diff --git a/commands/completion.go b/commands/completion.go
index 7fa80223..61d79713 100644
--- a/commands/completion.go
+++ b/commands/completion.go
@@ -11,6 +11,19 @@ import (
 	"github.com/upsun/cli/internal/config"
 )
 
+// completeCommandName is the hidden legacy (Symfony Console) command that the
+// generated completion scripts call to fetch suggestions.
+const completeCommandName = "_complete"
+
+// shellOptionReplacer rewrites the shell option of the generated completion
+// scripts from Symfony's glued short form (-szsh) to the long form
+// (--shell=zsh), which no argument parser can misread as a flag bundle.
+var shellOptionReplacer = strings.NewReplacer(
+	"-szsh", "--shell=zsh",
+	"-sbash", "--shell=bash",
+	"-sfish", "--shell=fish",
+)
+
 func newCompletionCommand(cnf *config.Config) *cobra.Command {
 	return &cobra.Command{
 		Use:           "completion",
@@ -44,7 +57,45 @@ func newCompletionCommand(cnf *config.Config) *cobra.Command {
 				filepath.Base(pharPath),
 				cnf.Application.Executable,
 			)
-			fmt.Fprintln(cmd.OutOrStdout(), completions)
+			fmt.Fprintln(cmd.OutOrStdout(), shellOptionReplacer.Replace(completions))
 		},
 	}
 }
+
+// newCompleteCommand proxies the hidden _complete command of the legacy CLI,
+// which the completion scripts call to fetch suggestions.
+//
+// It only exists to keep Cobra from parsing those arguments. The scripts pass
+// the shell as a glued short option (-szsh, -sbash, -sfish), which Cobra
+// splits into single-letter flags; as every supported shell name contains an
+// "h", that always produced a -h flag and the CLI printed help instead of
+// completions.
+func newCompleteCommand(cnf *config.Config) *cobra.Command {
+	return &cobra.Command{
+		Use:                completeCommandName,
+		Short:              "Internal command to provide shell completion suggestions",
+		Hidden:             true,
+		Args:               cobra.ArbitraryArgs,
+		DisableFlagParsing: true,
+		SilenceErrors:      true,
+		Run: func(cmd *cobra.Command, args []string) {
+			c := makeLegacyCLIWrapper(cnf, cmd.OutOrStdout(), cmd.ErrOrStderr(), cmd.InOrStdin())
+			if err := c.Exec(cmd.Context(), append([]string{completeCommandName}, args...)...); err != nil {
+				exitWithError(err)
+			}
+		},
+	}
+}
+
+// isCompletionRequest reports whether the command was run by a completion
+// script rather than by a user. Those runs must stay silent: the bash script
+// captures stderr along with stdout, so any extra message ends up in the
+// suggestions.
+func isCompletionRequest(cmd *cobra.Command) bool {
+	switch cmd.Name() {
+	case completeCommandName, cobra.ShellCompRequestCmd, cobra.ShellCompNoDescRequestCmd:
+		return true
+	}
+
+	return false
+}
diff --git a/commands/completion_test.go b/commands/completion_test.go
new file mode 100644
index 00000000..aef962d5
--- /dev/null
+++ b/commands/completion_test.go
@@ -0,0 +1,98 @@
+package commands
+
+import (
+	"io"
+	"testing"
+
+	"github.com/platformsh/platformify/vendorization"
+	"github.com/spf13/cobra"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestCompletionScriptShellOption checks that the generated completion scripts
+// request suggestions with the long --shell option, instead of the glued short
+// option that Symfony's templates use.
+func TestCompletionScriptShellOption(t *testing.T) {
+	cases := []struct {
+		name     string
+		template string
+		expected string
+	}{
+		{
+			name:     "zsh",
+			template: `requestComp="${words[0]} ${words[1]} _complete --no-interaction -szsh -a1 -c$((CURRENT-1))" i=""`,
+			expected: `requestComp="${words[0]} ${words[1]} _complete --no-interaction --shell=zsh -a1 -c$((CURRENT-1))" i=""`,
+		},
+		{
+			name:     "bash",
+			template: `local completecmd=("$sf_cmd" "_complete" "--no-interaction" "-sbash" "-c$cword" "-a1")`,
+			expected: `local completecmd=("$sf_cmd" "_complete" "--no-interaction" "--shell=bash" "-c$cword" "-a1")`,
+		},
+		{
+			name:     "fish",
+			template: `set completecmd "$sf_cmd[1]" "_complete" "--no-interaction" "-sfish" "-a1"`,
+			expected: `set completecmd "$sf_cmd[1]" "_complete" "--no-interaction" "--shell=fish" "-a1"`,
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			assert.Equal(t, c.expected, shellOptionReplacer.Replace(c.template))
+		})
+	}
+}
+
+// TestCompleteCommandPassesArgsThrough checks that the arguments of a
+// completion request reach the legacy CLI unchanged. Cobra used to split the
+// glued -s<shell> option into single-letter flags, and the bundled -h made it
+// print the help page instead of any completions.
