diff --git a/cmd/internal-tools/corpus-sweep/corpus.go b/cmd/internal-tools/corpus-sweep/corpus.go
new file mode 100644
index 0000000..cb8225b
--- /dev/null
+++ b/cmd/internal-tools/corpus-sweep/corpus.go
@@ -0,0 +1,174 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+)
+
+// The corpus is tastejs/todomvc. Its examples/ directory holds 48
+// implementations of one requirement; five are excluded and the 43 that remain
+// are this experiment's population.
+//
+// The population is named here rather than read from whatever directories
+// happen to exist, so a corpus at the wrong commit fails verifyCorpus instead
+// of quietly sweeping a different sample and reporting it as this one.
+var includedImplementations = []string{
+	"angular-dart", "angular2", "angular2_es2015", "angularjs", "angularjs_require",
+	"aurelia", "backbone", "backbone_marionette", "backbone_require", "binding-scala",
+	"canjs", "canjs_require", "closure", "dijon", "dojo",
+	"duel", "elm", "emberjs", "enyo_backbone", "exoskeleton",
+	"jquery", "js_of_ocaml", "jsblocks", "knockback", "knockoutjs",
+	"knockoutjs_require", "kotlin-react", "lavaca_require", "mithril", "polymer",
+	"ractive", "react", "react-alt", "react-backbone", "reagent",
+	"riotjs", "scalajs-react", "typescript-angular", "typescript-backbone", "typescript-react",
+	"vanilla-es6", "vanillajs", "vue",
+}
+
+// excludedImplementations are the five directories under examples/ that the
+// corpus survey dropped. They are listed rather than merely omitted so
+// verifyCorpus can insist the corpus holds exactly these 48 names: an example
+// added or renamed upstream then stops the sweep rather than silently shrinking
+// or growing the sample.
+var excludedImplementations = []string{
+	"cujo", "emberjs_require", "firebase-angular", "gwt", "react-hooks",
+}
+
+const examplesDirectory = "examples"
+
+// documentPath names the served document for the implementations that do not
+// keep an index.html at the root of their example directory.
+var documentPath = map[string]string{
+	"angular-dart": "examples/angular-dart/web/index.html",
+	"duel":         "examples/duel/www/index.html",
+}
+
+func documentFor(name string) string {
+	if override, ok := documentPath[name]; ok {
+		return override
+	}
+	return examplesDirectory + "/" + name + "/index.html"
+}
+
+// implementation is one member of the population, with the port it owns for the
+// whole sweep. The port is what keeps implementations apart: every one is
+// served on its own, so every one is its own web origin and localStorage keeps
+// their records in separate partitions. Four pairs in this corpus write the
+// same key, and one origin between them is one record between them.
+type implementation struct {
+	Name     string
+	Document string
+	Port     int
+}
+
+func (i implementation) Origin() string {
+	return fmt.Sprintf("http://127.0.0.1:%d", i.Port)
+}
+
+func (i implementation) URL() string {
+	return i.Origin() + "/" + i.Document
+}
+
+// verifyCorpus insists the corpus holds exactly the 48 example directories this
+// population was drawn from. A sweep against a different checkout would still
+// run, and its 43 arms would still be labelled, which is why the check is here
+// and not left to whoever reads the results.
+func verifyCorpus(corpusRoot string) error {
+	entries, err := os.ReadDir(filepath.Join(corpusRoot, examplesDirectory))
+	if err != nil {
+		return fmt.Errorf("--corpus: %w", err)
+	}
+	var found []string
+	for _, entry := range entries {
+		if entry.IsDir() {
+			found = append(found, entry.Name())
+		}
+	}
+	expected := slices.Concat(includedImplementations, excludedImplementations)
+	slices.Sort(expected)
+	slices.Sort(found)
+	if slices.Equal(expected, found) {
+		return nil
+	}
+	var missing, unexpected []string
+	for _, name := range expected {
+		if !slices.Contains(found, name) {
+			missing = append(missing, name)
+		}
+	}
+	for _, name := range found {
+		if !slices.Contains(expected, name) {
+			unexpected = append(unexpected, name)
+		}
+	}
+	return fmt.Errorf(
+		"%s/%s is not the corpus this population was drawn from: missing %v, unexpected %v",
+		corpusRoot,
+		examplesDirectory,
+		missing,
+		unexpected,
+	)
+}
+
+// selectImplementations resolves --implementations against the population. An
+// empty selection is the whole population, which is what a real sweep runs; a
+// named subset is for smoke runs and is recorded in the manifest like any other
+// intent.
+func selectImplementations(selection string) ([]string, error) {
+	if strings.TrimSpace(selection) == "" {
+		return slices.Clone(includedImplementations), nil
+	}
+	var names []string
+	seen := map[string]bool{}
+	for _, part := range strings.Split(selection, ",") {
+		name := strings.TrimSpace(part)
+		if name == "" {
+			return nil, fmt.Errorf("empty implementation in %q", selection)
+		}
+		if !slices.Contains(includedImplementations, name) {
+			return nil, fmt.Errorf(
+				"%q is not one of the %d implementations in this population",
+				name,
+				len(includedImplementations),
+			)
+		}
+		if seen[name] {
+			return nil, fmt.Errorf("duplicate implementation %q", name)
+		}
+		seen[name] = true
+		names = append(names, name)
+	}
+	return names, nil
+}
+
+// planImplementations gives every selected implementation its document and its
+// own port, in population order so the manifest can name the URL each arm was
+// served from before anything has been served.
