MCP tool optional input fields are surfaced in VS Code/Copilot as required

[bseddon]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

Copilot Agent Mode

Body

An MCP server is exposing tools whose input schema fields are optional, but the callable tool signature shown to the model in VS Code/Copilot presents those same fields as if they are all required. This loses semantic information and can push models into worse tool-use behavior.

In a created tool it is registered with optional fields:

inputSchema: {
  schemas: z.array(z.string().min(1)).optional(),
  objectTypes: z.array(z.enum(["table", "view"])).optional(),
  query: z.string().min(1).optional(),
  limit: z.number().int().min(1).max(100).optional(),
}

A definition tool is also registered with optional fields:

inputSchema: {
  schemaName: z.string().min(1).optional(),
  objectName: z.string().min(1).optional(),
  fullName: z.string().min(3).optional(),
}

Expected behavior

The tool contract exposed to the model should preserve optionality.

For the list tool, the model should understand:

all fields are optional
omitting 'query' is valid
omitting 'schemas' is valid

For the definition tool, the model should understand:

'fullName' alone is valid
or 'schemaName' plus 'objectName' is valid

Observed behavior

The live tool surface shown to the model in VS Code/Copilot appears as:

schemas: Array<string>
objectTypes: Array<"table" | "view">
query: string
limit: integer

and

schemaName: string
objectName: string
fullName: string

This presentation makes optional fields look required.

Why this appears to be client-side

The MCP SDK conversion path in
mcp.js uses 'zodToJsonSchema' on the registered Zod schema.

An independent 'zodToJsonSchema' check on the same shape produced JSON Schema with:

'properties'
no top-level required array

That indicates the server-side schema emission is preserving optionality correctly.

Impact

This causes models to over-constrain tool calls and make poorer decisions, for example:

assuming 'schemas' must always be supplied
assuming 'query' must always be supplied
assuming 'fullName', 'schemaName', and 'objectName' must all be supplied together

In practice, this led to unnecessary exploratory failures and forced prompt-side mitigations in descriptions to compensate for the incorrect callable signature.

Expected vs actual

Expected:

Optional JSON Schema fields remain visibly optional in the model-facing tool contract.

Actual:

Optional JSON Schema fields are rendered as plain typed fields with no indication of optionality, effectively appearing required.
Suggested investigation

Please inspect the VS Code/Copilot MCP client layer that transforms 'tools/list' responses into the tool signature shown to the model. The likely issue is in the rendering or intermediate normalization of JSON Schema object properties, where absence of 'required' is not being preserved semantically.

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