diff --git a/Makefile b/Makefile
index ee47bc8e..d02cdd32 100644
--- a/Makefile
+++ b/Makefile
@@ -293,6 +293,15 @@ test-vod:
 dev-setup:
 	$(MAKE) -j16 app-cached dev-setup-meson
 
+# Fast alternative to dev-setup for fresh worktrees: seed the build dir from an
+# already-built checkout (defaults to the main checkout this worktree came
+# from) instead of doing a cold meson/GStreamer build. Only reads the donor.
+# SEED_FROM=/path/to/built/checkout to pick a different donor.
+SEED_FROM?=
+.PHONY: dev-setup-fast
+dev-setup-fast:
+	bash hack/seed-build-dir.sh $(SEED_FROM)
+
 .PHONY: dev
 dev: app-cached $(LEXICON_STAMP)
 	if [ ! -d $(BUILDDIR) ]; then $(MAKE) dev-setup; fi
diff --git a/hack/seed-build-dir.sh b/hack/seed-build-dir.sh
new file mode 100755
index 00000000..f05803d6
--- /dev/null
+++ b/hack/seed-build-dir.sh
@@ -0,0 +1,166 @@
+#!/usr/bin/env bash
+# seed-build-dir.sh — make a fresh worktree Go-test-ready in seconds instead of
+# a ~1h cold `make dev-setup`.
+#
+# The dev (shared-library) build installs everything into the build dir itself
+# (meson prefix == build dir). The cgo deps that `go test` links against —
+# GStreamer, FFmpeg, glib, x264, libiroh_streamplace — are a pure function of
+# the meson inputs (meson.build, subprojects/*.wrap, rust/, Cargo.lock), NOT of
+# the branch's Go code. So a new worktree can copy the installed tree from an
+# already-built checkout, rewrite the absolute prefix paths in the .pc files,
+# and immediately run `go build` / `go test` against it.
+#
+# Usage:
+#   hack/seed-build-dir.sh [DONOR_CHECKOUT] [--force]
+#
+# DONOR_CHECKOUT defaults to the main checkout this worktree was created from
+# (derived from `git rev-parse --git-common-dir`). The donor is only ever read.
+#
+# After seeding, run tests with hack/test-env.sh, e.g.:
+#   hack/test-env.sh go test -run TestStreamTranscoder -v ./pkg/media
+#
+# What this does NOT cover:
+# - Changes to the C/rust dependency stack itself (meson.build, subprojects,
+#   rust/, Cargo.lock). The manifest check below catches those and tells you to
+#   run a real `make dev-setup`.
+# - Changes to the meson option blocks in the Makefile (BASE_OPTS/SHARED_OPTS).
+#   Those are rare and not covered by the manifest; if you're editing them,
+#   you're doing a real build anyway.
+# - The real frontend. js/app/dist gets a placeholder index.html so that
+#   packages embedding js/app compile; run `make app` if you need the actual UI.
+set -euo pipefail
+
+FORCE=0
+DONOR_ROOT=""
+for arg in "$@"; do
+  case "$arg" in
+    --force) FORCE=1 ;;
+    -h|--help) grep '^#' "$0" | sed 's/^# \{0,1\}//' | head -30; exit 0 ;;
+    *) DONOR_ROOT="$arg" ;;
+  esac
+done
+
+WT_ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+
+BUILDOS="${BUILDOS:-$(uname -s | tr '[:upper:]' '[:lower:]')}"
+BUILDARCH="${BUILDARCH:-$(uname -m | tr '[:upper:]' '[:lower:]')}"
+case "$BUILDARCH" in
+  aarch64) BUILDARCH=arm64 ;;
+  x86_64) BUILDARCH=amd64 ;;
+esac
+BUILDDIR="${BUILDDIR:-build-$BUILDOS-$BUILDARCH}"
+
+if [ -z "$DONOR_ROOT" ]; then
+  COMMON_DIR="$(cd "$WT_ROOT" && git rev-parse --path-format=absolute --git-common-dir)"
+  DONOR_ROOT="$(dirname "$COMMON_DIR")"
+fi
