Codex, a coding agent, is a lot of fun when you are deep in the terminal. Point it at a repository, give it a focused task, and it gets to work fast. It reads the code, proposes a fix, runs commands, and helps you move from idea to code without leaving the flow. That developer experience is a big part of the appeal of agentic coding tools, and it is also what made our recent Claude Code tutorial work so well.But writing the code is only the first step. You still need the issue, the merge request, the CI/CD pipeline, the review, and the final human decision to ship the change. Writing code and shipping software are not the same thing, and that gap becomes more obvious as coding agents get faster.That is where GitLab comes in. In this tutorial, we will walk through the following three use cases with Codex and GitLab Duo Agent Platform:Fix a Rust WebSocket bug locally to get started with CodexEnrich the context with GitLab MCP to fix the Rust bug aligned to issue requirementsUse Codex within GitLab Duo Agent Platform as an external agent to help address review feedback in the merge requestWe are using the Tanuki IoT Platform project for all three use cases. The Rust metrics backend provides two practical bugs to work with: a WebSocket metric filter bug and a REST API validation bug. Let’s get started!PrerequisitesCodex in the terminal, configured and running.A GitLab project with bug reports and feature proposal issues, for example, the Tanuki Iot Platform projectFor specific use cases: GitLab MCP server and GitLab Duo Agent Platform with external agents.For building code: Cargo and Rust compiler, for example, rustup.Prepare the GitLab projectIf you want to repeat the workflow in your own environment, start by importing and cloning the project and opening Codex in the repository root:Import the Tanuki Iot Platform project into your GitLab environment, including all open issues.Clone the project into your local environment, and navigate into it.Open a terminal and run Codex with codex. git clone https://gitlab.example.com/examplegroup/tanuki-iot-platform.git