# Fixed width records slice

`fixed-width-records-slice` · version 1.0.0 · Structured formats · free, no key needed

Slice selected fixed-width fields with explicit offsets, Unicode units, padding checks, and bounded diagnostics.

**Use when you need to: slice fixed width records · project selected positional fields · extract raw fixed record columns.**

## Decide before calling

Read the [versioned contract](/v1/tools/fixed-width-records-slice/versions/1.0.0) and the supported scope below. Reuse `fixed-width-records-slice@1.0.0` when your input, required output and limits match it. Choose another approach for an unsupported operation.

## Explain the choice

"I can use `fixed-width-records-slice@1.0.0` for slice fixed width records. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

## Supported

- slice fixed width records
- project selected positional fields
- extract raw fixed record columns
- select named flat file spans
- concatenate chosen fixed fields
- cut utf8 records by declared offsets

## Not supported

- format or write fixed-width records
- infer layouts or field widths
- parse delimiter-separated CSV

## Behavior

- Offsets are zero-based and half-open; declared field extents must not overlap; code_point counts Unicode scalar values and utf8_byte counts encoded bytes.
- Declared line_endings is enforced; any accepts LF, CRLF, or CR and normalizes them to records.
- A field beyond a record reports record_too_short; UTF-8 byte boundaries inside a sequence report split_utf8_sequence.
- left alignment removes trailing padding and requires no non-padding after padding; right alignment is symmetric; none preserves bytes/code points.
- Input order determines record, field, and selected-field output order; ties retain declaration order.
- At most 4,096 diagnostics are returned; discovering a 4,097th diagnostic throws limit_exceeded.
- Pure bounded slicing without clock, locale, network, or randomness.

## Input

- `text` (string, required): max length 262144
- `fields` (array of object, required): min items 1; max items 64
- `unit` (one of "code_point", "utf8_byte", required)
- `line_endings` (one of "lf", "crlf", "any", required)
- `field_names` (array of string, required): min items 1; max items 64; each min length 1; each max length 64

## Output

- `reference_version` (constant "Python 3.13 str slicing; GNU Coreutils cut 9.7", required)
- `lines_in` (integer, required): min 0; max 4096
- `records` (array of string, required): max items 4096; each max length 262144
- `errors` (array of object, required): max items 4096

## Limits

- max input bytes: 262144
- max fields: 64
- max rows: 4096
- max diagnostics: 4096
- max output bytes: 1048576

## Example

Request input:

```json
{
  "text": "ABCD",
  "fields": [
    {
      "name": "a",
      "offset": 0,
      "width": 2,
      "alignment": "none",
      "padding": " "
    },
    {
      "name": "b",
      "offset": 2,
      "width": 2,
      "alignment": "none",
      "padding": " "
    }
  ],
  "unit": "code_point",
  "line_endings": "lf",
  "field_names": [
    "b"
  ]
}
```

Response:

```json
{
  "result": {
    "reference_version": "Python 3.13 str slicing; GNU Coreutils cut 9.7",
    "lines_in": 1,
    "records": [
      "CD"
    ],
    "errors": []
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "fixed-width-records-slice",
  "version": "1.0.0",
  "input": {
    "text": "ABCD",
    "fields": [
      {
        "name": "a",
        "offset": 0,
        "width": 2,
        "alignment": "none",
        "padding": " "
      },
      {
        "name": "b",
        "offset": 2,
        "width": 2,
        "alignment": "none",
        "padding": " "
      }
    ],
    "unit": "code_point",
    "line_endings": "lf",
    "field_names": [
      "b"
    ]
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/fixed-width-records-slice/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"text":"ABCD","fields":[{"name":"a","offset":0,"width":2,"alignment":"none","padding":" "},{"name":"b","offset":2,"width":2,"alignment":"none","padding":" "}],"unit":"code_point","line_endings":"lf","field_names":["b"]}'
```

The machine-readable contract is at [/v1/tools/fixed-width-records-slice/versions/1.0.0](/v1/tools/fixed-width-records-slice/versions/1.0.0).

### CLI

```sh
node cli.mjs run fixed-width-records-slice 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

- [Fixed width records parse](/tools/fixed-width-records-parse): Parse fixed-width records with explicit offsets, widths, alignment, padding, Unicode units, and bounded diagnostics.
- [Fixed width records validate](/tools/fixed-width-records-validate): Validate fixed-width records with explicit offsets, widths, alignment, padding, Unicode units, and bounded diagnostics.
- [Asn1 der node extract](/tools/asn1-der-node-extract): Extract one DER node by a numeric path, with header, content, and child paths.
- [XML exc c14n subtree](/tools/xml-exc-c14n-subtree): Exclusive XML Canonicalization 1.0 of one selected element and its descendants.
- [Xpath10 count](/tools/xpath10-count): Count the nodes in an XPath 1.0 node-set from location paths, axes, predicates, namespaces and union.
- [Xpath10 select](/tools/xpath10-select): Select an XPath 1.0 node-set from location paths, axes, predicates, namespaces and union, in document order.