+DONOR_ROOT="$(cd "$DONOR_ROOT" && pwd)"
+
+if [ "$DONOR_ROOT" = "$WT_ROOT" ]; then
+  echo "error: donor checkout resolves to this checkout ($WT_ROOT)." >&2
+  echo "Pass the path to an already-built checkout explicitly:" >&2
+  echo "  hack/seed-build-dir.sh /path/to/built/streamplace" >&2
+  exit 1
+fi
+
+DONOR_BUILD="$DONOR_ROOT/$BUILDDIR"
+DEST_BUILD="$WT_ROOT/$BUILDDIR"
+
+if [ ! -f "$DONOR_BUILD/lib/pkgconfig/streamplacedeps.pc" ]; then
+  echo "error: $DONOR_BUILD doesn't look like a completed dev build" >&2
+  echo "(missing lib/pkgconfig/streamplacedeps.pc). Run 'make dev-setup' in the" >&2
+  echo "donor checkout first, or pass a different donor." >&2
+  exit 1
+fi
+
+if [ -e "$DEST_BUILD" ]; then
+  if [ "$FORCE" = 1 ]; then
+    echo "--force: removing existing $DEST_BUILD"
+    rm -rf "$DEST_BUILD"
+  else
+    echo "error: $DEST_BUILD already exists; rm -rf it or pass --force." >&2
+    exit 1
+  fi
+fi
+
+# Hash the inputs that determine the native dependency build. If these differ
+# between donor and worktree, the donor's libs may not match what this branch
+# expects and you need a real build.
+# Only tracked files count: builds promote extra gitignored .wrap files into
+# subprojects/, which must not make two otherwise-identical checkouts mismatch.
+manifest() {
+  local root="$1" out
+  out="$(
+    cd "$root"
+    git ls-files -z -- meson.build 'subprojects/*.wrap' Cargo.toml Cargo.lock rust |
+      LC_ALL=C sort -z | xargs -0 -r sha256sum
+  )"
+  if [ -z "$out" ]; then
+    echo "error: git ls-files found no native build inputs in $root" >&2
+    echo "(not a streamplace checkout, or git can't read it here?)" >&2
+    return 1
+  fi
+  printf '%s' "$out" | sha256sum | cut -d' ' -f1
+}
+
+DONOR_MANIFEST="$(manifest "$DONOR_ROOT")"
+WT_MANIFEST="$(manifest "$WT_ROOT")"
+if [ "$DONOR_MANIFEST" != "$WT_MANIFEST" ]; then
+  echo "error: native build inputs differ between this checkout and the donor:" >&2
+  echo "  donor    ($DONOR_ROOT): $DONOR_MANIFEST" >&2
+  echo "  worktree ($WT_ROOT): $WT_MANIFEST" >&2
+  echo "meson.build / subprojects/*.wrap / rust/ / Cargo.lock changed, so the" >&2
+  echo "donor's libraries may not match this branch. Run 'make dev-setup' for a" >&2
+  echo "real build, or re-run with --force if you know the change is irrelevant." >&2
+  if [ "$FORCE" != 1 ]; then
+    exit 1
+  fi
+  echo "--force: continuing anyway." >&2
+fi
+
+echo "Seeding $DEST_BUILD from $DONOR_BUILD ..."
+mkdir -p "$DEST_BUILD"
+# --reflink=auto: instant CoW copy on filesystems that support it, plain copy
+# otherwise. Deliberately NOT hardlinks: a later `meson install` in the donor
+# overwrites files in place, which would corrupt hardlinked copies.
+for d in lib include libexec bin; do
+  if [ -e "$DONOR_BUILD/$d" ]; then
+    cp -a --reflink=auto "$DONOR_BUILD/$d" "$DEST_BUILD/$d"
+  fi
+done
+
+# The .pc files carry the donor's absolute prefix; retarget them here.
+grep -rlF --include='*.pc' "$DONOR_ROOT" "$DEST_BUILD" | while read -r pc; do
+  sed -i "s|$DONOR_ROOT|$WT_ROOT|g" "$pc"
+done
+
+# Placeholder frontend + lexicon stamp so `go build` (go:embed js/app/dist) and
+# `make dev` work without a pnpm build. Run `make app` for the real UI.