+func planImplementations(
+	corpusRoot string,
+	names []string,
+	basePort int,
+) ([]implementation, error) {
+	if basePort+len(names)-1 > 65535 {
+		return nil, fmt.Errorf(
+			"--base-port %d leaves no room for %d implementations",
+			basePort,
+			len(names),
+		)
+	}
+	planned := make([]implementation, 0, len(names))
+	for index, name := range names {
+		document := documentFor(name)
+		if _, err := os.Stat(filepath.Join(corpusRoot, filepath.FromSlash(document))); err != nil {
+			return nil, fmt.Errorf("%s: %w", name, err)
+		}
+		planned = append(planned, implementation{
+			Name:     name,
+			Document: document,
+			Port:     basePort + index,
+		})
+	}
+	return planned, nil
+}
diff --git a/cmd/internal-tools/corpus-sweep/corpus_test.go b/cmd/internal-tools/corpus-sweep/corpus_test.go
new file mode 100644
index 0000000..1d77ceb
--- /dev/null
+++ b/cmd/internal-tools/corpus-sweep/corpus_test.go
@@ -0,0 +1,161 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+)
+
+// writeCorpus builds a tree with the shape the sweep expects: every example
+// directory the population was drawn from, each holding the document that
+// implementation is served at.
+func writeCorpus(t *testing.T, names []string) string {
+	t.Helper()
+	root := t.TempDir()
+	for _, name := range names {
+		if err := os.MkdirAll(filepath.Join(root, examplesDirectory, name), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		document := filepath.Join(root, filepath.FromSlash(documentFor(name)))
+		if err := os.MkdirAll(filepath.Dir(document), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		body := "<!doctype html><title>" + name + "</title><ul class=\"todo-list\"></ul>"
+		if err := os.WriteFile(document, []byte(body), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	return root
+}
+
+func wholeCorpus(t *testing.T) string {
+	t.Helper()
+	return writeCorpus(
+		t,
+		slices.Concat(includedImplementations, excludedImplementations),
+	)
+}
+
+func TestPopulation_IsFortyThreeNamesDisjointFromTheExclusions(t *testing.T) {
+	if len(includedImplementations) != 43 {
+		t.Errorf(
+			"population size: got %d, want 43",
+			len(includedImplementations),
+		)
+	}
+	seen := map[string]bool{}
+	for _, name := range includedImplementations {
+		if seen[name] {
+			t.Errorf("%q appears twice in the population", name)
+		}
+		seen[name] = true
+	}
+	for _, name := range excludedImplementations {
+		if seen[name] {
+			t.Errorf("%q is both included and excluded", name)
+		}
+	}
+}
+
+func TestVerifyCorpus_RejectsATreeThatIsNotThisCorpus(t *testing.T) {
+	if err := verifyCorpus(wholeCorpus(t)); err != nil {
+		t.Fatalf(
+			"the corpus this population was drawn from should verify: %v",
+			err,
+		)
+	}
+
+	short := writeCorpus(
+		t,
+		slices.Concat(includedImplementations[1:], excludedImplementations),
+	)
+	err := verifyCorpus(short)
+	if err == nil {
+		t.Fatal(
+			"a corpus missing an implementation swept 42 arms and reported them as 43",
+		)
+	}
+	if !strings.Contains(err.Error(), includedImplementations[0]) {
+		t.Errorf("the error should name what is missing: %v", err)
+	}
+
+	extra := writeCorpus(
+		t,
+		slices.Concat(
+			includedImplementations,
+			excludedImplementations,
+			[]string{"svelte"},
+		),
+	)
+	err = verifyCorpus(extra)
+	if err == nil {
+		t.Fatal(
+			"a corpus with an implementation this population never drew from verified",
+		)
+	}
+	if !strings.Contains(err.Error(), "svelte") {
+		t.Errorf("the error should name what is unexpected: %v", err)
+	}
+}
+
+func TestDocumentFor_ServesTheOverriddenPathForImplementationsWithoutARootIndex(
+	t *testing.T,
+) {
+	for name, want := range map[string]string{
+		"angular-dart": "examples/angular-dart/web/index.html",
+		"duel":         "examples/duel/www/index.html",
+		"vanillajs":    "examples/vanillajs/index.html",
+		"react":        "examples/react/index.html",
+	} {
+		if got := documentFor(name); got != want {
+			t.Errorf("%s document: got %q, want %q", name, got, want)
+		}
+	}
+}
+
+func TestPlanImplementations_RefusesAnImplementationWhoseDocumentIsMissing(
+	t *testing.T,
+) {
+	root := wholeCorpus(t)
+	if err := os.Remove(filepath.Join(root, filepath.FromSlash(documentFor("duel")))); err != nil {
+		t.Fatal(err)
+	}
+	_, err := planImplementations(root, []string{"duel"}, 5400)
+	if err == nil {
+		t.Fatal(
+			"an implementation whose document is missing would be swept as a 404 page",
+		)
+	}
+	if !strings.Contains(err.Error(), "duel") {
+		t.Errorf("the error should name the implementation: %v", err)
+	}
+}
+
+func TestSelectImplementations_DefaultsToThePopulationAndRejectsNamesOutsideIt(
+	t *testing.T,
+) {
+	all, err := selectImplementations("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(all, includedImplementations) {
+		t.Errorf(
+			"an empty selection should be the whole population, got %d names",
+			len(all),
+		)
+	}
+	subset, err := selectImplementations("react, angular2_es2015")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(subset, []string{"react", "angular2_es2015"}) {
+		t.Errorf("subset: got %v", subset)
+	}
+	for _, selection := range []string{"cujo", "svelte", "react,react"} {
+		if _, err := selectImplementations(selection); err == nil {
+			t.Errorf("selection %q should have been rejected", selection)
+		}
+	}
+}
diff --git a/cmd/internal-tools/corpus-sweep/end_to_end_test.go b/cmd/internal-tools/corpus-sweep/end_to_end_test.go
new file mode 100644
index 0000000..e7179ed
--- /dev/null
+++ b/cmd/internal-tools/corpus-sweep/end_to_end_test.go
@@ -0,0 +1,468 @@
+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 fetcher the stub campaign uses, so the URL a
+// campaign is handed is really requested while the sweep is serving, and what
+// came back is on disk for the test to read.
