agentvet

Validate LLM-generated tool args before execution. Wrap your tools with a schema; throws ToolArgError before the tool runs when the model hallucinates wrong types or misses required fields. Carries a structured message you can feed back to the model as a tool_result for next-turn correction. Zero runtime dependencies.

npm install @mukundakatta/agentvet

import { vet, adapters, ToolArgError } from '@mukundakatta/agentvet';
import { z } from 'zod';

const searchSchema = z.object({
  query: z.string(),
  limit: z.number().int().min(1).max(100),
});

const search = vet({
  name: 'search',
  schema: adapters.zod(searchSchema),
  fn: async ({ query, limit }) => doSearch(query, limit),
});

// Inside your agent loop, when the LLM emits a tool_use block:
try {
  const result = await search(toolUse.input);
  // send `result` back as tool_result
} catch (err) {
  if (err instanceof ToolArgError) {
    // send `err.toLLMFeedback()` back as tool_result with is_error: true
    // The model sees what went wrong and retries with corrected args
  }
}

If the model passes search({ query: 42 }), the tool never runs — the side effect is prevented before it can happen, and the LLM gets a structured retry hint.

TypeScript types ship in the box.

See it in action

git clone https://github.com/MukundaKatta/agentvet && cd agentvet
node examples/demo-reject.js

A simulated agent loop where the model hallucinates a number for a string arg, agentvet rejects it before book_flight runs, feedback is sent back, and the model "corrects" on retry. The dangerous side effect (booking) only happens with validated args.

Why

LLMs hallucinate tool arguments. They pass:

• A number where a string was expected (flight_id: 12345 instead of "UA123")
• A string where an enum was expected (seat: "window" when the enum is 'window' | 'aisle' | 'middle')
• Forget required fields entirely
• Pass extra fields that confuse downstream code

If your tool runs anyway, it either crashes deep in your code (with a stack trace the LLM can't act on) or — worse — it actually does something with the bad args. Booking the wrong flight. Sending the wrong email. Charging the wrong card.

vet() makes the tool refuse before any side effect, with an error message designed to feed back to the model so the next turn can correct itself.

API

vet({name, schema, fn, onError?}) → wrapped fn

Wrap a tool function with arg validation.

const wrapped = vet({
  name: 'search',           // required: tool name (surfaces in errors)
  schema: validator,        // required: validator function
  fn: async (args) => ...,  // required: the underlying tool
  onError: (err, args) => undefined, // optional: see below
});

If onError returns a non-undefined value, that value is used as the tool's return value (suppresses the throw). Useful for falling back to a default response instead of erroring up the agent loop.

validate(name, schema, args) → { valid, value | error }

One-shot check without wrapping a tool. Useful for inline validation or when you want to inspect the error before deciding what to do.

const r = validate('search', adapters.zod(schema), userArgs);
if (!r.valid) console.error(r.error.toLLMFeedback());

adapters.zod(schema)

Bridge for any validator with a safeParse() method (zod, valibot, etc.).

import { z } from 'zod';
const validate = adapters.zod(z.object({ query: z.string() }));

adapters.fn(predicate, errorBuilder?)

Ad-hoc predicate validation, no schema lib needed.

const validate = adapters.fn(
  (a) => typeof a?.query === 'string' && a.query.length > 0,
  'query must be a non-empty string'
);

adapters.shape(spec)

Tiny built-in shape checker for zero-dep end-to-end validation.

const validate = adapters.shape({
  query: 'string',
  limit: 'number?',  // suffix '?' for optional
  filters: 'object?',
});

Not a full JSON Schema validator — just enough to gate basic tool-arg shapes.

ToolArgError

import { ToolArgError } from '@mukundakatta/agentvet';

try {
  await tool(args);
} catch (err) {
  if (err instanceof ToolArgError) {
    err.tool;            // tool name
    err.validationError; // string from the validator
    err.args;            // the rejected args
    err.message;         // human-readable
    err.toLLMFeedback(); // string formatted for the LLM to read + retry with
  }
}

Recipes

Anthropic SDK tool loop

import Anthropic from '@anthropic-ai/sdk';
import { vet, adapters, ToolArgError } from '@mukundakatta/agentvet';

const tools = {
  search: vet({
    name: 'search',
    schema: adapters.shape({ query: 'string', limit: 'number?' }),
    fn: async ({ query, limit = 10 }) => api.search(query, limit),
  }),
};

const client = new Anthropic();
const messages = [{ role: 'user', content: 'find me coffee shops' }];

while (true) {
  const r = await client.messages.create({ model: 'claude-sonnet-4-6', tools: schemas, messages, max_tokens: 1024 });
  if (r.stop_reason !== 'tool_use') break;

  const toolUses = r.content.filter((b) => b.type === 'tool_use');
  const results = await Promise.all(toolUses.map(async (use) => {
    try {
      const data = await tools[use.name](use.input);
      return { type: 'tool_result', tool_use_id: use.id, content: JSON.stringify(data) };
    } catch (err) {
      if (err instanceof ToolArgError) {
        return {
          type: 'tool_result',
          tool_use_id: use.id,
          content: err.toLLMFeedback(),
          is_error: true,
        };
      }
      throw err;
    }
  }));

  messages.push({ role: 'assistant', content: r.content });
  messages.push({ role: 'user', content: results });
}

Compose with agentsnap's traceTool

import { traceTool } from '@mukundakatta/agentsnap';
import { vet, adapters } from '@mukundakatta/agentvet';

const search = traceTool('search', vet({
  name: 'search',
  schema: adapters.shape({ query: 'string' }),
  fn: async ({ query }) => api.search(query),
}));

// Now: invalid args are rejected (vet) and the rejection is recorded (traceTool).
// Inside record(), you'll see the failed tool attempt in the trace.

Fallback instead of throwing

const summarize = vet({
  name: 'summarize',
  schema: adapters.shape({ text: 'string' }),
  fn: async ({ text }) => llm.summarize(text),
  onError: (err, args) => ({ summary: '', error: err.validationError }),
});

CLI

@mukundakatta/agentvet ships an agentvet binary for one-off validation and lint checks (handy when sanity-checking an LLM tool definition or eyeballing a single tool_use blob):

# Validate a JSON args blob against a shape (exit 1 if invalid)
echo '{"query":42}' | npx -p @mukundakatta/agentvet agentvet validate - \
  --shape '{"query":"string","limit":"number"}' --name search --pretty

# Lint a tool definition (exits 1 on hard errors like missing input_schema)
npx -p @mukundakatta/agentvet agentvet lint tool.json --pretty

Output is JSON to stdout (use --pretty for indented). When validation fails, the JSON includes a retry_hint you can pipe straight back to the model as tool_result content with is_error: true. Exit code is 0 when valid, 1 when invalid, 2 on usage errors. Run agentvet --help for the full subcommand reference.

What this is not

• Not an output validator. This validates what the LLM passes IN to a tool. For validating what the LLM returns at the end of a generation, use @mukundakatta/agentcast.
• Not a network firewall. This stops bad args from reaching your function. For stopping bad URLs from reaching the wire, use @mukundakatta/agentguard.
• Not a schema generator. It runs your existing schema as a validator; it doesn't generate the JSON Schema you pass to the model. Pair with zod-to-json-schema (or your tool format generator of choice).

Sibling libraries

Part of the agent reliability stack — all @mukundakatta/* scoped, all zero-dep:

• @mukundakatta/agentfit — fit messages to budget. Fit it.
• @mukundakatta/agentsnap — snapshot tests for tool-call traces. Test it.
• @mukundakatta/agentguard — network egress firewall. Sandbox it.
• @mukundakatta/agentvet — tool-arg validator. Vet it.
• @mukundakatta/agentcast — structured output enforcer. Validate it.

Natural pipeline: fit the prompt → guard the network → snap the run → vet the tool args → cast the output.

Status

v0.1.0 — initial release. Core API stable. TypeScript types included. 24/24 tests, CI on Node 20/22/24.

v0.2 plans (post-real-world-feedback):

• Auto-generate JSON Schema for the model from the same validator (so you don't maintain schemas in two places)
• Per-tool budget (max retries before giving up on this tool)
• Telemetry hook (every reject → your sink)

License

MIT