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Remote Methods and Arguments
Last update:
May 18, 2026
Each tool is a remote callable method: the client sends a JSON payload with a tool name and arguments, and the MCP server executes the tool implementation and returns a structured result.
Overview
Each tool is a remote callable method: the client sends a JSON payload with a tool name and arguments, and the MCP server executes the tool implementation and returns a structured result.
Tool Call Structure
A typical tool call over MCP looks like:
{
"jsonrpc": "2.0",
"id": 42,
"method": "tools/call",
"params": {
"name": "getWeather",
"arguments": {
"city": "London",
"units": "metric"
}
}
}
The server responds with:
{
"jsonrpc": "2.0",
"id": 42,
"result": {
"content": [
{
"type": "text",
"text": "Current weather in London: 16°C, cloudy"
}
],
"isError": false
}
}
Defining Arguments
Arguments are defined in the tool’s
inputSchema. Users and agents rely on this schema to construct valid calls. You can expect:
- Required vs optional fields – The schema will specify which arguments are mandatory and which are optional.
- Types – Strings, numbers, booleans, arrays, and objects are clearly declared.
- Descriptions – Each argument should include a short, human-readable description.
Example input schema:
"inputSchema": {
"type": "object",
"properties": {
"city": {
"type": "string",
"description": "City name or postal code"
},
"units": {
"type": "string",
"description": "Temperature units (metric or imperial)",
"enum": ["metric", "imperial"]
}
},
"required": ["city"]
}
What Happens with Invalid Arguments
If arguments do not match the schema:
- The server will reject the call with a tool-level error, not a crash.
- The response will indicate
isError: trueand include an explanation such as:- Missing required field
- Invalid type
- Unsupported enum value
Users should expect:
- Clear error messages for incorrect inputs, not silent failures.
- No partial side effects for rejected calls (inputs that fail validation should not update anything).
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