VS Code via Xcode build server (sourcekit-xcode-bsp)

Status: ready for early adopters, uncommitted · 2026-10-14 Parent: projects/vscode-migration/vscode-migration Tech design: projects/vscode-migration/tech-design-xcode-build-server

An alternative to the projects/vscode-migration/tech-design-spm-browser-package: keep the Xcode projects as they are and give VS Code/Cursor what it needs from them.

Code intelligence: sourcekit-lsp ← buildServer.json ← sourcekit-xcode-bsp (BSP server on top of swift-build, Xcode's build engine; early-stage 0.1.0, Apache 2.0), patched by us.
Build/test/debug: xcodebuild tasks + lldb-dap launch configs, macOS and iOS simulator.
Builds and index shared with Xcode through the worktree's DerivedData folder.

Where things are

What	Where
VS Code setup	worktree `vscode-xcode-bsp`, branch `alex/vscode-xcode-bsp` — `DuckDuckGo.code-workspace`, `scripts/vscode/` (uncommitted)
BSP fork	`~/Developer/sourcekit-xcode-bsp`, branch `configuration-name` (uncommitted), base upstream `726ce706`; identical to `scripts/vscode/sourcekit-xcode-bsp.patch`
Installed BSP	`~/.local/opt/sourcekit-xcode-bsp` (by `install-build-server.sh`, with `patch.sha256` stamp); brew 0.1.0 also installed but unused

Files (`scripts/vscode/`)

setup-build-server.py [macos|ios] — writes buildServer.json on folder open. Keeps the current platform without an argument. Build root = the worktree's Xcode DerivedData, derivedDataLayout: xcode, configuration: Debug, target filter (app project + *Tests-product, no App Store variants, no other-platform app/local packages). Warns when the installed BSP was built from an older patch.
xcode_derived_data.py — Xcode's DerivedData folder for the worktree (MD5 → 28 letters; honors a custom location).
sourcekit-xcode-bsp.sh — argv of buildServer.json. Editor's server stops a running background indexer and waits for it; the indexer's server refuses to start next to an editor's. Pid files in .build/bsp/, checked to be sourcekit processes.
xcodebuild.sh <build|test> <scheme> [destination] — Debug into Xcode's DerivedData with Xcode's settings (no -derivedDataPath: passing it changes build parameters and makes Xcode and VS Code rebuild after each other). DDG_WORKTREE_CACHE=1: .build/xcodebuild + shared CAS with /^wt /^dd maps. .build/products → products folder. xcbeautify + raw path:line: error: lines for failed tests.
simulator.sh [--build-only|--no-build] [device] — builds iOS Browser for a concrete simulator, installs, simctl launch --wait-for-debugger, writes process attach --pid N to .build/simulator-attach.lldb.
index-in-background.py [--all] [--if-idle] — headless indexing; background-indexing.sh install = launchd agent every 30 min.
install-build-server.sh + sourcekit-xcode-bsp.patch + sourcekit-xcode-bsp.Package.resolved — reproducible build of the patched BSP.

