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`kern run` steps through the forward that takes `--rows` rows and adds `out.len() - 1` to its drafted-rows tally. A forward that writes no `tokens` output (the protocol only needs some forward to sample, not the one the step picks) hands back nothing: the subtraction underflows, a panic in debug, and in release the cursor never advances, so the loop never ends. The step now fails naming the forward. Reproduced with Qwen3-4B whose `decode` has its `sample` call removed (`decode_batch` still samples, so the protocol accepts it) and `kern run --rows 1`: the release build captured the step's graph and spun until a 90 s timeout killed it; with this change it exits 1 at once with "`decode` handed back no token".
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The protocol requires some forward to sample, not the one
kern runsteps through. When the step's forward writes notokensoutput,emittedreturns nothing andout.len() - 1underflows: a panic in debug, and in release a loop that never advances. The step now fails, naming the forward.Verification
Qwen3-4B with
sampleremoved fromdecode(decode_batchstill samples, so the protocol accepts it) andkern run --rows 1: before, the release build spun until a 90 s timeout killed it; after, it exits 1 with`decode` handed back no token; kern run steps through a forward that samples.