diff --git a/cmd/internal-tools/implementation-sweep/end_to_end_test.go b/cmd/internal-tools/implementation-sweep/end_to_end_test.go
new file mode 100644
index 0000000..3817520
--- /dev/null
+++ b/cmd/internal-tools/implementation-sweep/end_to_end_test.go
@@ -0,0 +1,482 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"math/rand"
+	"net"
+	"net/http"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+	"time"
+)
+
+// The test binary doubles as the stub implementation's server and as the
+// fetcher the stub campaign uses, so the sweep drives a real preview process
+// over a real port and the served URL is answered by a real HTTP server.
+func TestMain(m *testing.M) {
+	switch {
+	case os.Getenv("SWEEP_TEST_SERVE_PORT") != "":
+		serveUntilKilled(os.Getenv("SWEEP_TEST_SERVE_PORT"))
+	case os.Getenv("SWEEP_TEST_FETCH_URL") != "":
+		recordFetch(
+			os.Getenv("SWEEP_TEST_FETCH_URL"),
+			os.Getenv("SWEEP_TEST_FETCH_LOG"),
+		)
+	default:
+		os.Exit(m.Run())
+	}
+}
+
+func serveUntilKilled(port string) {
+	handler := http.HandlerFunc(
+		func(writer http.ResponseWriter, request *http.Request) {
+			fmt.Fprintf(writer, "%s %s", port, request.URL.RequestURI())
+		},
+	)
+	http.ListenAndServe("localhost:"+port, handler)
+}
+
+func recordFetch(url, logPath string) {
+	line := ""
+	response, err := http.Get(url)
+	if err != nil {
+		line = fmt.Sprintf("%s -> error %v\n", url, err)
+	} else {
+		body, _ := io.ReadAll(response.Body)
+		response.Body.Close()
+		line = fmt.Sprintf("%s -> %d %s\n", url, response.StatusCode, body)
+	}
+	logFile, err := os.OpenFile(
+		logPath,
+		os.O_CREATE|os.O_WRONLY|os.O_APPEND,
+		0o644,
+	)
+	if err != nil {
+		return
+	}
+	logFile.WriteString(line)
+	logFile.Close()
+}
+
+// stubBun answers install, fails to build impl-02, and serves the preview from
+// the test binary on the port it was given.
+const stubBun = `#!/bin/sh
+echo "$PWD $*" >> "%[1]s"
+if [ "$1" = "run" ] && [ "$2" = "build" ]; then
+  case "$PWD" in *impl-02) echo "TS2322: type error" >&2; exit 1 ;; esac
+  exit 0
+fi
+if [ "$1" = "run" ] && [ "$2" = "preview" ]; then
+  port=""
+  previous=""
+  for argument in "$@"; do
+    if [ "$previous" = "--port" ]; then port="$argument"; fi
+    previous="$argument"
+  done
+  SWEEP_TEST_SERVE_PORT="$port" exec "%[2]s"
+fi
+exit 0
+`
+
+// stubCampaign records the argv it was handed and whether the sweep manifest
+// was already on disk when it ran, fetches the URL it was told to drive, writes
+// the campaign directory the real tool would write, and fails impl-03 seed 4.
+const stubCampaign = `#!/bin/sh
+output=""
+seed=""
+url=""
+previous=""
+for argument in "$@"; do
+  case "$previous" in
+    --output) output="$argument" ;;
+    --seeds) seed="$argument" ;;
+    --bundle-id) url="$argument" ;;
+  esac
+  previous="$argument"
+done
+manifest=missing
+if [ -f "%[1]s" ]; then manifest=present; fi
+echo "manifest=$manifest argv: $*" >> "%[2]s"
+SWEEP_TEST_FETCH_URL="$url" SWEEP_TEST_FETCH_LOG="%[3]s" "%[4]s"
+mkdir -p "$output"
+printf '{"arm":"stub","seeds":[%%s]}\n' "$seed" > "$output/campaign.json"
+echo "stub campaign seed=$seed url=$url"
+case "$output" in
+  *impl-03/seed-4) exit 1 ;;
+esac
+exit 0
+`
+
+func TestRun_EndToEndAgainstStubBunAndCampaign(t *testing.T) {
+	root := t.TempDir()
+	implementations := filepath.Join(root, "implementations")
+	for _, name := range []string{"impl-01", "impl-02", "impl-03"} {
+		if err := os.MkdirAll(filepath.Join(implementations, name), 0o755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	specPath := filepath.Join(root, "relay.ts")
+	if err := os.WriteFile(specPath, []byte("export const properties = [];\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	output := filepath.Join(root, "campaigns")
+	bunLog := filepath.Join(root, "bun.log")
+	campaignLog := filepath.Join(root, "campaign.log")
+	fetchLog := filepath.Join(root, "fetch.log")
+	testBinary, err := filepath.Abs(os.Args[0])
+	if err != nil {
+		t.Fatal(err)
+	}
+	bunPath := writeScript(
+		t,
+		filepath.Join(root, "stub-bun"),
+		fmt.Sprintf(stubBun, bunLog, testBinary),
+	)
+	campaignPath := writeScript(
+		t,
+		filepath.Join(root, "stub-campaign"),
+		fmt.Sprintf(
+			stubCampaign,
+			filepath.Join(output, manifestFileName),
+			campaignLog,
+			fetchLog,
+			testBinary,
+		),
+	)
+	sanderlingPath := writeScript(
+		t,
+		filepath.Join(root, "stub-sanderling"),
+		"#!/bin/sh\nexit 0\n",
+	)
+	basePort := freePortRange(t, 3)
+
+	var stdout bytes.Buffer
+	err = run([]string{
+		"--implementations", implementations,
+		"--spec", specPath,
+		"--seeds", "4-5",
+		"--max-steps", "40",
+		"--duration", "30s",
+		"--concurrency", "2",
+		"--base-port", fmt.Sprint(basePort),
+		"--output", output,
+		"--bun", bunPath,
+		"--campaign", campaignPath,
+		"--sanderling", sanderlingPath,
+	}, &stdout, io.Discard)
+	if err == nil ||
+		!strings.Contains(err.Error(), "1 of 3 implementations never ran") {
+		t.Fatalf(
+			"expected the failed build and the failed campaign to be reported, got %v",
+			err,
+		)
+	}
+
+	var recorded manifest
+	manifestBody, err := os.ReadFile(filepath.Join(output, manifestFileName))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := json.Unmarshal(manifestBody, &recorded); err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(recorded.Seeds, []int64{4, 5}) {
+		t.Errorf("intended seeds: got %v", recorded.Seeds)
+	}
+	if len(recorded.Implementations) != 3 {
+		t.Fatalf("intended implementations: got %v", recorded.Implementations)
+	}
+	for index, planned := range recorded.Implementations {
+		wantPort := basePort + index
+		if planned.Port != wantPort {
+			t.Errorf(
+				"%s port: got %d, want %d",
+				planned.Name,
+				planned.Port,
+				wantPort,
+			)
+		}
+		wantURL := fmt.Sprintf("http://localhost:%d/?seed={seed}", wantPort)
+		if planned.URLTemplate != wantURL {
+			t.Errorf(
+				"%s url template: got %q, want %q",
+				planned.Name,
+				planned.URLTemplate,
+				wantURL,
+			)
+		}
+	}
+	if recorded.Generator != "seeded" || recorded.MaxSteps != 40 {
+		t.Errorf(
+			"manifest generator/budget: got %q/%d",
+			recorded.Generator,
+			recorded.MaxSteps,
+		)
+	}
+
+	// impl-02 fails its build, so the two implementations either side of it
+	// still have to reach the campaign tool with both seeds.
