diff --git a/go/README.md b/go/README.md
index 374d7d270..b2bffc36a 100644
--- a/go/README.md
+++ b/go/README.md
@@ -179,6 +179,11 @@ So a global is offered inside a subcommand, a redeclared name shadows the
 inherited one, a hidden flag binds without being advertised, and past a `--`
 nothing is offered at all.
 
+`argv.RenderAnswer` writes the result in the protocol each shell reads — bash
+takes values, fish, nu and PowerShell take a description after a tab, and zsh
+takes a third field with the text to insert, because what it displays and what it
+types are not always the same string.
+
 ## Errors
 
 `argv.Render` turns a failure into what a CLI should print to stderr:
diff --git a/go/argv/complete.go b/go/argv/complete.go
index 2d25428c0..e7be2f945 100644
--- a/go/argv/complete.go
+++ b/go/argv/complete.go
@@ -310,14 +310,12 @@ func choicesFor(key uint64, meta Metadata) []string {
 }
 
 func describe(key uint64, help HelpTable) string {
-	h := help.Lookup(key)
-	if h == nil {
-		return ""
+	if h := help.Lookup(key); h != nil {
+		// Whole, breaks included. Putting a description on one line is the
+		// renderer's job — see oneLine — because it is the line-based protocols
+		// that need it, and collapsing there keeps both halves of a two-line
+		// description instead of dropping the second.
+		return h.Short
 	}
-	// The first line only: a shell shows one line beside a candidate, and a
-	// description that wraps turns a completion menu into a wall.
-	if at := strings.IndexByte(h.Short, '\n'); at >= 0 {
-		return h.Short[:at]
-	}
-	return h.Short
+	return ""
 }
diff --git a/go/argv/complete_shell.go b/go/argv/complete_shell.go
new file mode 100644
index 000000000..96fc0cf2e
--- /dev/null
+++ b/go/argv/complete_shell.go
@@ -0,0 +1,185 @@
+package argv
+
+import "strings"
+
+// Writing an answer the way a shell reads it.
+//
+// One line per candidate, in the shape the shell's own completion machinery
+// expects — which is where the five differ. bash reads values only; fish, nu and
+// PowerShell take a description after a tab; zsh takes a third field, the text to
+// insert, because what it displays and what it types are not always the same
+// string.
+
+// Shell is a completion protocol, named for the shell that reads it.
+type Shell uint8
+
+const (
+	Bash Shell = iota
+	Zsh
+	Fish
+	Nu
+	PowerShell
+)
+
+// Files says whether paths belong at this position as well as the candidates.
+type Files uint8
+
+const (
+	// NoFiles means the position takes only what the CLI named.
+	NoFiles Files = iota
+	// AnyFile means files, directories, whatever the shell shows for a path.
+	AnyFile
+	// Dirs means directories only.
+	Dirs
+)
+
+// The line a shell reads to mean "paths belong here too".
+//
+// A whole line rather than a flag on the protocol, because every one of the five
+// shells can already split output into lines and look at the last one. `\x01`
+// opens it because no candidate can contain a control character — the parser's
+// values are escaped before they are rendered anywhere — so it cannot be mistaken
+// for one.
+const (
+	FilesMarker = "\x01files"
+	DirsMarker  = "\x01dirs"
+)
+
+// Answer is everything a shell needs to resolve one Tab.
+type Answer struct {
+	Candidates []Candidate
+	Files      Files
+}
+
+// RenderAnswer writes an answer in the protocol `shell` reads.
+//
+// Named for what it renders rather than just `Render`, because [Render] already
+// belongs to failures. Two things in one package both turning a value into text
+// for a terminal is reason enough to say which.
+func RenderAnswer(a Answer, shell Shell) string {
+	var out strings.Builder
+
+	// Descriptions are all-or-nothing per answer: a column that appears on some
+	// rows and not others reads as missing data rather than as an absent
+	// description.
+	//
+	// Over the rows that will actually be written, not over every candidate. A
+	// description sitting on a row that gets dropped below would otherwise put an
+	// empty column on all the survivors — the rule broken by the answer it was
+	// deciding for.
