fix(server): return -32602 for resource not found (SEP-2164)

docs/migration.md

@@ -487,6 +487,10 @@ await ctx.log(level="info", data="hello")
 
 Positional calls (`await ctx.info("hello")`) are unaffected.
 
+### `ResourceManager.get_resource()` and `ResourceTemplate.create_resource()` raise typed exceptions
+
+`ResourceManager.get_resource()` now raises `ResourceNotFoundError` (instead of `ValueError`) when no resource or template matches the URI. `ResourceTemplate.create_resource()` now raises `ResourceError` (instead of `ValueError`) when the template function fails. Neither subclasses `ValueError`, so callers catching `ValueError` should switch to `ResourceNotFoundError` / `ResourceError` (both importable from `mcp.server.mcpserver.exceptions`). `MCPServer.read_resource()` continues to raise `ResourceError` and is unaffected.
+
 ### Replace `RootModel` by union types with `TypeAdapter` validation
 
 The following union types are no longer `RootModel` subclasses:

src/mcp/server/mcpserver/context.py

@@ -113,6 +113,10 @@ async def read_resource(self, uri: str | AnyUrl) -> Iterable[ReadResourceContent
 
  Returns:
  The resource content as either text or bytes
+
+ Raises:
+ ResourceNotFoundError: If no resource or template matches the URI.
+ ResourceError: If template creation or resource reading fails.
  """
  assert self._mcp_server is not None, "Context is not available outside of a request"
  return await self._mcp_server.read_resource(uri, self)

src/mcp/server/mcpserver/exceptions.py

@@ -13,6 +13,15 @@ class ResourceError(MCPServerError):
  """Error in resource operations."""
 
 
+class ResourceNotFoundError(ResourceError):
+ """Resource does not exist.
+
+ Raise this from a resource template handler to signal that the requested
+ instance does not exist; clients receive ``-32602`` (invalid params) per
+ SEP-2164.
+ """
+
+
 class ToolError(MCPServerError):
  """Error in tool operations."""
 

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
@@ -79,7 +80,12 @@ def add_template(
  return template
 
  async def get_resource(self, uri: AnyUrl | str, context: Context[LifespanContextT, RequestT]) -> Resource:
- """Get resource by URI, checking concrete resources first, then templates."""
+ """Get resource by URI, checking concrete resources first, then templates.
+
+ Raises:
+ ResourceNotFoundError: If no resource or template matches the URI.
+ ResourceError: If a matching template fails to create the resource.
+ """
  uri_str = str(uri)
  logger.debug("Getting resource", extra={"uri": uri_str})
 
@@ -90,12 +96,9 @@ 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

@@ -11,12 +11,16 @@
 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
+from mcp.server.mcpserver.utilities.logging import get_logger
 from mcp.shared._callable_inspection import is_async_callable
 from mcp.types import Annotations, Icon
 
+logger = get_logger(__name__)
+
 if TYPE_CHECKING:
  from mcp.server.context import LifespanContextT, RequestT
  from mcp.server.mcpserver.context import Context
@@ -106,7 +110,7 @@ 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
@@ -129,5 +133,8 @@ async def create_resource(
  meta=self.meta,
  fn=lambda: result, # Capture result in closure
  )
+ except ResourceError:
+ raise
  except Exception as e:
- raise ValueError(f"Error creating resource from template: {e}")
+ logger.exception("Error creating resource from template")
+ raise ResourceError(f"Error creating resource from template: {e}") from e

