diff --git a/README.md b/README.md
index 13f1790..23edf69 100644
--- a/README.md
+++ b/README.md
@@ -94,7 +94,9 @@ an hour.
 provenance predicate of that run - the workflow at its ref as the builder, the repository at its
 commit and every versions.yaml pin as the inputs, and no clock - for `cosign attest-blob --type
 slsaprovenance1 --predicate <file>` to sign over the files. Package `provenance` is the same, for
-a build that adds inputs of its own (`Build`'s extra).
+a build that adds inputs of its own (`Build`'s extra). In a reusable workflow, `-builder
+.github/workflows/<file>` (`Predicate.BuiltBy`) makes that workflow the builder, as the run's
+certificate names it; GitHub's environment names only the caller there.
 
 `attestgate.Gate`, run from a test like the versions gate, fails on a workflow step that
 publishes (a release action, `oras push`, `gh release upload`, `docker push`, `aws s3 cp`, a task
@@ -102,7 +104,9 @@ named in `UploadTasks`) without a step before it in its job that attests (`cosig
 --type slsaprovenance1`, or a task in `AttestTasks`) unconditionally and with `id-token: write`.
 `NotPublished` excuses a step by `<file>/<job>/<step>`, with its reason, and an excuse that matches
 nothing fails; `MainOnly` holds release workflows to no push trigger and every job on
-`refs/heads/main`. The YAML cannot say which files a glob covered, so the step right before an
+`refs/heads/main`. `Builder` names a reusable workflow (`on: workflow_call` alone) that every
+upload must be in, called only from a `MainOnly` workflow whose job grants `id-token: write`: the
+certificate then names the builder, which a verifier pins. The YAML cannot say which files a glob covered, so the step right before an
 upload runs `go tool attestgate files -require provenance [-except checksums.txt] <dir>`, which
 fails on a file there with no `<file>.provenance.sigstore.json` beside it.
 
diff --git a/attestgate/attestgate.go b/attestgate/attestgate.go
index 9a840b2..9ac308b 100644
--- a/attestgate/attestgate.go
+++ b/attestgate/attestgate.go
@@ -39,6 +39,19 @@ type Gate struct {
 	// MainOnly are the workflow files that cut a release: none has a push trigger, and every job
 	// in one runs only on refs/heads/main.
 	MainOnly []string
+	// Builder is the reusable workflow every publishing step is in, by its file name: on:
+	// workflow_call and nothing else, so it runs only as a job of a workflow that calls it with
+	// uses:, and only a MainOnly one may. The certificate a step in it signs with names the
+	// builder, not the caller, so a verifier pinning it pins the code that built and attested; a
+	// caller that published by itself would sign as itself and be outside the pin. Empty: no
+	// builder, and a release workflow publishes itself.
+	Builder string
+}
+
+// callsBuilder is whether a job's uses: is the builder, as a workflow of the same repository
+// names it.
+func (g Gate) callsBuilder(j job) bool {
+	return g.Builder != "" && j.Uses == "./.github/workflows/"+g.Builder
 }
 
 type workflow struct {
@@ -204,6 +217,9 @@ func (g Gate) Check() error {
 			problems = append(problems, fmt.Sprintf("%s: MainOnly names a workflow there is not", f))
 		}
 	}
+	if g.Builder != "" && !files[g.Builder] {
+		problems = append(problems, fmt.Sprintf("%s: Builder names a workflow there is not", g.Builder))
+	}
 	for _, k := range slices.Sorted(maps.Keys(g.NotPublished)) {
 		if strings.TrimSpace(g.NotPublished[k]) == "" {
 			problems = append(problems, fmt.Sprintf("%s: NotPublished with no reason", k))
@@ -218,22 +234,44 @@ func (g Gate) Check() error {
 	return nil
 }
 
+// checkJob is what is wrong with job j at where, as a whole: a release job that may run off main,
+// and a call to the builder from where it may not be called or with no token to sign.
