Two mutants the cut-2 pass recorded as equivalent were not. Both notes were
reasoning, and both were wrong in the same way: they argued from the shape of
the code instead of trying the input that breaks it.
The decoder's length refusal reads `payloadLength < 0 || expected !== length`.
The note said the first clause could never fire on its own, because a negative
payload length makes the sum smaller than the frame. It does not: the sum also
carries the three identity lengths, and a negative payload cancels against
them. A 37-byte frame declaring `channelLength=65535, payloadLength=-65535`
sums to exactly 37, matches its own byte length, and without the clause decodes
to three blank identities and an empty payload where `CultMeshRealtimeWireProtocol`
refuses it. Two smaller spellings do the same, `ch=1, pl=-1` over 37 bytes and
`ch=2, pl=-1` over 38. No test covered that shape; one does now, and the
mutation is listed and killed rather than excused.
The payload copy was worse, because the fix and the bug looked identical. The
decoder used `bytes.slice(...)`, which copies a `Uint8Array` — and the existing
test passes one, so it was green — but Node's `Buffer` overrides `slice` with
an alias of `subarray`. A `Buffer` is what a socket read hands a Node caller,
which is exactly how the QUIC plane in cut 4 will reach this function, so every
decoded payload would have been a live view of a frame buffer the transport is
free to reuse.
`Uint8Array.prototype.slice.call(bytes, ...)` was the obvious repair and does
copy, but `%TypedArray%.prototype.slice` takes its result constructor from
`@@species`, and `Buffer`'s is `FastBuffer`: the payload then comes back a
`Buffer` from a `Buffer` frame and a `Uint8Array` from a `Uint8Array` one, so
the same frame compares equal or unequal under `deepStrictEqual` depending on
how it arrived. `new Uint8Array(bytes.subarray(...))` copies and is one type on
both. Three mutations now guard the line — return a view, return a `Buffer`
view, return a species-typed copy — and the first two die only on their own
input type.
Mutation runner: 75 mutations, 75 killed, 0 survivors, three green controls.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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