[Bug]: tools/call without `arguments` is rejected with -32602 even when the tool has no required fields

[ondraulehla]

Server type

Hosted (mcp.apify.com) and local (stdio via @apify/actors-mcp-server)

MCP Client

Other: raw JSON-RPC 2.0 probes (newline-delimited over stdio, plain POSTs
over streamable HTTP), no MCP client library, so the reproduction below is
exact requests

Operating System

macOS

What happened?

tools/call with the arguments key omitted is rejected with JSON-RPC
-32602 before validation runs. The MCP spec marks
CallToolRequest.params.arguments as optional (the SDK's
CallToolRequestSchema has arguments: z.ZodOptional<...>), so omitting
the key for a tool with no required fields is spec-conformant, but every
such call fails. search-actors declares "required": [] and every field
has a default; the identical call with arguments: {} succeeds.

I reproduced this on hosted mcp.apify.com (serverInfo 0.15.4, streamable
HTTP), on the published npm 0.15.4 over stdio, and on master 4bf6ecb
over stdio. All three behave the same:

Call	Result
search-actors, no arguments key	-32602 "Missing arguments for tool"
search-actors, arguments: {}	ok, 5 Actors
fetch-actor-details, no arguments key	-32602 "Missing arguments for tool"
fetch-actor-details, arguments: {}	-32602 "root: must have required property 'actor'"

The fourth row shows a secondary cost: for a tool that does have required
fields, the blanket guard masks the specific validation message that #337
standardized. With {} the client learns which property is missing; without
the key it gets the generic text plus the fetch-actor-details pointer,
which is dead advice for internal tools (that pointer is #1214).

The guard is if (!args) in src/mcp/tool_call_engine.ts:171, surfaced as
McpError(InvalidParams) in src/mcp/legacy_server.ts:469. It predates the
shared validation (git log -S traces it to #28), and both protocol
adapters run it through prepareToolCall (legacy_server.ts:426,
stateless_server.ts:213); I verified the stateless path through the unit
harness with the same result.

This is not a synthetic client shape. A stock mark3labs/mcp-go v1.0.0
client with Arguments unset sends (captured on the wire via tee):

{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"search-actors"}}

because the field is declared with json:"arguments,omitempty", and every
such call fails with the -32602 above; with an empty map it succeeds.
(The official modelcontextprotocol/go-sdk is unaffected: its CallTool
normalizes unset arguments to {}.) On 0.15.4, four tools declare no
required fields and each fails this way: search-actors,
get-actor-run-list, get-dataset-list, get-key-value-store-list.

Steps to reproduce

1. Start the server with no token:
   node dist/stdio.js --tools search-actors,fetch-actor-details --telemetry-enabled false
   (env APIFY_TOKEN="").
2. Complete initialize and send notifications/initialized.
3. Send {"method":"tools/call","params":{"name":"search-actors"}} (no
   arguments key).
4. Send the same call with "arguments": {}.

Expected result

The two calls behave the same: an omitted arguments validates as an empty
object, so search-actors runs with its defaults, and a tool with required
fields returns the specific -32602 from the shared validation.

Actual result

Step 3 returns:

{"code":-32602,"message":"MCP error -32602: Missing arguments for tool \"search-actors\".\nPlease provide the required arguments for this tool. Check the tool's input schema using fetch-actor-details tool to see what parameters are required."}

Step 4 returns the search results (5 Actors).

Node.js version

v24.2.0

Apify MCP Server version

Hosted mcp.apify.com (serverInfo 0.15.4), published npm 0.15.4, and
master 4bf6ecb built from source

MCP server configuration

{
  "command": "node",
  "args": ["dist/stdio.js", "--tools", "search-actors,fetch-actor-details", "--telemetry-enabled", "false"],
  "env": { "APIFY_TOKEN": "", "TELEMETRY_ENABLED": "false" }
}

Error logs

No transport or server error. The failure is the JSON-RPC error response
above.

Additional context

Defaulting an absent arguments to {} before validation would fix both
rows at once: no-required tools run, and required-field tools get the
specific validation error from #337.

AI assistance: I used Claude Code to search the codebase and to script and
run the JSON-RPC probes. I reviewed the requests, the cited source lines,
and this text myself; every result above comes from those runs.