src/mcp/server/mcpserver/server.py

@@ -31,7 +31,7 @@
 from mcp.server.lowlevel.server import LifespanResultT, Server
 from mcp.server.lowlevel.server import lifespan as default_lifespan
 from mcp.server.mcpserver.context import Context
-from mcp.server.mcpserver.exceptions import ResourceError
+from mcp.server.mcpserver.exceptions import ResourceError, ResourceNotFoundError
 from mcp.server.mcpserver.prompts import Prompt, PromptManager
 from mcp.server.mcpserver.resources import FunctionResource, Resource, ResourceManager
 from mcp.server.mcpserver.tools import Tool, ToolManager
@@ -44,6 +44,8 @@
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
+ INTERNAL_ERROR,
+ INVALID_PARAMS,
  Annotations,
  BlobResourceContents,
  CallToolRequestParams,
@@ -341,7 +343,12 @@ async def _handle_read_resource(
  self, ctx: ServerRequestContext[LifespanResultT], params: ReadResourceRequestParams
  ) -> ReadResourceResult:
  context = Context(request_context=ctx, mcp_server=self)
- results = await self.read_resource(params.uri, context)
+ try:
+ 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):
@@ -442,13 +449,15 @@ async def list_resource_templates(self) -> list[MCPResourceTemplate]:
  async def read_resource(
  self, uri: AnyUrl | str, context: Context[LifespanResultT, Any] | None = None
  ) -> Iterable[ReadResourceContents]:
- """Read a resource by URI."""
+ """Read a resource by URI.
+
+ Raises:
+ ResourceNotFoundError: If no resource or template matches the URI.
+ ResourceError: If template creation or resource reading fails.
+ """
  if context is None:
  context = Context(mcp_server=self)
- try:
- resource = await self._resource_manager.get_resource(uri, context)
- except ValueError:
- raise ResourceError(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

@@ -13,13 +13,15 @@
 from mcp.server.context import ServerRequestContext
 from mcp.server.experimental.request_context import Experimental
 from mcp.server.mcpserver import Context, MCPServer
-from mcp.server.mcpserver.exceptions import ToolError
+from mcp.server.mcpserver.exceptions import ResourceNotFoundError, ToolError
 from mcp.server.mcpserver.prompts.base import Message, UserMessage
 from mcp.server.mcpserver.resources import FileResource, FunctionResource
 from mcp.server.mcpserver.utilities.types import Audio, Image
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
+ INTERNAL_ERROR,
+ INVALID_PARAMS,
  AudioContent,
  BlobResourceContents,
  Completion,
@@ -727,13 +729,16 @@ def get_text():
  assert result.contents[0].text == "Hello, world!"
 
  async def test_read_unknown_resource(self):
- """Test that reading an unknown resource raises MCPError."""
+ """Test that reading an unknown resource returns -32602 with uri in data (SEP-2164)."""
  mcp = MCPServer()
 
  async with Client(mcp) as client:
- with pytest.raises(MCPError, match="Unknown resource: unknown://missing"):
+ with pytest.raises(MCPError, match="Unknown resource: unknown://missing") as exc_info:
  await client.read_resource("unknown://missing")
 
+ assert exc_info.value.error.code == INVALID_PARAMS
+ assert exc_info.value.error.data == {"uri": "unknown://missing"}
+
  async def test_read_resource_error(self):
  """Test that resource read errors are properly wrapped in MCPError."""
  mcp = MCPServer()
@@ -1516,3 +1521,35 @@ async def test_report_progress_passes_related_request_id():
  message="halfway",
  related_request_id="req-abc-123",
  )
+
+
+async def test_read_resource_template_error():
+ """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_read_resource_template_not_found():
+ """A template handler raising ResourceNotFoundError must surface as INVALID_PARAMS per SEP-2164."""
+ mcp = MCPServer()
+
+ @mcp.resource("resource://users/{user_id}")
+ def get_user(user_id: str) -> str:
+ raise ResourceNotFoundError(f"no user {user_id}")
+
+ async with Client(mcp) as client:
+ with pytest.raises(MCPError, match="no user 999") as exc_info:
+ await client.read_resource("resource://users/999")
+
+ assert exc_info.value.error.code == INVALID_PARAMS
+ assert exc_info.value.error.data == {"uri": "resource://users/999"}