Return -32602 for resource not found (SEP-2164) (#2920)

Author: Kludex

.github/actions/conformance/expected-failures.2026-07-28.yml

@@ -117,7 +117,5 @@ server:
   # These scenarios emit no FAILURE checks, only SHOULD-level WARNINGs, but
   # the expected-failures evaluator counts WARNINGs as failures. Same entries
   # as the draft suite in expected-failures.yml.
-  # SEP-2164: server returns -32600 (not -32602) and omits error.data.uri.
-  - sep-2164-resource-not-found
   # SEP-2322 SHOULD-level behaviour (re-request missing inputResponses).
   - input-required-result-missing-input-response

.github/actions/conformance/expected-failures.yml

@@ -70,8 +70,6 @@ server:
   - http-header-validation
   # WARNING-only entries: these scenarios emit no FAILURE checks, only SHOULD-level
   # WARNINGs, but the expected-failures evaluator counts WARNINGs as failures.
-  # SEP-2164: server returns -32600 (not -32602) and omits error.data.uri.
-  - sep-2164-resource-not-found
   # SEP-2322 SHOULD-level behaviour (re-request missing inputResponses).
   - input-required-result-missing-input-response
   # SEP-2322 negative-case scenarios: input-required-result-validate-input is

docs/migration.md

@@ -480,6 +480,12 @@ async def my_tool(x: int, ctx: Context) -> str:
 
 The internal layers (`ToolManager.call_tool`, `Tool.run`, `Prompt.render`, `ResourceTemplate.create_resource`, etc.) now require `context` as a positional argument.
 
+### Resource not found returns `-32602` and resource lookups raise typed exceptions (SEP-2164)
+
+Reading a missing resource now returns JSON-RPC error code `-32602` (invalid params) with the requested URI in `error.data` (`{"uri": ...}`), per [SEP-2164](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2164). Previously the server returned code `0` with no `data`. Clients can now reliably distinguish not-found from other errors; a template handler that raises `ResourceNotFoundError` (from `mcp.server.mcpserver.exceptions`) produces this same response.
+
+The underlying lookups now raise typed exceptions instead of `ValueError`. `ResourceManager.get_resource()` raises `ResourceNotFoundError` when no resource or template matches the URI, and `ResourceTemplate.create_resource()` raises `ResourceError` when the template function fails. Neither subclasses `ValueError`, so callers catching `ValueError` should switch to `ResourceNotFoundError` / `ResourceError` (both importable from `mcp.server.mcpserver.exceptions`; `ResourceNotFoundError` subclasses `ResourceError`).
+
 ### Registering lowlevel handlers from `MCPServer`
 
 `MCPServer` does not expose public APIs for `subscribe_resource`, `unsubscribe_resource`, or `set_logging_level` handlers. In v1, the workaround was to reach into the private lowlevel server and use its decorator methods:

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](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/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 Exception as e:
-            raise ValueError(f"Error creating resource from template: {e}")
+        except ResourceError:
+            raise
+        except Exception as exc:
+            logger.exception(f"Error creating resource from template {uri}")
+            raise ResourceError(f"Error creating resource from template {uri}") from exc

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 as exc:
-            raise ResourceError(f"Unknown resource: {uri}") from exc
+        resource = await self._resource_manager.get_resource(uri, context)
 
         try:
             content = await resource.read()

tests/interaction/_requirements.py

@@ -1142,8 +1142,10 @@ def __post_init__(self) -> None:
     ),
     "mcpserver:resource:read-throws-surfaced": Requirement(
         source="sdk",
-        behavior="A resource function that raises is surfaced to the caller as a JSON-RPC error response.",
-        arm_exclusions=(ArmExclusion(reason="modern-error-surface", spec_version="2026-07-28"),),
+        behavior=(
+            "A resource function that raises is surfaced to the caller as a JSON-RPC error response "
+            "(-32603 Internal error), with the original exception text withheld."
+        ),
     ),
     "mcpserver:resource:static": Requirement(
         source="sdk",
@@ -1161,14 +1163,10 @@ def __post_init__(self) -> None:
     ),
     "mcpserver:resource:unknown-uri": Requirement(
         source=f"{SPEC_BASE_URL}/server/resources#error-handling",
-        behavior="resources/read for a URI matching no registered resource returns JSON-RPC error -32002.",
-        divergence=Divergence(
-            note=(
-                "The spec reserves -32002 for resource-not-found; MCPServer raises ResourceError, which "
-                "the low-level server converts to error code 0."
-            ),
+        behavior=(
+            "resources/read for a URI matching no registered resource returns JSON-RPC error -32602 "
+            "(invalid params) with the requested URI in error.data, per SEP-2164."
         ),
-        arm_exclusions=(ArmExclusion(reason="modern-error-surface", spec_version="2026-07-28"),),
     ),
     # ═══════════════════════════════════════════════════════════════════════════
     # Prompts

tests/interaction/mcpserver/test_resources.py

@@ -112,10 +112,7 @@ def user_profile(user_id: str) -> str:
 
 @requirement("mcpserver:resource:unknown-uri")
 async def test_read_unknown_uri_is_error(connect: Connect) -> None:
-    """Reading a URI that matches no registered resource fails with a JSON-RPC error.
-
-    The spec reserves -32002 for resource-not-found; see the divergence note on the requirement.
-    """
+    """Reading a URI that matches no registered resource fails with -32602 and the URI in data (SEP-2164)."""
     mcp = MCPServer("library")
 
     @mcp.resource("config://app")
@@ -127,16 +124,17 @@ def app_config() -> str:
         with pytest.raises(MCPError) as exc_info:
             await client.read_resource("config://missing")
 
-    assert exc_info.value.error == snapshot(ErrorData(code=0, message="Unknown resource: config://missing"))
+    assert exc_info.value.error == snapshot(
+        ErrorData(code=-32602, message="Unknown resource: config://missing", data={"uri": "config://missing"})
+    )
 
 
 @requirement("mcpserver:resource:read-throws-surfaced")
 async def test_resource_function_that_raises_is_surfaced_as_a_jsonrpc_error(connect: Connect) -> None:
     """An exception raised by a resource function reaches the caller as a JSON-RPC error.
 
-    MCPServer wraps the failure in a generic error that names only the URI, so the original
-    exception text is not leaked to the client. The wrapped exception becomes error code 0 the
-    same way every other unhandled server-side exception does.
+    MCPServer wraps the failure in a generic ResourceError that names only the URI, so the original
+    exception text is not leaked to the client. The wrapped exception surfaces as -32603 Internal error.
     """
     mcp = MCPServer("library")
 
@@ -148,7 +146,9 @@ def boom() -> str:
         with pytest.raises(MCPError) as exc_info:
             await client.read_resource("res://boom")
 
-    assert exc_info.value.error == snapshot(ErrorData(code=0, message="Error reading resource res://boom"))
+    assert exc_info.value.error == snapshot(
+        ErrorData(code=-32603, message="Error reading resource res://boom", data={"uri": "res://boom"})
+    )
 
 
 @requirement("mcpserver:resource:duplicate-name")