+func (g Gate) checkJob(where string, j job, mainOnly, token bool) []string {
+	var problems []string
+	if mainOnly && !mainRef.MatchString(j.If) {
+		problems = append(problems, where+": a release job with no if: github.ref == 'refs/heads/main'")
+	}
+	if !g.callsBuilder(j) {
+		return problems
+	}
+	if !mainOnly {
+		problems = append(problems, where+": calls the builder from a workflow that is not MainOnly")
+	}
+	// A called workflow has at most its caller's permissions, whatever it declares.
+	if !token {
+		problems = append(problems, where+": calls the builder with no id-token: write, so it cannot sign")
+	}
+	return problems
+}
+
 func (g Gate) check(file string, w workflow, excused map[string]bool) []string {
 	var problems []string
 	mainOnly := slices.Contains(g.MainOnly, file)
 	if mainOnly && slices.Contains(triggers(w.On), "push") {
 		problems = append(problems, fmt.Sprintf("%s: a release workflow with a push trigger: it runs on whatever was pushed", file))
 	}
+	builder := g.Builder != "" && file == g.Builder
+	if on := triggers(w.On); builder && (len(on) != 1 || on[0] != "workflow_call") {
+		problems = append(problems, fmt.Sprintf("%s: the builder is on: %v, not workflow_call alone: it runs without a release workflow calling it", file, on))
+	}
 	for _, jn := range slices.Sorted(maps.Keys(w.Jobs)) {
 		j := w.Jobs[jn]
-		if mainOnly && !mainRef.MatchString(j.If) {
-			problems = append(problems, fmt.Sprintf("%s/%s: a release job with no if: github.ref == 'refs/heads/main'", file, jn))
-		}
 		// A job's permissions replace the workflow's whole, they do not add to them.
 		token := idToken(w.Permissions)
 		if j.Permissions.Kind != 0 {
 			token = idToken(j.Permissions)
 		}
+		problems = append(problems, g.checkJob(file+"/"+jn, j, mainOnly, token)...)
 		attested := covers{}
 		for _, s := range j.Steps {
 			where := file + "/" + jn + "/" + s.key()
@@ -255,6 +293,8 @@ func (g Gate) check(file string, w workflow, excused map[string]bool) []string {
 			}
 			has := attested.or(self)
 			switch {
+			case g.Builder != "" && !builder:
+				problems = append(problems, fmt.Sprintf("%s: publishes outside the builder %s: a release workflow calls it with uses:", where, g.Builder))
 			case !has.provenance:
 				problems = append(problems, fmt.Sprintf("%s: publishes with no provenance attested before it in its job", where))
 			case g.RequireSBOM && !has.sbom:
diff --git a/attestgate/attestgate_test.go b/attestgate/attestgate_test.go
index 7146b11..4961f7a 100644
--- a/attestgate/attestgate_test.go
+++ b/attestgate/attestgate_test.go
@@ -173,6 +173,83 @@ func TestAnUploadNeedsAnAttestationBeforeIt(t *testing.T) {
 	}
 }
 
+// With a builder, what is published is published from it alone: a reusable workflow that runs only
+// when called, called only by a release workflow from main that gives it a token to sign with, and
+// held inside to the rules every publishing job is held to.