+	campaignLines := readLines(t, campaignLog)
+	if len(campaignLines) != 4 {
+		t.Fatalf(
+			"campaign invocations: got %d, want 4:\n%s",
+			len(campaignLines),
+			strings.Join(campaignLines, "\n"),
+		)
+	}
+	seen := map[string]bool{}
+	for _, line := range campaignLines {
+		if !strings.HasPrefix(line, "manifest=present") {
+			t.Errorf(
+				"a campaign ran before the sweep manifest was written: %q",
+				line,
+			)
+		}
+		arguments := strings.Fields(strings.SplitN(line, "argv: ", 2)[1])
+		arm := argumentValue(arguments, "--arm")
+		seed := argumentValue(arguments, "--seeds")
+		bundle := argumentValue(arguments, "--bundle-id")
+		port := basePort + slices.Index(
+			[]string{"impl-01", "impl-02", "impl-03"},
+			arm,
+		)
+		wantBundle := fmt.Sprintf("http://localhost:%d/?seed=%s", port, seed)
+		if bundle != wantBundle {
+			t.Errorf(
+				"%s seed %s: bundle id %q, want %q",
+				arm,
+				seed,
+				bundle,
+				wantBundle,
+			)
+		}
+		if got := argumentValue(arguments, "--output"); got != filepath.Join(
+			output,
+			arm,
+			"seed-"+seed,
+		) {
+			t.Errorf("%s seed %s: campaign output %q", arm, seed, got)
+		}
+		if got := argumentValue(arguments, "--sanderling"); got != sanderlingPath {
+			t.Errorf("%s seed %s: sanderling path %q", arm, seed, got)
+		}
+		seen[arm+"/"+seed] = true
+	}
+	for _, want := range []string{"impl-01/4", "impl-01/5", "impl-03/4", "impl-03/5"} {
+		if !seen[want] {
+			t.Errorf("%s never reached the campaign tool", want)
+		}
+	}
+
+	// What the served page actually saw: the right port for the
+	// implementation, carrying the same seed the campaign was given.
+	fetched := readLines(t, fetchLog)
+	for _, want := range []string{
+		fmt.Sprintf("http://localhost:%d/?seed=4 -> 200 %d /?seed=4", basePort, basePort),
+		fmt.Sprintf("http://localhost:%d/?seed=5 -> 200 %d /?seed=5", basePort, basePort),
+		fmt.Sprintf("http://localhost:%d/?seed=4 -> 200 %d /?seed=4", basePort+2, basePort+2),
+		fmt.Sprintf("http://localhost:%d/?seed=5 -> 200 %d /?seed=5", basePort+2, basePort+2),
+	} {
+		if !slices.Contains(fetched, want) {
+			t.Errorf(
+				"the served page never saw %q:\n%s",
+				want,
+				strings.Join(fetched, "\n"),
+			)
+		}
+	}
+
+	records := readRecords(t, filepath.Join(output, recordsFileName))
+	if len(records) != 3 {
+		t.Fatalf("implementation records: got %d, want 3", len(records))
+	}
+	byName := map[string]implementationRecord{}
+	for _, record := range records {
+		byName[record.Name] = record
+	}
+	failed := byName["impl-02"]
+	if failed.FailedStage != stageBuild || len(failed.Runs) != 0 {
+		t.Errorf(
+			"impl-02: got stage %q with %d runs, want a build failure and no runs",
+			failed.FailedStage,
+			len(failed.Runs),
+		)
+	}
+	if !strings.Contains(failed.Error, "build.log") {
+		t.Errorf("impl-02 error should point at its log: %q", failed.Error)
+	}
+	buildLog, err := os.ReadFile(
+		filepath.Join(output, "impl-02", stageBuild+".log"),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(buildLog), "TS2322") {
+		t.Errorf("impl-02 build log lost the compiler error: %q", buildLog)
+	}
+	for _, name := range []string{"impl-01", "impl-03"} {
+		record := byName[name]
+		if record.FailedStage != "" || len(record.Runs) != 2 {
+			t.Errorf(
+				"%s: stage %q with %d runs, want no failure and 2 runs",
+				name,
+				record.FailedStage,
+				len(record.Runs),
+			)
+		}
+		if record.MonotonicMillis <= 0 {
+			t.Errorf(
+				"%s took %d ms, so nothing timed how long it worked",
+				name,
+				record.MonotonicMillis,
+			)
+		}
+		for _, run := range record.Runs {
+			if run.MonotonicMillis <= 0 {
+				t.Errorf(
+					"%s seed %d took %d ms, so nothing timed the campaign",
+					name,
+					run.Seed,
+					run.MonotonicMillis,
+				)
+			}
+		}
+	}
+	if exit := byName["impl-03"].Runs[0].ExitCode; exit != 1 {
+		t.Errorf("impl-03 seed 4 exit code: got %d, want 1", exit)
+	}
+	if exit := byName["impl-03"].Runs[1].ExitCode; exit != 0 {
+		t.Errorf(
+			"impl-03 seed 5 ran after seed 4 failed and should have exited 0, got %d",
+			exit,
+		)
+	}
+
+	for _, name := range []string{"impl-01", "impl-03"} {
+		for _, seed := range []string{"4", "5"} {
+			directory := filepath.Join(output, name, "seed-"+seed)
+			if _, err := os.Stat(filepath.Join(directory, "campaign.json")); err != nil {
+				t.Errorf(
+					"%s seed %s: no campaign directory: %v",
+					name,
+					seed,
+					err,
+				)
+			}
+			log, err := os.ReadFile(filepath.Join(directory, "campaign.log"))
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !strings.Contains(string(log), "stub campaign seed="+seed) {
+				t.Errorf(
+					"%s seed %s: campaign output was not captured: %q",
+					name,
+					seed,
+					log,
+				)
+			}
+		}
+	}
+
+	installed := readLines(t, bunLog)
+	for _, name := range []string{"impl-01", "impl-02", "impl-03"} {
+		if !slices.Contains(
+			installed,
+			filepath.Join(implementations, name)+" install",
+		) {
+			t.Errorf(
+				"%s was never installed:\n%s",
+				name,
+				strings.Join(installed, "\n"),
+			)
+		}
+	}
+
+	// Every preview server the sweep started is gone with it: a leaked one
+	// holds its port, and the next sweep would be served by the old build.
