Routing coding agent sessions across Claude Code, Codex, and Ollama in one harness — model picked per session

Spent the last few months building an agentic coding setup for my team. The design decision I'd defend hardest is refusing to marry a single provider, mostly because every model I've committed to has been obsoleted roughly six weeks later. Everything runs through one session abstraction. Underneath, three execution engines: Claude Code CLI — Opus/Sonnet, does the heavy lifting on real refactors Codex CLI — GPT-5.6 variants Ollama — minimax-m3 and glm-5.2 via cloud, same path works fully local Twelve models, picked per session from a dropdown. The session, its history, and its working directory don't know or care which engine is behind it. Why it was worth it: routing by task value. Renaming a variable does not require a frontier model, no matter how much the frontier model would enjoy it. Cheap model for config tweaks, frontier for the multi-file refactors, local for anything that can't leave the box. New model drops, it's a config entry instead of a weekend. The genuinely annoying part is that the three CLIs agree on nothing. Session resumption, streaming format, approval prompts, token reporting — all different, all confidently so. Roughly 80% of the work was normalizing that into one interface. The other 20% was the fun part I originally started this for. 1,851 sessions through it so far, 15-person team. Anyone else running multi-engine? Still picking models by hand like an animal — curious if anyone's automated the routing.