MCP tools/list flattens anyOf array-or-null filter schema to scalar string

[alexfazio]

Minimal repro

Tool: flywheel_list_nodes

Input used by affected clients:

• owners: "me"

Expected canonical shape:

• owners: ["me"]

Observed behavior:

• Client flattens array-or-null schema (anyOf: [{"type":"array"}, {"type":"null"}]) into scalar-string guidance.
• Server rejects scalar with list validation (owners must be an array of strings).

Why this matters

This causes first-call tool failures and requires users/agents to manually discover array wrapping despite correct server schema.

Request

Please preserve array-or-null input schema shape in surfaced MCP tool manifests/instructions for list filters.

[github-actions]

Potential duplicates detected. Please review them and close your issue if it is a duplicate.

• MCP tool schema mismatch: structured args are surfaced as strings for Hindsight retain #16841

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