diff --git a/go/buildstep/merge.go b/go/buildstep/merge.go
new file mode 100644
index 0000000..6bc8df9
--- /dev/null
+++ b/go/buildstep/merge.go
@@ -0,0 +1,112 @@
+package buildstep
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"git.novox.be/novox/mesh-sdk/go/forgekey"
+)
+
+// MergeResult is what MergeInto made of a head and a trunk commit: the trunk commit (Base) and the head, whole, and the
+// merge commit and the paths the merge changes against the trunk commit; or, when the head does not merge cleanly, the
+// files in conflict, with Merge and Changed empty.
+type MergeResult struct {
+	Base, Head, Merge string
+	Conflicts         []string
+	Changed           []string
+}
+
+// MergeInto makes, in the workspace dir that FetchCommit left at commit, the merge of commit into the trunk commit
+// base, as the forge's default merge would (a merge commit, never a fast-forward), and leaves the workspace at the merge
+// (novox/hq ADR 0484 clause 3, ADR 0323). It is the one merge the build modules' fetch steps make, with the identity
+// and flags of the controller's check merge (mesh-controller internal/builder/check_merge.go): a name nobody owns, no
+// signature, no hook, nothing of the machine's git configuration that changes a merge. Nothing is pushed.
+//
+// A merge needs both histories, so it makes the fetch whole (the shallow clone FetchCommit made is deepened) and
+// fetches base by its sha. A head that does not merge cleanly is an answer, not an error: the files in conflict, the
+// merge undone and the workspace left at base. A head already in base answers a Merge equal to Base and no changed
+// paths. An error is that the merge could not be tried: a base or commit that is not a full sha, no base, a workspace
+// not at commit, or a fetch or git command that failed. remote and the options are those of FetchCommit; the runner's
+// bound must allow a whole fetch.
+func MergeInto(ctx context.Context, run forgekey.Runner, dir, remote, base, commit string, opts ...Option) (MergeResult, error) {
+	var o fetchOptions
+	for _, opt := range opts {
+		opt(&o)
+	}
+	res := MergeResult{Base: base, Head: commit}
+	if base == "" {
+		return res, fmt.Errorf("buildstep: no base: a merge needs the trunk commit it is made onto")
+	}
+	if err := checkFetch(o, dir, remote, base, ""); err != nil {
+		return res, err
+	}
+	if err := checkFetch(o, dir, remote, commit, ""); err != nil {
+		return res, err
+	}
+	env := []string{"GIT_TERMINAL_PROMPT=0"}
+	git := func(args ...string) (string, error) {
+		full := append([]string{"-c", "credential.helper=", "-C", dir}, args...)
+		r := run(ctx, env, "git", full...)
+		if r.Status != 0 || r.Err != "" {
+			return "", fmt.Errorf("git %s: %s", args[0], firstLine(r.Err+" "+r.Stderr))
+		}
+		return r.Stdout, nil
+	}
+	at, err := git("rev-parse", "HEAD")
+	if err != nil {
+		return res, err
+	}
+	if head := strings.TrimSpace(at); head != commit {
+		return res, fmt.Errorf("buildstep: the workspace is at %s, not at the commit %s", head, commit)
+	}
+	shallow, err := git("rev-parse", "--is-shallow-repository")
+	if err != nil {
+		return res, err
+	}
+	fetch := []string{"fetch", "--no-tags", "origin", base}
+	if strings.TrimSpace(shallow) == "true" {
+		fetch = []string{"fetch", "--no-tags", "--unshallow", "origin", base}
+	}
+	if _, err := git(fetch...); err != nil {
+		return res, fmt.Errorf("the trunk commit %s: %w", base, err)
+	}
+	if _, err := git("checkout", "-q", "--detach", base); err != nil {
+		return res, fmt.Errorf("the trunk commit %s cannot be checked out: %w", base, err)
+	}
+	message := fmt.Sprintf("Check the merge of %s into %s (made for a check, never pushed)", short(commit), short(base))
+	args := append(append([]string{}, mergeIdentity...), "merge", "--no-ff", "--no-edit", "--no-verify", "--quiet", "-m", message, commit)
+	if _, err := git(args...); err != nil {
+		out, _ := git("diff", "--name-only", "--diff-filter=U")
+		res.Conflicts = strings.Fields(out)
+		_, _ = git("merge", "--abort")
+		if len(res.Conflicts) == 0 {
+			return res, fmt.Errorf("the head %s could not be merged into the trunk commit %s: %w", short(commit), short(base), err)
+		}
+		return res, nil
+	}
+	merge, err := git("rev-parse", "HEAD")
+	if err != nil {
+		return res, fmt.Errorf("the merge cannot be read: %w", err)
+	}
+	res.Merge = strings.TrimSpace(merge)
+	changed, err := git("diff", "--name-only", "--no-renames", base, res.Merge)
+	if err != nil {
+		return res, fmt.Errorf("the paths the merge changes cannot be read: %w", err)
+	}
+	res.Changed = strings.Fields(changed)
+	return res, nil
+}
+
+// mergeIdentity is who a check's merge is made as: it is never pushed, so it names nobody's account. No hook of the
+// clone's or the build machine's git configuration runs, and no setting there changes the merge (the controller's
+// check merge, internal/builder/check_merge.go).
