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Make $ref dereferencing optional via FastMCP(dereference_refs=...) (#3151)

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Co-authored-by: marvin-context-protocol[bot] <225465937+marvin-context-protocol[bot]@users.noreply.github.com>
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Jeremiah Lowin 2026-02-11 13:45:37 -05:00 • committed by GitHub
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---
title: dereference
sidebarTitle: dereference
---
# `fastmcp.server.middleware.dereference`
Middleware that dereferences $ref in JSON schemas before sending to clients.
## Classes
### `DereferenceRefsMiddleware` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/middleware/dereference.py#L15" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
Dereferences $ref in component schemas before sending to clients.
Some MCP clients (e.g., VS Code Copilot) don't handle JSON Schema $ref
properly. This middleware inlines all $ref definitions so schemas are
self-contained. Enabled by default via ``FastMCP(dereference_schemas=True)``.
**Methods:**
#### `on_list_tools` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/middleware/dereference.py#L24" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
on_list_tools(self, context: MiddlewareContext[mt.ListToolsRequest], call_next: CallNext[mt.ListToolsRequest, Sequence[Tool]]) -> Sequence[Tool]
```
#### `on_list_resource_templates` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/middleware/dereference.py#L33" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
on_list_resource_templates(self, context: MiddlewareContext[mt.ListResourceTemplatesRequest], call_next: CallNext[mt.ListResourceTemplatesRequest, Sequence[ResourceTemplate]]) -> Sequence[ResourceTemplate]
```