+func TestABuilderIsWhereEveryUploadIs(t *testing.T) {
+	const main = "github.ref == 'refs/heads/main'"
+	caller := func(on, perms, jobIf string) string {
+		w := "name: release\non: " + on + "\n"
+		if perms != "" {
+			w += "permissions:\n  " + perms + "\n"
+		}
+		w += "jobs:\n  release:\n"
+		if jobIf != "" {
+			w += "    if: " + jobIf + "\n"
+		}
+		return w + "    uses: ./.github/workflows/build-release.yml\n"
+	}
+	good := caller("workflow_dispatch", "id-token: write", main)
+	builder := workflow("workflow_call", "id-token: write", "", "", build, attest, release)
+	gate := attestgate.Gate{MainOnly: []string{"release.yml"}, Builder: "build-release.yml"}
+	for name, tc := range map[string]struct {
+		files    map[string]string
+		gate     attestgate.Gate
+		problems []string
+	}{
+		"called from main":  {files: map[string]string{"release.yml": good, "build-release.yml": builder}, gate: gate},
+		"no builder at all": {files: map[string]string{"release.yml": workflow("workflow_dispatch", "id-token: write", main, "", attest, release)}, gate: attestgate.Gate{MainOnly: []string{"release.yml"}}},
+		"an upload in the caller": {files: map[string]string{"release.yml": workflow("workflow_dispatch", "id-token: write", main, "", attest, release), "build-release.yml": builder},
+			gate: gate, problems: []string{"release.yml/release/Release: publishes outside the builder build-release.yml"}},
+		"an upload in another workflow": {files: map[string]string{"release.yml": good, "build-release.yml": builder,
+			"nightly.yml": workflow("schedule", "id-token: write", "", "", attest, orasPush)},
+			gate: gate, problems: []string{"nightly.yml/release/Push: publishes outside the builder"}},
+		"an excused upload elsewhere": {files: map[string]string{"release.yml": good, "build-release.yml": builder,
+			"ci.yml": workflow("push", "", "", "", "      - name: Logs\n        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1\n")},
+			gate: attestgate.Gate{MainOnly: []string{"release.yml"}, Builder: "build-release.yml", NotPublished: map[string]string{"ci.yml/release/Logs": "the run's logs"}}},
+		"a builder that runs by itself": {files: map[string]string{"release.yml": good,
+			"build-release.yml": workflow("[workflow_call, workflow_dispatch]", "id-token: write", "", "", attest, release)},
+			gate: gate, problems: []string{"build-release.yml: the builder is on: [workflow_call workflow_dispatch]"}},
+		"a builder on something else": {files: map[string]string{"release.yml": good,
+			"build-release.yml": workflow("push", "id-token: write", "", "", attest, release)},
+			gate: gate, problems: []string{"build-release.yml: the builder is on: [push]"}},
+		"a builder there is not": {files: map[string]string{"release.yml": good}, gate: gate,
+			problems: []string{"build-release.yml: Builder names a workflow there is not"}},
+		"a caller that is not main only": {files: map[string]string{"release.yml": good, "build-release.yml": builder},
+			gate: attestgate.Gate{Builder: "build-release.yml"}, problems: []string{"release.yml/release: calls the builder from a workflow that is not MainOnly"}},
+		"a caller from any ref": {files: map[string]string{"release.yml": caller("workflow_dispatch", "id-token: write", ""), "build-release.yml": builder},
+			gate: gate, problems: []string{"release.yml/release: a release job with no if"}},
+		"a caller with no token to give": {files: map[string]string{"release.yml": caller("workflow_dispatch", "contents: write", main), "build-release.yml": builder},
+			gate: gate, problems: []string{"release.yml/release: calls the builder with no id-token: write"}},
+		"another workflow's uses is not the builder": {files: map[string]string{"release.yml": good, "build-release.yml": builder,
+			"lint.yml": strings.Replace(caller("push", "", ""), "build-release.yml", "lint-reusable.yml", 1)}, gate: gate},
+		"a builder that publishes unattested": {files: map[string]string{"release.yml": good,
+			"build-release.yml": workflow("workflow_call", "id-token: write", "", "", build, release)},
+			gate: gate, problems: []string{"build-release.yml/release/Release: publishes with no provenance"}},
+	} {
+		t.Run(name, func(t *testing.T) {
+			root := t.TempDir()
+			dir := filepath.Join(root, ".github", "workflows")
+			require.NoError(t, os.MkdirAll(dir, 0o755))
+			for f, body := range tc.files {
+				require.NoError(t, os.WriteFile(filepath.Join(dir, f), []byte(body), 0o600))
+			}
+			tc.gate.Root = root
+			err := tc.gate.Check()
+			if len(tc.problems) == 0 {
+				require.NoError(t, err)
+				return
+			}
+			require.Error(t, err)
+			lines := strings.Split(err.Error(), "\n")[1:]
+			require.Len(t, lines, len(tc.problems), err.Error())
+			for i, p := range tc.problems {
+				assert.Contains(t, lines[i], p)
+			}
+		})
+	}
+}
+
 // A repository with no workflows, or one that is not YAML, is no clean answer.
 func TestWorkflowsItCannotReadAreAnError(t *testing.T) {
 	require.Error(t, attestgate.Gate{Root: t.TempDir()}.Check())
diff --git a/cmd/provenance/main.go b/cmd/provenance/main.go
index 23daf7f..901a6cb 100644
--- a/cmd/provenance/main.go
+++ b/cmd/provenance/main.go
@@ -3,7 +3,7 @@
 // cosign attest-blob --type slsaprovenance1 --predicate to sign over the files a release
 // publishes. Outside a workflow it refuses.
 //
-//	go tool provenance -version v20261013.1 -out _output/provenance.json
+//	go tool provenance -version v20261013.1 -out _output/provenance.json [-builder .github/workflows/build-release.yml]
 package main
 
 import (
@@ -27,6 +27,7 @@ func run(env func(string) string, args []string, stderr io.Writer) int {
 	version := fs.String("version", "", "the release the run builds")
 	out := fs.String("out", "", "where to write the predicate")
 	pins := fs.String("versions", versions.File, "the repository's versions.yaml")
+	builder := fs.String("builder", "", "the reusable workflow the run signs in (.github/workflows/<file>); the dispatched one when empty")
 	if err := fs.Parse(args); err != nil {
 		return 2
 	}
@@ -40,6 +41,9 @@ func run(env func(string) string, args []string, stderr io.Writer) int {
 		return 2
 	}
 	p, err := provenance.Build(env, *version, v, nil)
+	if err == nil && *builder != "" {
+		p, err = p.BuiltBy(*builder)
+	}
 	if err != nil {
 		say(stderr, err)
 		return 2
diff --git a/cmd/provenance/main_test.go b/cmd/provenance/main_test.go
index 65c3b1f..c0786d2 100644
--- a/cmd/provenance/main_test.go
+++ b/cmd/provenance/main_test.go
@@ -40,6 +40,7 @@ func TestItWritesTheRunsPredicateOrNothing(t *testing.T) {
 		args    func(dir string) []string
 		pins    string
 		written bool
+		builder string
 	}{
 		"in a run": {env: run1, pins: pins, written: true,
 			args: func(dir string) []string { return []string{"-version", "v1", "-out", filepath.Join(dir, "p.json")} }},
@@ -49,6 +50,14 @@ func TestItWritesTheRunsPredicateOrNothing(t *testing.T) {
 			args: func(dir string) []string { return []string{"-version", "v1", "-out", filepath.Join(dir, "p.json")} }},
 		"no versions.yaml": {env: run1,
 			args: func(dir string) []string { return []string{"-version", "v1", "-out", filepath.Join(dir, "p.json")} }},
+		"built by a reusable workflow": {env: run1, pins: pins, written: true, builder: "https://github.com/spin-stack/machine/.github/workflows/build-release.yml@refs/heads/main",
+			args: func(dir string) []string {
+				return []string{"-version", "v1", "-builder", ".github/workflows/build-release.yml", "-out", filepath.Join(dir, "p.json")}
+			}},
+		"a builder that is no workflow": {env: run1, pins: pins,
+			args: func(dir string) []string {
+				return []string{"-version", "v1", "-builder", "build-release.yml", "-out", filepath.Join(dir, "p.json")}
+			}},
 		"an unknown flag": {env: run1, pins: pins,
 			args: func(string) []string { return []string{"-subject", "x"} }},
 		"an -out it cannot write": {env: run1, pins: pins,
@@ -76,6 +85,11 @@ func TestItWritesTheRunsPredicateOrNothing(t *testing.T) {
 			require.NoError(t, json.Unmarshal(got, &p))
 			assert.Equal(t, "v1", p.BuildDefinition.ExternalParameters.Version)