+var mergeIdentity = []string{"-c", "user.name=mesh merge check", "-c", "user.email=merge-check@mesh.invalid",
+	"-c", "commit.gpgsign=false", "-c", "core.hooksPath=/dev/null", "-c", "merge.verifySignatures=false"}
+
+func short(commit string) string {
+	if len(commit) > 12 {
+		return commit[:12]
+	}
+	return commit
+}
diff --git a/go/buildstep/merge_test.go b/go/buildstep/merge_test.go
new file mode 100644
index 0000000..4c59610
--- /dev/null
+++ b/go/buildstep/merge_test.go
@@ -0,0 +1,232 @@
+package buildstep
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+
+	"git.novox.be/novox/mesh-sdk/go/forgekey"
+)
+
+func realGit(t *testing.T, dir string, args ...string) string {
+	t.Helper()
+	cmd := exec.Command("git", append([]string{"-C", dir, "-c", "user.name=t", "-c", "user.email=t@example.invalid", "-c", "commit.gpgsign=false"}, args...)...)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("git %v: %v\n%s", args, err, out)
+	}
+	return strings.TrimSpace(string(out))
+}
+
+func writeFile(t *testing.T, dir, name, content string) {
+	t.Helper()
+	p := filepath.Join(dir, name)
+	if err := os.MkdirAll(filepath.Dir(p), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(p, []byte(content), 0o644); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// mergeRig is a local bare repository standing in for the forge, a work repository that pushes to it, and an empty
+// workspace: the fetch step's shape (FetchCommit of the head into the workspace, then MergeInto).
+type mergeRig struct {
+	t              *testing.T
+	bare, work, ws string
+	fork, trunk    string // the commit both histories share, and the trunk's head
+	remote         string
+}
+
+func newMergeRig(t *testing.T) *mergeRig {
+	root := t.TempDir()
+	r := &mergeRig{t: t, bare: filepath.Join(root, "r.git"), work: filepath.Join(root, "work"), ws: filepath.Join(root, "ws")}
+	for _, d := range []string{r.bare, r.work, r.ws} {
+		if err := os.MkdirAll(d, 0o755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	r.remote = "file://" + r.bare
+	realGit(t, r.bare, "init", "-q", "--bare")
+	realGit(t, r.work, "init", "-q", "-b", "main")
+	writeFile(t, r.work, "a.txt", "one\ntwo\nthree\n")
+	writeFile(t, r.work, "modules/m/module.json", "{}\n")
+	realGit(t, r.work, "add", ".")
+	realGit(t, r.work, "commit", "-q", "-m", "fork point")
+	r.fork = realGit(t, r.work, "rev-parse", "HEAD")
+	return r
+}
+
+// head makes a commit on a branch off the fork point, pushes it as the pull request's head ref and answers its sha.
+func (r *mergeRig) head(files map[string]string) string {
+	realGit(r.t, r.work, "checkout", "-q", "-B", "pr", r.fork)
+	for n, c := range files {
+		writeFile(r.t, r.work, n, c)
+	}
+	realGit(r.t, r.work, "add", ".")
+	realGit(r.t, r.work, "commit", "-q", "-m", "head")
+	h := realGit(r.t, r.work, "rev-parse", "HEAD")
+	realGit(r.t, r.work, "push", "-q", r.bare, "HEAD:refs/pull/1/head")
+	return h
+}
+
+// advance moves the trunk past the fork point with a commit changing files, pushes it and answers its sha.
