fix: use correct wt.exe split-pane flags (-H/-V) instead of invalid --direction

README.md

@@ -253,7 +253,7 @@ Configuration is stored in the platform-specific config directory:
 | `agent` | string | `""` | Pass `--agent <name>` to Copilot CLI |
 | `model` | string | `""` | Pass `--model <name>` to Copilot CLI |
 | `launch_mode` | string | `"tab"` | How to open sessions: `in-place`, `tab`, `window`, `pane` |
-| `pane_direction` | string | `"auto"` | Split direction for pane mode: `auto`, `right`, `down`, `left`, `up` |
+| `pane_direction` | string | `"auto"` | Split direction for pane mode: `auto`, `right`, `down`, `left`, `up` (see note below) |
 | `custom_command` | string | `""` | Custom launch command (`{sessionId}` is replaced) |
 | `excluded_dirs` | array | `[]` | Directory paths to hide from session list |
 | `theme` | string | `"auto"` | Color scheme: `auto` or a named scheme |
@@ -262,6 +262,20 @@ Configuration is stored in the platform-specific config directory:
 | `hiddenSessions` | array | `[]` | Session IDs hidden from the main list |
 | `favoriteSessions` | array | `[]` | Session IDs starred as favorites |
 
+#### Pane Direction Semantics
+
+When `launch_mode` is `"pane"`, the `pane_direction` value maps to Windows Terminal's `-H` / `-V` split-pane flags:
+
+| Direction | WT Flag | Meaning |
+|-----------|---------|---------|
+| `down` | `-H` | Horizontal split — divider runs horizontally, new pane below |
+| `up` | `-H` | Horizontal split — WT controls actual placement (closest available) |
+| `right` | `-V` | Vertical split — divider runs vertically, new pane to the right |
+| `left` | `-V` | Vertical split — WT controls actual placement (closest available) |
+| `auto` | *(none)* | Windows Terminal decides automatically |
+
+> **Note:** `-H` and `-V` control split *orientation* only (the direction the divider runs). Windows Terminal decides actual pane placement based on available space.
+
 ### Example config.json
 
 ```json

internal/platform/coverage_test.go

@@ -155,6 +155,57 @@ func TestLaunchStyleConstants(t *testing.T) {
 	}
 }
 
+// ---------------------------------------------------------------------------
+// appendWTPaneDirFlags
+// ---------------------------------------------------------------------------
+
+func TestAppendWTPaneDirFlags(t *testing.T) {
+	tests := []struct {
+		name string
+		dir  string
+		want []string
+	}{
+		{"down", "down", []string{"-H"}},
+		{"up", "up", []string{"-H"}},
+		{"right", "right", []string{"-V"}},
+		{"left", "left", []string{"-V"}},
+		{"auto", "auto", nil},
+		{"empty", "", nil},
+		{"unknown", "diagonal", nil},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			base := []string{"-w", "0", "split-pane"}
+			got := appendWTPaneDirFlags(base, tc.dir)
+
+			// base should never be modified in place.
+			extra := got[len(base):]
+			if tc.want == nil {
+				if len(extra) != 0 {
+					t.Errorf("dir=%q: got extra args %v, want none", tc.dir, extra)
+				}
+				return
+			}
+			if len(extra) != len(tc.want) {
+				t.Fatalf("dir=%q: got %v extra args, want %v", tc.dir, extra, tc.want)
+			}
+			for i, w := range tc.want {
+				if extra[i] != w {
+					t.Errorf("dir=%q: extra[%d]=%q, want %q", tc.dir, i, extra[i], w)
+				}
+			}
+		})
+	}
+}
+
+func TestAppendWTPaneDirFlags_PreservesExistingArgs(t *testing.T) {
+	base := []string{"existing", "args"}
+	got := appendWTPaneDirFlags(base, "down")
+	if len(got) != 3 || got[0] != "existing" || got[1] != "args" || got[2] != "-H" {
+		t.Errorf("got %v, want [existing args -H]", got)
+	}
+}
+
 // ---------------------------------------------------------------------------
 // TerminalInfo and ShellInfo
 // ---------------------------------------------------------------------------

internal/platform/shell.go

@@ -537,6 +537,34 @@ func defaultWindowsShell() ShellInfo {
 	return ShellInfo{Name: "Command Prompt", Path: p}
 }
 
+// appendWTPaneDirFlags translates a dispatch pane direction string into
+// the correct wt.exe split-pane flags.
