1 files changed, 42 insertions, 0 deletions
diff --git a/internal/tmuxedit/pane.go b/internal/tmuxedit/pane.go
new file mode 100644
index 0000000..aae2d69
--- /dev/null
+++ b/internal/tmuxedit/pane.go
@@ -0,0 +1,42 @@
+package tmuxedit
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+// runCommand is the seam for exec.Command().Output(). Override in tests.
+var runCommand = func(name string, args ...string) ([]byte, error) {
+	return exec.Command(name, args...).Output()
+}
+
+// resolveTargetPane determines which tmux pane to target using a fallback
+// chain: explicit flag > HEXAI_TMUX_PANE env var > tmux query for active pane.
+// Returns the pane ID (e.g. "%5") or an error.
+func resolveTargetPane(flagPane string) (string, error) {
+	// 1. Explicit --pane flag
+	if p := strings.TrimSpace(flagPane); p != "" {
+		return p, nil
+	}
+	// 2. Environment variable
+	if p := strings.TrimSpace(os.Getenv("HEXAI_TMUX_PANE")); p != "" {
+		return p, nil
+	}
+	// 3. Query tmux for the active pane in the current window
+	return queryActivePane()
+}
+
+// queryActivePane asks tmux for the active pane ID using display-message.
+func queryActivePane() (string, error) {
+	out, err := runCommand("tmux", "display-message", "-p", "#{pane_id}")
+	if err != nil {
+		return "", fmt.Errorf("cannot determine tmux pane: %w", err)
+	}
+	pane := strings.TrimSpace(string(out))
+	if pane == "" {
+		return "", fmt.Errorf("tmux returned empty pane ID")
+	}
+	return pane, nil
+}