+func TestMain(m *testing.M) {
+	if url := os.Getenv("CORPUS_SWEEP_TEST_FETCH_URL"); url != "" {
+		recordFetch(url, os.Getenv("CORPUS_SWEEP_TEST_FETCH_LOG"))
+		return
+	}
+	os.Exit(m.Run())
+}
+
+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()
+}
+
+// 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 one seed.
+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: $*" >> "%[2]s"
+CORPUS_SWEEP_TEST_FETCH_URL="$url" CORPUS_SWEEP_TEST_FETCH_LOG="%[3]s" "%[4]s"
+mkdir -p "$output"
+printf '{"seeds":[%%s]}\n' "$seed" > "$output/campaign.json"
+echo "stub campaign seed=$seed url=$url"
+case "$output" in
+  *angular2_es2015/seed-5) exit 1 ;;
+esac
+exit 0
+`
+
+func TestRun_EndToEndAgainstAServedCorpusAndAStubCampaign(t *testing.T) {
+	root := t.TempDir()
+	corpus := wholeCorpus(t)
+	// dojo's document is served but cannot be read, so it stalls at serve and
+	// the two implementations either side of it still have to reach the
+	// campaign tool with both seeds.
+	unreadable := filepath.Join(corpus, filepath.FromSlash(documentFor("dojo")))
+	if err := os.Chmod(unreadable, 0o000); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { os.Chmod(unreadable, 0o644) })
+
+	specPath := filepath.Join(root, "todo.ts")
+	if err := os.WriteFile(specPath, []byte("export const properties = [];\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	output := filepath.Join(root, "campaigns")
+	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)
+	}
+	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",
+	)
+	selected := []string{"angular2", "angular2_es2015", "dojo"}
+	basePort := freePortRange(t, len(selected))
+
+	var stdout bytes.Buffer
+	err = run([]string{
+		"--corpus", corpus,
+		"--spec", specPath,
+		"--implementations", strings.Join(selected, ","),
+		"--seeds", "4-5",
+		"--max-steps", "40",
+		"--duration", "30s",
+		"--concurrency", "2",
+		"--base-port", fmt.Sprint(basePort),
+		"--output", output,
+		"--campaign", campaignPath,
+		"--sanderling", sanderlingPath,
+	}, &stdout, io.Discard)
+	if err == nil ||
+		!strings.Contains(err.Error(), "1 of 3 implementations never ran") {
+		t.Fatalf(
+			"expected the stalled implementation and the failed campaign to be reported, got %v",
+			err,
+		)
+	}
+
+	recorded := readManifest(t, filepath.Join(output, manifestFileName))
+	if !slices.Equal(recorded.Seeds, []int64{4, 5}) {
+		t.Errorf("intended seeds: got %v", recorded.Seeds)
+	}
+	if len(recorded.Implementations) != len(selected) {
+		t.Fatalf("intended implementations: got %v", recorded.Implementations)
+	}
+	for index, planned := range recorded.Implementations {
+		if planned.Name != selected[index] {
+			t.Errorf(
+				"implementation %d: got %q, want %q",
+				index,
+				planned.Name,
+				selected[index],
+			)
+		}
+		wantPort := basePort + index
+		wantURL := fmt.Sprintf(
+			"http://127.0.0.1:%d/examples/%s/index.html",
+			wantPort,
+			planned.Name,
+		)
+		if planned.Port != wantPort || planned.URL != wantURL {
+			t.Errorf(
+				"%s: port %d url %q, want %d %q",
+				planned.Name,
+				planned.Port,
+				planned.URL,
+				wantPort,
+				wantURL,
+			)
+		}
+	}
+	if recorded.Generator != "seeded" || recorded.Platform != "web" ||
+		recorded.MaxSteps != 40 {
+		t.Errorf(
+			"manifest generator/platform/budget: %q/%q/%d",
+			recorded.Generator,
+			recorded.Platform,
+			recorded.MaxSteps,
+		)
+	}
+	if recorded.CorpusRoot != corpus {
+		t.Errorf(
+			"manifest corpus root: got %q, want %q",
+			recorded.CorpusRoot,
+			corpus,
+		)
+	}
+
+	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")
+		port := basePort + slices.Index(selected, arm)
+		wantBundle := fmt.Sprintf(
+			"http://127.0.0.1:%d/examples/%s/index.html",
+			port,
+			arm,
+		)
+		if got := argumentValue(arguments, "--bundle-id"); got != wantBundle {
+			t.Errorf(
+				"%s seed %s: bundle id %q, want %q",
+				arm,
+				seed,
+				got,
+				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{"angular2/4", "angular2/5", "angular2_es2015/4", "angular2_es2015/5"} {
+		if !seen[want] {
+			t.Errorf("%s never reached the campaign tool", want)
+		}
+	}
+
+	// What the served page actually returned: each port answered with its own
+	// implementation's document, so no arm was driven against another's.
