CSV anti join

csv-anti-join · version 1.0.0 · CSV & tables · free, no key needed

Keep left CSV rows whose exact key tuples do not appear in the right document.

Use when you need to: csv anti join · keep unmatched left csv rows · csv rows absent from right keys.

Supported

• csv anti join
• keep unmatched left csv rows
• csv rows absent from right keys

Not supported

• inner join
• left join
• outer join
• fuzzy keys
• numeric keys
• suffix collision renaming
• read files
• fetch urls

Behavior

• keys names unique existing headers on both sides; equality uses exact string tuples with no trimming, case folding, or type coercion.
• Output CSV uses left headers and left cells only, in original left data-row order.
• Duplicate left keys are kept when unmatched; duplicate right keys only populate the set of right key tuples.
• Empty documents without headers are rejected. Header-only right keeps every left data row. Header-only left yields header-only output.
• rows_in is the number of left data rows; rows_out is the number of unmatched left data rows kept.
• The first left header name may not begin with U+FEFF because it is ambiguous with a CSV byte-order mark.

Input

• left (string, required): max length 256000
• right (string, required): max length 256000
• keys (array of string, required): min items 1; max items 32; each min length 1

Output

• csv (string, required): max length 256000
• rows_in (integer, required)
• rows_out (integer, required)

Limits

• max input bytes: 256000
• max data rows: 5000
• max columns: 256
• max header name bytes: 256
• max keys: 32
• max output bytes: 256000

Example

Request input:

{
  "left": "id,name\n1,Ada\n2,Lin\n3,Grace\n",
  "right": "id,city\n2,London\n4,Paris\n",
  "keys": [
    "id"
  ]
}

Response:

{
  "result": {
    "csv": "id,name\n1,Ada\n3,Grace\n",
    "rows_in": 3,
    "rows_out": 2
  }
}

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

{
  "id": "csv-anti-join",
  "version": "1.0.0",
  "input": {
    "left": "id,name\n1,Ada\n2,Lin\n3,Grace\n",
    "right": "id,city\n2,London\n4,Paris\n",
    "keys": [
      "id"
    ]
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/csv-anti-join/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"left":"id,name\n1,Ada\n2,Lin\n3,Grace\n","right":"id,city\n2,London\n4,Paris\n","keys":["id"]}'

The machine-readable contract is at /v1/tools/csv-anti-join/versions/1.0.0.

CLI

node cli.mjs run csv-anti-join 1.0.0 --input input.json --base-url https://computefirst.net

Get the client at /clients/cli/.

Related tools

• CSV left join: Left join two CSV documents on exact named key columns.
• CSV inner join: Inner join two CSV documents on exact named key columns.
• CSV row diff: Compare two CSV documents with identical headers by unique key tuples and report added, removed, and changed rows.
• CSV add row number: Prepend a 1-based row_number column of canonical integer strings.
• CSV concat: Concatenate CSV documents that share identical headers, preserving document and row order.
• CSV dedupe: Remove duplicate CSV data rows while preserving the first occurrence.