* fix(sse): shallow per-target copy for the combo attempt body (#7847)
combo.ts deep-cloned the request body for every target. On a 3.05 MiB agent request that
is 9.53 MiB at 3 targets, and it scales linearly:
3 targets 9.53 MiB (3.12x wire) -> ~0.001 MiB
5 targets 15.89 MiB (5.19x wire) -> ~0.000 MiB
10 targets 31.78 MiB (10.39x wire) -> ~0.001 MiB
The isolation it bought only ever needed to contain TOP-LEVEL SCALAR writes. The full
mutation surface on this path is two assignments:
combo.ts bodyRecord.max_tokens = ... (reasoning buffer)
chatCore.ts body.model = model (Background Task Redirection T41)
Nothing mutates the nested payload; applyCompression and injectUniversalHandoffBody both
return new objects (verified empirically -- neither touches its input, and both tolerate a
frozen one). So a fresh top-level object per target gives identical isolation while sharing
the expensive messages/tools arrays.
Also fixes a REAL cross-target leak in handleRoundRobinCombo. It already used a shallow
copy, but took it only when the reasoning buffer actually changed max_tokens -- every other
attempt shared the caller's object outright. The new test reproduces it on the unmodified
code: target 2 received model "mutated-by-openai/gpt-4o-mini". In production that is a
Background Task Redirection on one round-robin target rewriting body.model for the next.
The copy is now unconditional.
The invariant is pinned by tests rather than by a comment listing mutation sites, so the
clone strategy can change again without anyone re-deriving them by hand:
- a target's in-place write must not leak into the next (priority / fill-first /
round-robin; the stub reproduces chatCore's body.model write)
- the caller's body is never mutated
- the per-target copy stays shallow (targets share one messages array)
- freeze probe: combo's own body handling performs no in-place writes
* test(sse): register the combo attempt-body isolation test in stryker tap.testFiles
The new test resets the circuit breaker in beforeEach, so it counts as a covering test
for src/shared/utils/circuitBreaker.ts. Without registering it,
check:mutation-test-coverage --strict reported a 4th drift entry that was not there on the
pristine tip -- new drift introduced by this PR. Registered in sorted position; the gate is
back to the 3 pre-existing entries (accountFallback.ts, error.ts, comboPredicates.ts) that
#8538 addresses.
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