Improve GitLab Duo MR Review error handling for schema formatting

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Summary

When a .gitlab/duo/mr-review-instructions.yaml file contains YAML that doesn't match the expected schema, GitLab Duo MR Review fails with a generic "I have encountered some problems while I was reviewing. Please try again later." message instead of providing helpful validation feedback about the malformed configuration.

The underlying cause is a NoMethodError: undefined method 'strip' for nil:NilClass in ee/lib/gitlab/llm/templates/review_merge_request.rb:73 when the instructions field is missing or nil due to schema mismatch.

This was reported by a customer in this zendesk ticket

Steps to reproduce

  1. Create a .gitlab/duo/mr-review-instructions.yaml file with an incorrect schema format, for example: 
    version: 1

instructions:
  - name: "View Files Validation"
    description: "Validate .viewYml files for structural integrity"
    applies_to:
      file_patterns:
        - "**/*.viewYml"
    rules:
      - name: "No Development Folder References"
        severity: error
        description: "Views must not reference items in Development folders"
        check: |
          Search for cache dependencies containing 'development' in their paths.
  2. Create a merge request in the project
  3. Trigger GitLab Duo MR Review

Example Project

What is the current bug behavior?

GitLab Duo posts a comment with:

I have encountered some problems while I was reviewing. Please try again later.

Sidekiq logs show:

"exception.class":"NoMethodError"
"exception.message":"undefined method `strip' for nil:NilClass"
"exception.backtrace":["ee/lib/gitlab/llm/templates/review_merge_request.rb:73:in `block in format_custom_instructions_list'"...]

What is the expected correct behavior?

GitLab Duo should:

  1. Validate the YAML schema before attempting to process custom instructions
  2. Provide a clear, user-friendly error message indicating the configuration file format is incorrect
  3. Point users to documentation showing the correct schema format

Expected schema format:

version: 1
instructions:
  - name: "View Files Validation"
    instructions: |
      Search for cache dependencies containing 'development' in their paths.
      Flag any references to Development folders as policy violations.
    fileFilters:
      - "**/*.viewYml"

Relevant logs and/or screenshots 

{
  "severity": "ERROR",
  "time": "2026-01-26T19:52:30.949Z",
  "correlation_id": "01KFXXTPEE85M42BX2PZQTQ0JH",
  "meta.caller_id": "Llm::CompletionWorker",
  "meta.feature_category": "ai_abstraction_layer",
  "exception.class": "NoMethodError",
  "exception.message": "undefined method `strip' for nil:NilClass",
  "exception.backtrace": [
    "ee/lib/gitlab/llm/templates/review_merge_request.rb:73:in `block in format_custom_instructions_list'",
    "ee/lib/gitlab/llm/templates/review_merge_request.rb:71:in `map'",
    "ee/lib/gitlab/llm/templates/review_merge_request.rb:71:in `format_custom_instructions_list'",
    "ee/lib/gitlab/llm/templates/review_merge_request.rb:56:in `format_custom_instructions_section'",
    "ee/lib/gitlab/llm/templates/review_merge_request.rb:25:in `to_prompt_inputs'",
    "ee/lib/gitlab/llm/ai_gateway/completions/review_merge_request.rb:375:in `prepare_prompt_inputs'",
    "..."
  ],
  "extra.unit_primitive": "review_merge_request"
}

Output of checks

This bug happens on GitLab.com

Possible fixes

The issue is in ee/lib/gitlab/llm/templates/review_merge_request.rb at line 73:

"#{instruction[:instructions].strip}\n"

When the YAML uses an unexpected schema (e.g., rules[].check instead of instructions), the instruction[:instructions] value is nil, causing the NoMethodError.

Suggested fixes:

  1. Add schema validation when parsing the .gitlab/duo/mr-review-instructions.yaml file (likely in the code that populates custom_instructions)
  2. Add nil-safe handling: instruction[:instructions]&.strip with appropriate fallback
  3. Return a user-friendly error message when validation fails, pointing to documentation

Patch release information for backports

N/A - This is a UX improvement for error handling, not a critical security or data loss issue.

Edited by 🤖 GitLab Bot 🤖