diff --git a/tests/tools/test_mcp_tool.py b/tests/tools/test_mcp_tool.py
index 7027d9e8ff..f9a333af52 100644
--- a/tests/tools/test_mcp_tool.py
+++ b/tests/tools/test_mcp_tool.py
@@ -553,6 +553,33 @@ class TestSchemaConversion:
         assert schema["name"] == "mcp__my_server__get_sum"
         assert "-" not in schema["name"]
 
+    def test_long_names_are_clamped_to_64_chars(self):
+        """Portable Agent Plugin names can push mcp__<server>__<tool> past the
+        64-char limit OpenAI-compatible providers enforce on function names
+        (issue #81331). The registry name must be clamped with a stable hash
+        suffix, distinct long names must not collide, and the same inputs
+        must always produce the same shortened name.
+        """
+        from tools.mcp_tool_schema import _convert_mcp_schema, mcp_prefixed_tool_name
+
+        server_name = "agent_plugin_my_server_997167c9__my_server"
+        mcp_tool = _make_mcp_tool(name="reply_communication_todo")
+        schema = _convert_mcp_schema(server_name, mcp_tool)
+
+        assert len(schema["name"]) <= 64
+        assert schema["name"] == mcp_prefixed_tool_name(server_name, "reply_communication_todo")
+
+        other_tool = _make_mcp_tool(name="reply_communication_task")
+        other_schema = _convert_mcp_schema(server_name, other_tool)
+        assert other_schema["name"] != schema["name"]
+        assert len(other_schema["name"]) <= 64
+
+        # Deterministic across repeated calls with the same inputs.
+        assert (
+            mcp_prefixed_tool_name(server_name, "reply_communication_todo")
+            == schema["name"]
+        )
+
 
 # ---------------------------------------------------------------------------
 # Check function
diff --git a/tools/mcp_tool_schema.py b/tools/mcp_tool_schema.py
index dba5de056d..e0967b0126 100644
--- a/tools/mcp_tool_schema.py
+++ b/tools/mcp_tool_schema.py
@@ -3,6 +3,7 @@ compatibility, mcp__server__tool naming, utility-tool schemas, include/exclude f
 description injection scanning."""
 
 import logging
+import hashlib
 import fnmatch
 import re
 from typing import Any, List
@@ -135,9 +136,28 @@ def sanitize_mcp_name_component(value: str) -> str:
 MCP_TOOL_NAME_PREFIX = "mcp__"
 
 
+# OpenAI-compatible providers validate function names against ``^[a-zA-Z0-9_-]{1,64}$`` and 400 the
+# whole request when one generated name is longer. Portable plugin server keys fold the plugin name in
+# several times, so ``mcp__<server>__<tool>`` routinely passes 64 chars there (#81331). Clamp with a
+# deterministic hash suffix (same idea as ``schema_sanitizer.sanitize_property_key``); dispatch is
+# unaffected because handlers close over the original unprefixed tool name.
+_MCP_TOOL_NAME_MAX_LENGTH = 64
+_MCP_TOOL_NAME_HASH_LENGTH = 8
+_clamped_names_warned: set[str] = set()
+
+
 def mcp_prefixed_tool_name(server_name: str, tool_name: str) -> str:
-    """Registry/wire name: ``mcp__<sanitizedServer>__<sanitizedTool>``."""
-    return f"{MCP_TOOL_NAME_PREFIX}{sanitize_mcp_name_component(server_name)}__{sanitize_mcp_name_component(tool_name)}"
+    """Registry/wire name: ``mcp__<sanitizedServer>__<sanitizedTool>``, clamped to 64 chars with a
+    stable hash suffix when the natural name is longer."""
+    full_name = f"{MCP_TOOL_NAME_PREFIX}{sanitize_mcp_name_component(server_name)}__{sanitize_mcp_name_component(tool_name)}"
+    if len(full_name) <= _MCP_TOOL_NAME_MAX_LENGTH:
+        return full_name
+    suffix = "_" + hashlib.sha256(full_name.encode("utf-8")).hexdigest()[:_MCP_TOOL_NAME_HASH_LENGTH]
+    if full_name not in _clamped_names_warned:  # recomputed on every health refresh; warn once
+        _clamped_names_warned.add(full_name)
+        logger.warning("MCP tool name %r (%d chars) exceeds the %d-char provider limit; shortened to a "
+                       "deterministic hash-suffixed name", full_name, len(full_name), _MCP_TOOL_NAME_MAX_LENGTH)
+    return full_name[:_MCP_TOOL_NAME_MAX_LENGTH - len(suffix)] + suffix
 
 
 def _convert_mcp_schema(server_name: str, mcp_tool) -> dict: