> ## Documentation Index
> Fetch the complete documentation index at: https://docs.narrative.io/llms.txt
> Use this file to discover all available pages before exploring further.

# MCP Tool Argument Validation Failed

> The MCP server reported invalid-params (JSON-RPC -32602) for the call arguments, or local arg-shape check failed.

**Surface:** `GET /agents/runs/{id}`, `error.type = "ToolValidation"`

## When this error occurs

The model produced arguments that didn't satisfy the tool's input schema:

* Required field missing.
* Field type didn't match (`string` expected, `integer` given).
* `additionalProperties: false` rejected an extra key.

The MCP server rejected the call with JSON-RPC `-32602`. (The platform doesn't carry a copy of
the input schema for MCP tools — it discovers `tools/list` per run and forwards the schema to
the model verbatim, then forwards model output to the server. Validation happens server-side.)

## How to fix

* **Improve the input\_schema** in the run's config to be tighter — required fields, strict
  enums, `additionalProperties: false`. Tighter schemas → fewer model mistakes.
* **Strengthen the tool description** with a concrete example so the model copies the shape.
* This error is **not retried** by the workflow; the activity tag is in
  `RetryOptions.setDoNotRetry`. The run fails fast and the caller can re-prompt.