+func (r *mergeRig) advance(files map[string]string) string {
+	realGit(r.t, r.work, "checkout", "-q", "-B", "main", r.fork)
+	for n, c := range files {
+		writeFile(r.t, r.work, n, c)
+	}
+	realGit(r.t, r.work, "add", ".")
+	realGit(r.t, r.work, "commit", "-q", "-m", "trunk moves")
+	r.trunk = realGit(r.t, r.work, "rev-parse", "HEAD")
+	realGit(r.t, r.work, "push", "-q", "-f", r.bare, "HEAD:refs/heads/main")
+	return r.trunk
+}
+
+func (r *mergeRig) fetch(head string) {
+	if err := FetchCommit(context.Background(), forgekey.ExecRunner, r.ws, r.remote, head, "refs/pull/1/head", AllowFileRemote()); err != nil {
+		r.t.Fatal(err)
+	}
+}
+
+// 1674/AC1: a clean merge answers the merge commit and the changed files, and leaves the workspace at the merge, which
+// holds both sides' changes.
+func TestMergeIntoACleanMergeAnswersTheMergeCommitAndTheChangedFiles(t *testing.T) {
+	r := newMergeRig(t)
+	head := r.head(map[string]string{"b.txt": "head\n"})
+	base := r.advance(map[string]string{"a.txt": "one\ntwo\nthree\nfour\n"})
+	r.fetch(head)
+	res, err := MergeInto(context.Background(), forgekey.ExecRunner, r.ws, r.remote, base, head, AllowFileRemote())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.Base != base || res.Head != head || len(res.Merge) != 40 || res.Merge == base || res.Merge == head || len(res.Conflicts) != 0 {
+		t.Fatalf("result = %+v", res)
+	}
+	if !reflect.DeepEqual(res.Changed, []string{"b.txt"}) {
+		t.Errorf("changed = %v, want only what the head adds to the trunk: b.txt", res.Changed)
+	}
+	if at := realGit(t, r.ws, "rev-parse", "HEAD"); at != res.Merge {
+		t.Errorf("the workspace is at %s, not at the merge %s", at, res.Merge)
+	}
+	for f, want := range map[string]string{"b.txt": "head\n", "a.txt": "one\ntwo\nthree\nfour\n"} {
+		if got, _ := os.ReadFile(filepath.Join(r.ws, f)); string(got) != want {
+			t.Errorf("%s = %q, want %q", f, got, want)
+		}
+	}
+	if parents := strings.Fields(realGit(t, r.ws, "rev-list", "--parents", "-n", "1", res.Merge)); len(parents) != 3 || parents[1] != base || parents[2] != head {
+		t.Errorf("the merge commit's parents = %v, want the trunk commit and the head", parents)
+	}
+	if author := realGit(t, r.ws, "log", "-1", "--format=%an <%ae>", res.Merge); author != "mesh merge check <merge-check@mesh.invalid>" {
+		t.Errorf("the merge is made as %q", author)
+	}
+}
+
+// 1674/AC2: a conflict is an answer: the files in conflict, no merge commit, and the workspace left at the trunk commit.
+func TestMergeIntoAConflictAnswersTheFilesAndLeavesTheTreeAtTheTrunk(t *testing.T) {
+	r := newMergeRig(t)
+	head := r.head(map[string]string{"a.txt": "one\nHEAD\nthree\n", "c.txt": "c\n"})
+	base := r.advance(map[string]string{"a.txt": "one\nTRUNK\nthree\n"})
+	r.fetch(head)
+	res, err := MergeInto(context.Background(), forgekey.ExecRunner, r.ws, r.remote, base, head, AllowFileRemote())
+	if err != nil {
+		t.Fatalf("a conflict is an answer, not an error: %v", err)
+	}
+	if !reflect.DeepEqual(res.Conflicts, []string{"a.txt"}) || res.Merge != "" || len(res.Changed) != 0 || res.Base != base || res.Head != head {
+		t.Fatalf("result = %+v", res)
+	}
+	if at := realGit(t, r.ws, "rev-parse", "HEAD"); at != base {
+		t.Errorf("the workspace is at %s, not at the trunk commit %s", at, base)
+	}
+	if got, _ := os.ReadFile(filepath.Join(r.ws, "a.txt")); string(got) != "one\nTRUNK\nthree\n" {
+		t.Errorf("a.txt = %q, want the trunk's", got)
+	}
+	if st := realGit(t, r.ws, "status", "--porcelain"); st != "" {
+		t.Errorf("the tree is not clean after the aborted merge: %q", st)
+	}
+	if _, err := os.Stat(filepath.Join(r.ws, "c.txt")); err == nil {
+		t.Error("a file of the head is in a tree left at the trunk")
+	}
+}
+
+// 1674/AC3: no base is refused before anything runs; so is a base or a commit that is not a full sha, a workspace that
+// is not at the commit, and a remote that is not ssh://.
