MCP error -32602: missing field 'id' when deferred-schema tool arguments contain non-ASCII UTF-8 characters

Summary

When invoking a deferred-schema MCP tool (one that requires ToolSearch before use) with parameter values containing non-ASCII UTF-8 characters (German umlauts, €, §, etc.), the entire arguments object is dropped from the JSON-RPC tools/call request. The MCP server receives arguments: null, which it correctly reports as a missing required field — producing the misleading error:

MCP error -32602: failed to deserialize parameters: missing field `id`

Environment

• Client: Claude Code (claude-sonnet-4-6)
• MCP server: mcp__paperless (stdio transport, Rust/rmcp 1.7.0)
• Tool schema loading: Deferred — schema fetched via ToolSearch before invocation

Steps to reproduce

1. Use a tool that requires ToolSearch to load its schema (a deferred tool).
2. Invoke it with a string parameter value containing non-ASCII characters, e.g.:

{
  "id": 7976,
  "custom_fields": [
    {"field": 5, "value": "Verspätungszuschläge zur Gewerbesteuer (925,00 €) nach § 227 AO"}
  ]
}

Expected: Tool executes normally; UTF-8 string stored verbatim.

Actual:

MCP error -32602: failed to deserialize parameters: missing field `id`

1. Repeat the identical call with all non-ASCII characters replaced by ASCII equivalents:

{
  "id": 7976,
  "custom_fields": [
    {"field": 5, "value": "Verspaetungszuschlaege zur Gewerbesteuer (925,00 EUR) nach Paragraph 227 AO"}
  ]
}

Result: Call succeeds.

Root cause analysis

Traced the server-side call chain in rmcp 1.7.0 (handler/server/tool.rs:186):

let arguments = context.arguments.take().unwrap_or_default();
serde_json::from_value(serde_json::Value::Object(arguments)).map_err(|e| {
    ErrorData::invalid_params(format!("failed to deserialize parameters: {}", e), None)
})

The error missing field 'id' is produced when arguments is None (no arguments received), which unwrap_or_default() converts to {}. Deserializing {} into a struct with a required id field produces exactly this error.

The server receives a syntactically valid JSON-RPC message, but with arguments: null or the arguments key absent. The server-side Rust/serde_json/rmcp stack handles UTF-8 correctly and is not the cause.

The defect is in the client-side deferred-schema tool invocation path: when Claude Code constructs the tools/call JSON-RPC message after fetching a schema via ToolSearch, the arguments serialization step drops the parameter object entirely if any string value contains multi-byte UTF-8 codepoints.

This does not reproduce with non-deferred tools (tools whose schema is available without a prior ToolSearch call).

Impact

Any document or entity whose natural-language metadata contains non-ASCII text (German, French, Spanish, CJK, currency symbols, legal symbols, etc.) cannot be updated via deferred MCP tools without manual ASCII transliteration. This is a significant regression for non-English workflows.

Workaround

Replace all non-ASCII characters with ASCII equivalents before passing string values. Functional but typographically incorrect.