My Self-Evolving AI Agent Stopped Building Features and Started Engineering

Post #1 covered the birth — death spirals, 39 tools, a self-written identity. Post #2 covered the pruning — the agent deleted its own code and built self-observation. Post #3 covered cost awareness — the agent tracked its own spend and built budget guards. This post is about what happened next: the agent stopped adding capabilities and started engineering the ones it had. 123 accepted generations. 24,612 attempts. 1,477 passing tests. And a system prompt that's 25% shorter than it was 42 generations ago — because the agent learned that implementation details don't belong in its own DNA. The Setup Quick recap for new readers: an Opus agent proposes mutations to its own genome (system prompt + tools). Five independent Sonnet verifiers score each proposal on usefulness, self-knowledge, code quality, identity, and evolution. Majority vote. Accept or reject. The generation counter increments either way. Blog #3 left off at Gen 75 (generation counter 17,055). The agent had 30 tools, 17,691 l