+func TestMergeIntoRefusesWhatIsNotAMerge(t *testing.T) {
+	f := &fakeGit{}
+	for name, c := range map[string]struct{ remote, base, commit string }{
+		"no base":          {"ssh://mesh-forge/o/r.git", "", sha},
+		"a short base":     {"ssh://mesh-forge/o/r.git", "abc123", sha},
+		"a branch as base": {"ssh://mesh-forge/o/r.git", "main", sha},
+		"a short commit":   {"ssh://mesh-forge/o/r.git", sha, "abc"},
+		"an https remote":  {"https://forge.example/o/r.git", sha, sha},
+	} {
+		_, err := MergeInto(context.Background(), f.run, "/w", c.remote, c.base, c.commit)
+		if err == nil {
+			t.Errorf("%s accepted", name)
+		}
+	}
+	if len(f.calls) != 0 {
+		t.Errorf("git ran for a refused merge: %v", f.calls)
+	}
+	g := &fakeGit{rule: func(l string) forgekey.Ran {
+		if strings.Contains(l, "rev-parse HEAD") {
+			return forgekey.Ran{Stdout: strings.Repeat("f", 40) + "\n"}
+		}
+		return forgekey.Ran{}
+	}}
+	_, err := MergeInto(context.Background(), g.run, "/w", "ssh://mesh-forge/o/r.git", strings.Repeat("a", 40), sha)
+	if err == nil || !strings.Contains(err.Error(), "not at the commit") || len(g.calls) != 1 {
+		t.Errorf("a workspace at another commit: %v after %d commands", err, len(g.calls))
+	}
+}
+
+// 1674/AC4: the merge is made with the check merge's identity and flags, hooks off, and fetches with no credential.
+func TestMergeIntoRunsTheCheckMergesCommands(t *testing.T) {
+	base := strings.Repeat("b", 40)
+	f := &fakeGit{rule: func(l string) forgekey.Ran {
+		switch {
+		case strings.HasSuffix(l, "rev-parse HEAD"):
+			return forgekey.Ran{Stdout: sha + "\n"}
+		case strings.Contains(l, "--is-shallow-repository"):
+			return forgekey.Ran{Stdout: "true\n"}
+		}
+		return forgekey.Ran{}
+	}}
+	if _, err := MergeInto(context.Background(), f.run, "/w", "ssh://mesh-forge/o/r.git", base, sha); err != nil {
+		t.Fatal(err)
+	}
+	all := strings.Join(f.calls, "\n")
+	for _, want := range []string{
+		pre + "fetch --no-tags --unshallow origin " + base,
+		pre + "checkout -q --detach " + base,
+		"user.name=mesh merge check", "user.email=merge-check@mesh.invalid", "commit.gpgsign=false", "core.hooksPath=/dev/null",
+		"merge --no-ff --no-edit --no-verify --quiet -m",
+		pre + "diff --name-only --no-renames " + base,
+	} {
+		if !strings.Contains(all, want) {
+			t.Errorf("the commands lack %q:\n%s", want, all)
+		}
+	}
+	for _, c := range f.calls {
+		if strings.Contains(c, "https://") || !strings.Contains(c, "credential.helper= ") {
+			t.Errorf("command %q carries a credential or https", c)
+		}
+	}
+}
+
+// A head already in the trunk merges into nothing: the merge is the trunk commit and no path changes.
+func TestMergeIntoAHeadAlreadyInTheTrunkChangesNothing(t *testing.T) {
+	r := newMergeRig(t)
+	head := r.head(map[string]string{"b.txt": "head\n"})
+	r.fork = head // the trunk continues from the head
+	base := r.advance(map[string]string{"a.txt": "one\ntwo\nthree\nfour\n"})
+	r.fetch(head)
+	res, err := MergeInto(context.Background(), forgekey.ExecRunner, r.ws, r.remote, base, head, AllowFileRemote())
+	if err != nil || res.Merge != base || len(res.Changed) != 0 || len(res.Conflicts) != 0 {
+		t.Errorf("result = %+v, %v", res, err)
+	}
+}