Trait ArgumentValidator 

pub trait ArgumentValidator: Send + Sync {
    // Required method
    fn validate(&self, args: &Value) -> Result<(), ArgumentRefusal>;
}

A per-tool rule about a COMBINATION of argument values, run strictly AFTER the declared inputSchema check (Phase 128, E2).

Ordering is the contract

A validator NEVER sees arguments that failed the declared schema. It runs inside the same decorator, after the schema check returns Ok and before the inner handler. tests/request_policy.rs’s counting row fails if that order inverts.

It also survives the schema opt-out: with [server.validation] enforce_input_schema = false the decorator skips only the SCHEMA CHECK, and a registered validator still runs. Turning off one enforcement must never silently turn off another.

Refuse-only: this signature cannot normalize

validate takes &Value and returns a refusal, so it cannot mutate the arguments. Normalizing an argument before dispatch — rewriting a code, casing a string — is therefore OUT OF SCOPE for this signature. That is a deliberate restriction, not an omission: a mutating validator has a different contract (idempotency, what the tool’s published schema then describes, and whether the schema check should re-run on the rewritten value), and that contract is recorded as an open question rather than settled by the shape of a first signature.

Example

use pmcp_server_toolkit::{ArgumentRefusal, ArgumentValidator};
use serde_json::Value;

struct EndAfterStart;

impl ArgumentValidator for EndAfterStart {
    fn validate(&self, args: &Value) -> Result<(), ArgumentRefusal> {
        let start = args.get("start").and_then(Value::as_i64);
        let end = args.get("end").and_then(Value::as_i64);
        match (start, end) {
            (Some(s), Some(e)) if e < s => Err(ArgumentRefusal::new(
                "`end` must not precede `start`",
            )),
            _ => Ok(()),
        }
    }
}

Required Methods

fn validate(&self, args: &Value) -> Result<(), ArgumentRefusal>

Allow or refuse one call’s already-schema-valid arguments.

Errors

Return ArgumentRefusal to refuse. The inner handler is then never invoked and no backend request is made.

Dyn Compatibility

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors