feat(tabs): tab-to-tab agent communication via short handles

Add send_to_tab / read_tab tools so an agent can message or read another
tab by a git-style short handle (shortest unique prefix of the tab UUID,
min 4 chars), shown in the tab bar.

- core/db/tabs: resolveTabPrefix + shortestUniquePrefix (open tabs only,
  LIKE-sanitized prefix matching)
- new tools read-tab.ts / send-to-tab.ts (+ tests) decoupled from the DB
  TabRow via a minimal ResolvedTabRef projection
- agent-manager: unified deliverMessage routing (busy -> queue, idle ->
  new turn) shared by POST /chat and send_to_tab; agent->agent auto-wake
  budget (MAX_AGENT_AUTO_WAKES) to bound ping-pong loops
- summon/loader: send_to_tab + read_tab as grantable tools
- frontend: shortHandleFor + handle badge in TabBar; perm toggles
- notes: tab-comm / user-agents / todo-redesign plans
- chore: biome format fixes (debug-logger, summon.test)

Refs notes/plan-tab-comm.md

20 files changed, 1442 insertions, 40 deletions

diff --git a/packages/api/src/agent-manager.ts b/packages/api/src/agent-manager.ts
index 111237c..517c661 100644
--- a/packages/api/src/agent-manager.ts
+++ b/packages/api/src/agent-manager.ts
@@ -15,8 +15,10 @@ import {
 	createListFilesTool,
 	createReadFileSliceTool,
 	createReadFileTool,
+	createReadTabTool,
 	createRetrieveTool,
 	createRunShellTool,
+	createSendToTabTool,
 	createSkillsWatcher,
 	createSummonTool,
 	createTaskListTool,
@@ -32,6 +34,8 @@ import {
 	getClaudeAccountsFromDB,
 	getMessagesForTab,
 	getSetting,
+	getTab,
+	listOpenTabs,
 	loadAgent,
 	loadAgents,
 	loadConfig,
@@ -41,8 +45,11 @@ import {
 	refreshAccountCredentials,
 	refreshAccountCredentialsAsync,
 	resolveApiKey,
+	resolveTabPrefix,
 	type SkillDefinition,
 	type SystemChunkKind,
+	shortestUniquePrefix,
+	type TabResolution,
 	type TabStatusSnapshot,
 	TaskList,
 	toAvailableSubagents,
@@ -73,6 +80,16 @@ const TOOL_DESCRIPTIONS: Record<string, string> = {
 		"Fetch the transcript/subtitles for a YouTube video. Set background=true to start in the background and get a job_id for later retrieval.",
 };
 
+/**
+ * Maximum number of CONSECUTIVE agent-to-agent auto-wakes a tab will accept
+ * before it stops auto-responding and waits for a human. Each `send_to_tab`
+ * that would wake an idle tab consumes one unit; any human-originated message
+ * (e.g. via `POST /chat`) refills the budget to full. This bounds runaway
+ * agent ping-pong loops (A wakes B wakes A ...) that would otherwise spend
+ * tokens unbounded with no human in the loop. See notes/plan-tab-comm.md.
+ */
+const MAX_AGENT_AUTO_WAKES = 6;
+
 const DEFAULT_SYSTEM_PROMPT =
 	"You are Dispatch, an agent designed to help with any task that the user asks for. Be helpful and concise.";
 
@@ -197,6 +214,14 @@ interface TabAgent {
 	 * rows. Set at the start of `processMessage`, cleared when the turn ends.
 	 */
 	currentTurnId: string | null;
+	/**
+	 * Remaining consecutive agent-to-agent auto-wakes this tab will accept
+	 * before requiring human intervention (see `MAX_AGENT_AUTO_WAKES`).
+	 * Refilled to the max by any human-originated `deliverMessage`; decremented
+	 * each time an agent-originated `send_to_tab` wakes this tab from idle. When
+	 * it hits 0, further agent messages are queued but do NOT start a turn.
+	 */
+	autoWakeBudget: number;
 }
 
 export class AgentManager {
@@ -343,6 +368,7 @@ export class AgentManager {
 				currentChunks: null,
 				currentAssistantId: null,
 				currentTurnId: null,
+				autoWakeBudget: MAX_AGENT_AUTO_WAKES,
 			};
 			this.tabAgents.set(tabId, tabAgent);
 		}
@@ -366,10 +392,12 @@ export class AgentManager {
 		const permBash = getSetting("perm_bash") === "allow";
 		const permSummon = getSetting("perm_summon") === "allow";
 		const permUserAgent = getSetting("perm_user_agent") === "allow";
+		const permSendToTab = getSetting("perm_send_to_tab") === "allow";
+		const permReadTab = getSetting("perm_read_tab") === "allow";
 		const permWebSearch = getSetting("perm_web_search") === "allow";
 		const permYoutubeTranscribe = getSetting("perm_youtube_transcribe") === "allow";
 		const sysPrompt = getSetting("system_prompt") ?? "";
-		const permKey = `${permRead}:${permEdit}:${permBash}:${permSummon}:${permUserAgent}:${permWebSearch}:${permYoutubeTranscribe}:${sysPrompt}`;
+		const permKey = `${permRead}:${permEdit}:${permBash}:${permSummon}:${permUserAgent}:${permSendToTab}:${permReadTab}:${permWebSearch}:${permYoutubeTranscribe}:${sysPrompt}`;
 
 		// If the override differs or permissions changed, invalidate the cached agent
 		if (
@@ -504,6 +532,12 @@ export class AgentManager {
 						}),
 					});
 				}
+				// Tab-to-tab communication — gated on the child whitelist.
+				if (allowed.has("send_to_tab") || allowed.has("read_tab")) {
+					for (const entry of this.buildTabCommToolEntries(tabId)) {
+						if (allowed.has(entry.name)) toolEntries.push(entry);
+					}
+				}
 			} else {
 				// Parent agent: use permission settings from DB
 				if (permRead) {
@@ -581,6 +615,14 @@ export class AgentManager {
 						}),
 					});
 				}
+				if (permSendToTab || permReadTab) {
+					const tabCommAllowed = new Set<string>();
+					if (permSendToTab) tabCommAllowed.add("send_to_tab");
+					if (permReadTab) tabCommAllowed.add("read_tab");
+					for (const entry of this.buildTabCommToolEntries(tabId)) {
+						if (tabCommAllowed.has(entry.name)) toolEntries.push(entry);
+					}
+				}
 			}
 
 			const tools = toolEntries.map((e) => e.tool);
@@ -971,14 +1013,18 @@ export class AgentManager {
 			if (!agentDef) {
 				const allDefs = loadAgents(parentEffectiveDir);
 				if (options.topLevel) {
-					const userAgents = allDefs.filter((d) => !d.is_subagent).map((d) => `${d.slug} (${d.name})`);
+					const userAgents = allDefs
+						.filter((d) => !d.is_subagent)
+						.map((d) => `${d.slug} (${d.name})`);
 					const hint =
 						userAgents.length > 0
 							? ` Available user agents: ${userAgents.join(", ")}.`
 							: " No user agent definitions exist yet.";
 					throw new Error(`Agent definition not found: "${options.agentSlug}".${hint}`);
 				} else {
-					const subagents = allDefs.filter((d) => d.is_subagent).map((d) => `${d.slug} (${d.name})`);
+					const subagents = allDefs
+						.filter((d) => d.is_subagent)
+						.map((d) => `${d.slug} (${d.name})`);
 					const hint =
 						subagents.length > 0
 							? ` Available subagents: ${subagents.join(", ")}.`
@@ -1147,7 +1193,8 @@ export class AgentManager {
 			if (tabAgent.status === "running") {
 				return {
 					status: "error",
-					error: "This is a user agent (top-level tab) and cannot be retrieved. User agents are fire-and-forget.",
+					error:
+						"This is a user agent (top-level tab) and cannot be retrieved. User agents are fire-and-forget.",
 				};
 			}
 			return {
@@ -1159,6 +1206,214 @@ export class AgentManager {
 		return tabAgent.completionPromise;
 	}
 
+	// ─── Tab-to-tab communication ───────────────────────────────────
+	//
+	// `send_to_tab` / `read_tab` let an agent message a peer tab by its short
+	// handle (a git-style prefix of the tab UUID). Delivery reuses the exact
+	// running→queue / idle→new-turn routing that `POST /chat` uses (see
+	// `deliverMessage`), so an agent message behaves identically to a user one.
+
+	/**
+	 * Build the `send_to_tab` + `read_tab` tool entries for `tabId`. Shared by
+	 * both tool-construction paths (child whitelist + permission-gated parent).
+	 * `selfHandle` is computed once so the calling tab can stamp provenance and
+	 * reject self-sends.
+	 */
+	private buildTabCommToolEntries(
+		tabId: string,
+	): Array<{ name: string; tool: ReturnType<typeof createSendToTabTool> }> {
+		const selfHandle = shortestUniquePrefix(tabId);
+		return [
+			{
+				name: "send_to_tab",
+				tool: createSendToTabTool({
+					resolveShortId: (prefix) => this.resolveTabHandle(prefix),
+					// origin: "agent" subjects this to the receiver's auto-wake
+					// budget so agent↔agent loops are bounded (see deliverMessage).
+					deliver: (targetId, message) =>
+						this.deliverMessage(targetId, message, { origin: "agent" }),
+					listOpenHandles: () => this.listOpenHandles(tabId),
+					self: { id: tabId, handle: selfHandle },
+				}),
+			},
+			{
+				name: "read_tab",
+				tool: createReadTabTool({
+					resolveShortId: (prefix) => this.resolveTabHandle(prefix),
+					getLastResponse: (targetId) => this.getLastTabResponse(targetId),
+					listOpenHandles: () => this.listOpenHandles(tabId),
+				}),
+			},
+		];
+	}
+
+	/**
+	 * Project a core `ResolveTabPrefixResult` down to the tool-facing
+	 * `TabResolution` (minimal `{ id, title, handle }` refs). Each match's
+	 * `handle` is recomputed via `shortestUniquePrefix` so the value the tool
+	 * echoes back always matches what the UI currently shows.
+	 */
+	private resolveTabHandle(prefix: string): TabResolution {
+		const res = resolveTabPrefix(prefix);
+		if (res.status === "none") return { status: "none" };
+		if (res.status === "ok") {
+			return {
+				status: "ok",
+				tab: {
+					id: res.tab.id,
+					title: res.tab.title,
+					handle: shortestUniquePrefix(res.tab.id),
+				},
+			};
+		}
+		return {
+			status: "ambiguous",
+			matches: res.matches.map((t) => ({
+				id: t.id,
+				title: t.title,
+				handle: shortestUniquePrefix(t.id),
+			})),
+		};
+	}
+
+	/** Snapshot of open tabs as `{ handle, title }`, excluding `exceptId`
+	 *  (typically the caller's own tab). Drives the "available tabs" hints. */
+	private listOpenHandles(exceptId?: string): Array<{ handle: string; title: string }> {
+		return listOpenTabs()
+			.filter((t) => t.id !== exceptId)
+			.map((t) => ({ handle: shortestUniquePrefix(t.id), title: t.title }));
+	}
+
+	/**
+	 * Return a tab's most recent COMPLETED assistant turn as flat text, plus
+	 * its current status. Reads the persisted chunk log (source of truth) and
+	 * grabs the last `role === "assistant"` group's text chunks. `text` is null
+	 * when no completed assistant turn exists yet.
+	 */
+	getLastTabResponse(tabId: string): { text: string | null; status: AgentStatus } {
+		const status = this.getTabStatus(tabId);
+		try {
+			const messages = getMessagesForTab(tabId);
+			for (let i = messages.length - 1; i >= 0; i--) {
+				const msg = messages[i];
+				if (!msg || msg.role !== "assistant") continue;
+				const text = msg.chunks
+					.filter((c): c is { type: "text"; text: string } => c.type === "text")
+					.map((c) => c.text)
+					.join("")
+					.trim();
+				if (text.length > 0) return { text, status };
+			}
+		} catch {
+			// DB unavailable / tab unknown — fall through to null.
+		}
+		return { text: null, status };
+	}
+
+	/**
+	 * Deliver `message` to `tabId`, choosing the SAME routing as `POST /chat`:
+	 *   - target running → queue it (consumed like a user interrupt).
+	 *   - target idle/errored → wake it and start a new turn.
+	 *
+	 * Returns quickly; does NOT block on the turn. Both the HTTP `/chat` path
+	 * and the `send_to_tab` tool call through here so the running/idle decision
+	 * lives in exactly one place.
+	 *
+	 * `opts` carries the per-request knobs `/chat` forwards (key/model, agent
+	 * fallback chain, reasoning effort, working dir, an explicit queue id). The
+	 * `send_to_tab` tool passes none of these — for a cold wake (a tab not in
+	 * `tabAgents`, e.g. after a server restart) the key/model are hydrated from
+	 * the live `TabAgent` if present, else from the persisted tab row. (A cold
+	 * tab keeps its stored key/model but not its full agent-definition fallback
+	 * chain — see plan notes.)
+	 */
+	deliverMessage(
+		tabId: string,
+		message: string,
+		opts: {
+			keyId?: string;
+			modelId?: string;
+			agentModels?: Array<{ key_id: string; model_id: string }>;
+			reasoningEffort?: "none" | "low" | "medium" | "high" | "max";
+			workingDirectory?: string;
+			queueId?: string;
+			/**
+			 * Who is sending this message. `"human"` (default) is unrestricted
+			 * and REFILLS the target's agent-to-agent auto-wake budget. `"agent"`
+			 * (from the `send_to_tab` tool) is governed by that budget: an
+			 * agent-originated wake of an idle tab consumes one unit, and once the
+			 * budget is exhausted the message is queued WITHOUT starting a turn
+			 * (returned as `suppressed`) so a runaway A↔B loop can't spend tokens
+			 * forever with no human in the loop.
+			 */
+			origin?: "human" | "agent";
+		} = {},
+	): { status: "queued"; messageId: string } | { status: "started" } | { status: "suppressed" } {
+		const origin = opts.origin ?? "human";
+
+		// A human touching the tab clears any accumulated agent-wake throttle:
+		// the conversation is back under human supervision, so peers get a fresh
+		// budget of auto-wakes again.
+		if (origin === "human") {
+			this._getOrCreateTabAgent(tabId).autoWakeBudget = MAX_AGENT_AUTO_WAKES;
+		}
+
+		if (this.getTabStatus(tabId) === "running") {
+			// Busy target → always queue (consumed like a user interrupt),
+			// regardless of origin. Queuing does not itself start a turn, so it
+			// can't drive a runaway loop; we don't spend budget here.
+			const { messageId } = this.queueMessage(tabId, message, opts.queueId);
+			return { status: "queued", messageId };
+		}
+
+		// Idle/errored target → this delivery would WAKE the tab (start a turn).
+		// For agent-originated wakes, enforce the auto-wake budget first.
+		if (origin === "agent") {
+			const target = this._getOrCreateTabAgent(tabId);
+			if (target.autoWakeBudget <= 0) {
+				// Budget exhausted: preserve the message (queue it, never drop)
+				// but do NOT wake the tab. A human message will refill the budget
+				// and the queued message will be seen on the next human turn.
+				this.queueMessage(tabId, message, opts.queueId);
+				const notice =
+					`Automatic agent-to-agent message limit reached for this tab ` +
+					`(${MAX_AGENT_AUTO_WAKES} consecutive). Further messages from other tabs ` +
+					`are held until you send a message here.`;
+				this.emit({ type: "notice", message: notice }, tabId);
+				this.routeSystemEventToTab(tabId, "notice", notice);
+				return { status: "suppressed" };
+			}
+			target.autoWakeBudget -= 1;
+		}
+
+		// Resolve key/model: explicit opts win, then the live tab agent's, then
+		// the persisted row's.
+		const tabAgent = this.tabAgents.get(tabId);
+		let keyId = opts.keyId ?? tabAgent?.keyId ?? undefined;
+		let modelId = opts.modelId ?? tabAgent?.modelId ?? undefined;
+		const agentModels = opts.agentModels ?? tabAgent?.agentModels;
+		if (!keyId || !modelId) {
+			const row = getTab(tabId);
+			if (row) {
+				keyId = keyId ?? row.keyId ?? undefined;
+				modelId = modelId ?? row.modelId ?? undefined;
+			}
+		}
+
+		this.processMessage(
+			tabId,
+			message,
+			keyId,
+			modelId,
+			opts.reasoningEffort,
+			opts.workingDirectory,
+			agentModels,
+		).catch((err) => {
+			console.error(`[dispatch] deliverMessage processMessage error for tab ${tabId}:`, err);
+		});
+		return { status: "started" };
+	}
+
 	async processMessage(
 		tabId: string,
 		message: string,
diff --git a/packages/api/src/app.ts b/packages/api/src/app.ts
index 73d3de5..19cc193 100644
--- a/packages/api/src/app.ts
+++ b/packages/api/src/app.ts
@@ -56,28 +56,33 @@ app.post("/chat", async (c) => {
 		return c.json({ error: "message must be a non-empty string" }, 400);
 	}
 
-	if (agentManager.getTabStatus(tabId) === "running") {
-		const queueId = typeof body.queueId === "string" ? body.queueId : undefined;
-		const { messageId } = agentManager.queueMessage(tabId, message, queueId);
-		return c.json({ status: "queued", messageId });
-	}
-
 	const keyId = typeof body.keyId === "string" ? body.keyId : undefined;
 	const modelId = typeof body.modelId === "string" ? body.modelId : undefined;
 	const agentModels = Array.isArray(body.agentModels) ? body.agentModels : undefined;
 	const workingDirectory =
 		typeof body.workingDirectory === "string" ? body.workingDirectory : undefined;
+	const queueId = typeof body.queueId === "string" ? body.queueId : undefined;
 	const validEfforts = ["none", "low", "medium", "high", "max"];
 	const reasoningEffort =
 		typeof body.reasoningEffort === "string" && validEfforts.includes(body.reasoningEffort)
 			? (body.reasoningEffort as "none" | "low" | "medium" | "high" | "max")
 			: undefined;
 
-	// Non-blocking — let the agent run in the background
-	agentManager
-		.processMessage(tabId, message, keyId, modelId, reasoningEffort, workingDirectory, agentModels)
-		.catch(console.error);
+	// Single routing decision (queue if busy, new turn if idle) shared with the
+	// `send_to_tab` tool via `AgentManager.deliverMessage`. Non-blocking — a
+	// started turn runs in the background.
+	const outcome = agentManager.deliverMessage(tabId, message, {
+		...(keyId ? { keyId } : {}),
+		...(modelId ? { modelId } : {}),
+		...(agentModels ? { agentModels } : {}),
+		...(reasoningEffort ? { reasoningEffort } : {}),
+		...(workingDirectory !== undefined ? { workingDirectory } : {}),
+		...(queueId ? { queueId } : {}),
+	});
 
+	if (outcome.status === "queued") {
+		return c.json({ status: "queued", messageId: outcome.messageId });
+	}
 	return c.json({ status: "ok" });
 });
 
diff --git a/packages/api/tests/agent-manager.test.ts b/packages/api/tests/agent-manager.test.ts
index 4415bbb..1358eb1 100644
--- a/packages/api/tests/agent-manager.test.ts
+++ b/packages/api/tests/agent-manager.test.ts
@@ -24,6 +24,39 @@ function resetFakeMessages(): void {
 function setFakeMessages(tabId: string, rows: FakeMessageRow[]): void {
 	fakeMessagesByTab.set(tabId, rows);
 }
+
+// Configurable stub for the tabs DB (getTab / listOpenTabs). Tests can seed
+// rows to exercise deliverMessage cold-hydration and handle resolution.
+interface FakeTabRow {
+	id: string;
+	title: string;
+	keyId: string | null;
+	modelId: string | null;
+	parentTabId: string | null;
+	status: string;
+	isOpen: boolean;
+	position: number;
+	createdAt: number;
+	updatedAt: number;
+}
+const fakeTabs = new Map<string, FakeTabRow>();
+function resetFakeTabs(): void {
+	fakeTabs.clear();
+}
+function setFakeTab(row: Partial<FakeTabRow> & { id: string }): void {
+	fakeTabs.set(row.id, {
+		title: "Tab",
+		keyId: null,
+		modelId: null,
+		parentTabId: null,
+		status: "idle",
+		isOpen: true,
+		position: 0,
+		createdAt: 0,
+		updatedAt: 0,
+		...row,
+	});
+}
 function makeRow(
 	tabId: string,
 	seq: number,
@@ -42,11 +75,22 @@ function makeRow(
 // because the production code reassigns `agent.messages =
 // rows.slice(...)` AFTER `new Agent()` returns — capturing a
 // reference at construction would yield a stale empty array.
-const constructedAgents: Array<{ initialMessages: unknown[] }> = [];
+const constructedAgents: Array<{ initialMessages: unknown[]; toolNames: string[] }> = [];
 function resetConstructedAgents(): void {
 	constructedAgents.length = 0;
 }
 
+// Configurable settings store so tests can toggle tool permissions
+// (perm_send_to_tab / perm_read_tab / ...) and assert which tools the
+// constructed Agent receives. Defaults to empty (getSetting → null).
+const fakeSettings = new Map<string, string>();
+function resetFakeSettings(): void {
+	fakeSettings.clear();
+}
+function setFakeSetting(key: string, value: string): void {
+	fakeSettings.set(key, value);
+}
+
 // Allow tests to swap in a custom `run` generator (e.g. to simulate
 // a fallback failure mid-stream). Returning to undefined restores
 // the default.
@@ -87,12 +131,19 @@ vi.mock("@dispatch/core", () => ({
 	Agent: class MockAgent {
 		status = "idle";
 		messages: unknown[] = [];
+		toolNames: string[] = [];
+		constructor(config: { tools?: Array<{ name: string }> }) {
+			this.toolNames = (config?.tools ?? []).map((t) => t.name);
+		}
 		async *run(message: string): AsyncGenerator<unknown> {
 			// Snapshot the post-construction pre-populated message list
 			// the first thing `run()` does, before the real `Agent.run`
 			// would push the current user message at line 546. Tests
 			// inspect this to verify history was loaded correctly.
-			constructedAgents.push({ initialMessages: [...this.messages] });
+			constructedAgents.push({
+				initialMessages: [...this.messages],
+				toolNames: [...this.toolNames],
+			});
 			if (runImpl) {
 				for await (const ev of runImpl(message)) yield ev;
 				return;
@@ -244,6 +295,41 @@ vi.mock("@dispatch/core", () => ({
 		};
 	},
 	createTab() {},
+	getTab(id: string) {
+		return fakeTabs.get(id) ?? null;
+	},
+	listOpenTabs() {
+		return [...fakeTabs.values()].filter((t) => t.isOpen);
+	},
+	resolveTabPrefix(prefix: string) {
+		const sanitized = (prefix ?? "").toLowerCase().replace(/[^0-9a-f-]/g, "");
+		if (sanitized.length < 4) return { status: "none" };
+		const matches = [...fakeTabs.values()].filter(
+			(t) => t.isOpen && t.id.toLowerCase().startsWith(sanitized),
+		);
+		if (matches.length === 0) return { status: "none" };
+		if (matches.length === 1) return { status: "ok", tab: matches[0] };
+		return { status: "ambiguous", matches };
+	},
+	shortestUniquePrefix(id: string) {
+		return (id ?? "").slice(0, 4);
+	},
+	createSendToTabTool(_callbacks: unknown): ToolDefinition {
+		return {
+			name: "send_to_tab",
+			description: "send to tab",
+			parameters: { _type: "z.ZodObject", shape: {} } as unknown as ToolDefinition["parameters"],
+			execute: async () => "mock",
+		};
+	},
+	createReadTabTool(_callbacks: unknown): ToolDefinition {
+		return {
+			name: "read_tab",
+			description: "read tab",
+			parameters: { _type: "z.ZodObject", shape: {} } as unknown as ToolDefinition["parameters"],
+			execute: async () => "mock",
+		};
+	},
 	getClaudeAccountsFromDB() {
 		return [];
 	},
@@ -256,8 +342,8 @@ vi.mock("@dispatch/core", () => ({
 	resolveApiKey() {
 		return null;
 	},
-	getSetting(_key: string) {
-		return null;
+	getSetting(key: string) {
+		return fakeSettings.get(key) ?? null;
 	},
 	appendChunks() {
 		return [];
@@ -316,6 +402,8 @@ describe("AgentManager", () => {
 	beforeEach(() => {
 		resetFakeMessages();
 		resetConstructedAgents();
+		resetFakeTabs();
+		resetFakeSettings();
 		setRunImpl(null);
 		appendEventToChunksSpy.mockClear();
 	});
@@ -849,4 +937,273 @@ describe("AgentManager", () => {
 		expect(snap["tab-early"]).not.toHaveProperty("currentChunks");
 		expect(snap["tab-early"]).not.toHaveProperty("currentAssistantId");
 	});
+
+	// ─── Tab-to-tab communication ─────────────────────────────────
+
+	describe("deliverMessage", () => {
+		it("starts a new turn when the target tab is idle", async () => {
+			const manager = new AgentManager();
+			const events: AgentEvent[] = [];
+			manager.onEvent((e) => events.push(e));
+
+			const outcome = manager.deliverMessage("tab-idle", "wake up");
+			expect(outcome.status).toBe("started");
+
+			// Let the background turn run to completion.
+			await new Promise<void>((r) => setTimeout(r, 60));
+			expect(events.some((e) => e.type === "text-delta")).toBe(true);
+			expect(manager.getTabStatus("tab-idle")).toBe("idle");
+		});
+
+		it("queues the message when the target tab is running", () => {
+			const manager = new AgentManager();
+			const inner = manager as unknown as {
+				tabAgents: Map<string, Record<string, unknown>>;
+			};
+			// Seed a running tab agent directly.
+			inner.tabAgents.set("tab-busy", {
+				agent: null,
+				status: "running",
+				keyId: null,
+				modelId: null,
+				taskList: { onChange: () => {} },
+				messageQueue: [],
+				queueListeners: [],
+				shellStore: {},
+				transcriptStore: {},
+				currentChunks: null,
+				currentAssistantId: null,
+				currentTurnId: null,
+			});
+
+			const outcome = manager.deliverMessage("tab-busy", "queued msg");
+			expect(outcome.status).toBe("queued");
+			if (outcome.status === "queued") {
+				expect(typeof outcome.messageId).toBe("string");
+			}
+			// The message landed on the running tab's queue.
+			const agent = inner.tabAgents.get("tab-busy") as { messageQueue: unknown[] };
+			expect(agent.messageQueue).toHaveLength(1);
+		});
+
+		it("hydrates key/model from the persisted tab row for a cold wake", () => {
+			const manager = new AgentManager();
+			setFakeTab({ id: "tab-cold", keyId: "persisted-key", modelId: "persisted-model" });
+
+			// Spy on processMessage to capture the key/model deliverMessage
+			// forwarded — asserting the hydration decision directly rather than
+			// downstream tabAgent state (which the mocked ModelRegistry rewrites).
+			const spy = vi.spyOn(manager, "processMessage").mockResolvedValue(undefined);
+
+			const outcome = manager.deliverMessage("tab-cold", "hello");
+			expect(outcome.status).toBe("started");
+
+			expect(spy).toHaveBeenCalledTimes(1);
+			const args = spy.mock.calls[0] ?? [];
+			expect(args[0]).toBe("tab-cold"); // tabId
+			expect(args[1]).toBe("hello"); // message
+			expect(args[2]).toBe("persisted-key"); // keyId hydrated from row
+			expect(args[3]).toBe("persisted-model"); // modelId hydrated from row
+		});
+
+		it("prefers explicit opts over the persisted row on a cold wake", () => {
+			const manager = new AgentManager();
+			setFakeTab({ id: "tab-cold2", keyId: "row-key", modelId: "row-model" });
+			const spy = vi.spyOn(manager, "processMessage").mockResolvedValue(undefined);
+
+			manager.deliverMessage("tab-cold2", "hello", {
+				keyId: "explicit-key",
+				modelId: "explicit-model",
+			});
+
+			const args = spy.mock.calls[0] ?? [];
+			expect(args[2]).toBe("explicit-key");
+			expect(args[3]).toBe("explicit-model");
+		});
+	});
+
+	describe("deliverMessage — agent auto-wake budget", () => {
+		it("allows up to 6 consecutive agent wakes, then suppresses further ones", () => {
+			const manager = new AgentManager();
+			const spy = vi.spyOn(manager, "processMessage").mockResolvedValue(undefined);
+
+			// 6 agent-originated wakes of an idle tab should all start turns.
+			for (let i = 0; i < 6; i++) {
+				const outcome = manager.deliverMessage("tab-pp", `msg ${i}`, { origin: "agent" });
+				expect(outcome.status).toBe("started");
+			}
+			expect(spy).toHaveBeenCalledTimes(6);
+
+			// The 7th is suppressed: no new turn, message preserved on the queue.
+			const seventh = manager.deliverMessage("tab-pp", "msg 7", { origin: "agent" });
+			expect(seventh.status).toBe("suppressed");
+			expect(spy).toHaveBeenCalledTimes(6); // unchanged — no wake
+
+			const inner = manager as unknown as {
+				tabAgents: Map<string, { messageQueue: unknown[]; autoWakeBudget: number }>;
+			};
+			const agent = inner.tabAgents.get("tab-pp");
+			expect(agent?.autoWakeBudget).toBe(0);
+			// Suppressed message is queued, not dropped.
+			expect(agent?.messageQueue).toHaveLength(1);
+		});
+
+		it("a human message refills the budget and re-enables agent wakes", () => {
+			const manager = new AgentManager();
+			vi.spyOn(manager, "processMessage").mockResolvedValue(undefined);
+
+			// Exhaust the budget with agent wakes.
+			for (let i = 0; i < 6; i++) {
+				manager.deliverMessage("tab-refill", `a${i}`, { origin: "agent" });
+			}
+			expect(manager.deliverMessage("tab-refill", "blocked", { origin: "agent" }).status).toBe(
+				"suppressed",
+			);
+
+			// A human message refills the budget...
+			const humanOutcome = manager.deliverMessage("tab-refill", "human here", {
+				origin: "human",
+			});
+			expect(humanOutcome.status).toBe("started");
+
+			const inner = manager as unknown as {
+				tabAgents: Map<string, { autoWakeBudget: number }>;
+			};
+			expect(inner.tabAgents.get("tab-refill")?.autoWakeBudget).toBe(6);
+
+			// ...so an agent can wake it again.
+			expect(manager.deliverMessage("tab-refill", "again", { origin: "agent" }).status).toBe(
+				"started",
+			);
+		});
+
+		it("does not consume budget when the message is merely queued (busy target)", () => {
+			const manager = new AgentManager();
+			const inner = manager as unknown as {
+				tabAgents: Map<string, Record<string, unknown>>;
+			};
+			inner.tabAgents.set("tab-busy-budget", {
+				agent: null,
+				status: "running",
+				keyId: null,
+				modelId: null,
+				taskList: { onChange: () => {} },
+				messageQueue: [],
+				queueListeners: [],
+				shellStore: {},
+				transcriptStore: {},
+				currentChunks: null,
+				currentAssistantId: null,
+				currentTurnId: null,
+				autoWakeBudget: 6,
+			});
+
+			const outcome = manager.deliverMessage("tab-busy-budget", "queued one", {
+				origin: "agent",
+			});
+			expect(outcome.status).toBe("queued");
+			// Budget untouched — queuing can't drive a runaway loop.
+			const agent = inner.tabAgents.get("tab-busy-budget") as { autoWakeBudget: number };
+			expect(agent.autoWakeBudget).toBe(6);
+		});
+
+		it("human-originated wakes are never throttled", () => {
+			const manager = new AgentManager();
+			const spy = vi.spyOn(manager, "processMessage").mockResolvedValue(undefined);
+
+			// Far more than the budget, all human-originated → all start turns.
+			for (let i = 0; i < 10; i++) {
+				const outcome = manager.deliverMessage("tab-human", `h${i}`, { origin: "human" });
+				expect(outcome.status).toBe("started");
+			}
+			expect(spy).toHaveBeenCalledTimes(10);
+		});
+
+		it("defaults origin to human when unspecified (POST /chat path)", () => {
+			const manager = new AgentManager();
+			const spy = vi.spyOn(manager, "processMessage").mockResolvedValue(undefined);
+			for (let i = 0; i < 8; i++) {
+				expect(manager.deliverMessage("tab-default", `d${i}`).status).toBe("started");
+			}
+			expect(spy).toHaveBeenCalledTimes(8);
+		});
+	});
+
+	describe("getLastTabResponse", () => {
+		it("returns the most recent assistant turn's text and current status", () => {
+			const manager = new AgentManager();
+			setFakeMessages("tab-hist", [
+				makeRow("tab-hist", 1, "user", [{ type: "text", text: "hi" }]),
+				makeRow("tab-hist", 2, "assistant", [{ type: "text", text: "first answer" }]),
+				makeRow("tab-hist", 3, "user", [{ type: "text", text: "again" }]),
+				makeRow("tab-hist", 4, "assistant", [
+					{ type: "text", text: "second " },
+					{ type: "text", text: "answer" },
+				]),
+			]);
+
+			const res = manager.getLastTabResponse("tab-hist");
+			expect(res.text).toBe("second answer");
+			expect(res.status).toBe("idle");
+		});
+
+		it("returns null text when the tab has no assistant turn yet", () => {
+			const manager = new AgentManager();
+			setFakeMessages("tab-empty", [
+				makeRow("tab-empty", 1, "user", [{ type: "text", text: "hi" }]),
+			]);
+			const res = manager.getLastTabResponse("tab-empty");
+			expect(res.text).toBeNull();
+		});
+
+		it("skips assistant turns that contain no text chunks", () => {
+			const manager = new AgentManager();
+			setFakeMessages("tab-toolonly", [
+				makeRow("tab-toolonly", 1, "assistant", [{ type: "text", text: "real answer" }]),
+				// A later assistant turn with only non-text chunks should be skipped.
+				makeRow("tab-toolonly", 2, "assistant", [{ type: "thinking", text: "hmm" }]),
+			]);
+			const res = manager.getLastTabResponse("tab-toolonly");
+			expect(res.text).toBe("real answer");
+		});
+	});
+
+	describe("send_to_tab / read_tab permission split", () => {
+		// Drives the real parent-path tool construction in getOrCreateAgentForTab
+		// by toggling the new split permissions and inspecting which tools the
+		// constructed Agent received.
+		async function toolsForPerms(tabId: string, perms: Record<string, string>): Promise<string[]> {
+			for (const [k, v] of Object.entries(perms)) setFakeSetting(k, v);
+			const manager = new AgentManager();
+			await manager.processMessage(tabId, "go");
+			return constructedAgents.at(-1)?.toolNames ?? [];
+		}
+
+		it("grants only send_to_tab when only perm_send_to_tab is allowed", async () => {
+			const tools = await toolsForPerms("tab-send-only", { perm_send_to_tab: "allow" });
+			expect(tools).toContain("send_to_tab");
+			expect(tools).not.toContain("read_tab");
+		});
+
+		it("grants only read_tab when only perm_read_tab is allowed", async () => {
+			const tools = await toolsForPerms("tab-read-only", { perm_read_tab: "allow" });
+			expect(tools).toContain("read_tab");
+			expect(tools).not.toContain("send_to_tab");
+		});
+
+		it("grants both when both permissions are allowed", async () => {
+			const tools = await toolsForPerms("tab-both", {
+				perm_send_to_tab: "allow",
+				perm_read_tab: "allow",
+			});
+			expect(tools).toContain("send_to_tab");
+			expect(tools).toContain("read_tab");
+		});
+
+		it("grants neither when both permissions are off", async () => {
+			const tools = await toolsForPerms("tab-neither", {});
+			expect(tools).not.toContain("send_to_tab");
+			expect(tools).not.toContain("read_tab");
+		});
+	});
 });
diff --git a/packages/api/tests/routes.test.ts b/packages/api/tests/routes.test.ts
index 4b8dd40..9ab2afe 100644
--- a/packages/api/tests/routes.test.ts
+++ b/packages/api/tests/routes.test.ts
@@ -166,6 +166,34 @@ vi.mock("@dispatch/core", () => ({
 		};
 	},
 	createTab() {},
+	getTab() {
+		return null;
+	},
+	listOpenTabs() {
+		return [];
+	},
+	resolveTabPrefix() {
+		return { status: "none" };
+	},
+	shortestUniquePrefix(id: string) {
+		return (id ?? "").slice(0, 4);
+	},
+	createSendToTabTool(_callbacks: unknown) {
+		return {
+			name: "send_to_tab",
+			description: "send to tab",
+			parameters: { _type: "z.ZodObject", shape: {} },
+			execute: async () => "mock",
+		};
+	},
+	createReadTabTool(_callbacks: unknown) {
+		return {
+			name: "read_tab",
+			description: "read tab",
+			parameters: { _type: "z.ZodObject", shape: {} },
+			execute: async () => "mock",
+		};
+	},
 	getClaudeAccountsFromDB() {
 		return [];
 	},
diff --git a/packages/core/src/agents/loader.ts b/packages/core/src/agents/loader.ts
index 333716e..f4a6c5a 100644
--- a/packages/core/src/agents/loader.ts
+++ b/packages/core/src/agents/loader.ts
@@ -108,8 +108,8 @@ export function expandAgentToolNames(tools: string[]): string[] {
 			default:
 				// Pass through tool names that aren't permission-group
 				// aliases (summon, retrieve, web_search, youtube_transcribe,
-				// todo, and the granular file tools themselves if a user
-				// hand-wrote them in a TOML).
+				// send_to_tab, read_tab, todo, and the granular file tools
+				// themselves if a user hand-wrote them in a TOML).
 				expanded.add(t);
 		}
 	}
diff --git a/packages/core/src/db/tabs.ts b/packages/core/src/db/tabs.ts
index 928f434..8b290d2 100644
--- a/packages/core/src/db/tabs.ts
+++ b/packages/core/src/db/tabs.ts
@@ -159,3 +159,78 @@ export function getDescendantIds(rootId: string): string[] {
 	}
 	return order.reverse();
 }
+
+/**
+ * Minimum length of a tab-handle prefix accepted by `resolveTabPrefix`.
+ * Mirrors the frontend's minimum DISPLAY length (4 hex chars). Anything
+ * shorter is rejected as too broad — an agent must echo at least the 4-char
+ * handle shown in the UI.
+ */
+export const MIN_TAB_PREFIX_LENGTH = 4;
+
+/**
+ * Outcome of resolving a short tab handle (a git-style prefix of a tab's
+ * UUID) back to a concrete open tab.
+ *
+ *  - `ok`        — exactly one open tab matched; `tab` is it.
+ *  - `none`      — no open tab matched (bad/stale handle, or too-short prefix).
+ *  - `ambiguous` — more than one open tab shares the prefix; `matches` lists
+ *                  them so the caller can ask for one more character (the same
+ *                  UX as `git checkout <ambiguous-sha>`).
+ */
+export type ResolveTabPrefixResult =
+	| { status: "ok"; tab: TabRow }
+	| { status: "none" }
+	| { status: "ambiguous"; matches: TabRow[] };
+
+/**
+ * Resolve a short tab handle to a single OPEN tab by prefix match — the
+ * git-short-hash model. The handle is NEVER stored: it is always derived from
+ * (and matched against) the canonical lowercase UUID in `tabs.id`.
+ *
+ * Sanitization is mandatory because the SQLite `LIKE` operator treats `%` and
+ * `_` as wildcards: an unsanitized prefix like `a%` would match broadly. We
+ * lowercase the input (UUIDs are canonical lowercase; SQLite `LIKE` is also
+ * ASCII-case-insensitive by default) and strip everything outside the UUID
+ * alphabet `[0-9a-f-]` so no wildcard can survive into the query.
+ *
+ * A prefix shorter than `MIN_TAB_PREFIX_LENGTH` after sanitization returns
+ * `none` rather than matching a large swath of tabs.
+ *
+ * Only OPEN tabs (`is_open = 1`) are addressable — a closed tab's UUID prefix
+ * must not cause phantom ambiguity or resolve to a dead conversation.
+ */
+export function resolveTabPrefix(prefix: string): ResolveTabPrefixResult {
+	const sanitized = (prefix ?? "").toLowerCase().replace(/[^0-9a-f-]/g, "");
+	if (sanitized.length < MIN_TAB_PREFIX_LENGTH) {
+		return { status: "none" };
+	}
+	const db = getDatabase();
+	const rows = db
+		.query("SELECT * FROM tabs WHERE is_open = 1 AND id LIKE $prefix ORDER BY position ASC")
+		.all({ $prefix: `${sanitized}%` }) as Array<Record<string, unknown>>;
+	if (rows.length === 0) return { status: "none" };
+	if (rows.length === 1) return { status: "ok", tab: rowToTab(rows[0] as Record<string, unknown>) };
+	return { status: "ambiguous", matches: rows.map(rowToTab) };
+}
+
+/**
+ * Compute the shortest unique prefix (minimum `MIN_TAB_PREFIX_LENGTH` chars)
+ * that identifies `tabId` among the currently OPEN tabs — the backend twin of
+ * the frontend's display helper. Used when a tool needs to echo a tab's own
+ * handle (e.g. provenance prefixes, "available tabs" hints) without trusting a
+ * value from the wire.
+ *
+ * Returns the full id if no shorter unique prefix exists (degenerate — only if
+ * two open tabs share an entire id, which UUID uniqueness precludes).
+ */
+export function shortestUniquePrefix(tabId: string): string {
+	const db = getDatabase();
+	const rows = db.query("SELECT id FROM tabs WHERE is_open = 1").all() as Array<{ id: string }>;
+	const others = rows.map((r) => r.id).filter((id) => id !== tabId);
+	for (let len = MIN_TAB_PREFIX_LENGTH; len < tabId.length; len++) {
+		const candidate = tabId.slice(0, len);
+		if (!others.some((id) => id.startsWith(candidate))) return candidate;
+	}
+	return tabId;
+}
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index a3816ea..b1b17cc 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -49,6 +49,10 @@ export {
 	createTab,
 	getTab,
 	listOpenTabs,
+	MIN_TAB_PREFIX_LENGTH,
+	type ResolveTabPrefixResult,
+	resolveTabPrefix,
+	shortestUniquePrefix,
 	type TabRow,
 	updateTabModel,
 	updateTabStatus,
@@ -78,9 +82,16 @@ export { prefix as bashArityPrefix } from "./tools/bash-arity.js";
 export { createListFilesTool } from "./tools/list-files.js";
 export { createReadFileTool } from "./tools/read-file.js";
 export { createReadFileSliceTool } from "./tools/read-file-slice.js";
+export { createReadTabTool, type ReadTabCallbacks } from "./tools/read-tab.js";
 export { createToolRegistry } from "./tools/registry.js";
 export { createRetrieveTool, type RetrieveCallbacks } from "./tools/retrieve.js";
 export { BackgroundShellStore, createRunShellTool } from "./tools/run-shell.js";
+export {
+	createSendToTabTool,
+	type ResolvedTabRef,
+	type SendToTabCallbacks,
+	type TabResolution,
+} from "./tools/send-to-tab.js";
 export { analyzeCommand } from "./tools/shell-analyze.js";
 export {
 	type AvailableAgent,
diff --git a/packages/core/src/llm/debug-logger.ts b/packages/core/src/llm/debug-logger.ts
index 2b7420c..072a7a1 100644
--- a/packages/core/src/llm/debug-logger.ts
+++ b/packages/core/src/llm/debug-logger.ts
@@ -281,11 +281,7 @@ export function logStepLifecycle(data: {
  * Log agent loop-level events (loop start, break conditions, etc.).
  * Only logs at verbosity >= 3.
  */
-export function logAgentLoop(data: {
-	tabId?: string;
-	event: string;
-	detail?: unknown;
-}): void {
+export function logAgentLoop(data: { tabId?: string; event: string; detail?: unknown }): void {
 	if (!ENABLED || VERBOSITY < 3) return;
 	const detail = data.detail !== undefined ? ` ${JSON.stringify(data.detail)}` : "";
 	console.error(`[dispatch-debug] AGENT tab=${data.tabId ?? "?"} ${data.event}${detail}`);
@@ -308,16 +304,14 @@ export function logAgentLoop(data: {
  * we just clone and read once via `.text()` — simpler and safe because
  * non-streaming bodies are bounded.
  */
-export function wrapFetchWithLogging<
-	F extends (...args: never[]) => Promise<Response> | Response,
->(baseFetch: F, opts: { tabId?: string; modelHint?: string }): F {
+export function wrapFetchWithLogging<F extends (...args: never[]) => Promise<Response> | Response>(
+	baseFetch: F,
+	opts: { tabId?: string; modelHint?: string },
+): F {
 	if (!ENABLED) return baseFetch;
 	const wrapped = async (...args: Parameters<F>) => {
 		const requestId = ++seq;
-		const [input, init] = args as unknown as [
-			RequestInfo | URL,
-			RequestInit | undefined,
-		];
+		const [input, init] = args as unknown as [RequestInfo | URL, RequestInit | undefined];
 		const url =
 			typeof input === "string"
 				? input
@@ -325,7 +319,8 @@ export function wrapFetchWithLogging<
 					? input.toString()
 					: (input as Request).url;
 		const method =
-			init?.method ?? (typeof input === "object" && "method" in input ? (input as Request).method : "POST");
+			init?.method ??
+			(typeof input === "object" && "method" in input ? (input as Request).method : "POST");
 
 		// Snapshot headers as a plain object for logging.
 		const headerObj: Record<string, string> = {};
diff --git a/packages/core/src/tools/read-tab.ts b/packages/core/src/tools/read-tab.ts
new file mode 100644
index 0000000..e80dbd0
--- /dev/null
+++ b/packages/core/src/tools/read-tab.ts
@@ -0,0 +1,95 @@
+import { z } from "zod";
+import type { AgentStatus, ToolDefinition } from "../types/index.js";
+import type { TabResolution } from "./send-to-tab.js";
+
+export interface ReadTabCallbacks {
+	/** Resolve a (possibly short) handle to one open tab. */
+	resolveShortId(prefix: string): TabResolution;
+	/**
+	 * Return the target tab's most recent COMPLETED assistant turn as plain
+	 * text, plus its current status. `text` is null when the tab has no
+	 * completed assistant turn yet.
+	 */
+	getLastResponse(tabId: string): { text: string | null; status: AgentStatus };
+	/** Snapshot of currently-open tabs, for "available tabs" error hints. */
+	listOpenHandles(): Array<{ handle: string; title: string }>;
+}
+
+/** Render the "available tabs" hint shared by the none/ambiguous branches. */
+function renderOpenHandles(handles: Array<{ handle: string; title: string }>): string {
+	if (handles.length === 0) return "No other tabs are currently open.";
+	const lines = handles.map((h) => `  - ${h.handle}: ${h.title}`);
+	return ["Currently open tabs:", ...lines].join("\n");
+}
+
+export function createReadTabTool(callbacks: ReadTabCallbacks): ToolDefinition {
+	return {
+		name: "read_tab",
+		description: [
+			"Read the most recent completed response from another tab (agent) by its short ID.",
+			"",
+			"Returns a SNAPSHOT — it does NOT block or wait for the target to finish.",
+			"  - If the target is idle, you get its just-finished turn.",
+			"  - If the target is still running, you get its PREVIOUS completed turn (if any);",
+			"    call read_tab again later to get the newest one.",
+			"",
+			"Use this after send_to_tab to collect another agent's reply. IDs are git-style",
+			"prefixes: pass any length that uniquely identifies the target (min 4 chars).",
+		].join("\n"),
+		parameters: z.object({
+			tab_id: z
+				.string()
+				.describe(
+					"The short ID (handle) of the tab to read, as shown in the tab bar. Any unique-length prefix works (min 4 chars).",
+				),
+		}),
+		execute: async (args: Record<string, unknown>): Promise<string> => {
+			const rawId = (args.tab_id as string | undefined)?.trim() ?? "";
+
+			if (!rawId) {
+				return `Error: tab_id is required.\n\n${renderOpenHandles(callbacks.listOpenHandles())}`;
+			}
+
+			const resolution = callbacks.resolveShortId(rawId);
+
+			if (resolution.status === "none") {
+				return [
+					`Error: no open tab matches the ID "${rawId}".`,
+					"",
+					renderOpenHandles(callbacks.listOpenHandles()),
+				].join("\n");
+			}
+			if (resolution.status === "ambiguous") {
+				const matches = resolution.matches.map((m) => `  - ${m.handle}: ${m.title}`).join("\n");
+				return [
+					`Error: the ID "${rawId}" is ambiguous — it matches multiple open tabs:`,
+					matches,
+					"",
+					"Add one or more characters to disambiguate.",
+				].join("\n");
+			}
+
+			const target = resolution.tab;
+			const { text, status } = callbacks.getLastResponse(target.id);
+
+			const runningNote =
+				status === "running"
+					? " (this tab is still running; the response below is its previous completed turn — read again later for the newest)"
+					: "";
+
+			if (text === null) {
+				const reason =
+					status === "running"
+						? "it is still working on its first turn"
+						: "it has no assistant responses yet";
+				return `Tab ${target.handle} (${target.title}) has no completed response — ${reason}.`;
+			}
+
+			return [
+				`<tab_response tab="${target.handle}" status="${status}"${runningNote}>`,
+				text,
+				"</tab_response>",
+			].join("\n");
+		},
+	};
+}
diff --git a/packages/core/src/tools/send-to-tab.ts b/packages/core/src/tools/send-to-tab.ts
new file mode 100644
index 0000000..eb86b7e
--- /dev/null
+++ b/packages/core/src/tools/send-to-tab.ts
@@ -0,0 +1,147 @@
+import { z } from "zod";
+import type { ToolDefinition } from "../types/index.js";
+
+/**
+ * A tab reference surfaced to the `send_to_tab` / `read_tab` tools. The tools
+ * are intentionally decoupled from the DB `TabRow` shape — the AgentManager
+ * maps `resolveTabPrefix(...)` results down to this minimal projection so the
+ * tools (and their unit tests) never depend on the persistence layer.
+ */
+export interface ResolvedTabRef {
+	/** The tab's canonical full UUID. */
+	id: string;
+	/** The tab's display title (for disambiguation hints). */
+	title: string;
+	/** The tab's current short handle (shortest unique prefix). */
+	handle: string;
+}
+
+/**
+ * Outcome of resolving a short tab handle. Mirrors core's
+ * `ResolveTabPrefixResult` but over the minimal `ResolvedTabRef` projection.
+ */
+export type TabResolution =
+	| { status: "ok"; tab: ResolvedTabRef }
+	| { status: "none" }
+	| { status: "ambiguous"; matches: ResolvedTabRef[] };
+
+export interface SendToTabCallbacks {
+	/** Resolve a (possibly short) handle to one open tab. */
+	resolveShortId(prefix: string): TabResolution;
+	/**
+	 * Deliver `message` to `tabId`. If the target is mid-turn the message is
+	 * queued (same path as a user message); if idle/errored it wakes the tab
+	 * and starts a new turn. Returns quickly — does NOT block on the turn.
+	 */
+	deliver(
+		tabId: string,
+		message: string,
+	):
+		| Promise<{ status: "queued" | "started" | "suppressed" }>
+		| { status: "queued" | "started" | "suppressed" };
+	/** Snapshot of currently-open tabs, for "available tabs" error hints. */
+	listOpenHandles(): Array<{ handle: string; title: string }>;
+	/** The calling tab's own id + handle — used to block self-sends and to
+	 *  stamp provenance onto the delivered message. */
+	self: { id: string; handle: string };
+}
+
+/** Render the "available tabs" hint shared by the none/ambiguous branches. */
+function renderOpenHandles(handles: Array<{ handle: string; title: string }>): string {
+	if (handles.length === 0) return "No other tabs are currently open.";
+	const lines = handles.map((h) => `  - ${h.handle}: ${h.title}`);
+	return ["Currently open tabs:", ...lines].join("\n");
+}
+
+export function createSendToTabTool(callbacks: SendToTabCallbacks): ToolDefinition {
+	return {
+		name: "send_to_tab",
+		description: [
+			"Send a message to another tab (agent) by its short ID — the handle shown in the tab bar.",
+			"",
+			"Behaviour mirrors a user sending a message:",
+			"  - If the target tab is mid-turn (busy), your message is QUEUED and picked up next.",
+			"  - If the target tab is idle, your message WAKES it and starts a new turn.",
+			"",
+			"This is fire-and-forget: it returns immediately and does NOT wait for a reply.",
+			"Use the 'read_tab' tool with the same ID later to read the target's latest response.",
+			"",
+			"Your tab ID is auto-added to the top of the message so the recipient can reply to you.",
+			"IDs are git-style prefixes: pass any length that uniquely identifies the target (min 4 chars).",
+			"If the ID is ambiguous you'll be asked to add a character.",
+		].join("\n"),
+		parameters: z.object({
+			tab_id: z
+				.string()
+				.describe(
+					"The short ID (handle) of the target tab, as shown in the tab bar. Any unique-length prefix of the tab's id works (min 4 chars).",
+				),
+			message: z
+				.string()
+				.describe("The message to deliver to the target tab, exactly as a user would type it."),
+		}),
+		execute: async (args: Record<string, unknown>): Promise<string> => {
+			const rawId = (args.tab_id as string | undefined)?.trim() ?? "";
+			const message = (args.message as string | undefined) ?? "";
+
+			if (!rawId) {
+				return `Error: tab_id is required.\n\n${renderOpenHandles(callbacks.listOpenHandles())}`;
+			}
+			if (!message.trim()) {
+				return "Error: message must not be empty.";
+			}
+
+			const resolution = callbacks.resolveShortId(rawId);
+
+			if (resolution.status === "none") {
+				return [
+					`Error: no open tab matches the ID "${rawId}".`,
+					"",
+					renderOpenHandles(callbacks.listOpenHandles()),
+				].join("\n");
+			}
+			if (resolution.status === "ambiguous") {
+				const matches = resolution.matches.map((m) => `  - ${m.handle}: ${m.title}`).join("\n");
+				return [
+					`Error: the ID "${rawId}" is ambiguous — it matches multiple open tabs:`,
+					matches,
+					"",
+					"Add one or more characters to disambiguate.",
+				].join("\n");
+			}
+
+			const target = resolution.tab;
+
+			if (target.id === callbacks.self.id) {
+				return "Error: cannot send a message to your own tab.";
+			}
+
+			// Stamp provenance so the recipient (and the watching user) can see
+			// which tab the message came from and reply back via its handle.
+			const delivered = `[message from tab ${callbacks.self.handle}]\n\n${message}`;
+
+			try {
+				const result = await callbacks.deliver(target.id, delivered);
+				if (result.status === "suppressed") {
+					// The target hit its automatic agent-to-agent wake limit. The
+					// message was preserved (queued) but did NOT start a turn — a
+					// human must step in. Tell the sender plainly so it stops
+					// hammering the target and creating a runaway loop.
+					return [
+						`Message HELD for tab ${target.handle} (${target.title}) — it was NOT delivered as a wake.`,
+						`That tab has reached its automatic agent-to-agent message limit, so it will not`,
+						`auto-respond again until a human sends it a message. Do not keep resending:`,
+						`your message is already queued and will be seen when a human resumes that tab.`,
+					].join("\n");
+				}
+				const verb =
+					result.status === "queued"
+						? "queued (target is busy; it will be picked up next turn)"
+						: "delivered (target was idle; a new turn has started)";
+				return `Message ${verb}. Target tab: ${target.handle} (${target.title}). Use read_tab with "${target.handle}" to read its reply later.`;
+			} catch (err) {
+				return `Error delivering message: ${err instanceof Error ? err.message : String(err)}`;
+			}
+		},
+	};
+}
diff --git a/packages/core/src/tools/summon.ts b/packages/core/src/tools/summon.ts
index d40f0ca..4820e89 100644
--- a/packages/core/src/tools/summon.ts
+++ b/packages/core/src/tools/summon.ts
@@ -177,6 +177,8 @@ export function createSummonTool(
 		"  - retrieve: Collect results from its children (required if summon is given)",
 		"  - web_search: Search the web",
 		"  - youtube_transcribe: Fetch YouTube video transcripts",
+		"  - send_to_tab: Send a message to another tab/agent by its ID",
+		"  - read_tab: Read another tab/agent's latest response by its ID",
 		"",
 		"The 'agent' parameter is required — every spawned agent must use a definition.",
 		"Tools default to the agent definition's tools, intersected with your own tools (you can't grant capabilities you don't have).",
@@ -232,6 +234,8 @@ export function createSummonTool(
 					"retrieve",
 					"web_search",
 					"youtube_transcribe",
+					"send_to_tab",
+					"read_tab",
 				]),
 			)
 			.optional()
@@ -262,7 +266,9 @@ export function createSummonTool(
 			const tools = args.tools as string[] | undefined;
 			const workingDirectory = args.working_directory as string | undefined;
 			const background = (args.background as boolean | undefined) ?? false;
-			const topLevel = userAgentEnabled ? ((args.top_level as boolean | undefined) ?? false) : false;
+			const topLevel = userAgentEnabled
+				? ((args.top_level as boolean | undefined) ?? false)
+				: false;
 
 			try {
 				const agentId = await callbacks.spawn({
diff --git a/packages/core/tests/agents/loader.test.ts b/packages/core/tests/agents/loader.test.ts
index 88173ea..35ab0cf 100644
--- a/packages/core/tests/agents/loader.test.ts
+++ b/packages/core/tests/agents/loader.test.ts
@@ -22,10 +22,34 @@ describe("expandAgentToolNames", () => {
 		expect(out).toContain("run_shell");
 	});
 
+	it("passes through the tab tools as independent names (no tab_comm group)", () => {
+		const out = expandAgentToolNames(["send_to_tab", "read_tab"]);
+		expect(out).toContain("send_to_tab");
+		expect(out).toContain("read_tab");
+		// Granting only one must not pull in the other.
+		const onlySend = expandAgentToolNames(["send_to_tab"]);
+		expect(onlySend).toContain("send_to_tab");
+		expect(onlySend).not.toContain("read_tab");
+	});
+
 	it("passes through non-group tool names unchanged", () => {
-		const out = expandAgentToolNames(["summon", "retrieve", "web_search", "youtube_transcribe"]);
+		const out = expandAgentToolNames([
+			"summon",
+			"retrieve",
+			"web_search",
+			"youtube_transcribe",
+			"send_to_tab",
+			"read_tab",
+		]);
 		expect(out).toEqual(
-			expect.arrayContaining(["summon", "retrieve", "web_search", "youtube_transcribe"]),
+			expect.arrayContaining([
+				"summon",
+				"retrieve",
+				"web_search",
+				"youtube_transcribe",
+				"send_to_tab",
+				"read_tab",
+			]),
 		);
 	});
 
diff --git a/packages/core/tests/db/tabs.test.ts b/packages/core/tests/db/tabs.test.ts
index e1c9bf8..67533dc 100644
--- a/packages/core/tests/db/tabs.test.ts
+++ b/packages/core/tests/db/tabs.test.ts
@@ -73,6 +73,22 @@ class FakeDatabase {
 			return [{ max_pos: maxPos }];
 		}
 
+		// resolveTabPrefix: open tabs whose id starts with a sanitized prefix.
+		// The production query binds `$prefix` as `<sanitized>%`; emulate SQLite
+		// LIKE prefix semantics here (case-insensitive, `%` = "rest of string").
+		if (norm === "SELECT * FROM tabs WHERE is_open = 1 AND id LIKE $prefix ORDER BY position ASC") {
+			const raw = String(params?.$prefix ?? "");
+			const needle = raw.endsWith("%") ? raw.slice(0, -1) : raw;
+			return this.rows
+				.filter((r) => r.is_open === 1 && r.id.toLowerCase().startsWith(needle.toLowerCase()))
+				.sort((a, b) => a.position - b.position);
+		}
+
+		// shortestUniquePrefix: all open tab ids.
+		if (norm === "SELECT id FROM tabs WHERE is_open = 1") {
+			return this.rows.filter((r) => r.is_open === 1).map((r) => ({ id: r.id }));
+		}
+
 		throw new Error(`FakeDatabase: unsupported SELECT: ${norm}`);
 	}
 
@@ -134,7 +150,8 @@ vi.mock("../../src/db/index.js", () => ({
 // Dynamic import AFTER `vi.mock` registers (vitest hoists `vi.mock` to
 // the very top of the file, so by the time this line runs the mock is
 // active for `./index.js` resolution inside `tabs.ts`).
-const { archiveTab, createTab, getDescendantIds, getTab } = await import("../../src/db/tabs.js");
+const { archiveTab, createTab, getDescendantIds, getTab, resolveTabPrefix, shortestUniquePrefix } =
+	await import("../../src/db/tabs.js");
 
