Why a program did not produce a value.

A failure is data, never an exception. A program that throws, loops forever, floods stdout, or dies with its substrate is a normal outcome of running model-authored code: the model gets told what happened and writes a better program. Raising would make the tool call an error and lose that loop.

The kinds are distinct because the model — and the operator reading a log — can act differently on each:

• :exception — the program raised. Its own bug; the message is the guest's.
• :timeout — the wall-clock budget expired. Possibly an infinite loop, possibly just slow; the program ran and may have had side effects.
• :abort — cancelled from outside (Nous.CodeRuntime.cancel/2), e.g. the agent run was cancelled. Not the program's fault.
• :substrate_exit — the runtime died underneath us. Ours to fix, not the model's, and the one kind that should page somebody.
• :invalid_output — the program returned something unrepresentable.
• :output_limit — it produced more output than the ledger allows. The retained prefix is still in Result.logs, because truncated evidence beats none.