+	client := &http.Client{Timeout: 2 * time.Second}
+	for _, port := range []int{basePort, basePort + 2} {
+		if response, err := client.Get(readinessURL(port)); err == nil {
+			response.Body.Close()
+			t.Errorf("port %d is still served after the sweep finished", port)
+		}
+	}
+	if !strings.Contains(stdout.String(), "failed at build") {
+		t.Errorf(
+			"progress output does not name the build failure: %q",
+			stdout.String(),
+		)
+	}
+}
+
+func writeScript(t *testing.T, path, body string) string {
+	t.Helper()
+	if err := os.WriteFile(path, []byte(body), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	return path
+}
+
+func readLines(t *testing.T, path string) []string {
+	t.Helper()
+	body, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var lines []string
+	scanner := bufio.NewScanner(strings.NewReader(string(body)))
+	for scanner.Scan() {
+		if line := strings.TrimSpace(scanner.Text()); line != "" {
+			lines = append(lines, line)
+		}
+	}
+	return lines
+}
+
+func readRecords(t *testing.T, path string) []implementationRecord {
+	t.Helper()
+	var records []implementationRecord
+	for _, line := range readLines(t, path) {
+		var record implementationRecord
+		if err := json.Unmarshal([]byte(line), &record); err != nil {
+			t.Fatalf("%s: %v", line, err)
+		}
+		records = append(records, record)
+	}
+	return records
+}
+
+// freePortRange finds count consecutive free ports, which is what the sweep
+// hands out: one port per implementation from --base-port upwards.
+func freePortRange(t *testing.T, count int) int {
+	t.Helper()
+	for range 100 {
+		base := 20000 + rand.Intn(20000)
+		if portsAreFree(base, count) {
+			return base
+		}
+	}
+	t.Fatalf("no run of %d free ports", count)
+	return 0
+}
+
+func portsAreFree(base, count int) bool {
+	for offset := range count {
+		listener, err := net.Listen(
+			"tcp",
+			fmt.Sprintf("localhost:%d", base+offset),
+		)
+		if err != nil {
+			return false
+		}
+		listener.Close()
+	}
+	return true
+}
diff --git a/cmd/internal-tools/implementation-sweep/main.go b/cmd/internal-tools/implementation-sweep/main.go
new file mode 100644
index 0000000..7cf2502
--- /dev/null
+++ b/cmd/internal-tools/implementation-sweep/main.go
@@ -0,0 +1,284 @@
+// Command implementation-sweep runs one identical campaign against every model
+// implementation of a single requirement. It installs, builds and serves each
+// implementation on its own port, then hands the campaign tool the same seed
+// slice, the same step budget and the same generator for all of them, so a
+// difference between implementations is not a difference in exploration.
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"os/signal"
+	"path/filepath"
+	"strconv"
+	"syscall"
+	"time"
+
+	"github.com/priyanshujain/sanderling/internal/seedspec"
+)
+
+// The generator and the platform are fixed rather than exposed: the
+// pre-registration runs the seeded policy against a served web build, and a
+// sweep that could quietly run something else records a comparison nobody made.
+const (
+	generator = "seeded"
+	platform  = "web"
+)
+
+// defaultConcurrency is how many implementations are built, served and swept at
+// once. fleet.md measured eight concurrent web campaigns clean at about 1.1 GB
+// resident each, parallel efficiency 0.83 at eight against 0.87 at six, and a
+// knee at twelve to sixteen, on a contended laptop it says to re-measure before
+// trusting anything above eight. Six sits at the better efficiency, costs about
+// 7 GB of a 64 GB host, and leaves the Android emulator farm that shares that
+// host its four to six slots. Each worker here also carries a vite server the
+// fleet measurement did not include.
+const defaultConcurrency = 6
+
+// defaultBasePort is the first port handed out. Vite's own defaults are 5173
+// and 4173, so a sweep starting here does not collide with a dev server someone
+// left running.
+const defaultBasePort = 5300
+
+type config struct {
+	implementationsDirectory string
+	specPath                 string
+	outputDirectory          string
+	seeds                    []int64
+	maxSteps                 int
+	duration                 time.Duration
+	concurrency              int
+	basePort                 int
+	bunPath                  string
+	campaignPath             string
+	sanderlingPath           string
+	extraArguments           []string
+}
+
+const usage = `implementation-sweep runs one seeded campaign per model implementation.
+
+Usage:
+  implementation-sweep --implementations <dir> --spec <path> --seeds <spec>
+                       --max-steps <n> --output <dir> [flags]
+                       [-- <sanderling test flags>]
+
+Each impl-* directory under --implementations is installed, built and served on
+its own port, and every one is swept with the same seeds and the same step
+budget. An implementation that fails to install, build or serve is recorded and
+the sweep moves on to the next one.
+
+Everything after a bare -- reaches every sanderling test call through the
+campaign tool.
+`
+
+func parseArguments(arguments []string, stderr io.Writer) (config, error) {
+	flagSet := flag.NewFlagSet("implementation-sweep", flag.ContinueOnError)
+	flagSet.SetOutput(stderr)
+	flagSet.Usage = func() {
+		fmt.Fprint(stderr, usage)
+		flagSet.PrintDefaults()
+	}
+	var configuration config
+	var seedSpecification string
+	flagSet.StringVar(
+		&configuration.implementationsDirectory,
+		"implementations",
+		"",
+		"directory holding impl-01 to impl-NN (required)",
+	)
+	flagSet.StringVar(
+		&configuration.specPath,
+		"spec",
+		"",
+		"path to the property set every implementation is run against (required)",
+	)
+	flagSet.StringVar(
+		&seedSpecification,
+		"seeds",
+		"",
+		"seeds every implementation runs: ranges and lists, e.g. 1-10,20 (required)",
+	)
+	flagSet.IntVar(
+		&configuration.maxSteps,
+		"max-steps",
+		0,
+		"per-run step budget, identical across implementations (required, must be positive)",
+	)
+	flagSet.DurationVar(
+		&configuration.duration,
+		"duration",
+		5*time.Minute,
+		"per-run wall-clock ceiling passed to each campaign",
+	)
+	flagSet.IntVar(
+		&configuration.concurrency,
+		"concurrency",
+		defaultConcurrency,
+		"implementations built, served and swept at once",
+	)
+	flagSet.IntVar(
+		&configuration.basePort,
+		"base-port",
+		defaultBasePort,
+		"first port served; each implementation takes the next one in name order",
+	)
+	flagSet.StringVar(
+		&configuration.outputDirectory,
+		"output",
+		"",
+		"campaign tree to create (required)",
+	)
+	flagSet.StringVar(
+		&configuration.bunPath,
+		"bun",
+		"bun",
+		"bun binary that installs, builds and serves each implementation",
+	)
+	flagSet.StringVar(
+		&configuration.campaignPath,
+		"campaign",
+		"campaign",
+		"campaign binary to invoke per implementation and seed",
+	)
+	flagSet.StringVar(
+		&configuration.sanderlingPath,
+		"sanderling",
+		"sanderling",
+		"sanderling binary each campaign invokes",
+	)
+	if err := flagSet.Parse(arguments); err != nil {
+		return config{}, err
+	}
+	configuration.extraArguments = flagSet.Args()
+
+	for name, value := range map[string]string{
+		"--implementations": configuration.implementationsDirectory,
+		"--spec":            configuration.specPath,
+		"--seeds":           seedSpecification,
+		"--output":          configuration.outputDirectory,
+	} {
+		if value == "" {
+			return config{}, fmt.Errorf("%s is required", name)
+		}
+	}
+	if configuration.maxSteps <= 0 {
+		return config{}, fmt.Errorf(
+			"--max-steps must be positive: every implementation needs the same step budget",
+		)
+	}
+	if configuration.duration <= 0 {
+		return config{}, fmt.Errorf(
+			"--duration must be positive: %s",
+			configuration.duration,
+		)
+	}
+	if configuration.concurrency <= 0 {
+		return config{}, fmt.Errorf(
+			"--concurrency must be positive: %d",
+			configuration.concurrency,
+		)
+	}
+	if configuration.basePort < 1024 || configuration.basePort > 65535 {
+		return config{}, fmt.Errorf(
+			"--base-port %d is outside 1024-65535",
+			configuration.basePort,
+		)
+	}
+	seeds, err := seedspec.Parse(seedSpecification)
+	if err != nil {
+		return config{}, fmt.Errorf("--seeds: %w", err)
+	}
+	configuration.seeds = seeds
+	for name, value := range map[string]*string{
+		"--implementations": &configuration.implementationsDirectory,
+		"--spec":            &configuration.specPath,
+		"--output":          &configuration.outputDirectory,
+	} {
+		absolute, err := filepath.Abs(*value)
+		if err != nil {
+			return config{}, fmt.Errorf("%s: %w", name, err)
+		}
+		*value = absolute
+	}
+	return configuration, nil
+}
+
+// servedURL is the one place a seed becomes a URL. The same seed is also handed
+// to the campaign as --seeds, which reaches sanderling as --seed and fixes the
+// exploration, while the scaffold reads ?seed= and fixes the latency and the
+// outcome of every send. A violation replays only when both carry the same
+// number, so both come from the seed argument here and never from two flags.
