fix: return resource-not-found code from MCPServer

src/mcp/server/mcpserver/exceptions.py

@@ -13,6 +13,10 @@ class ResourceError(MCPServerError):
     """Error in resource operations."""
 
 
+class ResourceNotFoundError(ResourceError):
+    """Requested resource does not exist."""
+
+
 class ToolError(MCPServerError):
     """Error in tool operations."""
 

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 +44,7 @@
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
+    RESOURCE_NOT_FOUND,
     Annotations,
     BlobResourceContents,
     CallToolRequestParams,
@@ -341,7 +342,10 @@ 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 exc:
+            raise MCPError(RESOURCE_NOT_FOUND, str(exc)) from exc
         contents: list[TextResourceContents | BlobResourceContents] = []
         for item in results:
             if isinstance(item.content, bytes):
@@ -448,7 +452,7 @@ async def read_resource(
         try:
             resource = await self._resource_manager.get_resource(uri, context)
         except ValueError as exc:
-            raise ResourceError(f"Unknown resource: {uri}") from exc
+            raise ResourceNotFoundError(f"Unknown resource: {uri}") from exc
 
         try:
             content = await resource.read()

src/mcp/types/__init__.py

@@ -153,6 +153,7 @@
     METHOD_NOT_FOUND,
     PARSE_ERROR,
     REQUEST_TIMEOUT,
+    RESOURCE_NOT_FOUND,
     URL_ELICITATION_REQUIRED,
     ErrorData,
     JSONRPCError,
@@ -320,6 +321,7 @@
     "METHOD_NOT_FOUND",
     "PARSE_ERROR",
     "REQUEST_TIMEOUT",
+    "RESOURCE_NOT_FOUND",
     "URL_ELICITATION_REQUIRED",
     "ErrorData",
     "JSONRPCError",

src/mcp/types/jsonrpc.py

@@ -37,6 +37,9 @@ class JSONRPCResponse(BaseModel):
 
 
 # MCP-specific error codes in the range [-32000, -32099]
+RESOURCE_NOT_FOUND = -32002
+"""Error code indicating that a requested resource does not exist."""
+
 URL_ELICITATION_REQUIRED = -32042
 """Error code indicating that a URL mode elicitation is required before the request can be processed."""
 

tests/interaction/_requirements.py

@@ -901,12 +901,6 @@ 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."
-            ),
-        ),
     ),
     # ═══════════════════════════════════════════════════════════════════════════
     # Prompts

tests/interaction/mcpserver/test_resources.py

@@ -6,6 +6,7 @@
 from mcp import MCPError
 from mcp.server.mcpserver import MCPServer
 from mcp.types import (
+    RESOURCE_NOT_FOUND,
     ErrorData,
     ListResourcesResult,
     ListResourceTemplatesResult,
@@ -114,7 +115,7 @@ def user_profile(user_id: str) -> str:
 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.
+    The spec reserves -32002 for resource-not-found.
     """
     mcp = MCPServer("library")
 
@@ -127,7 +128,9 @@ 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=RESOURCE_NOT_FOUND, message="Unknown resource: config://missing")
+    )
 
 
 @requirement("mcpserver:resource:read-throws-surfaced")

tests/server/mcpserver/test_server.py

@@ -19,6 +19,7 @@
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
+    RESOURCE_NOT_FOUND,
     AudioContent,
     BlobResourceContents,
     Completion,
@@ -730,9 +731,11 @@ async def test_read_unknown_resource(self):
         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 == RESOURCE_NOT_FOUND
+
     async def test_read_resource_error(self):
         """Test that resource read errors are properly wrapped in MCPError."""
         mcp = MCPServer()
@@ -742,9 +745,11 @@ def failing_resource():
             raise ValueError("Resource read failed")
 
         async with Client(mcp) as client:
-            with pytest.raises(MCPError, match="Error reading resource resource://failing"):
+            with pytest.raises(MCPError, match="Error reading resource resource://failing") as exc_info:
                 await client.read_resource("resource://failing")
 
+        assert exc_info.value.error.code == 0
+
     async def test_binary_resource(self):
         mcp = MCPServer()