| # Lessons Learned | |
| In this document we briefly collect what we have learned while developing and using Serena, | |
| what works well and what doesn't. | |
| ## What Worked | |
| ### Separate Tool Logic From MCP Implementation | |
| MCP is just another protocol, one should let the details of it creep into the application logic. | |
| The official docs suggest using function annotations to define tools and prompts. While that may be | |
| useful for small projects to get going fast, it is not wise for more serious projects. In Serena, | |
| all tools are defined independently and then converted to instances of `MCPTool` using our `make_tool` | |
| function. | |
| ### Autogenerated PromptFactory | |
| Prompt templates are central for most LLM applications, so one needs good representations of them in the code, | |
| while at the same time they often need to be customizable and exposed to users. In Serena we address these conflicting | |
| needs by defining prompt templates (in jinja format) in separate yamls that users can easily modify and by autogenerated | |
| a `PromptFactory` class with meaningful method and parameter names from these yamls. The latter is committed to our code. | |
| We separated out the generation logic into the [interprompt](/src/interprompt/README.md) subpackage that can be used as a library. | |
| ### Tempfiles and Snapshots for Testing of Editing Tools | |
| We test most aspects of Serena by having a small "project" for each supported language in `tests/resources`. | |
| For the editing tools, which would change the code in these projects, we use tempfiles to copy over the code. | |
| The pretty awesome [syrupy](https://github.com/syrupy-project/syrupy) pytest plugin helped in developing | |
| snapshot tests. | |
| ### Dashboard and GUI for Logging | |
| It is very useful to know what the MCP Server is doing. We collect and display logs in a GUI or a web dashboard, | |
| which helps a lot in seeing what's going on and in identifying any issues. | |
| ### Unrestricted Bash Tool | |
| We know it's not particularly safe to permit unlimited shell commands outside a sandbox, but we did quite some | |
| evaluations and so far... nothing bad has happened. Seems like the current versions of the AI overlords rarely want to execute `sudo rm - rf /`. | |
| Still, we are working on a safer approach as well as better integration with sandboxing. | |
| ### Multilspy | |
| The [multilspy](https://github.com/microsoft/multilspy/) project helped us a lot in getting started and stands at the core of Serena. | |
| Many more well known python implementations of language servers were subpar in code quality and design (for example, missing types). | |
| ### Developing Serena with Serena | |
| We clearly notice that the better the tool gets, the easier it is to make it even better | |
| ## Prompting | |
| ### Shouting and Emotive Language May Be Needed | |
| When developing the `ReplaceRegexTool` we were initially not able to make Claude 4 (in Claude Desktop) use wildcards to save on output tokens. Neither | |
| examples nor explicit instructions helped. It was only after adding | |
| ``` | |
| IMPORTANT: REMEMBER TO USE WILDCARDS WHEN APPROPRIATE! I WILL BE VERY UNHAPPY IF YOU WRITE LONG REGEXES WITHOUT USING WILDCARDS INSTEAD! | |
| ``` | |
| to the initial instructions and to the tool description that Claude finally started following the instructions. | |
| ## What Didn't Work | |
| ### Lifespan Handling by MCP Clients | |
| The MCP technology is clearly very green. Even though there is a lifespan context in the MCP SDK, | |
| many clients, including Claude Desktop, fail to properly clean up, leaving zombie processes behind. | |
| We mitigate this through the GUI window and the dashboard, so the user sees whether Serena is running | |
| and can terminate it there. | |
| ### Trusting Asyncio | |
| Running multiple asyncio apps led to non-deterministic | |
| event loop contamination and deadlocks, which were very hard to debug | |
| and understand. We solved this with a large hammer, by putting all asyncio apps into a separate | |
| process. It made the code much more complex and slightly enhanced RAM requirements, but it seems | |
| like that was the only way to reliably overcome asyncio deadlock issues. | |
| ### Cross-OS Tkinter GUI | |
| Different OS have different limitations when it comes to starting a window or dealing with Tkinter | |
| installations. This was so messy to get right that we pivoted to a web-dashboard instead | |
| ### Editing Based on Line Numbers | |
| Not only are LLMs notoriously bad in counting, but also the line numbers change after edit operations, | |
| and LLMs are also often too dumb to understand that they should update the line numbers information they had | |
| received before. We pivoted to string-matching and symbol-name based editing. |