+func servedURL(port int, seed string) string {
+	return fmt.Sprintf("http://localhost:%d/?seed=%s", port, seed)
+}
+
+func readinessURL(port int) string {
+	return fmt.Sprintf("http://localhost:%d/", port)
+}
+
+func campaignDirectory(
+	configuration config,
+	target implementation,
+	seed string,
+) string {
+	return filepath.Join(
+		configuration.outputDirectory,
+		target.Name,
+		"seed-"+seed,
+	)
+}
+
+// campaignArguments builds one campaign invocation. The seed is a string
+// because it lands in two arguments, --seeds and the ?seed= of --bundle-id,
+// and passing it once keeps them from drifting apart.
+func campaignArguments(
+	configuration config,
+	target implementation,
+	seed string,
+) []string {
+	arguments := []string{
+		"--spec", configuration.specPath,
+		"--bundle-id", servedURL(target.Port, seed),
+		"--platform", platform,
+		"--arm", target.Name,
+		"--generator", generator,
+		"--max-steps", strconv.Itoa(configuration.maxSteps),
+		"--duration", configuration.duration.String(),
+		"--seeds", seed,
+		"--sanderling", configuration.sanderlingPath,
+		"--output", campaignDirectory(configuration, target, seed),
+	}
+	if len(configuration.extraArguments) > 0 {
+		arguments = append(arguments, "--")
+		arguments = append(arguments, configuration.extraArguments...)
+	}
+	return arguments
+}
+
+func run(arguments []string, stdout, stderr io.Writer) error {
+	configuration, err := parseArguments(arguments, stderr)
+	if err != nil {
+		return err
+	}
+	ctx, cancel := signal.NotifyContext(
+		context.Background(),
+		os.Interrupt,
+		syscall.SIGTERM,
+	)
+	defer cancel()
+	return runSweep(ctx, configuration, stdout)
+}
+
+func main() {
+	if err := run(os.Args[1:], os.Stdout, os.Stderr); err != nil {
+		if errors.Is(err, flag.ErrHelp) {
+			return
+		}
+		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		os.Exit(1)
+	}
+}
diff --git a/cmd/internal-tools/implementation-sweep/main_test.go b/cmd/internal-tools/implementation-sweep/main_test.go
new file mode 100644
index 0000000..f9d4d6d
--- /dev/null
+++ b/cmd/internal-tools/implementation-sweep/main_test.go
@@ -0,0 +1,214 @@
+package main
+
+import (
+	"io"
+	"net/url"
+	"path/filepath"
+	"slices"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+)
+
+func baseArguments() []string {
+	return []string{
+		"--implementations", "/e4/implementations",
+		"--spec", "/e4/relay.ts",
+		"--seeds", "1-3",
+		"--max-steps", "400",
+		"--output", "/campaigns/e4",
+	}
+}
+
+func TestParseArguments_DefaultsAndSeeds(t *testing.T) {
+	configuration, err := parseArguments(baseArguments(), io.Discard)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(configuration.seeds, []int64{1, 2, 3}) {
+		t.Errorf("seeds: got %v", configuration.seeds)
+	}
+	if configuration.concurrency != defaultConcurrency {
+		t.Errorf(
+			"concurrency default: got %d, want %d",
+			configuration.concurrency,
+			defaultConcurrency,
+		)
+	}
+	if configuration.basePort != defaultBasePort {
+		t.Errorf(
+			"base port default: got %d, want %d",
+			configuration.basePort,
+			defaultBasePort,
+		)
+	}
+	if configuration.duration != 5*time.Minute {
+		t.Errorf("duration default: got %s", configuration.duration)
+	}
+	for name, got := range map[string]string{
+		"bun":        configuration.bunPath,
+		"campaign":   configuration.campaignPath,
+		"sanderling": configuration.sanderlingPath,
+	} {
+		if got != name {
+			t.Errorf("%s path default: got %q", name, got)
+		}
+	}
+}
+
+func TestParseArguments_Rejections(t *testing.T) {
+	cases := []struct {
+		name      string
+		arguments []string
+		want      string
+	}{
+		{
+			"missing implementations",
+			[]string{
+				"--spec",
+				"s",
+				"--seeds",
+				"1",
+				"--max-steps",
+				"10",
+				"--output",
+				"o",
+			},
+			"--implementations is required",
+		},
+		{
+			"missing spec",
+			[]string{
+				"--implementations",
+				"i",
+				"--seeds",
+				"1",
+				"--max-steps",
+				"10",
+				"--output",
+				"o",
+			},
+			"--spec is required",
+		},
+		{
+			"missing output",
+			[]string{
+				"--implementations",
+				"i",
+				"--spec",
+				"s",
+				"--seeds",
+				"1",
+				"--max-steps",
+				"10",
+			},
+			"--output is required",
+		},
+		{
+			"zero max steps",
+			append(baseArguments(), "--max-steps", "0"),
+			"--max-steps must be positive",
+		},
+		{
+			"zero concurrency",
+			append(baseArguments(), "--concurrency", "0"),
+			"--concurrency must be positive",
+		},
+		{
+			"privileged base port",
+			append(baseArguments(), "--base-port", "80"),
+			"outside 1024-65535",
+		},
+		{
+			"seed zero",
+			append(baseArguments(), "--seeds", "0,1"),
+			"not reproducible",
+		},
+	}
+	for _, testCase := range cases {
+		_, err := parseArguments(testCase.arguments, io.Discard)
+		if err == nil {
+			t.Errorf("%s: expected error", testCase.name)
+			continue
+		}
+		if !strings.Contains(err.Error(), testCase.want) {
+			t.Errorf(
+				"%s: got %q, want it to contain %q",
+				testCase.name,
+				err,
+				testCase.want,
+			)
+		}
+	}
+}
+
+// The seed reaches two independent things, the campaign's own seed and the
+// scaffold's failure stream, and a replay reproduces neither unless they carry
+// the same number.
