fix(server): don't raise when tool_use is not followed by tool_result

Closes #2960

Per SEP-1577, tool_result blocks MUST be preceded by a tool_use block, but
there is no requirement that a tool_use MUST be followed by a tool_result. A
plain text response after a tool_use is valid.

The ID-matching check in validate_tool_use_result_messages ran whenever the
previous message contained a tool_use, regardless of whether the last message
actually had any tool_result blocks. With no tool_result blocks present,
tool_result_ids was empty and never matched the non-empty tool_use_ids,
raising a false 'ids ... do not match' ValueError.

Gate the check on has_tool_results so it only runs when the last message
actually contains tool_result blocks.

Adds a regression test that fails without the fix.

Author: Bartok9

src/mcp/server/validation.py

@@ -80,7 +80,7 @@ def validate_tool_use_result_messages(messages: list[SamplingMessage]) -> None:
         if not has_previous_tool_use:
             raise ValueError("tool_result blocks do not match any tool_use in the previous message")
 
-    if has_previous_tool_use and previous_content:
+    if has_tool_results and has_previous_tool_use and previous_content:
         tool_use_ids = {c.id for c in previous_content if c.type == "tool_use"}
         tool_result_ids = {c.tool_use_id for c in last_content if c.type == "tool_result"}
         if tool_use_ids != tool_result_ids:

tests/server/test_validation.py

@@ -159,3 +159,27 @@ def test_validate_tool_use_result_messages_no_error_when_tool_result_matches_too
         ),
     ]
     validate_tool_use_result_messages(messages)  # Should not raise
+
+
+def test_validate_tool_use_result_messages_no_error_when_tool_use_not_followed_by_tool_result() -> None:
+    """No error when a tool_use is followed by a plain (non-tool_result) response.
+
+    Regression test for #2960: per SEP-1577, tool_result blocks MUST be preceded
+    by a tool_use block, but there is NO requirement that a tool_use MUST be
+    followed by a tool_result. A plain text response after a tool_use is valid and
+    must not raise a false "ids do not match" ValueError.
+    """
+    messages = [
+        SamplingMessage(
+            role="assistant",
+            content=[
+                TextContent(type="text", text="Hold on..."),
+                ToolUseContent(type="tool_use", id="abc", name="search", input={"q": "test"}),
+            ],
+        ),
+        SamplingMessage(
+            role="user",
+            content=TextContent(type="text", text="Thanks, no results needed"),
+        ),
+    ]
+    validate_tool_use_result_messages(messages)  # Should not raise