Tool use responds to mid-training, not just post-training: MidTool synthesizes supervision from real APIs and MCP skills

arXiv 2608.20314·medium signalXBlueskyLinkedInCopy link

MidTool builds a data pipeline that mixes web text, PDFs and code with supervision synthesized from real tool APIs, MCP skills and document-grounded workflows, targeting four competencies: recognizing tool affordances, grounding arguments from context, composing call chains, and recovering from incomplete information. Applied to Qwen3-4B-Base and Qwen3-8B-Base ahead of SFT and RL, it improved BFCL, tau2-Bench and MCP Universe consistently. The claim worth testing is structural, that tool use behaves like math and science reasoning and benefits from a dedicated mid-training stage.