+func TestCampaignArguments_OneSeedReachesBothTheCampaignAndTheURL(
+	t *testing.T,
+) {
+	configuration, err := parseArguments(
+		append(baseArguments(), "--", "--clear-data=false"),
+		io.Discard,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	target := implementation{
+		Name:      "impl-07",
+		Directory: "/e4/implementations/impl-07",
+		Port:      5306,
+	}
+	for _, seed := range []string{"1", "42"} {
+		arguments := campaignArguments(configuration, target, seed)
+		if got := argumentValue(arguments, "--seeds"); got != seed {
+			t.Errorf("--seeds: got %q, want %q", got, seed)
+		}
+		bundle := argumentValue(arguments, "--bundle-id")
+		parsed, err := url.Parse(bundle)
+		if err != nil {
+			t.Fatalf("--bundle-id %q: %v", bundle, err)
+		}
+		if got := parsed.Query().Get("seed"); got != seed {
+			t.Errorf(
+				"served URL seed: got %q, want %q (from %q)",
+				got,
+				seed,
+				bundle,
+			)
+		}
+		if parsed.Host != "localhost:"+strconv.Itoa(target.Port) {
+			t.Errorf(
+				"served host: got %q, want the implementation's own port %d",
+				parsed.Host,
+				target.Port,
+			)
+		}
+		for flagName, want := range map[string]string{
+			"--arm":       "impl-07",
+			"--platform":  "web",
+			"--generator": "seeded",
+			"--max-steps": "400",
+			"--spec":      "/e4/relay.ts",
+			"--output":    filepath.Join("/campaigns/e4", "impl-07", "seed-"+seed),
+		} {
+			if got := argumentValue(arguments, flagName); got != want {
+				t.Errorf("%s: got %q, want %q", flagName, got, want)
+			}
+		}
+		if arguments[len(arguments)-2] != "--" ||
+			arguments[len(arguments)-1] != "--clear-data=false" {
+			t.Errorf("passthrough flags lost: %v", arguments)
+		}
+	}
+}
+
+func argumentValue(arguments []string, name string) string {
+	index := slices.Index(arguments, name)
+	if index < 0 || index+1 >= len(arguments) {
+		return ""
+	}
+	return arguments[index+1]
+}
diff --git a/cmd/internal-tools/implementation-sweep/manifest.go b/cmd/internal-tools/implementation-sweep/manifest.go
new file mode 100644
index 0000000..e1f377e
--- /dev/null
+++ b/cmd/internal-tools/implementation-sweep/manifest.go
@@ -0,0 +1,117 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+const (
+	manifestFileName = "sweep.json"
+	recordsFileName  = "implementations.jsonl"
+	seedPlaceholder  = "{seed}"
+)
+
+// plannedImplementation is one implementation the sweep intends to run, with
+// the port it is served on and the URL every seed is served at.
+type plannedImplementation struct {
+	Name        string `json:"name"`
+	Directory   string `json:"directory"`
+	Port        int    `json:"port"`
+	URLTemplate string `json:"url_template"`
+}
+
+// manifest is sweep.json: what the sweep INTENDED to run, written before the
+// first install so a host that dropped an implementation or a seed shows up as
+// a missing run rather than as a smaller sample.
+type manifest struct {
+	Generator       string                  `json:"generator"`
+	Platform        string                  `json:"platform"`
+	SpecPath        string                  `json:"spec_path"`
+	MaxSteps        int                     `json:"max_steps"`
+	DurationMillis  int64                   `json:"duration_millis"`
+	Seeds           []int64                 `json:"seeds"`
+	Implementations []plannedImplementation `json:"implementations"`
+	Concurrency     int                     `json:"concurrency"`
+	Host            string                  `json:"host"`
+	BunPath         string                  `json:"bun_path"`
+	CampaignPath    string                  `json:"campaign_path"`
+	SanderlingPath  string                  `json:"sanderling_path"`
+	StartedAt       time.Time               `json:"started_at"`
+}
+
+func buildManifest(
+	configuration config,
+	implementations []implementation,
+	host string,
+	startedAt time.Time,
+) manifest {
+	planned := make([]plannedImplementation, 0, len(implementations))
+	for _, target := range implementations {
+		planned = append(planned, plannedImplementation{
+			Name:        target.Name,
+			Directory:   target.Directory,
+			Port:        target.Port,
+			URLTemplate: servedURL(target.Port, seedPlaceholder),
+		})
+	}
+	return manifest{
+		Generator:       generator,
+		Platform:        platform,
+		SpecPath:        configuration.specPath,
+		MaxSteps:        configuration.maxSteps,
+		DurationMillis:  configuration.duration.Milliseconds(),
+		Seeds:           configuration.seeds,
+		Implementations: planned,
+		Concurrency:     configuration.concurrency,
+		Host:            host,
+		BunPath:         configuration.bunPath,
+		CampaignPath:    configuration.campaignPath,
+		SanderlingPath:  configuration.sanderlingPath,
+		StartedAt:       startedAt,
+	}
+}
+
+func writeManifest(directory string, value manifest) error {
+	body, err := json.MarshalIndent(value, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshal manifest: %w", err)
+	}
+	return os.WriteFile(
+		filepath.Join(directory, manifestFileName),
+		append(body, '\n'),
+		0o644,
+	)
+}
+
+// runRecord is one campaign, which is one implementation at one seed.
+type runRecord struct {
+	Seed              int64  `json:"seed"`
+	URL               string `json:"url"`
+	ExitCode          int    `json:"exit_code"`
+	LaunchError       string `json:"launch_error,omitempty"`
+	CampaignDirectory string `json:"campaign_directory"`
+	MonotonicMillis   int64  `json:"monotonic_millis"`
+}
+
+// implementationRecord is one line of implementations.jsonl. FailedStage names
+// the step that stopped this implementation, and an implementation that never
+// got past install, build or serve carries no runs at all.
+type implementationRecord struct {
+	Name            string      `json:"implementation"`
+	Directory       string      `json:"directory"`
+	Port            int         `json:"port"`
+	FailedStage     string      `json:"failed_stage,omitempty"`
+	Error           string      `json:"error,omitempty"`
+	StartedAt       time.Time   `json:"started_at"`
+	MonotonicMillis int64       `json:"monotonic_millis"`
+	Runs            []runRecord `json:"runs"`
+}
+
+const (
+	stageInstall = "install"
+	stageBuild   = "build"
+	stageServe   = "serve"
+)
diff --git a/cmd/internal-tools/implementation-sweep/serve.go b/cmd/internal-tools/implementation-sweep/serve.go
new file mode 100644
index 0000000..666aa2a
--- /dev/null
+++ b/cmd/internal-tools/implementation-sweep/serve.go
@@ -0,0 +1,109 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"os/exec"
+	"syscall"
+	"time"
+)
+
+const (
+	// serverStartTimeout covers a cold vite start on a host already running
+	// five other implementations.
