Serena

What it is

When your agent needs to change a function, it usually searches your files for the text, reads big chunks of them, and edits by matching lines. That works, but it's clumsy on a large project, and easy to miss a spot.

Your code editor does this differently. It runs a language server: a background program that understands your code's structure, so it knows where every function is defined and everywhere it's used. That's what powers "go to definition", "rename everywhere" and the red squiggles under errors.

Serena hands those abilities to your agent. Its semantic code tools let the agent say "find the createInvoice function", "show me everything that calls it" or "rename it everywhere", instead of hunting through text. It plugs into Claude Code, Codex, Cursor and other agents as an MCP server, a plug-in that adds tools. It's free and open source, made by Oraios AI, which also sells an optional paid JetBrains plugin with extra tools.

What actually happens when you use it

1. It starts with your agent. When you open a project, Serena starts the right language server for your code. It supports over 40 languages and installs some of the servers itself.
2. The agent gets new tools. Things like "find symbol", "find references", "replace this function's body", "insert after this function" and "rename". In Claude Code and Codex, Serena switches off its own duplicate file and shell tools by default.
3. The agent looks things up by name. Instead of reading a 900-line file, it asks for an outline, then reads only the function it needs.
4. It keeps notes. Serena has a simple memory system for facts about your project between sessions. You can switch it off.
5. A dashboard runs alongside. A small local web page shows logs and which tools were used. It opens on launch unless you turn that off.

Version 2, released on 13 October 2026, added a new mode where the agent writes small scripts against Serena instead of calling dozens of separate tools. It's only ten days old, so there's no independent feedback on it yet.

Does it still help with today's top models?

This is the real question, and the honest answer is: less than it used to, especially in Claude Code.

• Claude Code now has some of this built in. Anthropic's official code intelligence plugins connect language servers for about a dozen languages, including TypeScript and Python. They show Claude its own errors after each edit and let it look up definitions and references. They're read-only, so Serena still adds things like renaming across files and editing whole functions by name.
• Claude tends to ignore it. Serena's own docs say recent Claude Code and Opus updates made it almost impossible to get Opus 4.7 to use Serena without replacing Claude Code's system prompt. A maintainer said the team is at the mercy of Anthropic here. Serena now ships hooks that nudge the agent back to its tools, but the docs still suggest the system-prompt swap.
• Agents in general prefer plain text search. A small August 2026 study found Claude models picked language-server tools only 0 to 6% of the time when finding code, and forcing them didn't raise success. It didn't test Serena itself, and the authors call it a preliminary pilot.
• Other agents may get more out of it. Several of the upbeat 2026 reports come from people using Codex or Gemini, though one person struggled to get Kimi to use it.
• Serena's own evaluation is the agents grading themselves. The README quotes Opus and GPT models praising it. The method has each agent judge its own experience, with no measured success rates or token savings. It does admit the agent's built-in tools are better for small edits and text search.

What people like

• Big, cross-file changes. When someone on HN asked why agents have no tool for moving a function between files, the reply was simply Serena. One user called it a secret weapon alongside Superpowers and Context7.
• One tool for every language. A September HN user runs it as the MCP for all their language servers and says it works great.
• It helps weaker setups. One user found Gemini mostly works and loops less once given Serena's tools.
• Possible token savings in Codex. An August r/codex post credited Serena plus config changes with using noticeably less of the weekly limit. Treat that as one person's report: another user spent a day trying to replicate it without success.

What people run into

• Getting Claude to actually use it. One user reminds Claude Code to use Serena every turn, and it still drifts back to its own tools mid-task. On HN, another said it takes arcane instructions to make Claude Code comply.
• It does a lot by default. The author of a smaller rival tool said they used Serena first and didn't like how much it does out of the box. In r/codex, one of the first replies to a happy user was you are still using Serena?
• Memory and setup trouble. A January issue reported Serena eating about 30GB of memory until Claude Code froze. A fix landed within a week, but someone reported it again in July. Language servers that hang on startup are a recurring theme in the issue tracker.
• Pop-ups. The dashboard opening a browser tab or app every session is a frequent complaint. Running several agents at once once meant several extra icons in the macOS dock until the team changed it.

Before you install

• Update to version 2. Three advisories in 2026 described ways for someone else's code to run on your machine. Two only needed you to open a booby-trapped repository, through the symbol cache and project prompt files. The third let a malicious web page reach the dashboard. All are fixed in current versions, and newer releases limit what an untrusted project can do. Still, don't point it at code you don't trust.
• Try the built-in option first. If you use Claude Code and write TypeScript, Python or another supported language, install the official LSP plugin and see if that's enough.
• The licence changed. Version 2 moved from MIT to GPL-3.0. That doesn't affect using it on your own projects. It only matters if you redistribute Serena itself. Older versions stay MIT.
• Turn off what you don't need. Set web_dashboard: false to stop the dashboard appearing, and disable memories if you already use CLAUDE.md or AGENTS.md.
• It needs Python tooling. You'll need uv, and some languages need extra installs. Expect a first-run indexing step on big projects.

The short version

Serena is a clever idea: give agents the same code-understanding tools human developers rely on. On large codebases, unusual languages and big refactors, it can still earn its place, particularly outside Claude Code. But Claude Code now covers the basics itself, the newest Claude models mostly prefer their own tools, version 2 is brand new, and the tool has had a rough year for security. If your app is small or medium and you're not hitting walls, you won't miss it.