+	described := false
+	for _, c := range a.Candidates {
+		if c.Describe != "" && travels(c.Value) {
+			described = true
+			break
+		}
+	}
+
+	for _, c := range a.Candidates {
+		// The protocols are lines with tab-separated fields, so a value carrying
+		// either would be read as more rows or more fields. A candidate normally
+		// comes from a spec and contains neither, but a `complete` script can
+		// produce anything.
+		//
+		// Such a candidate is dropped rather than repaired. A value is the text
+		// that gets typed onto the command line, so collapsing a tab inside it
+		// would insert an argument nobody offered — the shell would report success
+		// while the CLI received something else, which is the confusing half of
+		// the two. A missing candidate is the honest failure: the user types the
+		// value themselves and it works.
+		if !travels(c.Value) {
+			continue
+		}
+		value := c.Value
+		// The description is prose, and collapsing prose onto one line is what a
+		// one-line protocol asks for. Nothing is typed from it.
+		description := oneLine(c.Describe)
+		switch shell {
+		case Bash:
+			out.WriteString(value)
+		case Zsh:
+			// Display, then description, then what to type: a candidate containing
+			// a space or a quote has to reach the command line intact.
+			out.WriteString(value + "\t" + description + "\t" + zshQuote(value))
+		default:
+			out.WriteString(value)
+			if described {
+				out.WriteString("\t" + description)
+			}
+		}
+		out.WriteString("\n")
+	}
+
+	switch a.Files {
+	case AnyFile:
+		out.WriteString(FilesMarker + "\n")
+	case Dirs:
+		out.WriteString(DirsMarker + "\n")
+	}
+	return out.String()
+}
+
+// travels reports whether a value can be written into these protocols as itself.
+//
+// Every control character, not only the three that delimit: the marker lines
+// that say "files belong here too" open with `\x01`, and a candidate beginning
+// with one would be read as a marker rather than as a candidate. The comment on
+// FilesMarker says no candidate can contain a control character; this is what
+// makes that true.
+func travels(value string) bool {
+	for _, r := range value {
+		if r < 0x20 || r == 0x7f {
+			return false
+		}
+	}
+	return true
+}
+
+// oneLine collapses text onto one line, because the protocols are line-based
+// with tab-separated fields: a break or a tab inside either field would be read
+// as another row or another column.
+//
+// Collapsed rather than truncated, so a two-line description still says both
+// halves. A run of breaks becomes one space, and never a leading or trailing one.
+// Every other control character goes the same way: a description is displayed by
+// the shell, and an escape sequence displayed is an escape sequence run.
+func oneLine(text string) string {
+	var out strings.Builder
+	spaced := false
+	for _, r := range text {
+		if r < 0x20 || r == 0x7f {
+			if !spaced && out.Len() > 0 {
+				out.WriteByte(' ')
+				spaced = true
+			}
+			continue
+		}
+		out.WriteRune(r)
+		spaced = false
+	}
+	return strings.TrimRight(out.String(), " ")
+}
+
+// zshQuote makes a value safe to insert on the command line.
+func zshQuote(value string) string {
+	safe := func(r rune) bool {
+		switch {
+		case r >= 'a' && r <= 'z', r >= 'A' && r <= 'Z', r >= '0' && r <= '9':
+			return true
+		}
+		return strings.ContainsRune("_-./:@+=%,", r)
+	}
+	if value != "" {
+		all := true
+		for _, r := range value {
+			if !safe(r) {
+				all = false
+				break
+			}
+		}
+		if all {
+			return value
+		}
+	}
+	return "'" + strings.ReplaceAll(value, "'", `'\''`) + "'"
+}
diff --git a/go/argv/complete_shell_test.go b/go/argv/complete_shell_test.go
new file mode 100644
index 000000000..dd30c1b69
--- /dev/null
+++ b/go/argv/complete_shell_test.go
@@ -0,0 +1,162 @@
+package argv
+
+import (
+	"strings"
+	"testing"
+)
+
+func answer() Answer {
+	return Answer{Candidates: []Candidate{
+		{Kind: CandidateCommand, Value: "use", Describe: "Installs a tool"},
+		{Kind: CandidateFlag, Value: "--global"},
+	}}
+}
+
+// Each shell reads a different shape, and the differences are the whole point of
+// having five.