 			assert.Equal(t, "debian-snapshot", p.BuildDefinition.ResolvedDependencies[1].Name)
+			builder := tc.builder
+			if builder == "" {
+				builder = "https://github.com/spin-stack/machine/.github/workflows/release.yml@refs/heads/main"
+			}
+			assert.Equal(t, builder, p.RunDetails.Builder.ID)
 		})
 	}
 }
diff --git a/provenance/provenance.go b/provenance/provenance.go
index dd472f3..08f8d67 100644
--- a/provenance/provenance.go
+++ b/provenance/provenance.go
@@ -137,6 +137,19 @@ func Build(env func(string) string, version string, pins *versions.Versions, ext
 	}, nil
 }
 
+// BuiltBy is p built by the reusable workflow at path (.github/workflows/<file>) of the same
+// repository, at the same ref: what the run's certificate names when the steps that sign are that
+// workflow's. GitHub sets GITHUB_WORKFLOW_REF to the caller in a called workflow too, so the run
+// cannot say this itself; the caller stays the build's workflow, which is what was dispatched.
+func (p Predicate) BuiltBy(path string) (Predicate, error) {
+	if !strings.HasPrefix(path, ".github/workflows/") || strings.Contains(path, "@") {
+		return Predicate{}, fmt.Errorf("provenance: builder %q is not a workflow file of the repository", path)
+	}
+	w := p.BuildDefinition.ExternalParameters.Workflow
+	p.RunDetails.Builder.ID = w.Repository + "/" + path + "@" + w.Ref
+	return p, nil
+}
+
 // Pinned is a versions.yaml entry as an input: its name, where it is, and the digest that holds
 // it - none for a module, which the checksum database holds by its version, or a date.
 func Pinned(e versions.Entry) ResourceDescriptor {
diff --git a/provenance/provenance_test.go b/provenance/provenance_test.go
index f3700af..5cf5b15 100644
--- a/provenance/provenance_test.go
+++ b/provenance/provenance_test.go
@@ -96,6 +96,23 @@ func TestARunsPredicateIsTheGolden(t *testing.T) {
 	assert.Equal(t, p, again)
 }
 
+// Built by a reusable workflow, the builder is that workflow at the run's ref, and the dispatched
+// one stays the build's workflow; a builder that is not a workflow file of the repository is no
+// builder.
+func TestABuilderIsTheReusableWorkflowAtTheRunsRef(t *testing.T) {
+	p, err := provenance.Build(getenv(run), "v20261013.1", nil, nil)
+	require.NoError(t, err)
+	built, err := p.BuiltBy(".github/workflows/build-release.yml")
+	require.NoError(t, err)
+	assert.Equal(t, "https://github.com/spin-stack/spin/.github/workflows/build-release.yml@refs/heads/main", built.RunDetails.Builder.ID)
+	assert.Equal(t, ".github/workflows/release.yml", built.BuildDefinition.ExternalParameters.Workflow.Path)
+	assert.Equal(t, "https://github.com/spin-stack/spin/.github/workflows/release.yml@refs/heads/main", p.RunDetails.Builder.ID)
+	for _, path := range []string{"hack/build.yml", "build-release.yml", ".github/workflows/build-release.yml@refs/heads/x"} {
+		_, err := p.BuiltBy(path)
+		assert.Error(t, err, path)
+	}
+}
+
 // An environment that is not a workflow run's, or a workflow ref that is not the repository's,
 // is refused: provenance with no builder in it names nobody.
 func TestAnEnvironmentThatIsNotARunIsRefused(t *testing.T) {