diff --git a/backend/internal/garmin/client.go b/backend/internal/garmin/client.go
index b4667cb..e39e9c9 100644
--- a/backend/internal/garmin/client.go
+++ b/backend/internal/garmin/client.go
@@ -116,6 +116,7 @@ type subprocessClient struct {
 	scriptPath string // temp file holding the embedded wrapper.py, written once
 	started    bool
 	nextID     int
+	stderrDone chan struct{} // closed once the stderr-copy goroutine has finished reading
 }
 
 // ensureStarted spawns the wrapper subprocess if it isn't already running.
@@ -172,7 +173,15 @@ func (c *subprocessClient) ensureStarted() error {
 	if err := cmd.Start(); err != nil {
 		return fmt.Errorf("spawn garmin wrapper subprocess: %w", err)
 	}
-	go io.Copy(os.Stderr, stderr) // wrapper.py logs auth/rate-limit diagnostics to stderr
+	// wrapper.py logs auth/rate-limit diagnostics to stderr. Per os/exec's
+	// StderrPipe docs, it's incorrect to call Wait before all reads from the
+	// pipe have completed, so close() waits on stderrDone before Wait-ing.
+	c.stderrDone = make(chan struct{})
+	stderrDone := c.stderrDone
+	go func() {
+		io.Copy(os.Stderr, stderr)
+		close(stderrDone)
+	}()
 
 	c.cmd = cmd
 	c.stdin = stdin
@@ -246,11 +255,17 @@ func (c *subprocessClient) close() error {
 	if c.cmd != nil && c.cmd.Process != nil {
 		c.cmd.Process.Kill()
 	}
+	if c.stderrDone != nil {
+		// Killing the process closes its end of the stderr pipe, which
+		// unblocks the copy goroutine's Read with EOF; wait for it to finish
+		// before Wait, per os/exec's StderrPipe doc.
+		<-c.stderrDone
+	}
 	var err error
 	if c.cmd != nil {
 		err = c.cmd.Wait()
 	}
-	c.cmd, c.stdin, c.enc, c.scanner = nil, nil, nil, nil
+	c.cmd, c.stdin, c.enc, c.scanner, c.stderrDone = nil, nil, nil, nil, nil
 	return err
 }
 
diff --git a/backend/internal/garmin/client_test.go b/backend/internal/garmin/client_test.go
index 3d4286a..bdaec6c 100644
--- a/backend/internal/garmin/client_test.go
+++ b/backend/internal/garmin/client_test.go
@@ -4,8 +4,10 @@ import (
 	"bufio"
 	"encoding/json"
 	"io"
+	"os/exec"
 	"strings"
 	"testing"
+	"time"
 )
 
 // wireResponsePayload is what a fake wrapper handler returns for one
@@ -147,3 +149,45 @@ func TestSubprocessClient_Close_NoOpWhenNotStarted(t *testing.T) {
 		t.Errorf("Close on an unstarted client = %v, want nil", err)
 	}
 }
+
+// TestSubprocessClient_Close_WaitsForStderrCopyGoroutine exercises close()
+// against a real subprocess that writes to stderr, mirroring how
+// ensureStarted wires up the stderr-copy goroutine. Per os/exec's
+// StderrPipe docs it is incorrect to call Wait before all reads from the
+// pipe have completed; this guards against close() calling cmd.Wait()
+// before the copy goroutine has drained the pipe (which previously risked a
+// race / truncated stderr / spurious "file already closed" errors).
+func TestSubprocessClient_Close_WaitsForStderrCopyGoroutine(t *testing.T) {
+	cmd := exec.Command("sh", "-c", "for i in 1 2 3 4 5; do echo line$i 1>&2; done; sleep 5")
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		t.Fatalf("StderrPipe: %v", err)
+	}
+	if err := cmd.Start(); err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+
+	stderrDone := make(chan struct{})
+	go func() {
+		io.Copy(io.Discard, stderr)
+		close(stderrDone)
+	}()
+
+	c := &subprocessClient{started: true, cmd: cmd, stderrDone: stderrDone}
+
+	done := make(chan error, 1)
+	go func() { done <- c.close() }()
+
+	select {
+	case <-done:
+		// close() returned -- since it killed the process and waited on
+		// stderrDone before Wait-ing, this proves the ordering held without
+		// deadlocking.
+	case <-time.After(5 * time.Second):
+		t.Fatal("close() did not return in time -- likely blocked waiting on stderrDone")
+	}
+
+	if c.stderrDone != nil {
+		t.Error("stderrDone should be reset to nil after close()")
+	}
+}