+	serverStartTimeout  = 90 * time.Second
+	serverPollInterval  = 250 * time.Millisecond
+	serverShutdownGrace = 10 * time.Second
+)
+
+// server is one implementation's preview server. It runs in its own process
+// group so that stopping it takes the whole vite tree with it: a leaked server
+// holds its port, and the next sweep against that implementation would be
+// served by the previous build.
+type server struct {
+	command *exec.Cmd
+	logFile *os.File
+	exited  chan struct{}
+}
+
+func startServer(
+	ctx context.Context,
+	configuration config,
+	target implementation,
+	logPath string,
+) (*server, error) {
+	logFile, err := os.Create(logPath)
+	if err != nil {
+		return nil, err
+	}
+	command := exec.CommandContext(ctx, configuration.bunPath,
+		"run", "preview", "--port", fmt.Sprint(target.Port), "--strictPort")
+	command.Dir = target.Directory
+	command.Stdout = logFile
+	command.Stderr = logFile
+	command.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	if err := command.Start(); err != nil {
+		logFile.Close()
+		return nil, err
+	}
+	running := &server{
+		command: command,
+		logFile: logFile,
+		exited:  make(chan struct{}),
+	}
+	go func() {
+		command.Wait()
+		close(running.exited)
+	}()
+	return running, nil
+}
+
+// waitReady polls the served page until it answers. A server that exits first
+// is reported as such rather than waited on for the full timeout, because the
+// usual cause is a port already taken and that answer is in the log.
+func (s *server) waitReady(ctx context.Context, url string) error {
+	client := &http.Client{Timeout: 5 * time.Second}
+	deadline := time.Now().Add(serverStartTimeout)
+	for {
+		select {
+		case <-s.exited:
+			return fmt.Errorf("server exited before it answered %s", url)
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+		}
+		response, err := client.Get(url)
+		if err == nil {
+			io.Copy(io.Discard, response.Body)
+			response.Body.Close()
+			if response.StatusCode == http.StatusOK {
+				return nil
+			}
+		}
+		if time.Now().After(deadline) {
+			return fmt.Errorf(
+				"server did not answer %s within %s",
+				url,
+				serverStartTimeout,
+			)
+		}
+		time.Sleep(serverPollInterval)
+	}
+}
+
+func (s *server) stop() {
+	if s.command.Process != nil {
+		group := -s.command.Process.Pid
+		syscall.Kill(group, syscall.SIGTERM)
+		select {
+		case <-s.exited:
+		case <-time.After(serverShutdownGrace):
+			syscall.Kill(group, syscall.SIGKILL)
+			<-s.exited
+		}
+	}
+	s.logFile.Close()
+}
diff --git a/cmd/internal-tools/implementation-sweep/serve_test.go b/cmd/internal-tools/implementation-sweep/serve_test.go
new file mode 100644
index 0000000..070935b
--- /dev/null
+++ b/cmd/internal-tools/implementation-sweep/serve_test.go
@@ -0,0 +1,119 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"time"
+)
+
+// bunSpawningAServer serves from a child process and then waits, which is the
+// shape of `bun run preview`: the port belongs to something below the process
+// the sweep started.
+const bunSpawningAServer = `#!/bin/sh
+port=""
+previous=""
+for argument in "$@"; do
+  if [ "$previous" = "--port" ]; then port="$argument"; fi
+  previous="$argument"
+done
+SWEEP_TEST_SERVE_PORT="$port" "%[1]s" &
+wait
+`
+
+func TestServerStop_TakesTheProcessBelowItWithTheServer(t *testing.T) {
+	directory := t.TempDir()
+	testBinary, err := filepath.Abs(os.Args[0])
+	if err != nil {
+		t.Fatal(err)
+	}
+	bunPath := writeScript(
+		t,
+		filepath.Join(directory, "stub-bun"),
+		fmt.Sprintf(bunSpawningAServer, testBinary),
+	)
+	port := freePortRange(t, 1)
+	target := implementation{Name: "impl-01", Directory: directory, Port: port}
+
+	// Background rather than the test context: only stop() may end this
+	// server, or a leak would be hidden by the context being cancelled.
+	running, err := startServer(
+		context.Background(),
+		config{bunPath: bunPath},
+		target,
+		filepath.Join(directory, "serve.log"),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(
+		func() { syscall.Kill(-running.command.Process.Pid, syscall.SIGKILL) },
+	)
+	if err := running.waitReady(context.Background(), readinessURL(port)); err != nil {
+		t.Fatal(err)
+	}
+
+	running.stop()
+
+	client := &http.Client{Timeout: time.Second}
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		response, err := client.Get(readinessURL(port))
+		if err != nil {
+			return
+		}
+		response.Body.Close()
+		time.Sleep(100 * time.Millisecond)
+	}
+	t.Fatalf(
+		"port %d is still served after stop(): the server below bun outlived the sweep and holds the port",
+		port,
+	)
+}
+
+func TestServerWaitReady_ReportsAServerThatExited(t *testing.T) {
+	directory := t.TempDir()
+	bunPath := writeScript(
+		t,
+		filepath.Join(directory, "stub-bun"),
+		"#!/bin/sh\necho 'port is already in use' >&2\nexit 1\n",
+	)
+	port := freePortRange(t, 1)
+
+	running, err := startServer(
+		context.Background(),
+		config{bunPath: bunPath},
+		implementation{
+			Name:      "impl-01",
+			Directory: directory,
+			Port:      port,
+		},
+		filepath.Join(directory, "serve.log"),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer running.stop()
+
+	started := time.Now()
+	err = running.waitReady(context.Background(), readinessURL(port))
+	if err == nil {
+		t.Fatal(
+			"a server that exited should not be waited on until the start timeout",
+		)
+	}
+	if elapsed := time.Since(started); elapsed > 30*time.Second {
+		t.Errorf("waited %s for a server that had already exited", elapsed)
+	}
+	log, err := os.ReadFile(filepath.Join(directory, "serve.log"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(log) == "" {
+		t.Error("serve.log did not capture why the server exited")
+	}
+}
diff --git a/cmd/internal-tools/implementation-sweep/sweep.go b/cmd/internal-tools/implementation-sweep/sweep.go
new file mode 100644
index 0000000..defba0e
--- /dev/null
+++ b/cmd/internal-tools/implementation-sweep/sweep.go
@@ -0,0 +1,391 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+// implementation is one directory under --implementations, with the port it
+// owns for the whole sweep. The port comes from the implementation's position
+// in name order rather than from a pool, so the manifest can name the URL every
+// run was served from before anything has been served.
