The fork test asked the model to call subagent with fork: true and no agent, which the tool schema has never supported - it only passed when a model improvised. It now spawns a real fork-mode agent (test-fork with session-mode: fork). The harness default model is now openai-codex/gpt-5.6-luna. The previous active-session-model fallback broke under pi -ne, where extension-registered providers do not resolve.