Reviewed the diff — this looks good. Small, well-scoped, and correctly implemented:

• Placement is right. The PaJack_SetClientName() call sits inside the if (!m_paInitialized) block immediately before Pa_Initialize(), which satisfies the pa_jack contract, and it re-runs correctly if the sink is ever re-initialized after Pa_Terminate() in the destructor.
• Lifetime is right. static const char[] satisfies pa_jack's reference-not-copy semantics; the comment documents why, which future readers will appreciate.
• The "only PortAudio user" claim is accurate. I verified CwSidetonePortAudioSink.{h,cpp} is the only code touching portaudio.h/Pa_Initialize in the tree, so naming the process-global JACK client here is unambiguous as stated.
• Guard works. Q_OS_LINUX is defined by the time the preprocessor reaches the check (the header pulls in QtCore via QString/CwSidetoneSinkBackend.h), and macOS/Windows builds are unaffected.

One non-blocking note: the __has_include(<pa_jack.h>) guard tests header presence, not whether libportaudio was actually built with JACK support. A hand-built portaudio with JACK disabled but headers installed would fail at link time on PaJack_SetClientName. All mainstream distro packages build with JACK enabled (and CI uses pkg-config portaudio), so this is a theoretical edge case — just worth knowing if a user ever reports a link error against a custom portaudio. Not asking for a change; a CMake symbol check would be heavier than this 13-line fix deserves.

Ignoring the PaJack_SetClientName return value is fine — the pa_jack implementation just stores the pointer and always returns paNoError.

Nice test plan with the pw-dump A/B ground truth, and good continuity with #3438/#3440. Thanks for the contribution, K6OZY! 73

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Reviewed the diff against the surrounding code — this looks good. Verified the details that matter for this kind of change:

• Guard evaluates correctly. The new #if defined(Q_OS_LINUX) && __has_include(<pa_jack.h>) sits after #include "CwSidetonePortAudioSink.h", which pulls in <QString>, so Q_OS_LINUX is already defined at that point. No silent no-op on Linux.
• Call ordering and lifetime are right. PaJack_SetClientName is called inside the !m_paInitialized block immediately before Pa_Initialize(), and re-runs correctly on any Pa_Terminate/re-init cycle. The function-local static const char[] satisfies pa_jack's reference-not-copy contract.
• "Only PortAudio user" claim holds. Pa_Initialize appears only in this file; AudioEngine references HAVE_PORTAUDIO for backend selection but never initializes PortAudio itself. Naming the process-global JACK client here is unambiguous as stated.
• Build safety. PortAudio comes from pkg-config (system package), and pa_jack.h is only installed when portaudio was built with JACK support, so the __has_include guard doubles as a link-safety check in practice. macOS/Windows are excluded cleanly.

One optional nit, not blocking: PaJack_SetClientName returns a PaError that's silently discarded. A failure just leaves the default name (and the 21-char name is well under JACK's 64-char limit), so this is harmless — but a qCWarning(lcAudio) on non-paNoError would match the error-logging style of the rest of this file if you want it.

Nice, tightly-scoped fix with a solid pw-dump A/B test plan — thanks for continuing the patchbay-identifiability work from #3438, @Ozy311!

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Reviewed the diff, both commits, and the related patchbay-naming work (#3438, #3440).

Technical sign-off:

• Call ordering is correct — PaJack_SetClientName sits inside !m_paInitialized immediately before Pa_Initialize(), satisfying the pa_jack pre-init contract. Re-runs correctly on any Pa_Terminate/re-init cycle.
• static const char[] lifetime is correct — pa_jack references, doesn't copy; static storage outlives the PortAudio session.
• Verified independently that Pa_Initialize appears only in this file — the "only PortAudio user, naming the process-global JACK client is unambiguous" claim holds.
• Guard evaluates correctly on Linux — Q_OS_LINUX is defined before the #if via the existing Qt includes.
• Second commit fully resolves the bot's return-value nit; qCWarning(lcAudio) on non-paNoError now mirrors the Pa_Initialize error path in style.

One non-blocking note for the record: __has_include(<pa_jack.h>) guards header presence but not whether the installed PortAudio was compiled with JACK support. A hand-built PortAudio with JACK disabled but headers still on disk would fail at link time. All mainstream distros package portaudio with JACK enabled and CI uses pkg-config, so this is purely theoretical — not requesting a change, just noting it for anyone who hits a link error against a custom PortAudio build.

Principle XI — Fix is demonstrated: pw-dump A/B ground truth on a live FLEX-6700 is exactly the right evidence standard for a JACK node-naming change. Nice work continuing the patchbay-identifiability line from #3438.