createBranchedSession copies the conversation without switching
the current session, then tmux opens the fork in a separate target.
Resolves the full path to pi for tmux popup's minimal shell environment.

Assisted-by: Claude Opus 4.6 via pi

diff --git a/.pi/agent/extensions/fork-tmux.ts b/.pi/agent/extensions/fork-tmux.ts
new file mode 100644
index 00000000..e48cd2a1
--- /dev/null
+++ b/.pi/agent/extensions/fork-tmux.ts
@@ -0,0 +1,120 @@
+/**
+ * Fork Tmux - Fork the current session into a new tmux pane, window, or popup.
+ *
+ * Creates a branched copy of the current session and opens it
+ * in a new tmux target, leaving the current session untouched.
+ *
+ * Usage:
+ *   /fork-tmux              Horizontal split (right)
+ *   /fork-tmux -v           Vertical split (below)
+ *   /fork-tmux -w           New window
+ *   /fork-tmux -p           Popup (tmux 3.3+)
+ *   /fork-tmux -p 80%x80%  Popup with size
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+type Mode = "split-h" | "split-v" | "window" | "popup";
+
+function parseArgs(raw: string): { mode: Mode; extra: string[] } {
+	const parts = raw.trim().split(/\s+/).filter(Boolean);
+	let mode: Mode = "split-h";
+	const extra: string[] = [];
+
+	for (const p of parts) {
+		if (p === "-v") mode = "split-v";
+		else if (p === "-w") mode = "window";
+		else if (p === "-p") mode = "popup";
+		else extra.push(p);
+	}
+
+	return { mode, extra };
+}
+
+function shellEscape(s: string): string {
+	return "'" + s.replace(/'/g, "'\\''") + "'";
+}
+
+function findPi(): string {
+	// Resolve full path to pi so it works in minimal shell environments (e.g. tmux popups)
+	try {
+		const { execSync } = require("node:child_process");
+		return execSync("which pi", { encoding: "utf-8" }).trim();
+	} catch {
+		return "pi";
+	}
+}
+
+function buildShellCommand(mode: Mode, extra: string[], sessionFile: string): string {
+	const piCmd = `${findPi()} --session ${shellEscape(sessionFile)}`;
+
+	switch (mode) {
+		case "split-h":
+			return `tmux split-window -h -- ${piCmd}`;
+		case "split-v":
+			return `tmux split-window -v -- ${piCmd}`;
+		case "window":
+			return `tmux new-window -- ${piCmd}`;
+		case "popup": {
+			const sizeArgs: string[] = [];
+			if (extra[0]) {
+				const [w, h] = extra[0].split("x");
+				if (w) sizeArgs.push(`-w ${w}`);
+				if (h) sizeArgs.push(`-h ${h}`);
+			}
+			return `tmux display-popup -E ${sizeArgs.join(" ")} -- ${piCmd}`;
+		}
+	}
+}
+
+export default function (pi: ExtensionAPI) {
+	pi.registerCommand("fork-tmux", {
+		description: "Fork session into a new tmux pane/window/popup (-v, -w, -p [size])",
+		handler: async (args, ctx) => {
+			if (!ctx.hasUI) {
+				ctx.ui.notify("fork-tmux requires interactive mode", "error");
+				return;
+			}
+
+			if (!process.env.TMUX) {
+				ctx.ui.notify("Not inside a tmux session", "error");
+				return;
+			}
+
+			const sessionFile = ctx.sessionManager.getSessionFile();
+			if (!sessionFile) {
+				ctx.ui.notify("No session file (ephemeral session)", "error");
+				return;
+			}
+
+			const leafId = ctx.sessionManager.getLeafId();
+			if (!leafId) {
+				ctx.ui.notify("No entries to fork from", "error");
+				return;
+			}
+
+			const newSessionFile = ctx.sessionManager.createBranchedSession(leafId);
+			if (!newSessionFile) {
+				ctx.ui.notify("Failed to create branched session", "error");
+				return;
+			}
+
+			const { mode, extra } = parseArgs(args);
+			const cmd = buildShellCommand(mode, extra, newSessionFile);
+
+			const result = await pi.exec("bash", ["-c", cmd]);
+			if (result.code !== 0) {
+				ctx.ui.notify(`tmux failed (exit ${result.code}): ${result.stderr || result.stdout || "(no output)"}`, "error");
+				return;
+			}
+
+			const labels: Record<Mode, string> = {
+				"split-h": "pane (right)",
+				"split-v": "pane (below)",
+				"window": "window",
+				"popup": "popup",
+			};
+			ctx.ui.notify(`Forked into new ${labels[mode]}`, "success");
+		},
+	});
+}