+mkdir -p "$WT_ROOT/js/app/dist" "$WT_ROOT/.build"
+[ -f "$WT_ROOT/js/app/dist/index.html" ] || touch "$WT_ROOT/js/app/dist/index.html"
+touch "$WT_ROOT/.build/lexicon-stamp"
+
+# Canary: the donor's libs must be loadable in THIS environment. A build dir
+# made on the host doesn't load inside the builder container (newer glibc), and
+# vice versa can't be assumed either — catch that now, not at `go test` time.
+if [ -x "$DEST_BUILD/bin/gio" ]; then
+  if ! LD_LIBRARY_PATH="$DEST_BUILD/lib" "$DEST_BUILD/bin/gio" version >/dev/null 2>&1; then
+    echo "error: the seeded libraries don't load in this environment:" >&2
+    LD_LIBRARY_PATH="$DEST_BUILD/lib" "$DEST_BUILD/bin/gio" version 2>&1 | head -3 >&2 || true
+    echo "The donor ($DONOR_BUILD) was probably built against a different libc" >&2
+    echo "(e.g. built on the host, but you're seeding inside the builder" >&2
+    echo "container). Pick a donor checkout whose build ran in the same" >&2
+    echo "environment you'll run tests in." >&2
+    rm -rf "$DEST_BUILD"
+    exit 1
+  fi
+fi
+
+{
+  echo "seeded-from: $DONOR_BUILD"
+  echo "date: $(date -u +%Y-%m-%dT%H:%M:%SZ)"
+  echo "manifest: $WT_MANIFEST"
+} > "$DEST_BUILD/.seeded-from"
+
+echo "Done. Run Go tests with:"
+echo "  hack/test-env.sh go test -run <TestName> -v ./pkg/<pkg>"
diff --git a/hack/test-env.sh b/hack/test-env.sh
new file mode 100755
index 00000000..7dc0024a
--- /dev/null
+++ b/hack/test-env.sh
@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# test-env.sh — run a command with the env cgo Go tests need against this
+# checkout's dev (shared) build in build-<os>-<arch>/, e.g.:
+#
+#   hack/test-env.sh go test -run TestStreamTranscoder -v ./pkg/media
+#
+# With no arguments, prints export lines for eval:
+#
+#   eval "$(hack/test-env.sh)"
+#
+# Works with a build dir produced by `make dev-setup` or hack/seed-build-dir.sh.
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+
+BUILDOS="${BUILDOS:-$(uname -s | tr '[:upper:]' '[:lower:]')}"
+BUILDARCH="${BUILDARCH:-$(uname -m | tr '[:upper:]' '[:lower:]')}"
+case "$BUILDARCH" in
+  aarch64) BUILDARCH=arm64 ;;
+  x86_64) BUILDARCH=amd64 ;;
+esac
+BUILDDIR="${BUILDDIR:-build-$BUILDOS-$BUILDARCH}"
+B="$ROOT/$BUILDDIR"
+
+if [ ! -f "$B/lib/pkgconfig/streamplacedeps.pc" ]; then
+  echo "error: $B has no dev build; run 'make dev-setup' or 'hack/seed-build-dir.sh'." >&2
+  exit 1
+fi
+
+PKG_CONFIG_PATH="$B/lib/pkgconfig:$B/lib/gstreamer-1.0/pkgconfig:$B/meson-uninstalled"
+LD_LIBRARY_PATH="$B/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+CGO_LDFLAGS="${CGO_LDFLAGS:--lm}"
+# A seeded build dir contains libs whose compiled-in plugin path points at the
+# donor checkout; point GStreamer at this checkout's copies explicitly.
+GST_PLUGIN_PATH="$B/lib/gstreamer-1.0"
+
+if [ "$#" -eq 0 ]; then
+  echo "export PKG_CONFIG_PATH=\"$PKG_CONFIG_PATH\""
+  echo "export LD_LIBRARY_PATH=\"$LD_LIBRARY_PATH\""
+  echo "export CGO_LDFLAGS=\"$CGO_LDFLAGS\""
+  echo "export GST_PLUGIN_PATH=\"$GST_PLUGIN_PATH\""
+  exit 0
+fi
+
+export PKG_CONFIG_PATH LD_LIBRARY_PATH CGO_LDFLAGS GST_PLUGIN_PATH
+exec "$@"