+	fetched := readLines(t, fetchLog)
+	for index, name := range []string{"angular2", "angular2_es2015"} {
+		want := fmt.Sprintf(
+			"http://127.0.0.1:%d/examples/%s/index.html -> 200 <!doctype html><title>%s</title>",
+			basePort+index,
+			name,
+			name,
+		)
+		matched := 0
+		for _, line := range fetched {
+			if strings.HasPrefix(line, want) {
+				matched++
+			}
+		}
+		if matched != 2 {
+			t.Errorf(
+				"%s was served its own document %d times, want 2:\n%s",
+				name,
+				matched,
+				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
+	}
+	stalled := byName["dojo"]
+	if stalled.FailedStage != stageServe || len(stalled.Runs) != 0 {
+		t.Errorf(
+			"dojo: stage %q with %d runs, want a serve failure and no runs",
+			stalled.FailedStage,
+			len(stalled.Runs),
+		)
+	}
+	for _, name := range []string{"angular2", "angular2_es2015"} {
+		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,
+			)
+		}
+	}
+	if exit := byName["angular2_es2015"].Runs[1].ExitCode; exit != 1 {
+		t.Errorf("angular2_es2015 seed 5 exit code: got %d, want 1", exit)
+	}
+	if exit := byName["angular2_es2015"].Runs[0].ExitCode; exit != 0 {
+		t.Errorf("angular2_es2015 seed 4 exit code: got %d, want 0", exit)
+	}
+
+	for _, name := range []string{"angular2", "angular2_es2015"} {
+		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,
+				)
+			}
+		}
+	}
+
+	client := &http.Client{Timeout: 2 * time.Second}
+	for offset := range selected {
+		if response, err := client.Get(fmt.Sprintf("http://127.0.0.1:%d/", basePort+offset)); err == nil {
+			response.Body.Close()
+			t.Errorf(
+				"port %d is still served after the sweep finished",
+				basePort+offset,
+			)
+		}
+	}
+	if !strings.Contains(stdout.String(), "failed at serve") {
+		t.Errorf(
+			"progress output does not name the stalled implementation: %q",
+			stdout.String(),
+		)
+	}
+}
+
+func TestRun_RefusesAnOutputDirectoryThatAlreadyHoldsASweep(t *testing.T) {
+	root := t.TempDir()
+	output := filepath.Join(root, "campaigns")
+	if err := os.MkdirAll(output, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(output, manifestFileName), []byte("{}"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	specPath := filepath.Join(root, "todo.ts")
+	if err := os.WriteFile(specPath, []byte("export const properties = [];\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	err := run([]string{
+		"--corpus", wholeCorpus(t), "--spec", specPath, "--seeds", "1",
+		"--max-steps", "10", "--output", output,
+	}, io.Discard, io.Discard)
+	if err == nil || !strings.Contains(err.Error(), manifestFileName) {
+		t.Fatalf(
+			"two sweeps sharing a directory would interleave their records, got %v",
+			err,
+		)
+	}
+}
+
+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 readManifest(t *testing.T, path string) manifest {
+	t.Helper()
+	body, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var recorded manifest
+	if err := json.Unmarshal(body, &recorded); err != nil {
+		t.Fatal(err)
+	}
+	return recorded
+}
+
+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
+}
+
+func argumentValue(arguments []string, name string) string {
+	index := slices.Index(arguments, name)
+	if index < 0 || index+1 >= len(arguments) {
+		return ""
+	}
+	return arguments[index+1]
+}
+
+// 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("127.0.0.1:%d", base+offset),
+		)
+		if err != nil {
+			return false
+		}
+		listener.Close()
+	}
+	return true
+}
diff --git a/cmd/internal-tools/corpus-sweep/main.go b/cmd/internal-tools/corpus-sweep/main.go
new file mode 100644
index 0000000..c4dbc5d
--- /dev/null
+++ b/cmd/internal-tools/corpus-sweep/main.go
@@ -0,0 +1,275 @@
+// Command corpus-sweep runs one specification against every implementation in a
+// served corpus of independent implementations of the same requirement. It
+// serves each one on its own port, which is what keeps them apart: the corpus
+// holds pairs that write the same localStorage key, and one origin shared
+// between two of them is one stored record shared between them.
+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 the served corpus, 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 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 said to re-measure
+// before trusting anything above eight on a contended host. Six sits at the
+// better efficiency and leaves the emulator farm that shares the host its
+// slots. Serving costs nothing here: the corpus needs no build and no separate
+// server process, so a worker is one browser.
+const defaultConcurrency = 6
+
+// defaultBasePort starts above the range the model-implementation sweep hands
+// out, so the two can run on one host without either being served the other's
+// pages.
+const defaultBasePort = 5400
+
+type config struct {
+	corpusRoot      string
+	specPath        string
+	outputDirectory string
+	implementations []string
+	seeds           []int64
+	maxSteps        int
+	duration        time.Duration
+	concurrency     int
+	basePort        int
+	campaignPath    string
+	sanderlingPath  string
+	extraArguments  []string
+}
+
+const usage = `corpus-sweep runs one seeded campaign per implementation of a served corpus.
+
+Usage:
+  corpus-sweep --corpus <dir> --spec <path> --seeds <spec>
+               --max-steps <n> --output <dir> [flags]
+               [-- <sanderling test flags>]
+
+Every implementation is served on its own port, so every one is its own origin
+and none can read another's stored state, and every one is swept with the same
+seeds and the same step budget. Each run's arm is the implementation's name.
+
+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("corpus-sweep", flag.ContinueOnError)
+	flagSet.SetOutput(stderr)
+	flagSet.Usage = func() {
+		fmt.Fprint(stderr, usage)
+		flagSet.PrintDefaults()
+	}
+	var configuration config
+	var seedSpecification string
+	var selection string
+	flagSet.StringVar(
+		&configuration.corpusRoot,
+		"corpus",
+		"",
+		"root of the checked-out corpus, holding examples/ (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.StringVar(
+		&selection,
+		"implementations",
+		"",
+		"comma-separated subset of the population to sweep (default: all of it)",
+	)
+	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 swept at once",
+	)
+	flagSet.IntVar(
+		&configuration.basePort,
+		"base-port",
+		defaultBasePort,
+		"first port served; each implementation takes the next one in population order",
+	)
+	flagSet.StringVar(
+		&configuration.outputDirectory,
+		"output",
+		"",
+		"campaign tree to create (required)",
+	)
+	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{
+		"--corpus": configuration.corpusRoot,
+		"--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
+	names, err := selectImplementations(selection)
+	if err != nil {
+		return config{}, fmt.Errorf("--implementations: %w", err)
+	}
+	configuration.implementations = names
+	for name, value := range map[string]*string{
+		"--corpus": &configuration.corpusRoot,
+		"--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
+}
+
+func campaignDirectory(
+	configuration config,
+	target implementation,
+	seed string,
+) string {
+	return filepath.Join(
+		configuration.outputDirectory,
+		target.Name,
+		"seed-"+seed,
+	)
+}
+
+// campaignArguments builds one campaign invocation. The arm is the
+// implementation's name, so every run in the tree can be attributed to the
+// implementation it came from without reading back which port served it.
