Can verifiers monitor reasoning without slowing generation down?

Existing test-time verification sits at two unattractive extremes. Final-answer verification misses errors that happen early in a long trace. Branch-and-verify strategies explore multiple trajectories and pay a large compute multiplier for the privilege. interwhen's contribution is architectural: it decouples verification from generation so that verifiers run asynchronously alongside a single reasoning trajectory rather than being woven into generation or requiring branching.

The mechanism has two parts. First, instead of forcing the model to verify itself or prompting it into fixed steps (which constrains its reasoning strategy), a monitoring system periodically polls the trace and creates a forked execution that extracts the current verifiable state — the input variables a verifier needs. Second, the verifiers execute concurrently with generation and interrupt only when a violation is detected (or a write is attempted). On correct executions nothing fires, so the latency penalty is negligible; the cost is incurred only when it prevents an error.

The design choice that makes this work is treating verification as an out-of-band observer rather than an in-band participant. The model reasons freely; the verifier watches and intervenes surgically. This is the inverse of approaches that bake checking into the generation loop. It connects to a broader theme that process supervision is more informative than outcome supervision — since Why do standard process reward models fail on thinking traces?, any process-level checker must cope with the messy structure of real traces; interwhen sidesteps this by extracting clean state snapshots via the fork rather than scoring the raw trace. A counterpoint: the polling-and-forking adds engineering complexity and a small per-poll inference cost, so the "negligible overhead" claim holds in the common case but not adversarially. Why it matters: it offers a plug-and-play way to add formal checking to any reasoning agent at near-parity token cost — interwhen dominates CoT on every benchmark column at similar token budgets.