+type implementation struct {
+	Name      string
+	Directory string
+	Port      int
+}
+
+const implementationPrefix = "impl-"
+
+func discoverImplementations(
+	directory string,
+	basePort int,
+) ([]implementation, error) {
+	entries, err := os.ReadDir(directory)
+	if err != nil {
+		return nil, err
+	}
+	var found []implementation
+	for _, entry := range entries {
+		if !entry.IsDir() ||
+			!strings.HasPrefix(entry.Name(), implementationPrefix) {
+			continue
+		}
+		found = append(found, implementation{
+			Name:      entry.Name(),
+			Directory: filepath.Join(directory, entry.Name()),
+		})
+	}
+	if len(found) == 0 {
+		return nil, fmt.Errorf(
+			"no %s* directories in %s",
+			implementationPrefix,
+			directory,
+		)
+	}
+	sort.Slice(
+		found,
+		func(i, j int) bool { return found[i].Name < found[j].Name },
+	)
+	if basePort+len(found)-1 > 65535 {
+		return nil, fmt.Errorf(
+			"--base-port %d leaves no room for %d implementations",
+			basePort,
+			len(found),
+		)
+	}
+	for index := range found {
+		found[index].Port = basePort + index
+	}
+	return found, nil
+}
+
+// resolveBinaries turns bun, campaign and sanderling into absolute paths before
+// anything is installed. Each campaign runs from the sweep's own directory
+// rather than the implementation's, so a relative --sanderling would otherwise
+// resolve against the wrong one, and a binary that is missing altogether has to
+// stop the sweep here rather than fail once per implementation and seed.
+func resolveBinaries(configuration *config) error {
+	for name, value := range map[string]*string{
+		"--bun":        &configuration.bunPath,
+		"--campaign":   &configuration.campaignPath,
+		"--sanderling": &configuration.sanderlingPath,
+	} {
+		resolved, err := exec.LookPath(*value)
+		if err != nil {
+			return fmt.Errorf("%s: %w", name, err)
+		}
+		absolute, err := filepath.Abs(resolved)
+		if err != nil {
+			return fmt.Errorf("%s: %w", name, err)
+		}
+		*value = absolute
+	}
+	return nil
+}
+
+type sweep struct {
+	configuration config
+	stdout        io.Writer
+	records       io.Writer
+	mutex         sync.Mutex
+	stalled       int
+	failedRuns    int
+	totalRuns     int
+}
+
+func runSweep(
+	ctx context.Context,
+	configuration config,
+	stdout io.Writer,
+) error {
+	if _, err := os.Stat(filepath.Join(configuration.outputDirectory, manifestFileName)); err == nil {
+		return fmt.Errorf(
+			"%s already exists in %s: pick a fresh --output so two sweeps do not share a directory",
+			manifestFileName,
+			configuration.outputDirectory,
+		)
+	}
+	implementations, err := discoverImplementations(
+		configuration.implementationsDirectory,
+		configuration.basePort,
+	)
+	if err != nil {
+		return err
+	}
+	if err := resolveBinaries(&configuration); err != nil {
+		return err
+	}
+	if _, err := os.Stat(configuration.specPath); err != nil {
+		return fmt.Errorf("--spec: %w", err)
+	}
+	if err := os.MkdirAll(configuration.outputDirectory, 0o755); err != nil {
+		return fmt.Errorf("create sweep dir: %w", err)
+	}
+	host, _ := os.Hostname()
+	if err := writeManifest(configuration.outputDirectory, buildManifest(configuration, implementations, host, time.Now().UTC())); err != nil {
+		return fmt.Errorf("write %s: %w", manifestFileName, err)
+	}
+
+	recordsFile, err := os.OpenFile(
+		filepath.Join(configuration.outputDirectory, recordsFileName),
+		os.O_CREATE|os.O_WRONLY|os.O_APPEND,
+		0o644,
+	)
+	if err != nil {
+		return fmt.Errorf("open %s: %w", recordsFileName, err)
+	}
+	defer recordsFile.Close()
+
+	running := &sweep{
+		configuration: configuration,
+		stdout:        stdout,
+		records:       recordsFile,
+	}
+	fmt.Fprintf(
+		stdout,
+		"sweep: %d implementations, %d seeds each, %d at a time, %s\n",
+		len(
+			implementations,
+		),
+		len(configuration.seeds),
+		configuration.concurrency,
+		configuration.outputDirectory,
+	)
+	running.work(ctx, implementations)
+
+	fmt.Fprintf(
+		stdout,
+		"sweep complete: %d of %d implementations never ran, %d of %d campaigns failed\n",
+		running.stalled,
+		len(implementations),
+		running.failedRuns,
+		running.totalRuns,
+	)
+	if running.stalled > 0 || running.failedRuns > 0 {
+		return fmt.Errorf(
+			"%d of %d implementations never ran and %d of %d campaigns failed; see %s",
+			running.stalled,
+			len(implementations),
+			running.failedRuns,
+			running.totalRuns,
+			recordsFileName,
+		)
+	}
+	return nil
+}
+
+func (s *sweep) work(ctx context.Context, implementations []implementation) {
+	queue := make(chan implementation, len(implementations))
+	for _, target := range implementations {
+		queue <- target
+	}
+	close(queue)
+
+	workers := min(s.configuration.concurrency, len(implementations))
+	var waitGroup sync.WaitGroup
+	for range workers {
+		waitGroup.Add(1)
+		go func() {
+			defer waitGroup.Done()
+			for target := range queue {
+				if ctx.Err() != nil {
+					return
+				}
+				s.report(s.runImplementation(ctx, target))
+			}
+		}()
+	}
+	waitGroup.Wait()
+}
+
+// runImplementation carries one implementation from install to its last seed.
+// Every failure it can meet is returned in the record: one implementation that
+// cannot install, build or serve must not cost the other twenty-three their
+// runs.
+func (s *sweep) runImplementation(
+	ctx context.Context,
+	target implementation,
+) (record implementationRecord) {
+	record = implementationRecord{
+		Name:      target.Name,
+		Directory: target.Directory,
+		Port:      target.Port,
+		StartedAt: time.Now().UTC(),
+	}
+	started := time.Now()
+	defer func() { record.MonotonicMillis = time.Since(started).Milliseconds() }()
+
+	directory := filepath.Join(s.configuration.outputDirectory, target.Name)
+	if err := os.MkdirAll(directory, 0o755); err != nil {
+		record.FailedStage = stageInstall
+		record.Error = err.Error()
+		return record
+	}
+	for _, step := range []struct {
+		stage     string
+		arguments []string
+	}{
+		{stageInstall, []string{"install"}},
+		{stageBuild, []string{"run", "build"}},
+	} {
+		logPath := filepath.Join(directory, step.stage+".log")
+		exitCode, err := runCommand(
+			ctx,
+			target.Directory,
+			s.configuration.bunPath,
+			step.arguments,
+			logPath,
+		)
+		if err != nil {
+			record.FailedStage = step.stage
+			record.Error = err.Error()
+			return record
+		}
+		if exitCode != 0 {
+			record.FailedStage = step.stage
+			record.Error = fmt.Sprintf(
+				"bun %s exited %d, see %s",
+				strings.Join(step.arguments, " "),
+				exitCode,
+				logPath,
+			)
+			return record
+		}
+	}
+
+	running, err := startServer(
+		ctx,
+		s.configuration,
+		target,
+		filepath.Join(directory, "serve.log"),
+	)
+	if err != nil {
+		record.FailedStage = stageServe
+		record.Error = err.Error()
+		return record
+	}
+	defer running.stop()
+	if err := running.waitReady(ctx, readinessURL(target.Port)); err != nil {
+		record.FailedStage = stageServe
+		record.Error = err.Error()
+		return record
+	}
+
+	for _, seed := range s.configuration.seeds {
+		if ctx.Err() != nil {
+			return record
+		}
+		record.Runs = append(record.Runs, s.runSeed(ctx, target, seed))
+	}
+	return record
+}
+
+func (s *sweep) runSeed(
+	ctx context.Context,
+	target implementation,
+	seed int64,
+) (record runRecord) {
+	seedText := strconv.FormatInt(seed, 10)
+	directory := campaignDirectory(s.configuration, target, seedText)
+	record = runRecord{
+		Seed:              seed,
+		URL:               servedURL(target.Port, seedText),
+		CampaignDirectory: directory,
+	}
+	started := time.Now()
+	defer func() { record.MonotonicMillis = time.Since(started).Milliseconds() }()
+
+	if err := os.MkdirAll(directory, 0o755); err != nil {
+		record.ExitCode = -1
+		record.LaunchError = err.Error()
+		return record
+	}
+	exitCode, err := runCommand(
+		ctx,
+		"",
+		s.configuration.campaignPath,
+		campaignArguments(
+			s.configuration,
+			target,
+			seedText,
+		),
+		filepath.Join(directory, "campaign.log"),
+	)
+	record.ExitCode = exitCode
+	if err != nil {
+		record.LaunchError = err.Error()
+	}
+	return record
+}
+
+// runCommand runs one step of the pipeline with its output in logPath. An
+// empty directory keeps the sweep's own working directory, which is what the
+// campaign tool gets: it has no reason to run inside an implementation.