+func campaignArguments(
+	configuration config,
+	target implementation,
+	seed string,
+) []string {
+	arguments := []string{
+		"--spec", configuration.specPath,
+		"--bundle-id", target.URL(),
+		"--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/corpus-sweep/manifest.go b/cmd/internal-tools/corpus-sweep/manifest.go
new file mode 100644
index 0000000..31c84ac
--- /dev/null
+++ b/cmd/internal-tools/corpus-sweep/manifest.go
@@ -0,0 +1,132 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+const (
+	manifestFileName = "sweep.json"
+	recordsFileName  = "implementations.jsonl"
+)
+
+// plannedImplementation is one implementation the sweep intends to run, with
+// the origin that keeps its stored state its own and the URL every seed is
+// driven at.
+type plannedImplementation struct {
+	Name     string `json:"name"`
+	Document string `json:"document"`
+	Port     int    `json:"port"`
+	Origin   string `json:"origin"`
+	URL      string `json:"url"`
+}
+
+// manifest is sweep.json: what the sweep INTENDED to run, written before the
+// first run 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"`
+	CorpusRoot      string                  `json:"corpus_root"`
+	CorpusCommit    string                  `json:"corpus_commit"`
+	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"`
+	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,
+			Document: target.Document,
+			Port:     target.Port,
+			Origin:   target.Origin(),
+			URL:      target.URL(),
+		})
+	}
+	return manifest{
+		Generator:       generator,
+		Platform:        platform,
+		SpecPath:        configuration.specPath,
+		CorpusRoot:      configuration.corpusRoot,
+		CorpusCommit:    corpusCommit(configuration.corpusRoot),
+		MaxSteps:        configuration.maxSteps,
+		DurationMillis:  configuration.duration.Milliseconds(),
+		Seeds:           configuration.seeds,
+		Implementations: planned,
+		Concurrency:     configuration.concurrency,
+		Host:            host,
+		CampaignPath:    configuration.campaignPath,
+		SanderlingPath:  configuration.sanderlingPath,
+		StartedAt:       startedAt,
+	}
+}
+
+// corpusCommit records which checkout was swept. It is empty rather than fatal
+// for a corpus that is not a git working tree, because the population check has
+// already established the corpus holds the right examples.
+func corpusCommit(corpusRoot string) string {
+	command := exec.Command("git", "-C", corpusRoot, "rev-parse", "HEAD")
+	output, err := command.Output()
+	if err != nil {
+		return ""
+	}
+	return strings.TrimSpace(string(output))
+}
+
+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"`
+	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 one that never got served
+// carries no runs at all.
+type implementationRecord struct {
+	Name            string      `json:"implementation"`
+	Document        string      `json:"document"`
+	Port            int         `json:"port"`
+	Origin          string      `json:"origin"`
+	URL             string      `json:"url"`
+	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 stageServe = "serve"
diff --git a/cmd/internal-tools/corpus-sweep/origin_test.go b/cmd/internal-tools/corpus-sweep/origin_test.go
new file mode 100644
index 0000000..2e7f3b3
--- /dev/null
+++ b/cmd/internal-tools/corpus-sweep/origin_test.go
@@ -0,0 +1,194 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// collisionPairs are implementations in this corpus that write the same
+// localStorage key as each other. Served from one origin they share one stored
+// record: in the corpus survey angular2_es2015 crashed at bootstrap on a record
+// angular2 had written, which is a violation belonging to no implementation.
+var collisionPairs = [][2]string{
+	{"angular2", "angular2_es2015"},
+	{"backbone", "backbone_require"},
+	{"canjs", "canjs_require"},
+	{"react", "typescript-react"},
+}
+
+func originOf(t *testing.T, rawURL string) string {
+	t.Helper()
+	parsed, err := url.Parse(rawURL)
+	if err != nil {
+		t.Fatalf("parse %q: %v", rawURL, err)
+	}
+	return parsed.Scheme + "://" + parsed.Host
+}
+
+// The whole population is swept so the assertion covers every implementation
+// rather than the four pairs already known to collide: the survey found those
+// four, and an unexamined fifth would be just as damaging.
+func TestSweep_GivesEveryImplementationAnOriginNoOtherImplementationShares(
+	t *testing.T,
+) {
+	root := t.TempDir()
+	corpus := wholeCorpus(t)
+	specPath := filepath.Join(root, "todo.ts")
+	if err := os.WriteFile(specPath, []byte("export const properties = [];\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	output := filepath.Join(root, "campaigns")
+	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)
+	}
+	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",
+	)
+
+	err = run([]string{
+		"--corpus", corpus,
+		"--spec", specPath,
+		"--seeds", "1",
+		"--max-steps", "10",
+		"--duration", "30s",
+		"--concurrency", "8",
+		"--base-port", fmt.Sprint(freePortRange(t, len(includedImplementations))),
+		"--output", output,
+		"--campaign", campaignPath,
+		"--sanderling", sanderlingPath,
+	}, io.Discard, io.Discard)
+	if err != nil {
+		t.Fatalf("sweep: %v", err)
+	}
+
+	// What the sweep declared it would serve each implementation from.
