Autocomplete (FIM)

Fill-in-the-middle (FIM) generates code that fits between existing code. You send the code before the cursor as prompt (the prefix) and the code after the cursor as suffix; Mercury Edit 2 returns the text that belongs in between. This is what powers inline autocomplete in an editor.

FIM vs. Next Edit

	Autocomplete (FIM)	Next Edit
Endpoint	`POST /v1/fim/completions`	`POST /v1/edit/completions`
Question it answers	”What belongs between these two known points?"	"What should I change next in this file?”
Inputs	`prompt` prefix and optional `suffix`	Current file, editable region, edit history, and recently viewed snippets
Best for	Inline completion as the user types	Predictive multi-line edits informed by recent context

FIM has no separate fields for edit history or other open files. Add any extra context directly to prompt, such as imports, related types, or a short description of the intended code. Use Next Edit when the suggestion should incorporate edit history or recently viewed snippets.

Set your API key as an environment variable and install the SDK.

export INCEPTION_API_KEY="your_api_key_here"

set INCEPTION_API_KEY=your_api_key_here

$env:INCEPTION_API_KEY="your_api_key_here"

pip install inceptionai

npm install inceptionai

In the example below, the prefix defines factorial up to return n * and the suffix is the call that uses it. Mercury Edit 2 fills the gap — e.g. factorial(n - 1).

curl https://api.inceptionlabs.ai/v1/fim/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $INCEPTION_API_KEY" \
  -d '{
    "model": "mercury-edit-2",
    "prompt": "def factorial(n):\n    if n <= 1:\n        return 1\n    return n * ",
    "suffix": "\n\nprint(factorial(5))",
    "max_tokens": 1000
}'

from inceptionai import Inception

client = Inception()  # reads INCEPTION_API_KEY from the environment

completion = client.fim.completions.create(
    model="mercury-edit-2",
    prompt="def factorial(n):\n    if n <= 1:\n        return 1\n    return n * ",
    suffix="\n\nprint(factorial(5))",
    max_tokens=1000,
)
# The completion is the code that fills the gap, e.g. "factorial(n - 1)"
print(completion.choices[0].text)

import Inception from 'inceptionai';

const client = new Inception(); // reads INCEPTION_API_KEY from the environment

const completion = await client.fim.completions.create({
  model: 'mercury-edit-2',
  prompt: 'def factorial(n):\n    if n <= 1:\n        return 1\n    return n * ',
  suffix: '\n\nprint(factorial(5))',
  max_tokens: 1000,
});
// The completion is the code that fills the gap, e.g. "factorial(n - 1)"
console.log(completion.choices[0]?.text);

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FIM vs. Next Edit

​FIM vs. Next Edit

FIM vs. Next Edit