$ref and $defs in tool schemas not dereferenced before sending

6/10 Medium

Tool schemas with JSON Schema references ($ref, $defs) were not being inlined before being sent to MCP clients, violating spec requirements and causing client incompatibilities.

MCP
Category
compatibility
Workaround
solid
Stage
debug
Freshness
declining
Scope
framework
Upstream
open
Recurring
No
Buyer Type
team
Maintainer
active

Sources

Collection History

Query: “What are the most common pain points with FastMCP for developers in 2025?”4/8/2026

v2.14.4 backported `dereference_refs()` but never wired it into the tool schema pipeline — `$ref` and `$defs` were still sent to MCP clients. Now fixed: schemas are fully inlined before reaching clients.

Created: 4/8/2026Updated: 4/8/2026