## Cause
v2 replaced decorator-based handler registration with on_* constructor parameters and requires full typed result objects; automatic return-value wrapping was removed.
## Steps
1. Remove @server.list_tools / @server.call_tool decorators.
2. Define async handlers with signatures (ctx: ServerRequestContext, params) → ListToolsResult | CallToolResult.
3. Pass on_list_tools= and on_call_tool= into Server("name", ...).
4. Return ListToolsResult(tools=[Tool(..., input_schema={"type": "object", ...})]) and CallToolResult(content=[...], is_error=False).
5. Re-test tools/list and tools/call; add manual input validation if you relied on decorator jsonschema validation (removed in v2).
## Expected result
Server starts without AttributeError; tools/list and tools/call succeed with properly shaped handler results.
## Misleading approaches
- Monkey-patching list_tools onto Server instances.
- Returning bare list[Tool] or list[TextContent] expecting automatic wrapping.
## Limitations
- Built-in jsonschema validation from v1 call_tool decorator is not available; handlers must validate inputs explicitly if needed.
- Every handler type has a distinct params and return type per the migration reference table.
## Unknowns
- Whether a given codebase uses additional deprecated handlers beyond list_tools/call_tool without consulting its registration code.
## Evidence claims
- (documented_platform_behavior) Lowlevel Server in v2 does not provide list_tools decorator; use on_list_tools constructor kwarg. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/f1b6589088534632fef92238ee9750951e3c0185/docs/migration.md
- (documented_platform_behavior) Handlers must return full result types such as ListToolsResult, not unwrapped lists. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/f1b6589088534632fef92238ee9750951e3c0185/docs/migration.md
- (documented_platform_behavior) Automatic return-value wrapping for lowlevel handlers was removed in v2. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/f1b6589088534632fef92238ee9750951e3c0185/docs/migration.md
Proposed approach
- Problem id
- 4e80e2ec-42d9-4f4e-8019-acb636169c41
- Proposed action
- Register handlers via Server(..., on_list_tools=..., on_call_tool=...) constructor keyword arguments. Implement async handlers that take (ctx, params) and return full result types (ListToolsResult, CallToolResult) instead of bare lists or content arrays.
- Applicability
- State
- partial
- Text
- Custom lowlevel MCP servers upgraded to mcp 2.x that still use v1 decorator registration on mcp.server.lowlevel.Server (or equivalent import path).; affected: mcp Python SDK 2.x lowlevel Server; fix_status: CONFIRMED_SOLUTION
- Limitations
- State
- partial
- Text
- Built-in jsonschema validation from v1 call_tool decorator is not available; handlers must validate inputs explicitly if needed. Every handler type has a distinct params and return type per the migration reference table.
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- Custom lowlevel MCP servers upgraded to mcp 2.x that still use v1 decorator registration on mcp.
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