Thanks so much to /u/thelastlokean for raving about this.
I've been spending days writing my own custom scripts with grep, ast-grep, and writing tracing through instrumentation hooks and open telemetry to get Claude to understand the structure of the various api calls and function calls.... Wow. Then Serena MCP (+ Claude Code) seems to be built exactly to solve that.

Within a few moments of reading some of the docs and trying it out I can immediately see this is a game changer.

Don't take my word, try it out. Especially if your project is starting to become more complex.

https://github.com/oraios/serena

Claude 4, in particular Opus, is amazing for coding. It has only two main downsides: high cost and a relatively small context window.

Fortunately, there is a free, open-source (MIT licensed) solution to help with both: the Serena MCP server, a toolbox that uses language servers (and quite some code on top of them) to allow an LLM to perform symbolic operations, including edits, directly on your codebase. You may have seen my post on it a while ago, when we had just published the project. It turns a vanilla LLM into a capable coding agent, or improves existing coding agents if included into them

Now, a few weeks and 1k stars later, we are nearing a first stable version. I have started evaluating it, and I'm blown away by the results so far! When using it on its own in Claude Desktop, it turns Claude into a careful and token-frugal agent, capable of acting on enormous projects without running into token limits. As a complement to an existing agentic solution, like Claude Code or some other coding agent, Serena significantly reduced costs in all my experiments while keeping or increasing the quality of the output.

None of it is surprising, of course. If you give me an IDE, I will obviously be better and faster at coding than if I had to code in something like word and use pure file-reads and edits. Why shouldn't the same hold for an LLM?

A quantitative evaluation on SWE-verified is on its way, but to just give a taste of what Serena can do, I created one PR on a benchmark task from sympy, with Opus running on Claude Desktop. It demonstrates how Opus intelligently uses the tools to explore, read and edit the codebase in the most token-efficient manner possible. For complete transparency, the onboarding conversation and the solution conversation are included. The same holds for Sonnet, but for Opus it's particularly useful, since due to its high cost, token efficiency becomes key.

Since Claude Code is now included into the pro subscription, the file-read based MCPs are largely obsolete for coding purposes (for example, the codemcp dev said he now stops the project). Not so for Serena, since the symbolic tools it offers give a valuable addition to Claude Code, rather than being replaced by it.

Even though sympy is a huge repository, the Opus+Serena combo went through it like a breeze. For anyone wanting to have cheaper and faster coding agents, especially on larger projects, I highly recommend looking into Serena! We are still early in the journey, but I think the promise is very high.