Fix bugs with Codex and GitLab (opens in new tab)
Codex accelerates coding in the terminal, but producing a fix is only one part of shipping software. GitLab supplies the surrounding lifecycle: issues, merge requests, CI/CD, security scanning, code review, and human approval. The tutorial demonstrates this progression through a Rust WebSocket bug, first with local Codex, then with GitLab MCP for issue context, and finally with Codex as an external agent in GitLab Duo Agent Platform. ## Prerequisites and Project Setup - Configure Codex in the terminal, Rust/Cargo, and access to a GitLab project. - Import and clone the Tanuki IoT Platform project, then launch Codex from its repository root. - The tutorial focuses on `backend/`, where: - Sensors submit readings through a REST API. - Dashboards receive live readings through WebSocket streams. - `AGENTS.md` provides Codex with repository structure, toolchain instructions, build commands, and quality expectations. ## Reproducing the WebSocket Filtering Bug - Start the Rust metrics backend on port `9090`: ```bash PORT=9090 cargo run --manifest-path backend/rust-metrics-store/Cargo.toml ``` - Connect to a filtered WebSocket stream: ```bash websocat 'ws://localhost:9090/ws?sensor=arduino-iot-collector&metric=temperature_celsius' ``` - Submit both temperature and humidity readings for the same sensor through the REST API. - The stream incorrectly returns both metrics instead of only `temperature_celsius`, proving that the WebSocket handler does not apply the metric filter. ## Fixing the Bug with Codex - Give Codex a focused request to add metric filtering to `/ws`. - Codex examines the Rust source and identifies that the endpoint already supports `sensor` filtering but lacks an optional `metric` condition. - It updates the handler, adds tests, and keeps documentation aligned with the implementation. - Codex runs formatting, tests, and builds before creating a branch, committing, and pushing the change. - Once the merge request is created, GitLab handles: - CI/CD pipelines - Security scanning - GitLab Duo Code Review - A follow-up WebSocket test confirms that supplying both sensor and metric now returns only the requested metric. ## Adding GitLab Context with MCP - Local Codex can inspect repository files, but it cannot automatically see GitLab issues, requirements, implementation notes, merge-request discussions, or pipeline status. - The GitLab MCP server connects Codex to that development lifecycle context. - Codex can retrieve the existing issue directly instead of requiring the developer to copy its contents into the prompt. - The issue acts as the shared source of truth and includes: - The bug description - Functional behavior requirements - Non-functional requirements - Required tests - Updates to `README.md` and `AGENTS.md` - Implementation notes - This helps Codex produce a fix that satisfies the agreed requirements rather than merely addressing the symptom visible in the local code. ## Using Codex as an External GitLab Agent - The tutorial’s third workflow uses Codex inside GitLab Duo Agent Platform as an external agent. - This allows the agent to participate after the merge request is open, particularly when addressing review feedback. - GitLab remains the system coordinating issues, merge requests, pipelines, reviews, and deployment, while Codex contributes its terminal-oriented coding capabilities. - The overall workflow moves from bug report to implementation, automated validation, review feedback, revisions, and an informed human decision to ship. ## Practical Conclusion Use Codex for fast, repository-local implementation, but connect it to GitLab through MCP or Duo Agent Platform when requirements and review context matter. The strongest workflow combines Codex’s coding speed with GitLab’s issue-aware, automated, and human-governed delivery lifecycle.