+	recorded := readManifest(t, filepath.Join(output, manifestFileName))
+	if len(recorded.Implementations) != len(includedImplementations) {
+		t.Fatalf(
+			"intended implementations: got %d, want %d",
+			len(recorded.Implementations),
+			len(includedImplementations),
+		)
+	}
+	plannedOrigin := map[string]string{}
+	ownerOfOrigin := map[string]string{}
+	for _, planned := range recorded.Implementations {
+		if owner, taken := ownerOfOrigin[planned.Origin]; taken {
+			t.Errorf(
+				"%s and %s are both served from %s, so they share one localStorage",
+				owner,
+				planned.Name,
+				planned.Origin,
+			)
+		}
+		ownerOfOrigin[planned.Origin] = planned.Name
+		plannedOrigin[planned.Name] = planned.Origin
+		if got := originOf(t, planned.URL); got != planned.Origin {
+			t.Errorf(
+				"%s: url %q is not under the origin %q the manifest claims",
+				planned.Name,
+				planned.URL,
+				planned.Origin,
+			)
+		}
+	}
+	for _, pair := range collisionPairs {
+		if plannedOrigin[pair[0]] == plannedOrigin[pair[1]] {
+			t.Errorf(
+				"%s and %s write the same localStorage key and are both served from %s",
+				pair[0],
+				pair[1],
+				plannedOrigin[pair[0]],
+			)
+		}
+	}
+
+	// What actually reached the driver. The web driver parses the origin to
+	// clear out of --bundle-id, so two arms sharing a bundle-id origin clear
+	// and repopulate one another's storage however carefully they are labelled.
+	drivenOrigin := map[string]string{}
+	for _, line := range readLines(t, campaignLog) {
+		arguments := strings.Fields(strings.SplitN(line, "argv: ", 2)[1])
+		arm := argumentValue(arguments, "--arm")
+		if arm == "" {
+			t.Fatalf(
+				"a campaign ran with no arm, so its runs cannot be attributed: %q",
+				line,
+			)
+		}
+		drivenOrigin[arm] = originOf(t, argumentValue(arguments, "--bundle-id"))
+	}
+	if len(drivenOrigin) != len(includedImplementations) {
+		t.Fatalf(
+			"arms that reached the campaign tool: got %d, want %d",
+			len(drivenOrigin),
+			len(includedImplementations),
+		)
+	}
+	armOfOrigin := map[string]string{}
+	for arm, origin := range drivenOrigin {
+		if other, taken := armOfOrigin[origin]; taken {
+			t.Errorf(
+				"arms %s and %s were both driven at %s",
+				other,
+				arm,
+				origin,
+			)
+		}
+		armOfOrigin[origin] = arm
+		if origin != plannedOrigin[arm] {
+			t.Errorf(
+				"%s was driven at %s but the manifest promised %s",
+				arm,
+				origin,
+				plannedOrigin[arm],
+			)
+		}
+	}
+
+	// And what each origin answered with, which is the check that the ports
+	// are not merely distinct but each carries its own implementation.
+	served := map[string]string{}
+	for _, line := range readLines(t, fetchLog) {
+		requested, body, found := strings.Cut(line, " -> 200 ")
+		if !found {
+			t.Errorf("a served page did not answer: %q", line)
+			continue
+		}
+		served[originOf(t, requested)] = body
+	}
+	for arm, origin := range drivenOrigin {
+		if want := "<title>" + arm + "</title>"; !strings.Contains(
+			served[origin],
+			want,
+		) {
+			t.Errorf(
+				"%s at %s was served %q, which is not its own document",
+				arm,
+				origin,
+				served[origin],
+			)
+		}
+	}
+}
diff --git a/cmd/internal-tools/corpus-sweep/serve.go b/cmd/internal-tools/corpus-sweep/serve.go
new file mode 100644
index 0000000..d69730f
--- /dev/null
+++ b/cmd/internal-tools/corpus-sweep/serve.go
@@ -0,0 +1,178 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"net"
+	"net/http"
+	"path"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"time"
+)
+
+const (
+	readinessTimeout  = 5 * time.Second
+	readinessInterval = 100 * time.Millisecond
+)
+
+// stubScriptPath is served from every implementation's own origin in place of a
+// dependency that no longer answers.
+const stubScriptPath = "/__corpus-sweep__/stub.js"
+
+// deadDependency is polyfill.io, which was shut down after this corpus was
+// pinned. One implementation loads it from its index.html; the request fails
+// and the application works regardless, but it fails slowly, once per run, on
+// every run. The corpus is served from this process, so the reference is
+// rewritten to a local no-op on the way out.
+var deadDependency = regexp.MustCompile(`https?://polyfill\.io/[^"'\s>]*`)
+
+// staticServer serves the whole corpus tree on one implementation's port. The
+// tree rather than the implementation's own directory, because an example that
+// asks for a path above itself has to resolve the same way it would in the
+// upstream repository. Only one implementation is ever visited on this port, so
+// the origin still belongs to it alone.
+type staticServer struct {
+	implementation implementation
+	listener       net.Listener
+	server         *http.Server
+}
+
+func startStaticServer(
+	corpusRoot string,
+	target implementation,
+) (*staticServer, error) {
+	listener, err := net.Listen("tcp", fmt.Sprintf("127.0.0.1:%d", target.Port))
+	if err != nil {
+		return nil, fmt.Errorf("serve %s: %w", target.Name, err)
+	}
+	server := &http.Server{Handler: corpusHandler(corpusRoot)}
+	running := &staticServer{
+		implementation: target,
+		listener:       listener,
+		server:         server,
+	}
+	go server.Serve(listener)
+	return running, nil
+}
+
+func (s *staticServer) stop() {
+	s.server.Close()
+}
+
+// waitReady confirms the served document answers before any run drives it. A
+// wrong document path would otherwise reach the driver as a 404 page, and a
+// sweep of 404 pages produces clean runs for every implementation.
+//
+// Only a transport error is retried. The listener is bound before the sweep
+// starts, so a status that is not 200 is the server's answer about this
+// document and waiting will not change it.
