diff --git a/docs/docs/changelog.md b/docs/docs/changelog.md
index 562383f2..ba1ffc9a 100644
--- a/docs/docs/changelog.md
+++ b/docs/docs/changelog.md
@@ -5,6 +5,7 @@ title: Changelog
 
 ## [Unreleased](https://github.com/lets-cli/lets/releases/tag/v0.0.X)
 
+* `[Fixed]` `source <(lets completion -s zsh)` no longer hangs: skip terminal background-color detection when lets runs outside the terminal's foreground process group (e.g. inside process substitution), where the query would suspend the process with SIGTTIN/SIGTTOU.
 * `[Fixed]` Add spacing between the `EXAMPLES` help title and its examples.
 * `[Fixed]` Align styled error output to the left while preserving the padded error badge.
 * `[Docs]` Convert design specs into ADRs.
diff --git a/go.mod b/go.mod
index 4aa5c593..1e552c65 100644
--- a/go.mod
+++ b/go.mod
@@ -69,7 +69,7 @@ require (
 	github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51
 	github.com/lithammer/dedent v1.1.0
 	github.com/spf13/pflag v1.0.9
-	golang.org/x/sys v0.42.0 // indirect
+	golang.org/x/sys v0.42.0
 	gopkg.in/check.v1 v1.0.0-20200902074654-038fdea0a05b // indirect
 	gopkg.in/yaml.v3 v3.0.1
 )
diff --git a/internal/config/config/config.go b/internal/config/config/config.go
index 6d893d1b..90d0509b 100644
--- a/internal/config/config/config.go
+++ b/internal/config/config/config.go
@@ -259,7 +259,7 @@ func (c *Config) readMixin(mixin *Mixin) error {
 				// ensureRemoteConfig's behavior for the top-level remote config.
 				cachedData, readErr := rm.tryRead()
 				if readErr != nil {
-					return fmt.Errorf("download failed (%w); also failed to read cached version: %v", downloadErr, readErr)
+					return fmt.Errorf("download failed (%w); also failed to read cached version: %w", downloadErr, readErr)
 				}
 
 				if cachedData != nil {
diff --git a/internal/logging/formatter.go b/internal/logging/formatter.go
index 20508b7d..fc995437 100644
--- a/internal/logging/formatter.go
+++ b/internal/logging/formatter.go
@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"fmt"
 	"io"
-	"os"
 	"strings"
 	"unicode"
 
@@ -12,6 +11,7 @@ import (
 	"github.com/charmbracelet/x/term"
 	"github.com/fatih/color"
 	"github.com/lets-cli/fang"
+	"github.com/lets-cli/lets/internal/theme"
 	log "github.com/sirupsen/logrus"
 )
 
@@ -44,8 +44,7 @@ func newFormatter(errWriter io.Writer, cs fang.ColorSchemeFunc) *Formatter {
 		return f
 	}
 
-	isDark := lipgloss.HasDarkBackground(os.Stdin, file)
-	scheme := cs(lipgloss.LightDark(isDark))
+	scheme := cs(lipgloss.LightDark(theme.IsDarkBackground(file)))
 
 	w, _, err := term.GetSize(file.Fd())
 	if err != nil || w == 0 {
diff --git a/internal/theme/theme.go b/internal/theme/theme.go
index 599c9381..6cec9abd 100644
--- a/internal/theme/theme.go
+++ b/internal/theme/theme.go
@@ -8,6 +8,7 @@ import (
 	"github.com/charmbracelet/x/exp/charmtone"
 	"github.com/charmbracelet/x/term"
 	"github.com/lets-cli/fang"
+	"github.com/lets-cli/lets/internal/util"
 )
 
 // Supported theme names.
@@ -39,10 +40,22 @@ func ColorSchemeByName(name string) fang.ColorSchemeFunc {
 	}
 }
 
+// IsDarkBackground reports whether the terminal behind out has a dark background.
+// Detection queries the terminal (writes OSC 11 to out, reads the reply from stdin),
+// which suspends a background process group with SIGTTIN/SIGTTOU — so when the
+// process is not in the foreground, skip the query and assume dark (lipgloss's
+// own fallback).
+func IsDarkBackground(out term.File) bool {
+	if !util.IsForegroundProcess(out.Fd()) {
+		return true
+	}
+
+	return lipgloss.HasDarkBackground(os.Stdin, out)
+}
+
 // ProgressColorsByName resolves a theme name to fill and empty colors for download progress.
 func ProgressColorsByName(name string, out term.File) (color.Color, color.Color) {
-	isDark := lipgloss.HasDarkBackground(os.Stdin, out)
-	scheme := ColorSchemeByName(name)(lipgloss.LightDark(isDark))
+	scheme := ColorSchemeByName(name)(lipgloss.LightDark(IsDarkBackground(out)))
 
 	return ProgressColors(scheme)
 }
diff --git a/internal/util/foreground.go b/internal/util/foreground.go
new file mode 100644
index 00000000..a8600f6b
--- /dev/null
+++ b/internal/util/foreground.go
@@ -0,0 +1,19 @@
+package util
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// IsForegroundProcess reports whether this process belongs to the foreground
+// process group of the terminal referred to by fd. Reading or reconfiguring a
+// terminal from a background process group suspends the process with
+// SIGTTIN/SIGTTOU — e.g. inside process substitution:
+// `source <(lets completion -s zsh)`.
+func IsForegroundProcess(fd uintptr) bool {
+	pgrp, err := unix.IoctlGetInt(int(fd), unix.TIOCGPGRP)
+	if err != nil {
+		return false
+	}
+
+	return pgrp == unix.Getpgrp()
+}
diff --git a/tests/completion.bats b/tests/completion.bats
index df5dfbf4..200c968c 100644
--- a/tests/completion.bats
+++ b/tests/completion.bats
@@ -51,3 +51,13 @@ setup() {
     assert_line --index 0 "--debug[Run with debug]"
     assert_line --index 1 "--env[Set env]"
 }
+
+# Regression: querying the terminal background color from a background process
+# group suspends the process with SIGTTOU, so `source <(lets completion -s zsh)`
+# in an interactive (job-control) shell hung forever. `script` allocates a PTY,
+# `set -m` enables job control like an interactive shell.
+@test "completion: source <(lets completion -s zsh) must not hang under job control" {
+    run timeout 10 script -qec "zsh -c 'set -m; source <(lets completion -s zsh); echo SOURCED_OK'" /dev/null
+    assert_success
+    assert_output --partial "SOURCED_OK"
+}