Findings

Works for app code out of the box, no Package.swift. Swift extension 2.16.7 activates on buildServer.json.
Tests: nothing. BSP had no configuration → swift-build used defaultConfigurationName = Release → no testability → @testable import fails. Fix: configuration (default Debug; swift-build's own index build defaults to Debug too).
Stray .o, failed links, duplicate outputs. BSP configured all 977 targets in one build (DMG + App Store, iOS + macOS, each package library 3–4 times). Fix: targets filter → 86 roots, 288 in the graph, 0 failures.
Indexing ~2 h. swift-build computes sourceKitOptions per file (~0.8 s, single thread, whole build request each time). Swift args of a target differ only in the trailing -index-unit-output-path. Fix: per-target cache keyed by (target, purpose) + single flight. Fresh worktree full index ~5.5 min (M4 Max, low load).
Preparation (~250 serialized prepare builds) is now the critical path.
Shared CAS between worktrees (xcodebuild): needs SWIFT/CLANG_ENABLE_PREFIX_MAPPING + *_OTHER_PREFIX_MAPPINGS = <DerivedData>=/^dd <worktree>=/^wt. Clean build 179 s → 90–120 s, 0 misses in a new worktree. Xcode's own project prefix mapping (/^src per project) breaks lldb sources; /^wt + sourceMap works. Opt-in now, because it can't be shared with Xcode (unconfirmed how Xcode's debugger handles mapped paths).
CAS doesn't help the language server (prepare-for-index doesn't cache; cold vs warm the same).
Index shared with Xcode works: with Xcode's DerivedData layout (index store = Index.noindex/DataStore, INDEX_REGULAR_BUILD_* = Build/…), units written by an xcodebuild build count as up to date: after building macOS Browser, indexing skipped all app files (2,150 files left vs 3,972). BSP prepares in Index.noindex/SourceKitBuild so it never shares a build DB with an open Xcode.
Xcode-compatible builds: xcodebuild without -derivedDataPath and the wrapper don't rebuild after each other (no-op 26–31 s). With -derivedDataPath pointing to the same folder they do.
iOS simulator: generic destination fails (fix_libswiftWebkit_simulator.sh needs TARGET_DEVICE_OS_VERSION) → build for a concrete simulator. Debugging in one launch: preLaunchTask runs simulator.sh (launch --wait-for-debugger, writes the attach command), the config attaches with attachCommands: command source .build/simulator-attach.lldb (read at attach time, after the task). Earlier failures of attachCommands/program were caused by the wrong sourceMap (10a), not by lldb-dap. 10a. Gray breakpoints in VS Code (macOS app): the launch configs had sourceMap /^wt → worktree for all builds. lldb also applies it in reverse when setting breakpoints, so <worktree>/…/File.swift became /^wt/…/File.swift, which regular (Xcode-mode) debug info doesn't contain → "no locations". Fix: no sourceMap; xcodebuild.sh writes .build/debug.lldbinit per build (source map only for DDG_WORKTREE_CACHE=1), launch configs command source it in initCommands. Confirmed fixed in VS Code 2026-10-15.
Index sharing between worktrees: rejected (absolute paths + mtime-based staleness).
workspace/_pollIndex isn't in release sourcekit-lsp → indexer waits for "Indexing" progress end + 60 s quiet; gives up after 5 min without progress.
swift-build service runs in its own process group → stop children first.
Unrelated: main's SharedPackages/BrowserServicesKit/Package.resolved is stale (C-S-S 17.20 vs exact: 17.26.0) — separate task spawned.

Review (independent agent, 2026-10-14)

Verdict: "close, not ready" → fixed: iOS platform reset by setup/indexer, unpinned swift-tools-protocols (now Package.resolved + --force-resolved-versions), purpose not in cache key, empty include, sources not generation-checked, test gaps (58 tests now), editor/indexer race (refuse + wait), stop() killing unchecked pids, indexer hanging 1 h, spaces in prefix maps, in-place install over running binaries, stale-install detection, test failures not in Problems, Xcode 26/27 wording, re.escape, as_uri, editor detection, xcbeautify missing warning, files.watcherExclude, lldb-dap pinned, risky waitFor attach removed, mismatched scheme/destination task. Not changed: Debug default (documented; matches swift-build's index default).

Verified (Xcode 27.1)

Code intelligence: macOS app, macOS unit test (@testable, jump into app), package test, iOS app (ios mode) — in both layouts.
Test macOS Unit Tests task (33 tests pass), Build macOS Browser, iOS build for simulator, install, launch, lldb-dap attach + breakpoint; macOS launch + breakpoint (CLI lldb).
Background indexer: full run, skip when unchanged, editor takeover, no orphans.
Installer from scratch: patch applies, pinned build, stamp.

Not verified: VS Code/Cursor UI itself; Xcode IDE reading our index / no-rebuild after VS Code builds (xcodebuild was used as Xcode's stand-in).

Open items

Walk through VS Code and Cursor UI (tasks, Problems, launch/attach, workspace trust prompt).
Check Xcode IDE: open a worktree indexed/built from VS Code — no reindex, no rebuild.
Upstream PRs to slime-studio/sourcekit-xcode-bsp (public repo, needs OK); or a DDG fork.
Upstream issue: duplicate outputs/stray .o when all targets are configured; lldb-dap attach with program/attachCommands to simulator processes.
PR (needs Asana task link).

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