Routing Down Is Easy. Knowing When Not To Is Hard: Why Cheap Models Break Your Coding Agent

Disclosure: I maintain Lynkr , an open-source router whose design decisions this post explains. The failure modes described are patterns widely reported across router issue trackers and local-LLM forums — the examples are representative reconstructions, not captured transcripts. The problem is real either way; ask anyone who's routed a coding agent to a 7B model. Everyone who gets their first LLM router working does the same thing within the hour: point the expensive coding agent at a free local model and watch the bill drop to zero. Then the agent tries to edit a file. The graveyard of downgraded sessions If you browse the issue tracker of any Claude Code router — or r/LocalLLaMA on any given week — you'll find the same story in a hundred variations. The routing works perfectly. The session dies anyway. The killers, in rough order of frequency: 1. Malformed tool arguments. The agent decides to call Edit , and the model produces arguments that are almost JSON: { "file_path" : "src/auth.js" , "old_string" : "if (token) {" , "new_string" : "if (token && !expired) {" One missing brace. The harness rejects the call, the model retries, produces a different malformation, and you're three turns deep into fixing nothing. Frontier models emit structurally valid tool calls with boring reliability; sub-10B models do it most of the time — and "most of the time," at 30 tool calls per session, means every session breaks. 2. Stale string matching. Edit -style tools require the old_string to match the file exactly. Small models paraphrase from memory instead of quoting — they'll "remember" the line as if (token) { when the file says if (accessToken) { . The edit fails, the model re-reads the file, burns 2,000 tokens, tries again with a different paraphrase. This is the single most reported failure, because it looks like the router's fault and is actually a capability cliff. 3. Hallucinated context. Ask a small model to run tests and it may confidently call Bash with npm test -- --g