Isolation and repeatability

Each execution receives its own identity and trace. Trials are independent executions; they are not automatic retries. A server subprocess can still retain state for the lifetime of one connection, and an external HTTP service can retain state beyond the test.

Make direct contract tests deterministic: use fixed inputs, assert structured results, and reset owned state. Agent tests are inherently variable, so assert captured behavior and run multiple trials when the variance matters.

Managed harness runtimes isolate the selected executable and writable runtime state. They are not an operating-system sandbox. Workspace, terminal, and tool policies still define what the harness may do.