Mario Zechner explains why he stripped his coding agent to four tools

Zechner's coding agent, Pi, ships four tools: read, write, edit, and bash. No MCP, no sub-agents, no built-in plan mode, no background bash, no built-in todo list; his standard answer for each is to write it as a file or spawn it in tmux instead (18:15 to 22:37). He grounds the minimalism in TerminalBench, where Terminus, an agent whose only interface is keystrokes into a tmux session, performs near the top of the leaderboard, which he reads as evidence that feature surface may not be what makes an agent perform well (16:07 to 17:36). He names a specific harness failure mode worth watching: OpenCode's per-turn session compaction prunes tool results before the last 40,000 tokens, which destroys the prompt cache, and he ties that directly to friction between Anthropic and OpenCode over infrastructure use (11:37 to 12:40). He also argues against approval dialogues as a safety mechanism on the grounds that they cause fatigue and get rubber-stamped, preferring containerization instead (21:01 to 21:15), and against feeding LSP errors back to the model mid-edit, since an unfinished edit is not a bug (12:52 to 14:08). Auto-generated captions throughout; paraphrased rather than quoted.

Why it matters: Pi is a working counterexample to the assumption that more tool surface makes a harness more capable. If you are deciding what to build into your own harness, Zechner argues for a smaller default.