GitLab MR +0 Triggered ChatGPT Automatic Code Review

GitLab comment → n8n webhook → fetch MR changes → GPT review → post discussion reply

Who

Engineers who want automated code reviews or a second opinion on GitLab Merge Requests

You get

After typing +0 on an MR comment, automatically fetch code changes and have GPT generate a review, posted as a discussion reply on the relevant line

You need

n8n · gitlab

Steps

1. Configure webhook and credentials

   Create a webhook in GitLab project settings with note_events, pointing to the n8n Webhook node URL. Configure OpenAI API credentials (type openAiApi) in n8n. In the Need Review node, confirm the condition checks if comment content equals +0 (modifiable to other keywords).

2. Fetch and filter changed files

   Once triggered, the Get Changes node calls the GitLab API to retrieve the MR changes list. The Split Out node splits the changes array into individual items. The Skip File Changes node filters out entries where renamed_file=true, deleted_file=true, or diff does not start with @@, keeping only valid code changes.

3. Parse diff and extract old vs new code

   The Parse Last Diff Line node runs JavaScript to parse the last line of the diff, computing lastOldLine and lastNewLine for later positioning. The Code node splits the diff by line: lines starting with - go to originalCode, lines starting with + go to newCode, and all other lines are kept in both.

4. LLM review of changes

   The Basic LLM Chain node assembles file path, original code, and new code into a prompt, calling the OpenAI Chat Model (default gpt-4o-mini) to generate a review. The system message instructs the model to act as a senior programming expert, give an accept/reject decision, a 0-100 score, point out issues in concise and stern language, and optionally provide modified content, all in strict Markdown format.

   Execution prompt · Source text。Basic LLM Chain node text parameter (user message template)

   Copy prompt

   =File path：{{ $('Skip File Changes').item.json.new_path }}
   
   ```Original code
    {{ $json.originalCode }}
   ```
   change to
   ```New code
    {{ $json.newCode }}
   ```
   Please review the code changes in this section:

   Execution prompt · Source text。Basic LLM Chain node messages.messageValues[0].message (system message)

   Copy prompt

   # Overview:| You are a senior programming expert Bot, responsible for reviewing code changes and providing review recommendations. At the beginning of the suggestion, it is necessary to clearly make a decision to "reject" or "accept" the code change, and rate the change in the format "Change Score: Actual Score", with a score range of 0-100 points. Then, point out the existing problems in concise language and a stern tone. If you feel it is necessary, you can directly provide the modified content. Your review proposal must use rigorous Markdown format.

5. Post review back to MR discussion

   The Post Discussions node calls the GitLab API via POST with multipart-form-data to create a discussion on the MR. The body is the LLM-generated review text; position parameters include old_path, new_path, start_sha, head_sha, base_sha, and the parsed new_line/old_line to precisely anchor the comment. PRIVATE-TOKEN must be supplied in headerParameters.

You are setting up an n8n workflow: when a GitLab Merge Request receives a +0 comment, automatically fetch the MR's code changes, have GPT review them, and post the review as a discussion reply on the corresponding code line. First confirm you have: 1) an accessible n8n instance; 2) GitLab project webhook configuration permission; 3) GitLab Private Token; 4) OpenAI API key. Then import the template and replace credentials and URLs.