+func runCommand(
+	ctx context.Context,
+	directory, binary string,
+	arguments []string,
+	logPath string,
+) (int, error) {
+	logFile, err := os.Create(logPath)
+	if err != nil {
+		return -1, err
+	}
+	defer logFile.Close()
+	command := exec.CommandContext(ctx, binary, arguments...)
+	command.Dir = directory
+	command.Stdout = logFile
+	command.Stderr = logFile
+	err = command.Run()
+	if err == nil {
+		return 0, nil
+	}
+	var exitError *exec.ExitError
+	if errors.As(err, &exitError) {
+		return exitError.ExitCode(), nil
+	}
+	return -1, err
+}
+
+func (s *sweep) report(record implementationRecord) {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	if record.FailedStage != "" {
+		s.stalled++
+	}
+	s.totalRuns += len(record.Runs)
+	for _, run := range record.Runs {
+		if run.ExitCode != 0 {
+			s.failedRuns++
+		}
+	}
+	if err := json.NewEncoder(s.records).Encode(record); err != nil {
+		fmt.Fprintf(s.stdout, "warning: %s record: %v\n", record.Name, err)
+	}
+	elapsed := time.Duration(record.MonotonicMillis) * time.Millisecond
+	if record.FailedStage != "" {
+		fmt.Fprintf(s.stdout, "%s port=%d failed at %s: %s (%s)\n",
+			record.Name, record.Port, record.FailedStage, record.Error, elapsed)
+		return
+	}
+	failed := 0
+	for _, run := range record.Runs {
+		if run.ExitCode != 0 {
+			failed++
+		}
+	}
+	fmt.Fprintf(s.stdout, "%s port=%d campaigns=%d failed=%d elapsed=%s\n",
+		record.Name, record.Port, len(record.Runs), failed, elapsed)
+}
diff --git a/cmd/internal-tools/implementation-sweep/sweep_test.go b/cmd/internal-tools/implementation-sweep/sweep_test.go
new file mode 100644
index 0000000..f0e3521
--- /dev/null
+++ b/cmd/internal-tools/implementation-sweep/sweep_test.go
@@ -0,0 +1,114 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestDiscoverImplementations_NameOrderAndOnePortEach(t *testing.T) {
+	directory := t.TempDir()
+	for _, name := range []string{"impl-03", "impl-01", "impl-10", "impl-02", "scaffold", ".DS_Store"} {
+		if err := os.MkdirAll(filepath.Join(directory, name), 0o755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := os.WriteFile(filepath.Join(directory, "impl-notes.md"), []byte("not a directory"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	found, err := discoverImplementations(directory, 5300)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []implementation{
+		{Name: "impl-01", Port: 5300},
+		{Name: "impl-02", Port: 5301},
+		{Name: "impl-03", Port: 5302},
+		{Name: "impl-10", Port: 5303},
+	}
+	if len(found) != len(want) {
+		t.Fatalf(
+			"got %d implementations, want %d: %v",
+			len(found),
+			len(want),
+			found,
+		)
+	}
+	for index, target := range found {
+		if target.Name != want[index].Name || target.Port != want[index].Port {
+			t.Errorf(
+				"position %d: got %s on %d, want %s on %d",
+				index,
+				target.Name,
+				target.Port,
+				want[index].Name,
+				want[index].Port,
+			)
+		}
+		if target.Directory != filepath.Join(directory, want[index].Name) {
+			t.Errorf("%s directory: got %q", target.Name, target.Directory)
+		}
+	}
+}
+
+func TestDiscoverImplementations_EmptyDirectoryIsRefused(t *testing.T) {
+	_, err := discoverImplementations(t.TempDir(), 5300)
+	if err == nil || !strings.Contains(err.Error(), "no impl-* directories") {
+		t.Fatalf("got %v, want a refusal naming impl-*", err)
+	}
+}
+
+// A binary that is not there fails once, before anything is installed, rather
+// than twenty-four times after the sweep has spent its build time.
+func TestRunSweep_StopsBeforeItInstallsAnythingWhenABinaryIsMissing(
+	t *testing.T,
+) {
+	implementations := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(implementations, "impl-01"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	output := filepath.Join(t.TempDir(), "campaigns")
+	configuration := config{
+		implementationsDirectory: implementations,
+		outputDirectory:          output,
+		basePort:                 5300,
+		concurrency:              1,
+		bunPath:                  "bun",
+		campaignPath:             "campaign-that-is-not-installed",
+		sanderlingPath:           "sanderling",
+	}
+	err := runSweep(t.Context(), configuration, os.Stdout)
+	if err == nil || !strings.Contains(err.Error(), "--campaign") {
+		t.Fatalf("got %v, want the missing campaign binary named", err)
+	}
+	if _, err := os.Stat(output); !os.IsNotExist(err) {
+		t.Errorf(
+			"the sweep created %s before it checked it could run: %v",
+			output,
+			err,
+		)
+	}
+}
+
+func TestRunSweep_RefusesADirectoryThatAlreadyHoldsASweep(t *testing.T) {
+	implementations := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(implementations, "impl-01"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	output := t.TempDir()
+	if err := os.WriteFile(filepath.Join(output, manifestFileName), []byte("{}"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	configuration := config{
+		implementationsDirectory: implementations,
+		outputDirectory:          output,
+		basePort:                 5300,
+		concurrency:              1,
+	}
+	if err := runSweep(t.Context(), configuration, os.Stdout); err == nil ||
+		!strings.Contains(err.Error(), "already exists") {
+		t.Fatalf("got %v, want a refusal to reuse the directory", err)
+	}
+}