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Retail Rock Band 3 enforces one player per instrument: you can never have two guitarists or two drummers at once. In a twelve-day sprint I built that feature into the sixteen-year-old game — multiple local players on the same instrument, each with independent difficulty and scoring — and shipped it as a 1.0.0 release, verified on physical Xbox 360 hardware.

The work sits on top of RB3Enhanced, a community-maintained open-source (GPLv2) runtime mod. My contribution is a self-contained feature drop: 34 commits on master, centered on a 2,058-line hook layer that defeats the one-player-per-instrument rule at every engine layer it is baked into — UI part selection, part-claim arbitration, track binding, per-player note state, and audio mixing. Every hook is inert behind a default-off config flag, so existing installs see zero behavioral change.

The hardest stretch was a series of numbered diagnosis cycles debugging the closed-source retail binary live on a devkit console over the Xbox debug protocol. Rather than patching around a recurring crash, each cycle converted a symptom into a named root cause — culminating in tracing the crash loop to a slot-assignment function leaving an over-subscribed player at slot −1, and fixing it at the source. The two- and three-drummer crash was eliminated entirely, confirmed on hardware.

Two design problems stand out. First, the feature needed a patched ~7 MB game asset that is roughly 99% stock copyrighted content — unshippable for a GPL project. The solution: the mod embeds a 691-byte compressed splice plan and reconstructs the patched asset from the user’s own game files at first boot, with SHA-256 gates on input and output and an automated tripwire that rejects any shipped byte sequence matching the stock file. Not a single copyrighted byte is distributed. Second, to build without Microsoft’s proprietary SDK, I wrote a from-scratch open reconstruction of the Xbox 360 SDK headers (1,141 lines across nine headers) — reusable by any homebrew or preservation project targeting the platform.

The release shipped as “RB3 Overdrive” with full GPLv2 source. It is a compact demonstration of end-to-end delivery under constraint: reverse-engineering knowledge turned into a real feature, root-caused on real hardware, packaged with legal diligence, in twelve days.