Every session showed the same "π - dotfiles" pane title, so parallel
sessions were indistinguishable. After the first exchange, a cheap
claude-haiku-4-5 call blurbs the conversation into the session name,
which pi already renders into the title tmux shows. Set once per
session; a session that already has a name is left alone.

Assisted-by: Claude Opus 4.8 via pi

diff --git a/ai/pi/extensions/session-blurb.ts b/ai/pi/extensions/session-blurb.ts
new file mode 100644
index 00000000..74355ead
--- /dev/null
+++ b/ai/pi/extensions/session-blurb.ts
@@ -0,0 +1,209 @@
+/**
+ * Session Blurb - keep the tmux pane title describing the current session.
+ *
+ * Once pi settles after the first real exchange, a cheap side model
+ * condenses the conversation into a 3-6 word blurb and stores it as the
+ * session name. Pi renders the session name into the terminal title as
+ * `<APP> - <blurb> - <cwd>`, which tmux picks up as the pane title
+ * (shown via pane-border-format).
+ *
+ * The blurb is set once per session and then left alone. A session that
+ * already has a name (e.g. set via `/name`) is never touched.
+ */
+
+import { complete, getModel } from "@mariozechner/pi-ai/compat";
+import type { ExtensionAPI, ExtensionContext } from "@mariozechner/pi-coding-agent";
+
+// --- Configuration ---------------------------------------------------------
+
+const MODEL_PROVIDER = "anthropic";
+const MODEL_ID = "claude-haiku-4-5";
+const MAX_WORDS = 6;
+const MAX_CHARS = 40;
+// Cap how much transcript we send to the summarizer. The tail carries the
+// most relevant "what's happening now" signal, so we keep the last N chars.
+const MAX_TRANSCRIPT_CHARS = 6000;
+
+const PROMPT_PREAMBLE = [
+	`Summarize what this coding session is about in ${MAX_WORDS} words or fewer.`,
+	"Write a terse noun phrase, no punctuation, no quotes, lowercase unless a",
+	"proper noun. Describe the task, not the assistant. Reply with only the phrase.",
+	"",
+	"<conversation>",
+].join("\n");
+
+// --- Transcript extraction (mirrors the summarize.ts example) --------------
+
+type ContentBlock = {
+	type?: string;
+	text?: string;
+	name?: string;
+};
+
+type SessionEntry = {
+	type: string;
+	message?: {
+		role?: string;
+		content?: unknown;
+	};
+};
+
+function extractText(content: unknown): string[] {
+	if (typeof content === "string") return [content];
+	if (!Array.isArray(content)) return [];
+
+	const parts: string[] = [];
+	for (const part of content) {
+		if (!part || typeof part !== "object") continue;
+		const block = part as ContentBlock;
+		if (block.type === "text" && typeof block.text === "string") {
+			parts.push(block.text);
+		}
+	}
+	return parts;
+}
+
+function extractToolNames(content: unknown): string[] {
+	if (!Array.isArray(content)) return [];
+
+	const names: string[] = [];
+	for (const part of content) {
+		if (!part || typeof part !== "object") continue;
+		const block = part as ContentBlock;
+		if (block.type === "toolCall" && typeof block.name === "string") {
+			names.push(`(used ${block.name})`);
+		}
+	}
+	return names;
+}
+
+function buildTranscript(entries: SessionEntry[]): string {
+	const sections: string[] = [];
+
+	for (const entry of entries) {
+		const role = entry.type === "message" ? entry.message?.role : undefined;
+		if (role !== "user" && role !== "assistant") continue;
+
+		const lines: string[] = [];
+		const text = extractText(entry.message?.content).join("\n").trim();
+		if (text) lines.push(`${role === "user" ? "User" : "Assistant"}: ${text}`);
+		if (role === "assistant") lines.push(...extractToolNames(entry.message?.content));
+
+		if (lines.length > 0) sections.push(lines.join("\n"));
+	}
+
+	const full = sections.join("\n\n");
+	return full.length > MAX_TRANSCRIPT_CHARS ? full.slice(-MAX_TRANSCRIPT_CHARS) : full;
+}
+
+function countExchange(entries: SessionEntry[]): { users: number; assistants: number } {
+	let users = 0;
+	let assistants = 0;
+	for (const entry of entries) {
+		if (entry.type !== "message") continue;
+		if (entry.message?.role === "user") users++;
+		else if (entry.message?.role === "assistant") assistants++;
+	}
+	return { users, assistants };
+}
+
+// --- Blurb sanitizing ------------------------------------------------------
+
+function sanitizeBlurb(raw: string): string | undefined {
+	// Take the first non-empty line, drop surrounding quotes/backticks and
+	// trailing punctuation, collapse whitespace, and clamp length.
