FixAgent

The FixAgent is the most complex agent in the pipeline. It implements a full TDD (Test-Driven Development) cycle inside an isolated Sandbox container.

Responsibilities

• Clone the repository and create a fix branch
• Place the reproduction test and confirm the bug
• Generate a fix via LLM (up to 5 attempts with iterative refinement)
• Verify the fix passes the reproduction test
• Run a full regression check
• Lint, commit, push, and create a GitHub Pull Request

TDD Cycle

Step 1: Clone and Branch

git clone --depth 1 {repoUrl} /workspace/repo
git checkout -b sentinel/fix-{incidentId}
bun install

Step 2: Place Reproduction Test

The reproduction test is fetched from R2 (test-case.ts) and written to the repo. If the R2 artifact is unavailable, a fallback template is used.

Step 3: Baseline Check

Run the existing test suite to establish that it passes before the fix:

bun test

Step 4: Confirm Reproduction

Run the reproduction test in isolation. It must fail to confirm the bug exists:

bun test tests/sentinel-reproduction.test.ts

If the test passes (bug not reproduced), the incident is marked needs_human.

Step 5: Generate and Apply Fix (Loop)

Up to 5 attempts, each with iterative refinement:

1. Call LLM with: source code, root cause analysis, test output, and previous failure context
2. Write the generated fix to the file
3. Run the reproduction test — it should now pass
4. Run the full regression suite — all tests should still pass
5. If regression fails: revert the file (git checkout -- {file}) and try a different approach

Each subsequent attempt includes the error from the previous attempt, prompting the LLM to try a different strategy.

Step 6: Lint

bunx oxlint --fix .

Step 7: Commit and Push

git add -A
git commit -m "fix: {rootCauseExplanation}"
git push {authenticatedUrl} sentinel/fix-{incidentId}

Step 8: Create Pull Request

A GitHub PR is created via the REST API with a structured body:

• Incident ID and service name
• Root cause file, lines, and explanation
• Fix strategy and confidence level
• Affected files list
• Verification checklist (reproduction, regression, lint)

Artifacts

Artifact	R2 Key	Description
Patch	incidents/{id}/patch.diff	Git diff of the fix
PR body	incidents/{id}/pr-body.md	PR description markdown

Failure Handling

Condition	Result
No Sandbox binding	needs_human
No repository URL	needs_human
Reproduction test passes	needs_human (bug not confirmed)
5 fix attempts exhausted	needs_human
GitHub API error	needs_human + error logged

HTTP Endpoints

Method	Path	Description
POST	/fix	Attempt a fix from a queue message
GET	/status	Active fixes

Source

src/agents/fix.ts