> ## Documentation Index
> Fetch the complete documentation index at: https://iso8583sim.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Describe

> Rule-based, human-readable descriptions of ISO 8583 messages.

`iso8583sim.core.describe`

These helpers need no LLM. They power `iso8583sim explain --no-llm` and the
MCP server's explain\_message tool.

## Functions

### `field_name()`

```python theme={null}
def field_name(
    field_number: int,
    network: CardNetwork | None,
    version: ISO8583Version,
) -> str
```

Return a field's description, or "Unknown" if it is not defined.

### `field_entries()`

```python theme={null}
def field_entries(message: ISO8583Message) -> list[dict[str, Any]]
```

List a message's fields (excluding the MTI) as number, name and value entries.

### `mask_pan()`

```python theme={null}
def mask_pan(pan: str) -> str
```

Mask a PAN, keeping the first six and last four digits.

### `format_amount()`

```python theme={null}
def format_amount(amount: str, currency: str | None) -> str
```

Format a minor-unit amount (field 4) using the currency's exponent, e.g. "10.00 USD".

### `decode_emv_tags()`

```python theme={null}
def decode_emv_tags(data: str) -> list[dict[str, Any]]
```

Decode EMV TLV data into named tags, explaining the TVR (95) and CID (9F27).

### `describe_mti()`

```python theme={null}
def describe_mti(mti: str) -> dict[str, str]
```

Describe an MTI's message class, function and origin.

### `describe_message()`

```python theme={null}
def describe_message(message: ISO8583Message) -> dict[str, Any]
```

Describe a message in plain language without an LLM.

**Parameters**

<ResponseField name="message" type="ISO8583Message" required>
  Parsed message
</ResponseField>

<ResponseField name="Returns" type="dict[str, Any]">
  Dictionary with a one-paragraph "summary", the MTI breakdown, named fields
</ResponseField>

<ResponseField name="Returns" type="dict[str, Any]">
  and, when field 55 is present, decoded EMV tags
</ResponseField>


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