+//
+// Windows Terminal's -H and -V flags control the split *orientation* (the
+// direction the divider runs), not which side the new pane appears on.
+// WT itself decides actual pane placement based on available space.
+//
+// Mapping:
+//
+//	"down"  → -H  horizontal split — divider runs horizontally, new pane below
+//	"up"    → -H  horizontal split — WT picks closest available placement
+//	"right" → -V  vertical split   — divider runs vertically, new pane to the right
+//	"left"  → -V  vertical split   — WT picks closest available placement
+//	"auto"  → (no flag) WT default behavior
+//	""      → (no flag) WT default behavior
+//	unknown → (no flag) WT default behavior
+func appendWTPaneDirFlags(args []string, dir string) []string {
+	switch dir {
+	case "down", "up":
+		return append(args, "-H")
+	case "right", "left":
+		return append(args, "-V")
+	default:
+		// "auto" or empty — let Windows Terminal choose.
+		return args
+	}
+}
+
 func launchWindowsSession(shell ShellInfo, resumeCmd string, terminal string, cwd string, launchStyle string, paneDirection string) error {
 	// Use Windows Terminal when configured (or defaulted by LaunchSession).
 	if terminal == termWindowsTerminal {
@@ -549,9 +577,7 @@ func launchWindowsSession(shell ShellInfo, resumeCmd string, terminal string, cw
 			case LaunchStylePane:
 				// Open a split pane in the current tab.
 				args = append(args, "-w", "0", "split-pane")
-				if paneDirection != "" && paneDirection != "auto" {
-					args = append(args, "--direction", paneDirection)
-				}
+				args = appendWTPaneDirFlags(args, paneDirection)
 			default:
 				// Open a new tab in the most recently used window.
 				// Without -w 0, wt.exe opens a new window by default.
@@ -879,9 +905,7 @@ func buildWSLWTArgs(shell ShellInfo, resumeCmd, winCwd, distro, launchStyle, pan
 		args = append(args, "-w", "new", "new-tab")
 	case LaunchStylePane:
 		args = append(args, "-w", "0", "split-pane")
-		if paneDirection != "" && paneDirection != "auto" {
-			args = append(args, "--direction", paneDirection)
-		}
+		args = appendWTPaneDirFlags(args, paneDirection)
 	default:
 		args = append(args, "-w", "0", "new-tab")
 	}

internal/platform/wsl_test.go

@@ -454,8 +454,11 @@ func TestBuildWSLWTArgs_LaunchStyles(t *testing.T) {
 		{"explicit_tab", LaunchStyleTab, "", []string{"-w", "0", "new-tab"}},
 		{"window", LaunchStyleWindow, "", []string{"-w", "new", "new-tab"}},
 		{"pane_auto", LaunchStylePane, "auto", []string{"-w", "0", "split-pane"}},
-		{"pane_right", LaunchStylePane, "right", []string{"-w", "0", "split-pane", "--direction", "right"}},
-		{"pane_down", LaunchStylePane, "down", []string{"-w", "0", "split-pane", "--direction", "down"}},
+		{"pane_right", LaunchStylePane, "right", []string{"-w", "0", "split-pane", "-V"}},
+		{"pane_down", LaunchStylePane, "down", []string{"-w", "0", "split-pane", "-H"}},
+		{"pane_left", LaunchStylePane, "left", []string{"-w", "0", "split-pane", "-V"}},
+		{"pane_up", LaunchStylePane, "up", []string{"-w", "0", "split-pane", "-H"}},
+		{"pane_empty_dir", LaunchStylePane, "", []string{"-w", "0", "split-pane"}},
 	}
 
 	for _, tc := range tests {

web/src/pages/config.astro

@@ -37,7 +37,7 @@ import ConfigTable from '../components/ConfigTable.astro';
         { key: 'agent', type: 'string', default: '""', description: 'Pass --agent <name> to Copilot CLI.' },
         { key: 'model', type: 'string', default: '""', description: 'Pass --model <name> to Copilot CLI.' },
         { key: 'launch_mode', type: 'string', default: '"tab"', description: 'How to open sessions: in-place, tab, window, pane.' },
-        { key: 'pane_direction', type: 'string', default: '"auto"', description: 'Split direction for pane mode: auto, right, down, left, up.' },
