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[Bug] ResourceManager.add_template() ignores warn_on_duplicate_resources and silently overwrites existing templates #3813
Description
Initial Checks
- I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
- I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue
Release line
2.x (current stable)
Description
In mcp.server.mcpserver.resources.resource_manager.ResourceManager, the add_resource() method properly respects self.warn_on_duplicate_resources (configured via MCPServer(warn_on_duplicate_resources=True), which is True by default): when a duplicate URI is added, it logs a warning (Resource already exists: ...) and preserves the existing resource handler (return existing).
However, add_template() completely ignores warn_on_duplicate_resources:
- It does not check
self.warn_on_duplicate_resources. - It emits no log warning when an identical URI template is re-registered.
- It silently overwrites the existing template in
self._templates[template.uri_template] = template.
This creates divergent registration semantics between static resources and template resources registered via @server.resource(...), and leads to silent replacement of template handlers when multiple modules or extensions declare the same URI template.
Example Code
import logging
from mcp.server.mcpserver import MCPServer
logging.basicConfig(level=logging.WARNING)
server = MCPServer(warn_on_duplicate_resources=True)
# 1. Static resources: logs warning and preserves existing (doc1)
@server.resource("file://doc")
def doc1() -> str:
return "v1"
@server.resource("file://doc")
def doc2() -> str:
return "v2"
# 2. Template resources: emits no warning and silently overwrites template1 with template2
@server.resource("file://doc/{id}")
def template1(id: str) -> str:
return "template_v1"
@server.resource("file://doc/{id}")
def template2(id: str) -> str:
return "template_v2"
# Static resource warned and kept doc1:
assert server._resource_manager._resources["file://doc"].fn() == "v1"
# Template resource silently overwritten without warning:
assert server._resource_manager._templates["file://doc/{id}"].name == "template2"Expected output:
When template2 is registered for "file://doc/{id}", ResourceManager should emit:
WARNING:mcp.server.mcpserver.resources.resource_manager:Template already exists: file://doc/{id}
and retain or handle the duplicate consistently with add_resource / add_tool / add_prompt.
Python & MCP Python SDK
- Python 3.11 / 3.12 / 3.14
- MCP Python SDK: main branch (v2.x)
Activity
Shanmukh-l33t commentedon Nov 3, 2026
Confirmed independently and prototyped a fix (not opening a PR — I understand unassigned-contributor PRs get auto-closed here, and this is @Tanmay9223's report to own).
Root cause (
src/mcp/server/mcpserver/resources/resource_manager.py):add_template()builds theResourceTemplateand then doesself._templates[template.uri_template] = templateunconditionally — there is noexistinglookup at all, sowarn_on_duplicate_resourcesis never consulted.add_resource()(same file) andadd_tool()/add_prompt()(their managers) all do the lookup, warn, andreturn existing.Fix shape (mirror
add_resource()exactly):Tested branch, if useful to whoever lands this:
fix/3813-duplicate-template-registrationon Shanmukh-l33t/python-sdk (compare), commit02456738(+37/-0). Three regression tests mirroring the existingtest_add_duplicate_resource/test_warn_on_duplicate_resources/test_disable_warn_on_duplicate_resourcestrio: duplicate registration keeps the first template (second is first), warns with the URI template when enabled, and stays silent but still non-overwriting whenwarn_on_duplicate_resources=False.Evidence: the 3 new tests fail on
main@ af4c6f4 (3 failed) and pass with the change (test file 14 passed); full suite 6228 passed / 8 skipped / 1 xfailed with one failure,test_file_resources.py::test_permission_error, which fails identically on pristinemainin this environment (test runs as root);ruff check+ruff format --checkclean;pyrightclean on both touched files.One design note: the duplicate check runs after
ResourceTemplate.from_function(), so a re-registration with a malformed function still raises fromfrom_functionbefore the duplicate path — same failure surface as today; happy to move the lookup earlier if you'd prefer duplicates short-circuit before validation.(AI disclosure: this analysis, patch and tests were drafted with AI assistance and reviewed by me.)