+func TestCompleteCommandPassesArgsThrough(t *testing.T) {
+	cases := []struct {
+		name string
+		args []string
+	}{
+		{
+			name: "zsh",
+			args: []string{"--no-interaction", "-szsh", "-a1", "-c1", "-itest-cli-executable", "-ienv"},
+		},
+		{
+			name: "bash",
+			args: []string{"--no-interaction", "-sbash", "-c1", "-a1", "-itest-cli-executable", "-ienv"},
+		},
+		{
+			name: "fish",
+			args: []string{"--no-interaction", "-sfish", "-a1", "-itest-cli-executable", "-ienv"},
+		},
+		{
+			name: "long options",
+			args: []string{"--no-interaction", "--shell=zsh", "--api-version=1", "--current=1", "--input=test-cli-executable"},
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			cnf := testConfig()
+			root := newRootCommand(cnf, &vendorization.VendorAssets{Binary: cnf.Application.Executable})
+			root.SetOut(io.Discard)
+			root.SetErr(io.Discard)
+
+			var helpCalled bool
+			root.SetHelpFunc(func(_ *cobra.Command, _ []string) { helpCalled = true })
+
+			// Stub out the command so that the legacy CLI is not executed.
+			completeCmd, _, err := root.Find([]string{completeCommandName})
+			require.NoError(t, err)
+			require.Equal(t, completeCommandName, completeCmd.Name())
+
+			var got []string
+			completeCmd.Run = func(_ *cobra.Command, args []string) { got = args }
+
+			root.SetArgs(append([]string{completeCommandName}, c.args...))
+			require.NoError(t, root.Execute())
+
+			assert.False(t, helpCalled, "the help page was printed instead of completions")
+			assert.Equal(t, c.args, got)
+		})
+	}
+}
diff --git a/commands/root.go b/commands/root.go
index 49178829..31c3c7cf 100644
--- a/commands/root.go
+++ b/commands/root.go
@@ -53,6 +53,10 @@ func newRootCommand(cnf *config.Config, assets *vendorization.VendorAssets) *cob
 		SilenceUsage:       true,
 		SilenceErrors:      false,
 		PersistentPreRun: func(cmd *cobra.Command, _ []string) {
+			if isCompletionRequest(cmd) {
+				// Completions must be fast and quiet.
+				return
+			}
 			if viper.GetBool("quiet") && !viper.GetBool("debug") && !viper.GetBool("verbose") {
 				viper.Set("no-interaction", true)
 				cmd.SetErr(io.Discard)
@@ -93,6 +97,9 @@ func newRootCommand(cnf *config.Config, assets *vendorization.VendorAssets) *cob
 			}
 		},
 		PersistentPostRun: func(cmd *cobra.Command, _ []string) {
+			if isCompletionRequest(cmd) {
+				return
+			}
 			checkShellConfigLeftovers(cmd.ErrOrStderr(), cnf)
 			select {
 			case rel := <-updateMessageChan:
@@ -143,6 +150,7 @@ func newRootCommand(cnf *config.Config, assets *vendorization.VendorAssets) *cob
 
 	// Add subcommands.
 	cmd.AddCommand(
+		newCompleteCommand(cnf),
 		newConfigInstallCommand(),
 		newCompletionCommand(cnf),
 		newHelpCommand(cnf),
diff --git a/integration-tests/completion_test.go b/integration-tests/completion_test.go
new file mode 100644
index 00000000..19a925a8
--- /dev/null
+++ b/integration-tests/completion_test.go
@@ -0,0 +1,69 @@
+package tests
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestCompletionScript checks that the generated completion scripts request
+// suggestions with the long --shell option, rather than the glued short option
+// (-szsh) that Symfony's templates use.
+func TestCompletionScript(t *testing.T) {
+	f := newCommandFactory(t, "", "")
+
+	cases := []struct {
+		shell string
+	}{
+		{shell: "zsh"},
+		{shell: "bash"},
+		{shell: "fish"},
+	}
+
+	for _, c := range cases {
+		t.Run(c.shell, func(t *testing.T) {
+			script := f.Run("completion", c.shell)
+			assert.Contains(t, script, "--shell="+c.shell)
+			assert.NotContains(t, script, "-s"+c.shell)
+		})
+	}
+}
+
+// TestComplete checks that a completion request returns suggestions. The glued
+// short options of the completion scripts must reach the legacy CLI unparsed:
+// the bundled -h of -szsh used to make the CLI print its help page instead.
+func TestComplete(t *testing.T) {
+	f := newCommandFactory(t, "", "")
+
+	cases := []struct {
+		name     string
+		args     []string
+		expected string
+	}{
+		{
+			name:     "short options",
+			args:     []string{"_complete", "--no-interaction", "-szsh", "-a1", "-c1", "-iplatform-test", "-ienv"},
+			expected: "environment:list",
+		},
+		{
+			name: "long options",
+			args: []string{
+				"_complete", "--no-interaction", "--shell=zsh", "--api-version=1", "--current=1",
+				"--input=platform-test", "--input=env",
+			},
+			expected: "environment:list",
+		},
+		{
+			// An input token can contain an "h" too, as in "platform-test ssh --pro<TAB>".
+			name:     "input containing h",
+			args:     []string{"_complete", "--no-interaction", "-szsh", "-a1", "-c2", "-iplatform-test", "-issh", "-i--pro"},
+			expected: "--project",
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			assert.Contains(t, f.Run(c.args...), c.expected)
+		})
+	}
+}