Waiting Is Not a Tool Call: Making an MCP Server's Shell Event-Driven

One of our agents ran a test suite. The suite takes four minutes. The MCP client's idle timeout is sixty seconds. You can see where this is going. At second sixty the client cancelled the call. The process kept running — nobody told it to stop — while the model, holding a cancellation where its test results should be, did the reasonable thing and ran the suite again. Two test suites, same directory, racing each other over the same build artifacts. The second one failed with a locking error, the model reported the tests as broken, and the tests were fine. In another session the same model, burned before, developed a workaround: run the build, then call sleep 240 , then look. A tool call that does nothing, held open for four minutes, so that a different tool call might have something to show. The model had reinvented polling, badly, because we hadn't given it anything better. I build octofs , an open-source MCP filesystem server, and this incident set the agenda for eleven releases in two weeks (0.10.1 through 0.14.1). The principle behind them is one I keep coming back to: an MCP server's real interface is every string it hands back to the model. These releases apply it to the slowest string of all — the one the model waits for. The shell is now event-driven. Commands start in the foreground, move to the background on their own if they outlast ten seconds, and the client gets a notification when they finish. Nothing blocks, nothing gets killed, nothing runs twice. First fix: prove the call is alive The sixty-second cancellation had a shallow cause and a deep one. The shallow one: a shell call is silent by nature. A build that's compiling says nothing on the wire for minutes, and to an MCP client silence is indistinguishable from a hung server. So 0.10.2 added liveness heartbeats — while a command runs in the foreground, octofs emits a progress notification every ten seconds, well below any sane idle timeout, so a single missed beat can't cancel the call. That stopped