+func (s *staticServer) waitReady(ctx context.Context) error {
+	url := s.implementation.URL()
+	client := &http.Client{Timeout: 5 * time.Second}
+	deadline := time.Now().Add(readinessTimeout)
+	var lastErr error
+	for {
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+		response, err := client.Get(url)
+		if err == nil {
+			io.Copy(io.Discard, response.Body)
+			response.Body.Close()
+			if response.StatusCode == http.StatusOK {
+				return nil
+			}
+			return fmt.Errorf(
+				"%s answered %d, not the document",
+				url,
+				response.StatusCode,
+			)
+		}
+		lastErr = err
+		if time.Now().After(deadline) {
+			return fmt.Errorf(
+				"%s did not answer within %s: %w",
+				url,
+				readinessTimeout,
+				lastErr,
+			)
+		}
+		time.Sleep(readinessInterval)
+	}
+}
+
+func corpusHandler(corpusRoot string) http.Handler {
+	files := http.FileServer(http.Dir(corpusRoot))
+	return http.HandlerFunc(
+		func(writer http.ResponseWriter, request *http.Request) {
+			cleaned := path.Clean(
+				"/" + strings.TrimPrefix(request.URL.Path, "/"),
+			)
+			if cleaned == stubScriptPath {
+				writer.Header().Set("Content-Type", "application/javascript")
+				io.WriteString(
+					writer,
+					"/* corpus-sweep: dependency removed upstream */\n",
+				)
+				return
+			}
+			if !strings.HasSuffix(cleaned, ".html") {
+				files.ServeHTTP(writer, request)
+				return
+			}
+			body, modified, err := readDocument(corpusRoot, cleaned)
+			if err != nil {
+				// Never the file server's fallback: it answers a document it
+				// cannot read with a 200 directory listing, and a run driven at a
+				// listing explores nothing and comes back clean.
+				http.Error(writer, err.Error(), documentStatus(err))
+				return
+			}
+			rewritten := deadDependency.ReplaceAll(body, []byte(stubScriptPath))
+			http.ServeContent(
+				writer,
+				request,
+				path.Base(cleaned),
+				modified,
+				bytes.NewReader(rewritten),
+			)
+		},
+	)
+}
+
+func documentStatus(err error) int {
+	switch {
+	case errors.Is(err, fs.ErrNotExist):
+		return http.StatusNotFound
+	case errors.Is(err, fs.ErrPermission):
+		return http.StatusForbidden
+	default:
+		return http.StatusInternalServerError
+	}
+}
+
+func readDocument(corpusRoot, cleaned string) ([]byte, time.Time, error) {
+	file, err := http.Dir(corpusRoot).Open(cleaned)
+	if err != nil {
+		return nil, time.Time{}, err
+	}
+	defer file.Close()
+	info, err := file.Stat()
+	if err != nil || info.IsDir() {
+		return nil, time.Time{}, fmt.Errorf(
+			"%s is not a document",
+			filepath.FromSlash(cleaned),
+		)
+	}
+	body, err := io.ReadAll(file)
+	if err != nil {
+		return nil, time.Time{}, err
+	}
+	return body, info.ModTime(), nil
+}
diff --git a/cmd/internal-tools/corpus-sweep/serve_test.go b/cmd/internal-tools/corpus-sweep/serve_test.go
new file mode 100644
index 0000000..8aa1439
--- /dev/null
+++ b/cmd/internal-tools/corpus-sweep/serve_test.go
@@ -0,0 +1,214 @@
+package main
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+func get(t *testing.T, url string) (int, string) {
+	t.Helper()
+	client := &http.Client{Timeout: 5 * time.Second}
+	response, err := client.Get(url)
+	if err != nil {
+		t.Fatalf("GET %s: %v", url, err)
+	}
+	defer response.Body.Close()
+	body, err := io.ReadAll(response.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return response.StatusCode, string(body)
+}
+
+func TestStaticServer_ServesEachImplementationsDocumentFromItsOwnPort(
+	t *testing.T,
+) {
+	root := wholeCorpus(t)
+	planned, err := planImplementations(
+		root,
+		[]string{"angular-dart", "duel", "react"},
+		freePortRange(t, 3),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, target := range planned {
+		server, err := startStaticServer(root, target)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer server.stop()
+		if err := server.waitReady(context.Background()); err != nil {
+			t.Fatalf("%s: %v", target.Name, err)
+		}
+		status, body := get(t, target.URL())
+		if status != http.StatusOK ||
+			!strings.Contains(body, "<title>"+target.Name+"</title>") {
+			t.Errorf(
+				"%s at %s: got %d %q",
+				target.Name,
+				target.URL(),
+				status,
+				body,
+			)
+		}
+	}
+}
+
+func TestStaticServer_ReplacesTheDependencyThatNoLongerAnswers(t *testing.T) {
+	root := wholeCorpus(t)
+	document := filepath.Join(root, filepath.FromSlash(documentFor("aurelia")))
+	body := `<!doctype html><script src="https://polyfill.io/v3/polyfill.min.js?features=Promise"></script>`
+	if err := os.WriteFile(document, []byte(body), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	planned, err := planImplementations(
+		root,
+		[]string{"aurelia"},
+		freePortRange(t, 1),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	server, err := startStaticServer(root, planned[0])
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer server.stop()
+
+	status, served := get(t, planned[0].URL())
+	if status != http.StatusOK {
+		t.Fatalf("document answered %d", status)
+	}
+	if strings.Contains(served, "polyfill.io") {
+		t.Errorf(
+			"a request to a host that no longer answers reaches the browser once per run: %q",
+			served,
+		)
+	}
+	if !strings.Contains(served, `src="`+stubScriptPath+`"`) {
+		t.Errorf(
+			"the reference was removed rather than pointed at a local no-op: %q",
+			served,
+		)
+	}
+
+	stubStatus, stub := get(t, planned[0].Origin()+stubScriptPath)
+	if stubStatus != http.StatusOK || stub == "" {
+		t.Errorf("stub script: got %d %q", stubStatus, stub)
+	}
+}
+
+// An example that asks for a path above its own directory has to resolve the
+// same way it does upstream, which is why the whole tree is served rather than
+// the one directory.
