Function diagnose_args 

pub fn diagnose_args(
    tool: &str,
    args: &str,
    required_modes: &[&[&str]],
    example: &str,
) -> Result<Value, String>

Model-friendly tool-arguments validator.

Why this exists: serde’s “missing field X at line 1 column 793” error reads to weak models (GLM-5.1, Qwen) as a parser-position complaint and reliably triggers hallucinated “fixes” like “I should use positional arguments” — wasting a turn or six on the same tool call. See datalog atomgr-2d99b47d/2026-05-06_08-43-12.md Turns 64–75 for the failure mode this replaces.

What it returns instead, on failure:

• the keys the model actually provided
• the keys it’s missing for the closest mode
• a one-line example of a correct call

required_modes is a list of accepted key sets — any one fully matched passes. Single-mode tools pass &[&[required_keys]]. Multi-mode tools like edit_file pass one slice per mode; the diagnostic picks the mode with the fewest missing keys for the hint.

Returns the parsed Value on success so callers can avoid a second parse pass.