestParams, on_set_logging_level),
diff --git a/src/mcp/types/_types.py b/src/mcp/types/_types.py
index 82b4a084d5..f0b13f0bd0 100644
--- a/src/mcp/types/_types.py
+++ b/src/mcp/types/_types.py
@@ -16,9 +16,10 @@
     Field,
     FileUrl,
     TypeAdapter,
+    model_validator,
 )
 from pydantic.alias_generators import to_camel
-from typing_extensions import NotRequired, TypedDict
+from typing_extensions import NotRequired, Self, TypedDict
 
 from mcp.types.jsonrpc import RequestId
 
@@ -2052,7 +2053,7 @@ class InputRequiredResult(Result):
     (`tools/call`, `prompts/get`, `resources/read`). The client fulfills
     `input_requests` and retries the original request, carrying the responses
     and the echoed `request_state`. At least one of those two fields is
-    present on the wire (spec MUST; not enforced by the model).
+    present on the wire (spec MUST).
     """
 
     result_type: Literal["input_required"] = "input_required"
@@ -2064,6 +2065,12 @@ class InputRequiredResult(Result):
     request_state: str | None = None
     """Opaque state to pass back verbatim when the client retries the original request."""
 
+    @model_validator(mode="after")
+    def _require_one_field(self) -> Self:
+        if self.input_requests is None and self.request_state is None:
+            raise ValueError("InputRequiredResult requires at least one of input_requests or request_state")
+        return self
+
 
 # Forward refs to InputResponses; rebuild at import time rather than first use.
 InputResponseRequestParams.model_rebuild()
diff --git a/tests/test_types.py b/tests/test_types.py
index 3756bd893d..dffbc918dc 100644
--- a/tests/test_types.py
+++ b/tests/test_types.py
@@ -2,6 +2,7 @@
 
 import pytest
 from inline_snapshot import snapshot
+from pydantic import ValidationError
 
 from mcp.types import (
     LATEST_PROTOCOL_VERSION,
@@ -436,4 +437,13 @@ def test_empty_result_dumps_result_type_only_when_explicitly_tagged():
 
 
 def test_input_required_result_dumps_its_discriminating_tag():
-    assert _wire_dump(InputRequiredResult()) == snapshot({"resultType": "input_required"})
+    assert _wire_dump(InputRequiredResult(request_state="s")) == snapshot(
+        {"resultType": "input_required", "requestState": "s"}
+    )
+
+
+def test_input_required_result_requires_at_least_one_of_input_requests_or_request_state():
+    with pytest.raises(ValidationError):
+        InputRequiredResult()
+    assert InputRequiredResult(input_requests={}).request_state is None
+    assert InputRequiredResult(request_state="s").input_requests is None