Validating Tool Arguments against inputSchema

The problem

Every MCP (Model Context Protocol) tool declares an inputSchema: a description, in JSON (JavaScript Object Notation) Schema, of the arguments it accepts. A language model fills in the arguments, and it can get them wrong: a missing field, a number where a text is expected, a value that is not allowed. So the server must check the arguments before calling the tool, and, if they are wrong, tell the model what to fix. You will write the checker and the call.

The subset of JSON Schema

For an object schema:

"type": "object" at the top, with "properties" mapping names to {"type": ...}. The type is one of "string", "number" (int or float), "integer" (int only), "boolean", "array", "object". A property may also have "enum": a list of allowed values.
"required": names that must be present.
"additionalProperties": false: no other names allowed (when absent, extra names are allowed).

Validation

validate(arguments, schema) returns a list of error strings, empty if valid, in this order:

"missing required property '<name>'" for each required name, in required order;
then, for each argument in its own order, one of: "unexpected property '<name>'", "property '<name>' must be <type>" or "property '<name>' must be one of <enum list>".

In JSON Schema true is not a number: a bool matches only "boolean" (in Python True is an int, so check for it).

Example. The schema: an object with location (a string, required), days (an integer) and units (a string, one of "c" and "f"), and additionalProperties false.

{"days": 2.5, "units": "k", "x": 1} gives, in order: missing required property 'location', property 'days' must be integer, property 'units' must be one of ['c', 'f'], unexpected property 'x'.

{"location": "Paris", "days": 2} gives []: valid.

{"location": "Paris", "days": True} gives property 'days' must be integer.

Calling a tool

call_tool(tools, request): tools maps names to (schema, fn) pairs; request is a tools/call request. Return the JSON-RPC response:

an unknown tool is a protocol error: {"jsonrpc": "2.0", "id": <id>, "error": {"code": -32602, "message": "Unknown tool: <name>"}};
invalid arguments are a tool execution error: a result with "isError": True and one text content "Invalid arguments: " + "; ".join(errors), so the model can read what to fix;
valid arguments: call fn(**arguments) and return its str() as text with "isError": False.

Results carry "resultType": "complete" and the shape {"jsonrpc": "2.0", "id": ..., "result": {"resultType": "complete", "content": [{"type": "text", "text": ...}], "isError": ...}}.

Example. The tool weather with the schema above, called with {"location": "Paris", "days": "two"}: the result has isError true and the text Invalid arguments: property 'days' must be integer. The tool function is not called.

What to implement

validate(arguments, schema) and call_tool(tools, request).

Requirements

Never call the tool with invalid arguments.