Client drops typed additional properties from structured tool results

Finding: In an external report against FastMCP source main (commit e2fcc412001ff7414095c5dc282f94c70299a612; Python 3.12.14, Pydantic 2.13.4, MCP 2.0.0), Client.call_tool().data reportedly drops typed extra fields when a tool returns a Pydantic model that has named properties plus schema-valued additional properties (extra=allow with dict[str,int]). Source: https://github.com/PrefectHQ/fastmcp/issues/5655 Reported symptom: the server advertises an output schema with additionalProperties of type integer and a named 'name' property, and structured_content carries name, hits and misses. The hydrated .data object, however, is Root(name='cache'), and accessing .hits raises AttributeError: 'Root' object has no attribute 'hits'. The call itself succeeds. The reporter says the same loss affects nested and list-wrapped results. Reporter's scoping: mixed objects with additionalProperties: true reportedly keep extra fields, and schema-valued additionalProperties work for dictionaries that have no named properties. The failure is described as occurring only when named properties and schema-valued additional properties are combined. This is the reporter's characterization, not a maintainer-confirmed cause. Possible mitigation (inferred from the reporter's output, not maintainer-endorsed): since the values remain in structured_content, code that needs the extras can read result.structured_content directly rather than relying on result.data hydration. Status: the issue was open at collection time with no comments, no maintainer response, and no linked fix or release in the supplied data. Root cause is UNKNOWN. Limitations: this is a single external report with reporter-provided output; KFA did not reproduce it. Affected released versions are not identified, only the stated main commit. Comment coverage is partial, so later discussion may exist.