+func TestEachShellGetsItsOwnShape(t *testing.T) {
+	// bash reads values only.
+	if got := RenderAnswer(answer(), Bash); got != "use\n--global\n" {
+		t.Errorf("bash: got %q", got)
+	}
+
+	// zsh takes display, description, and the text to insert.
+	got := RenderAnswer(answer(), Zsh)
+	if !strings.HasPrefix(got, "use\tInstalls a tool\tuse\n") {
+		t.Errorf("zsh: got %q", got)
+	}
+
+	// fish, nu and PowerShell take a description after a tab.
+	for _, shell := range []Shell{Fish, Nu, PowerShell} {
+		got := RenderAnswer(answer(), shell)
+		if !strings.HasPrefix(got, "use\tInstalls a tool\n") {
+			t.Errorf("%v: got %q", shell, got)
+		}
+	}
+}
+
+// A column that appears on some rows and not others reads as missing data rather
+// than as an absent description, so it is all-or-nothing per answer.
+func TestDescriptionsAreAllOrNothing(t *testing.T) {
+	// One candidate has a description, so the other still gets the column.
+	got := RenderAnswer(answer(), Fish)
+	if !strings.Contains(got, "--global\t") && !strings.HasSuffix(got, "--global\t\n") {
+		if !strings.Contains(got, "--global\t\n") {
+			t.Errorf("the column should be present for every row: %q", got)
+		}
+	}
+	// None has one, so no row gets it.
+	bare := Answer{Candidates: []Candidate{{Value: "a"}, {Value: "b"}}}
+	if got := RenderAnswer(bare, Fish); got != "a\nb\n" {
+		t.Errorf("no descriptions means no column: %q", got)
+	}
+}
+
+// A description is collapsed onto one line rather than truncated, so a two-line
+// description still says both halves — the protocols are line-based, and a break
+// would look like another candidate.
+func TestADescriptionIsCollapsedNotTruncated(t *testing.T) {
+	a := Answer{Candidates: []Candidate{
+		{Value: "x", Describe: "first half\nsecond half"},
+	}}
+	got := RenderAnswer(a, Fish)
+	if strings.Count(got, "\n") != 1 {
+		t.Errorf("a candidate is one line: %q", got)
+	}
+	for _, want := range []string{"first half", "second half"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("want %q kept: %q", want, got)
+		}
+	}
+	// A run of breaks is one space, with none left at either end.
+	if got := oneLine("\n\na\n\n\nb\n\n"); got != "a b" {
+		t.Errorf("want %q, got %q", "a b", got)
+	}
+}
+
+// A candidate containing a space or a quote has to reach the command line intact.
+func TestZshQuotesWhatItMust(t *testing.T) {
+	for _, c := range []struct{ in, want string }{
+		{"use", "use"},
+		{"a/b-c.d:e@f+g=h%i,j_k", "a/b-c.d:e@f+g=h%i,j_k"},
+		{"two words", "'two words'"},
+		{"it's", `'it'\''s'`},
+		{"", "''"},
+	} {
+		if got := zshQuote(c.in); got != c.want {
+			t.Errorf("zshQuote(%q): want %q, got %q", c.in, c.want, got)
+		}
+	}
+}
+
+// The marker is a line because every shell can already look at the last one.
+func TestFilesAreAskedForOnTheirOwnLine(t *testing.T) {
+	a := answer()
+	a.Files = AnyFile
+	if got := RenderAnswer(a, Bash); !strings.HasSuffix(got, FilesMarker+"\n") {
+		t.Errorf("want the files marker last: %q", got)
+	}
+	a.Files = Dirs
+	if got := RenderAnswer(a, Bash); !strings.HasSuffix(got, DirsMarker+"\n") {
+		t.Errorf("want the dirs marker last: %q", got)
+	}
+	a.Files = NoFiles
+	if got := RenderAnswer(a, Bash); strings.Contains(got, "\x01") {
+		t.Errorf("no marker where paths do not belong: %q", got)
+	}
+}
+
+// A value carrying a tab or a newline would be read as more fields or more rows.
