Someone else's code – productive from day one

New project, 400,000 lines, half of them older than the team. The documentation: a README from 2019 and the sentence "Best ask Klaus". Unfortunately Klaus left two years ago.

That used to mean a week of reading, debugging and guessing. Today I start differently.

🗺️ Step 1: the map

Before I change anything, I ask questions. Only questions:

• "Explain the architecture of this project in ten sentences."
• "How does an order flow from the frontend to the database? Name the files."
• "Where is authentication checked? Are there places where it's bypassed?"

Claude reads the relevant files itself. In large codebases a subagent (part 1) does the digging so the main context stays clean.

📝 Step 2: capture the knowledge

/init has Claude analyse the project and create a first CLAUDE.md (part 4): build commands, structure, conventions. I revise it, add what I learned in step 1 and check it in. The next new colleague no longer starts from zero.

🔬 Step 3: the first small change

A small bug fix, in plan mode (part 10). Claude shows which files would be affected and which tests exist. I often learn more about the system this way than by reading.

A word of caution:

Claude explains things it's only guessing just as convincingly. Especially in legacy code full of dead code and historical detours. So always ask for the concrete files and lines and spot-check them. "Show me where" is the most important sentence in this part.

My rule of thumb:

• First hour: only ask, change nothing.
• First day: knowledge into CLAUDE.md.
• First week: small changes with plan mode, big ones only once there are tests.