+	const firstLine = raw
+		.split("\n")
+		.map((l) => l.trim())
+		.find((l) => l.length > 0);
+	if (!firstLine) return undefined;
+
+	let blurb = firstLine
+		.replace(/^[\s"'`*_]+/, "")
+		.replace(/[\s"'`*_.,;:!?]+$/, "")
+		.replace(/\s+/g, " ")
+		.trim();
+
+	if (!blurb) return undefined;
+
+	const words = blurb.split(" ");
+	if (words.length > MAX_WORDS) blurb = words.slice(0, MAX_WORDS).join(" ");
+	if (blurb.length > MAX_CHARS) blurb = blurb.slice(0, MAX_CHARS).trimEnd();
+
+	return blurb || undefined;
+}
+
+// --- Extension -------------------------------------------------------------
+
+export default function (pi: ExtensionAPI) {
+	// Closure state, reset naturally when extensions reload on session switch.
+	let blurbSet = false;
+	let generating = false;
+
+	async function generateBlurb(transcript: string, ctx: ExtensionContext): Promise<string | undefined> {
+		const model = getModel(MODEL_PROVIDER, MODEL_ID);
+		if (!model) return undefined;
+
+		const auth = await ctx.modelRegistry.getApiKeyAndHeaders(model);
+		if (!auth?.ok || !auth.apiKey) return undefined;
+
+		const response = await complete(
+			model,
+			{
+				messages: [
+					{
+						role: "user" as const,
+						content: [{ type: "text" as const, text: `${PROMPT_PREAMBLE}\n${transcript}\n</conversation>` }],
+						timestamp: Date.now(),
+					},
+				],
+			},
+			{ apiKey: auth.apiKey, headers: auth.headers, env: auth.env },
+		);
+
+		const text = response.content
+			.filter((c): c is { type: "text"; text: string } => c.type === "text")
+			.map((c) => c.text)
+			.join("\n");
+
+		return sanitizeBlurb(text);
+	}
+
+	pi.on("agent_settled", async (_event, ctx) => {
+		// Terminal title only matters in the interactive TUI.
+		if (ctx.mode !== "tui") return;
+		// Set the blurb once per session; keep retrying only until one lands.
+		if (blurbSet || generating) return;
+
+		const entries = ctx.sessionManager.getBranch() as SessionEntry[];
+
+		// Need at least one real exchange before a blurb means anything.
+		const { users, assistants } = countExchange(entries);
+		if (users < 1 || assistants < 1) return;
+
+		// Respect a manually chosen name: if the session is already named,
+		// leave it and never set a blurb for this session.
+		if (pi.getSessionName()) {
+			blurbSet = true;
+			return;
+		}
+
+		const transcript = buildTranscript(entries);
+		if (!transcript.trim()) return;
+
+		generating = true;
+		try {
+			const blurb = await generateBlurb(transcript, ctx);
+			if (blurb) {
+				pi.setSessionName(blurb);
+				blurbSet = true;
+			}
+			// On an empty/failed result, leave blurbSet false so the next
+			// settle retries; a successful set stops all future attempts.
+		} catch {
+			// Never surface summarizer failures into the session.
+		} finally {
+			generating = false;
+		}
+	});
+}