fix(server): raise distinct errors for not-found vs template failure

ResourceManager.get_resource() and ResourceTemplate.create_resource() now raise
ResourceNotFoundError and ResourceError respectively instead of generic ValueError.
This lets read_resource() drop its broad except-ValueError translation, so a
template whose user function throws is no longer reported to clients as -32602
(resource not found). It now propagates as ResourceError -> -32603 INTERNAL_ERROR,
matching static-resource read failures.

Author: pja-ant

src/mcp/server/mcpserver/resources/resource_manager.py

@@ -7,6 +7,7 @@
 
 from pydantic import AnyUrl
 
+from mcp.server.mcpserver.exceptions import ResourceNotFoundError
 from mcp.server.mcpserver.resources.base import Resource
 from mcp.server.mcpserver.resources.templates import ResourceTemplate
 from mcp.server.mcpserver.utilities.logging import get_logger
@@ -90,12 +91,9 @@ async def get_resource(self, uri: AnyUrl | str, context: Context[LifespanContext
         # Then check templates
         for template in self._templates.values():
             if params := template.matches(uri_str):
-                try:
-                    return await template.create_resource(uri_str, params, context=context)
-                except Exception as e:  # pragma: no cover
-                    raise ValueError(f"Error creating resource from template: {e}")
+                return await template.create_resource(uri_str, params, context=context)
 
-        raise ValueError(f"Unknown resource: {uri}")
+        raise ResourceNotFoundError(f"Unknown resource: {uri}")
 
     def list_resources(self) -> list[Resource]:
         """List all registered resources."""

src/mcp/server/mcpserver/resources/templates.py

@@ -11,6 +11,7 @@
 import anyio.to_thread
 from pydantic import BaseModel, Field, validate_call
 
+from mcp.server.mcpserver.exceptions import ResourceError
 from mcp.server.mcpserver.resources.types import FunctionResource, Resource
 from mcp.server.mcpserver.utilities.context_injection import find_context_parameter, inject_context
 from mcp.server.mcpserver.utilities.func_metadata import func_metadata
@@ -106,7 +107,7 @@ async def create_resource(
         """Create a resource from the template with the given parameters.
 
         Raises:
-            ValueError: If creating the resource fails.
+            ResourceError: If creating the resource fails.
         """
         try:
             # Add context to params if needed
@@ -130,4 +131,4 @@ async def create_resource(
                 fn=lambda: result,  # Capture result in closure
             )
         except Exception as e:
-            raise ValueError(f"Error creating resource from template: {e}")
+            raise ResourceError(f"Error creating resource from template: {e}")

src/mcp/server/mcpserver/server.py

@@ -44,6 +44,7 @@
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
+    INTERNAL_ERROR,
     INVALID_PARAMS,
     Annotations,
     BlobResourceContents,
@@ -346,6 +347,8 @@ async def _handle_read_resource(
             results = await self.read_resource(params.uri, context)
         except ResourceNotFoundError as err:
             raise MCPError(code=INVALID_PARAMS, message=str(err), data={"uri": str(params.uri)})
+        except ResourceError as err:
+            raise MCPError(code=INTERNAL_ERROR, message=str(err), data={"uri": str(params.uri)})
         contents: list[TextResourceContents | BlobResourceContents] = []
         for item in results:
             if isinstance(item.content, bytes):
@@ -449,10 +452,7 @@ async def read_resource(
         """Read a resource by URI."""
         if context is None:
             context = Context(mcp_server=self)
-        try:
-            resource = await self._resource_manager.get_resource(uri, context)
-        except ValueError:
-            raise ResourceNotFoundError(f"Unknown resource: {uri}")
+        resource = await self._resource_manager.get_resource(uri, context)
 
         try:
             content = await resource.read()

tests/server/mcpserver/resources/test_resource_manager.py

@@ -5,6 +5,7 @@
 from pydantic import AnyUrl
 
 from mcp.server.mcpserver import Context
+from mcp.server.mcpserver.exceptions import ResourceNotFoundError
 from mcp.server.mcpserver.resources import FileResource, FunctionResource, ResourceManager, ResourceTemplate
 
 
@@ -101,7 +102,7 @@ def greet(name: str) -> str:
 async def test_get_unknown_resource():
     """Test getting a non-existent resource."""
     manager = ResourceManager()
-    with pytest.raises(ValueError, match="Unknown resource"):
+    with pytest.raises(ResourceNotFoundError, match="Unknown resource"):
         await manager.get_resource(AnyUrl("unknown://test"), Context())
 
 

tests/server/mcpserver/resources/test_resource_template.py

@@ -6,6 +6,7 @@
 from pydantic import BaseModel
 
 from mcp.server.mcpserver import Context, MCPServer
+from mcp.server.mcpserver.exceptions import ResourceError
 from mcp.server.mcpserver.resources import FunctionResource, ResourceTemplate
 from mcp.types import Annotations
 
@@ -87,7 +88,7 @@ def failing_func(x: str) -> str:
             name="fail",
         )
 
-        with pytest.raises(ValueError, match="Error creating resource from template"):
+        with pytest.raises(ResourceError, match="Error creating resource from template"):
             await template.create_resource("fail://test", {"x": "test"}, Context())
 
     @pytest.mark.anyio

tests/server/mcpserver/test_server.py

@@ -20,6 +20,7 @@
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
+    INTERNAL_ERROR,
     INVALID_PARAMS,
     AudioContent,
     BlobResourceContents,
@@ -750,6 +751,21 @@ def failing_resource():
             with pytest.raises(MCPError, match="Error reading resource resource://failing"):
                 await client.read_resource("resource://failing")
 
+    async def test_read_resource_template_error(self):
+        """Template-creation failure must surface as INTERNAL_ERROR, not INVALID_PARAMS (not-found)."""
+        mcp = MCPServer()
+
+        @mcp.resource("resource://item/{item_id}")
+        def get_item(item_id: str) -> str:
+            raise RuntimeError("backend unavailable")
+
+        async with Client(mcp) as client:
+            with pytest.raises(MCPError, match="Error creating resource from template") as exc_info:
+                await client.read_resource("resource://item/42")
+
+            assert exc_info.value.error.code == INTERNAL_ERROR
+            assert exc_info.value.error.code != INVALID_PARAMS
+
     async def test_binary_resource(self):
         mcp = MCPServer()