+func TestStaticServer_ServesPathsAboveTheImplementationsOwnDirectory(
+	t *testing.T,
+) {
+	root := wholeCorpus(t)
+	shared := filepath.Join(root, "node_modules", "todomvc-common")
+	if err := os.MkdirAll(shared, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(shared, "base.css"), []byte("body{}"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	planned, err := planImplementations(
+		root,
+		[]string{"react"},
+		freePortRange(t, 1),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	server, err := startStaticServer(root, planned[0])
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer server.stop()
+
+	status, body := get(
+		t,
+		planned[0].Origin()+"/node_modules/todomvc-common/base.css",
+	)
+	if status != http.StatusOK || body != "body{}" {
+		t.Errorf("shared asset: got %d %q", status, body)
+	}
+}
+
+func TestStaticServerWaitReady_ReportsADocumentThatDoesNotAnswer(t *testing.T) {
+	root := wholeCorpus(t)
+	document := filepath.Join(root, filepath.FromSlash(documentFor("dojo")))
+	if err := os.Chmod(document, 0o000); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { os.Chmod(document, 0o644) })
+	planned, err := planImplementations(
+		root,
+		[]string{"dojo"},
+		freePortRange(t, 1),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	server, err := startStaticServer(root, planned[0])
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer server.stop()
+
+	started := time.Now()
+	err = server.waitReady(context.Background())
+	if err == nil {
+		t.Fatal(
+			"a document that does not answer would be swept as an error page and come back clean",
+		)
+	}
+	if elapsed := time.Since(started); elapsed > readinessTimeout {
+		t.Errorf(
+			"waited %s for a status that was never going to change",
+			elapsed,
+		)
+	}
+}
+
+func TestStaticServerStop_ReleasesThePort(t *testing.T) {
+	root := wholeCorpus(t)
+	planned, err := planImplementations(
+		root,
+		[]string{"vue"},
+		freePortRange(t, 1),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	server, err := startStaticServer(root, planned[0])
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := server.waitReady(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	server.stop()
+
+	client := &http.Client{Timeout: time.Second}
+	deadline := time.Now().Add(3 * time.Second)
+	for time.Now().Before(deadline) {
+		response, err := client.Get(planned[0].URL())
+		if err != nil {
+			return
+		}
+		response.Body.Close()
+		time.Sleep(50 * time.Millisecond)
+	}
+	t.Fatalf(
+		"port %d is still served after stop(), so the next sweep cannot bind it",
+		planned[0].Port,
+	)
+}
diff --git a/cmd/internal-tools/corpus-sweep/sweep.go b/cmd/internal-tools/corpus-sweep/sweep.go
new file mode 100644
index 0000000..39d56be
--- /dev/null
+++ b/cmd/internal-tools/corpus-sweep/sweep.go
@@ -0,0 +1,307 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"sync"
+	"time"
+)
+
+// resolveBinaries turns campaign and sanderling into absolute paths before
+// anything is served. Each campaign runs from the sweep's own directory, 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{
+		"--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
+	servers       map[string]*staticServer
+	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,
+		)
+	}
+	if err := verifyCorpus(configuration.corpusRoot); err != nil {
+		return err
+	}
+	implementations, err := planImplementations(
+		configuration.corpusRoot,
+		configuration.implementations,
+		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,
+	}
+	// Every port is bound before any run starts. A port already taken means
+	// that implementation cannot have an origin of its own, and continuing
+	// without one is what the separate origins are there to prevent.
+	if err := running.serveAll(implementations); err != nil {
+		return err
+	}
+	defer running.stopAll()
+
+	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) serveAll(implementations []implementation) error {
+	s.servers = make(map[string]*staticServer, len(implementations))
+	for _, target := range implementations {
+		server, err := startStaticServer(s.configuration.corpusRoot, target)
+		if err != nil {
+			s.stopAll()
+			return err
+		}
+		s.servers[target.Name] = server
+	}
+	return nil
+}
+
+func (s *sweep) stopAll() {
+	for _, server := range s.servers {
+		server.stop()
+	}
+}
+
+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 through all of its seeds. A
+// document that does not answer is recorded and the sweep moves on: one
+// implementation must not cost the other forty-two their runs.
+func (s *sweep) runImplementation(
+	ctx context.Context,
+	target implementation,
+) (record implementationRecord) {
+	record = implementationRecord{
+		Name:      target.Name,
+		Document:  target.Document,
+		Port:      target.Port,
+		Origin:    target.Origin(),
+		URL:       target.URL(),
+		StartedAt: time.Now().UTC(),
+	}
+	started := time.Now()
+	defer func() { record.MonotonicMillis = time.Since(started).Milliseconds() }()
+
+	if err := os.MkdirAll(filepath.Join(s.configuration.outputDirectory, target.Name), 0o755); err != nil {
+		record.FailedStage = stageServe
+		record.Error = err.Error()
+		return record
+	}
+	if err := s.servers[target.Name].waitReady(ctx); 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, 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
+}
+
+func runCommand(
+	ctx context.Context,
+	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.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)
+	failed := 0
+	for _, run := range record.Runs {
+		if run.ExitCode != 0 {
+			failed++
+		}
+	}
+	s.failedRuns += failed
+	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
+	}
+	fmt.Fprintf(s.stdout, "%s origin=%s campaigns=%d failed=%d elapsed=%s\n",
+		record.Name, record.Origin, len(record.Runs), failed, elapsed)
+}