+        { key: 'pane_direction', type: 'string', default: '"auto"', description: 'Split direction for pane mode: auto, right, down, left, up (see Pane Direction Semantics below).' },
         { key: 'custom_command', type: 'string', default: '""', description: 'Custom launch command. {sessionId} is replaced with the session ID.' },
         { key: 'excluded_dirs', type: 'array', default: '[]', description: 'Directory paths to hide from the session list.' },
         { key: 'favoriteSessions', type: 'array', default: '[]', description: 'Session IDs starred as favorites.' },
@@ -113,6 +113,29 @@ import ConfigTable from '../components/ConfigTable.astro';
       <pre><code>"custom_command": "my-tool resume {'{sessionId}'}"</code></pre>
     </div>
   </section>
+
+  <section class="section">
+    <div class="container">
+      <h2>Pane Direction Semantics</h2>
+      <p>
+        When <code>launch_mode</code> is <code>"pane"</code>, the
+        <code>pane_direction</code> value maps to Windows Terminal's
+        <code>-H</code> / <code>-V</code> split-pane flags:
+      </p>
+      <ul>
+        <li><code>down</code> → <code>-H</code> — horizontal split (divider runs horizontally, new pane below)</li>
+        <li><code>up</code> → <code>-H</code> — horizontal split (WT controls actual placement; closest available)</li>
+        <li><code>right</code> → <code>-V</code> — vertical split (divider runs vertically, new pane to the right)</li>
+        <li><code>left</code> → <code>-V</code> — vertical split (WT controls actual placement; closest available)</li>
+        <li><code>auto</code> / empty — no flag; Windows Terminal decides automatically</li>
+      </ul>
+      <p>
+        <strong>Note:</strong> <code>-H</code> and <code>-V</code> control
+        split <em>orientation</em> only (the direction the divider runs).
+        Windows Terminal decides actual pane placement based on available space.
+      </p>
+    </div>
+  </section>
 </Base>
 
 <style>

web/src/pages/features.astro

@@ -173,11 +173,28 @@ import Screenshot from '../components/Screenshot.astro';
           <ul>
             <li><kbd>Ctrl</kbd>+double-click forces a new window</li>
             <li><kbd>Shift</kbd>+double-click forces a new tab</li>
-            <li>Pane mode supports directional splits: auto, right, down, left, up</li>
+            <li>Pane mode supports directional splits: auto, right, down, left, up (see pane direction semantics below)</li>
             <li>Windows: uses <code>wt</code> commands for Windows Terminal</li>
             <li>macOS: supports Terminal.app and iTerm2</li>
             <li>Linux: detects and delegates to the active terminal emulator</li>
           </ul>
+          <h3>Pane Direction Semantics</h3>
+          <p>
+            The <code>pane_direction</code> config value maps to Windows Terminal's
+            <code>-H</code> / <code>-V</code> split-pane flags:
+          </p>
+          <ul>
+            <li><code>down</code> → <code>-H</code> — horizontal split (divider runs horizontally, new pane below)</li>
+            <li><code>up</code> → <code>-H</code> — horizontal split (WT controls actual placement)</li>
+            <li><code>right</code> → <code>-V</code> — vertical split (divider runs vertically, new pane right)</li>
+            <li><code>left</code> → <code>-V</code> — vertical split (WT controls actual placement)</li>
+            <li><code>auto</code> / empty — no flag; Windows Terminal decides automatically</li>
+          </ul>
+          <p>
+            <strong>Note:</strong> <code>-H</code> / <code>-V</code> control split
+            <em>orientation</em> only. Windows Terminal decides actual pane placement
+            based on available space.
+          </p>
         </div>
         <div class="feature-text">
           <p class="text-only-note">