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

# Coding Format Validation

> `coding_format_validation` deterministically validates the format of generated charges.

`coding_format_validation` deterministically validates the format of generated charges. It is part of the **Nanonets Coding Engine** tool group and implements the rules described in [coding-validator-format.md](/docs/tools/coding-validator-format).

This is a pure-compute tool (no LLM, no external calls). Run it after the verifier step and before optional write-back.

## Inputs

* `charges` (required): array of charges, each with `cpt_code` and optional `diagnosis_codes` and `modifiers`.

## Validation behavior

* CPT/HCPCS format: five digits (`99214`), four digits + one uppercase letter (`0509F`), or one letter + four digits (HCPCS Level II, e.g. `J3490`). Invalid format is an `error`.
  * Note: this is a superset of the original "five digits or four digits plus one uppercase letter" rule. HCPCS Level II (letter + four digits) is included because the CPT generation step legitimately emits drug J-codes; flagging them would block valid write-backs.
* ICD-10 format: a broad pattern accepting both dotted (`F33.1`) and dotless (`F329`) forms, including valid U-codes such as `U07.1`. Invalid format is an `error`.
* Modifier length: each modifier is expected to be 2 characters; anything else is a `warning` (does not invalidate the charge).

## Output

```json theme={null}
{
  "tool_name": "coding_format_validation",
  "success": true,
  "valid": false,
  "validation_issues": [{ "severity": "error", "source": "format", "code_in_question": "ABC", "problem": "invalid CPT/HCPCS format" }]
}
```

`valid` is false when any `error`-severity issue is present (warnings alone keep it true).

## Human-readable text

In the task feed the tool renders a `## Format Validation` table (valid, issue count) and an `## Issues` table (severity, source, code, problem). The full result struct stays in `structuredContent`.

## Agent Instruction Guidance

If `valid=false`, do not write back to the EHR without human review. Merge `validation_issues` into the final output's `validation_issues` array.

## Notes

* Deterministic tool. Free to call. `default_enabled` is false.