fix: return INVALID_PARAMS (-32602) error code for resource-not-found

Previously, when a resource was not found or failed to read, MCPServer
raised ResourceError (an MCPServerError subclass) which was caught by
the generic Exception handler in the low-level server and returned with
error code 0. This made it difficult for clients to programmatically
distinguish resource errors from other failures.

The fix raises MCPError with INVALID_PARAMS (-32602) directly, which is
consistent with the TypeScript SDK and the emerging spec consensus
(modelcontextprotocol/modelcontextprotocol#1545).

Closes #1579

Author: leoneperdigao

src/mcp/server/mcpserver/server.py

@@ -31,7 +31,6 @@
 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.prompts import Prompt, PromptManager
 from mcp.server.mcpserver.resources import FunctionResource, Resource, ResourceManager
 from mcp.server.mcpserver.tools import Tool, ToolManager
@@ -44,6 +43,7 @@
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
+    INVALID_PARAMS,
     Annotations,
     BlobResourceContents,
     CallToolRequestParams,
@@ -439,15 +439,15 @@ async def read_resource(
         try:
             resource = await self._resource_manager.get_resource(uri, context)
         except ValueError:
-            raise ResourceError(f"Unknown resource: {uri}")
+            raise MCPError(code=INVALID_PARAMS, message=f"Unknown resource: {uri}")
 
         try:
             content = await resource.read()
             return [ReadResourceContents(content=content, mime_type=resource.mime_type, meta=resource.meta)]
         except Exception as exc:
             logger.exception(f"Error getting resource {uri}")
             # If an exception happens when reading the resource, we should not leak the exception to the client.
-            raise ResourceError(f"Error reading resource {uri}") from exc
+            raise MCPError(code=INVALID_PARAMS, message=f"Error reading resource {uri}") from exc
 
     def add_tool(
         self,

tests/issues/test_141_resource_templates.py

@@ -2,7 +2,7 @@
 
 from mcp import Client
 from mcp.server.mcpserver import MCPServer
-from mcp.server.mcpserver.exceptions import ResourceError
+from mcp.shared.exceptions import MCPError
 from mcp.types import (
     ListResourceTemplatesResult,
     TextResourceContents,
@@ -55,10 +55,10 @@ def get_user_profile_missing(user_id: str) -> str:  # pragma: no cover
     assert result_list[0].mime_type == "text/plain"
 
     # Verify invalid parameters raise error
-    with pytest.raises(ResourceError, match="Unknown resource"):
+    with pytest.raises(MCPError, match="Unknown resource"):
         await mcp.read_resource("resource://users/123/posts")  # Missing post_id
 
-    with pytest.raises(ResourceError, match="Unknown resource"):
+    with pytest.raises(MCPError, match="Unknown resource"):
         await mcp.read_resource("resource://users/123/posts/456/extra")  # Extra path component
 
 

tests/issues/test_1579_resource_error_code.py

@@ -0,0 +1,51 @@
+"""Tests for issue #1579: FastMCP read_resource() returns incorrect error code.
+
+FastMCP previously returned error code 0 for resource-not-found because
+ResourceError (an MCPServerError subclass) was caught by the generic
+Exception handler in the low-level server, which defaults to code 0.
+
+The fix adds a dedicated handler for MCPServerError that maps it to
+INVALID_PARAMS (-32602), consistent with the TypeScript SDK and the
+emerging spec consensus.
+"""
+
+import pytest
+
+from mcp.client import Client
+from mcp.server.mcpserver import MCPServer
+from mcp.shared.exceptions import MCPError
+from mcp.types import INVALID_PARAMS
+
+pytestmark = pytest.mark.anyio
+
+
+async def test_unknown_resource_returns_invalid_params_error_code():
+    """Reading an unknown resource returns INVALID_PARAMS (-32602), not 0."""
+    mcp = MCPServer()
+
+    @mcp.resource("resource://exists")
+    def existing_resource():
+        return "data"
+
+    async with Client(mcp) as client:
+        with pytest.raises(MCPError) as exc_info:
+            await client.read_resource("resource://does-not-exist")
+
+        assert exc_info.value.code == INVALID_PARAMS
+        assert "Unknown resource" in exc_info.value.message
+
+
+async def test_resource_read_error_returns_invalid_params_error_code():
+    """A resource that raises during read returns INVALID_PARAMS (-32602)."""
+    mcp = MCPServer()
+
+    @mcp.resource("resource://failing")
+    def failing_resource():
+        raise RuntimeError("something broke")
+
+    async with Client(mcp) as client:
+        with pytest.raises(MCPError) as exc_info:
+            await client.read_resource("resource://failing")
+
+        assert exc_info.value.code == INVALID_PARAMS
+        assert "Error reading resource" in exc_info.value.message