fix: accept omitted arguments for zero-arg web_search_help prompt

[serhiizghama]

Problem

The web_search_help prompt declares no arguments, but prompts/get fails when a client omits params.arguments:

MCP error -32602: Invalid arguments for prompt web_search_help:
Invalid input: expected object, received undefined

Root cause is in src/mcp-handler.ts, where the prompt is registered with an empty args schema:

server.prompt(
  "web_search_help",
  "Get help with web search using Exa",
  {},            // ← empty args schema
  async () => { ... }
);

Passing {} selects the SDK's prompt(name, description, argsSchema, cb) overload. The SDK then builds z.object({}) and, in the prompts/get handler, runs argsSchema.parse(request.params.arguments). When arguments is omitted, that becomes z.object({}).parse(undefined) → "expected object, received undefined".

This breaks spec-compliant clients that drop empty maps. For example the official Go SDK:

type GetPromptParams struct {
  Arguments map[string]string `json:"arguments,omitempty"`
  Name      string            `json:"name"`
}

A nil/empty map is omitted under omitempty, so a zero-argument prompt request is serialized without arguments and is rejected. Per the MCP spec params.arguments is optional, so a no-argument prompt should accept its absence.

Fixes #358.

Solution

Drop the empty args schema so registration uses the prompt(name, description, cb) overload. With no args schema the SDK skips argument validation entirely (if (prompt.argsSchema) is false → the no-arg callback branch runs), and prompts/get succeeds whether arguments is omitted or {}.

The prompt takes no arguments, so removing the empty schema has no functional downside — there is nothing to validate.

Testing

• npm test — 91/91 pass, including a new regression assertion that web_search_help is registered without an args schema and that its handler resolves with no arguments.
• npm run typecheck — clean.
• npm run build — clean.

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[serhiizghama]

Rebased on main — only a comment conflict where the note landed next to the prompt registration. The fix (dropping the empty {} args schema so omitted arguments are accepted) and its tests are unchanged. 5/5 green.