+//
+// Dropped rather than collapsed: a value is what gets typed onto the command
+// line, so a repaired one inserts an argument nobody offered — the shell reports
+// success and the CLI receives something else. Values normally come from a spec
+// and contain none of this, but a `complete` script can produce anything.
+func TestAValueThatCannotTravelIsNotOffered(t *testing.T) {
+	a := Answer{Candidates: []Candidate{
+		{Value: "one\ttwo\nthree"},
+		{Value: "\x01files"}, // would be read as the marker line
+		{Value: "plain"},
+	}}
+	for _, shell := range []Shell{Bash, Zsh, Fish, Nu, PowerShell} {
+		got := RenderAnswer(a, shell)
+		if strings.Count(got, "\n") != 1 {
+			t.Errorf("%v: only the one that travels is offered, got %q", shell, got)
+		}
+		if !strings.HasPrefix(got, "plain") {
+			t.Errorf("%v: the candidate that travels should survive, got %q", shell, got)
+		}
+	}
+}
+
+// The description column is decided over the rows that survive.
+//
+// A description on a candidate that gets dropped would otherwise turn the column
+// on for everyone else, leaving an empty field on every row — the all-or-nothing
+// rule broken by the answer it was deciding for.
+func TestADroppedRowDoesNotTurnOnTheDescriptionColumn(t *testing.T) {
+	a := Answer{Candidates: []Candidate{
+		{Value: "bad\tvalue", Describe: "the only description"},
+		{Value: "plain"},
+	}}
+	for _, shell := range []Shell{Fish, Nu, PowerShell} {
+		if got := RenderAnswer(a, shell); got != "plain\n" {
+			t.Errorf("%v: no column where nothing written has a description: %q", shell, got)
+		}
+	}
+}
+
+// A description is prose, and prose collapses: nothing is typed from it, and a
+// two-line help still says both halves on one line.
+func TestADescriptionIsCollapsedRatherThanDropped(t *testing.T) {
+	a := Answer{Candidates: []Candidate{{Value: "run", Describe: "does a thing\nand another"}}}
+	got := RenderAnswer(a, Zsh)
+	if strings.Count(got, "\n") != 1 {
+		t.Errorf("one candidate is one row, got %q", got)
+	}
+	for _, want := range []string{"does a thing and another", "run"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("want %q kept in %q", want, got)
+		}
+	}
+}
diff --git a/go/argv/complete_test.go b/go/argv/complete_test.go
index 40c1ae1c2..12b9a0c31 100644
--- a/go/argv/complete_test.go
+++ b/go/argv/complete_test.go
@@ -145,14 +145,20 @@ func TestAHelpTopicOffersOnlyCommands(t *testing.T) {
 	}
 }
 
-// A description is one line: a shell shows one line beside a candidate, and a
-// wrapped description turns a completion menu into a wall.
-func TestDescriptionsAreOneLine(t *testing.T) {
+// A candidate carries the whole description; putting it on one line is the
+// renderer's job.
+//
+// Collapsing there rather than truncating here keeps both halves of a two-line
+// description — see TestADescriptionIsCollapsedNotTruncated.
+func TestACandidateCarriesTheWholeDescription(t *testing.T) {
 	root, help, meta := completionFixture()
 	help[1].Short = "run it\nand keep running it"
 	for _, c := range Candidates(Walk(root, nil), "run", help, meta) {
-		if strings.Contains(c.Describe, "\n") {
-			t.Errorf("description should be one line: %q", c.Describe)
+		if c.Value != "run" {
+			continue
+		}
+		if !strings.Contains(c.Describe, "and keep running it") {
+			t.Errorf("the second half should survive: %q", c.Describe)
 		}
 	}
 }