Percent-encoded local references lose hydrated types in retained output schemas

Finding: In an external report against FastMCP main (commit 5baeacfe20eca735cb949564b4915f93a622b916, Python 3.12.13, MCP 2.0.0, macOS 27.2 arm64, source checkout), a tool created with dereference_schemas=False and an output schema containing a percent-encoded local reference (#/$defs/Observation%20time) returns a date-time field as a plain string from Client.call_tool(...).data, rather than a datetime. Source: https://github.com/PrefectHQ/fastmcp/issues/5560 Reported details: the author's constructed example defines $defs key "Observation time" with type string and format date-time, and references it as Observation%20time, using the URI-fragment encoding from RFC 6901 section 6. The author expected the documented structured-result hydration to preserve the datetime type. The author prints type(result.data.observed_at) and gets str. The author also says the same example hydrates a datetime when schema dereferencing is left at its default (enabled). Source: https://github.com/PrefectHQ/fastmcp/issues/5560 Status: The issue was open when captured, with no maintainer response, no linked fix PR, and no release information in the supplied material. The root cause is UNKNOWN. A plausible hypothesis is that the retained-schema path does not percent-decode the fragment before resolving the pointer, but this is a model hypothesis, not evidence from the source. Practical reading: The only source-backed contrast is that default dereferencing reportedly hydrates correctly. That is an observation from the reporter, not a confirmed workaround for users who need dereference_schemas=False. Whether released packages are affected is unknown, since the report used a source checkout of main. Limitations: This is an external report of a constructed example, not a production incident. The author mentions AI assistance while investigating. KFA did not reproduce it. Comment coverage is partial and may omit middle comments. Freshness: updated 2026-10-06, within 14 days but older than 72 hours.