 beforeAll(() => {
 	fakeDb = new FakeDatabase();
@@ -234,3 +251,103 @@ describe("getDescendantIds", () => {
 		expect(ids2).toEqual(["b1", "a1"]);
 	});
 });
+
+// ---------------------------------------------------------------------------
+// resolveTabPrefix — git-style short-handle resolution
+// ---------------------------------------------------------------------------
+describe("resolveTabPrefix", () => {
+	it("returns none when the prefix is shorter than the minimum length", () => {
+		createTab("abcd1234-0000-4000-8000-000000000000", "A");
+		// 3 chars < MIN_TAB_PREFIX_LENGTH (4)
+		expect(resolveTabPrefix("abc").status).toBe("none");
+	});
+
+	it("returns none when no open tab matches", () => {
+		createTab("abcd1234-0000-4000-8000-000000000000", "A");
+		expect(resolveTabPrefix("ffff").status).toBe("none");
+	});
+
+	it("resolves a unique 4-char prefix to the single matching tab", () => {
+		createTab("abcd1234-0000-4000-8000-000000000000", "Alpha");
+		createTab("9999aaaa-0000-4000-8000-000000000000", "Beta");
+		const res = resolveTabPrefix("abcd");
+		expect(res.status).toBe("ok");
+		if (res.status === "ok") {
+			expect(res.tab.id).toBe("abcd1234-0000-4000-8000-000000000000");
+			expect(res.tab.title).toBe("Alpha");
+		}
+	});
+
+	it("resolves the full UUID (a maximal prefix)", () => {
+		createTab("abcd1234-0000-4000-8000-000000000000", "Alpha");
+		const res = resolveTabPrefix("abcd1234-0000-4000-8000-000000000000");
+		expect(res.status).toBe("ok");
+	});
+
+	it("reports ambiguity when multiple open tabs share the prefix", () => {
+		createTab("abcd1111-0000-4000-8000-000000000000", "One");
+		createTab("abcd2222-0000-4000-8000-000000000000", "Two");
+		const res = resolveTabPrefix("abcd");
+		expect(res.status).toBe("ambiguous");
+		if (res.status === "ambiguous") {
+			expect(res.matches).toHaveLength(2);
+			expect(res.matches.map((m) => m.title).sort()).toEqual(["One", "Two"]);
+		}
+	});
+
+	it("disambiguates when one more character is supplied", () => {
+		createTab("abcd1111-0000-4000-8000-000000000000", "One");
+		createTab("abcd2222-0000-4000-8000-000000000000", "Two");
+		const res = resolveTabPrefix("abcd1");
+		expect(res.status).toBe("ok");
+		if (res.status === "ok") expect(res.tab.title).toBe("One");
+	});
+
+	it("matches case-insensitively (UUIDs are lowercase; LIKE is ASCII-CI)", () => {
+		createTab("abcd1234-0000-4000-8000-000000000000", "Alpha");
+		const res = resolveTabPrefix("ABCD");
+		expect(res.status).toBe("ok");
+	});
+
+	it("sanitizes LIKE wildcards so they cannot broaden the match", () => {
+		createTab("abcd1234-0000-4000-8000-000000000000", "Alpha");
+		createTab("9999aaaa-0000-4000-8000-000000000000", "Beta");
+		// `%` would match everything if not stripped; after sanitization the
+		// query is effectively `abcd%` which matches only Alpha.
+		const res = resolveTabPrefix("ab%d");
+		// "ab%d" -> sanitized "abd" (3 chars) -> below min length -> none.
+		expect(res.status).toBe("none");
+	});
+
+	it("excludes archived (closed) tabs from matches", () => {
+		createTab("abcd1234-0000-4000-8000-000000000000", "Alpha");
+		archiveTab("abcd1234-0000-4000-8000-000000000000");
+		expect(resolveTabPrefix("abcd").status).toBe("none");
+	});
+});
+
+// ---------------------------------------------------------------------------
+// shortestUniquePrefix — display-handle derivation
+// ---------------------------------------------------------------------------
+describe("shortestUniquePrefix", () => {
+	it("returns a 4-char prefix when no other open tab collides", () => {
+		createTab("abcd1234-0000-4000-8000-000000000000", "Alpha");
+		expect(shortestUniquePrefix("abcd1234-0000-4000-8000-000000000000")).toBe("abcd");
+	});
+
+	it("grows the prefix one char at a time on a collision", () => {
+		createTab("abcd1111-0000-4000-8000-000000000000", "One");
+		createTab("abcd2222-0000-4000-8000-000000000000", "Two");
+		// First differing char is at index 4, so a 5-char prefix is unique.
+		expect(shortestUniquePrefix("abcd1111-0000-4000-8000-000000000000")).toBe("abcd1");
+		expect(shortestUniquePrefix("abcd2222-0000-4000-8000-000000000000")).toBe("abcd2");
+	});
+
+	it("ignores closed tabs when computing uniqueness", () => {
+		createTab("abcd1111-0000-4000-8000-000000000000", "One");
+		createTab("abcd2222-0000-4000-8000-000000000000", "Two");
+		archiveTab("abcd2222-0000-4000-8000-000000000000");
+		// With Two closed, One no longer collides → back to 4 chars.
+		expect(shortestUniquePrefix("abcd1111-0000-4000-8000-000000000000")).toBe("abcd");
+	});
+});
diff --git a/packages/core/tests/tools/read-tab.test.ts b/packages/core/tests/tools/read-tab.test.ts
new file mode 100644
index 0000000..71e419c
--- /dev/null
+++ b/packages/core/tests/tools/read-tab.test.ts
@@ -0,0 +1,101 @@
+import { describe, expect, it } from "vitest";
+import { createReadTabTool, type ReadTabCallbacks } from "../../src/tools/read-tab.js";
+import type { TabResolution } from "../../src/tools/send-to-tab.js";
+
+function makeCallbacks(overrides: Partial<ReadTabCallbacks> = {}): ReadTabCallbacks {
+	return {
+		resolveShortId: (): TabResolution => ({
+			status: "ok",
+			tab: { id: "target-id", title: "Target", handle: "targ" },
+		}),
+		getLastResponse: () => ({ text: "the answer is 42", status: "idle" }),
+		listOpenHandles: () => [{ handle: "targ", title: "Target" }],
+		...overrides,
+	};
+}
+
+describe("createReadTabTool — schema & description", () => {
+	it("is a non-blocking snapshot read", () => {
+		const tool = createReadTabTool(makeCallbacks());
+		expect(tool.name).toBe("read_tab");
+		expect(tool.description).toContain("SNAPSHOT");
+		expect(tool.description.toLowerCase()).toContain("does not block");
+	});
+});
+
+describe("createReadTabTool — execute()", () => {
+	it("returns the last assistant response wrapped in a tab_response tag", async () => {
+		const tool = createReadTabTool(makeCallbacks());
+		const out = await tool.execute({ tab_id: "targ" });
+		expect(out).toContain("<tab_response");
+		expect(out).toContain('tab="targ"');
+		expect(out).toContain('status="idle"');
+		expect(out).toContain("the answer is 42");
+		expect(out).toContain("</tab_response>");
+	});
+
+	it("notes that a running tab's response is its previous completed turn", async () => {
+		const tool = createReadTabTool(
+			makeCallbacks({
+				getLastResponse: () => ({ text: "older turn", status: "running" }),
+			}),
+		);
+		const out = await tool.execute({ tab_id: "targ" });
+		expect(out).toContain("still running");
+		expect(out).toContain("older turn");
+	});
+
+	it("explains when a tab has no completed response yet (idle)", async () => {
+		const tool = createReadTabTool(
+			makeCallbacks({
+				getLastResponse: () => ({ text: null, status: "idle" }),
+			}),
+		);
+		const out = await tool.execute({ tab_id: "targ" });
+		expect(out).toContain("no completed response");
+		expect(out).toContain("no assistant responses yet");
+	});
+
+	it("explains when a tab is still on its first turn (running, no prior text)", async () => {
+		const tool = createReadTabTool(
+			makeCallbacks({
+				getLastResponse: () => ({ text: null, status: "running" }),
+			}),
+		);
+		const out = await tool.execute({ tab_id: "targ" });
+		expect(out).toContain("no completed response");
+		expect(out).toContain("still working on its first turn");
+	});
+
+	it("rejects an empty tab_id and lists open handles", async () => {
+		const tool = createReadTabTool(makeCallbacks());
+		const out = await tool.execute({ tab_id: "" });
+		expect(out).toContain("Error");
+		expect(out).toContain("targ");
+	});
+
+	it("returns a helpful error when the id is unknown", async () => {
+		const tool = createReadTabTool(makeCallbacks({ resolveShortId: () => ({ status: "none" }) }));
+		const out = await tool.execute({ tab_id: "zzzz" });
+		expect(out).toContain("no open tab matches");
+		expect(out).toContain("Currently open tabs:");
+	});
+
+	it("asks for more characters when the id is ambiguous", async () => {
+		const tool = createReadTabTool(
+			makeCallbacks({
+				resolveShortId: () => ({
+					status: "ambiguous",
+					matches: [
+						{ id: "a1", title: "One", handle: "abcd1" },
+						{ id: "a2", title: "Two", handle: "abcd2" },
+					],
+				}),
+			}),
+		);
+		const out = await tool.execute({ tab_id: "abcd" });
+		expect(out).toContain("ambiguous");
+		expect(out).toContain("abcd1");
+		expect(out).toContain("abcd2");
+	});
+});
diff --git a/packages/core/tests/tools/send-to-tab.test.ts b/packages/core/tests/tools/send-to-tab.test.ts
new file mode 100644
index 0000000..4450fc5
--- /dev/null
+++ b/packages/core/tests/tools/send-to-tab.test.ts
@@ -0,0 +1,142 @@
+import { describe, expect, it, vi } from "vitest";
+import {
+	createSendToTabTool,
+	type SendToTabCallbacks,
+	type TabResolution,
+} from "../../src/tools/send-to-tab.js";
+
+function makeCallbacks(overrides: Partial<SendToTabCallbacks> = {}): SendToTabCallbacks {
+	return {
+		resolveShortId: (): TabResolution => ({
+			status: "ok",
+			tab: { id: "target-id", title: "Target", handle: "targ" },
+		}),
+		deliver: () => ({ status: "started" }),
+		listOpenHandles: () => [{ handle: "targ", title: "Target" }],
+		self: { id: "self-id", handle: "self" },
+		...overrides,
+	};
+}
+
+describe("createSendToTabTool — schema & description", () => {
+	it("exposes tab_id and message params and a fire-and-forget description", () => {
+		const tool = createSendToTabTool(makeCallbacks());
+		expect(tool.name).toBe("send_to_tab");
+		expect(tool.description).toContain("fire-and-forget");
+		expect(tool.description.toLowerCase()).toContain("queued");
+	});
+});
+
+describe("createSendToTabTool — execute()", () => {
+	it("delivers to a resolved target and reports the started status", async () => {
+		const deliver = vi.fn(() => ({ status: "started" as const }));
+		const tool = createSendToTabTool(makeCallbacks({ deliver }));
+		const out = await tool.execute({ tab_id: "targ", message: "hello there" });
+		expect(deliver).toHaveBeenCalledTimes(1);
+		const [targetId, delivered] = deliver.mock.calls[0] ?? [];
+		expect(targetId).toBe("target-id");
+		// Provenance prefix names the sending tab's handle.
+		expect(delivered).toContain("[message from tab self]");
+		expect(delivered).toContain("hello there");
+		expect(out).toContain("idle");
+		expect(out).toContain("targ");
+	});
+
+	it("reports the queued status when the target is busy", async () => {
+		const deliver = vi.fn(() => ({ status: "queued" as const }));
+		const tool = createSendToTabTool(makeCallbacks({ deliver }));
+		const out = await tool.execute({ tab_id: "targ", message: "ping" });
+		expect(out.toLowerCase()).toContain("queued");
+		expect(out.toLowerCase()).toContain("busy");
+	});
+
+	it("reports a HELD message when delivery is suppressed (auto-wake limit hit)", async () => {
+		const deliver = vi.fn(() => ({ status: "suppressed" as const }));
+		const tool = createSendToTabTool(makeCallbacks({ deliver }));
+		const out = await tool.execute({ tab_id: "targ", message: "ping again" });
+		expect(out).toContain("HELD");
+		expect(out.toLowerCase()).toContain("limit");
+		// It must steer the sender away from retrying in a loop.
+		expect(out.toLowerCase()).toContain("do not keep resending");
+		expect(out.toLowerCase()).toContain("human");
+	});
+
+	it("rejects an empty tab_id and lists open handles", async () => {
+		const tool = createSendToTabTool(makeCallbacks());
+		const out = await tool.execute({ tab_id: "  ", message: "hi" });
+		expect(out).toContain("Error");
+		expect(out).toContain("targ");
+	});
+
+	it("rejects an empty message", async () => {
+		const deliver = vi.fn(() => ({ status: "started" as const }));
+		const tool = createSendToTabTool(makeCallbacks({ deliver }));
+		const out = await tool.execute({ tab_id: "targ", message: "   " });
+		expect(out).toContain("Error");
+		expect(deliver).not.toHaveBeenCalled();
+	});
+
+	it("returns a helpful error and open-tab list when the id is unknown", async () => {
+		const deliver = vi.fn(() => ({ status: "started" as const }));
+		const tool = createSendToTabTool(
+			makeCallbacks({
+				resolveShortId: () => ({ status: "none" }),
+				deliver,
+			}),
+		);
+		const out = await tool.execute({ tab_id: "zzzz", message: "hi" });
+		expect(out).toContain("no open tab matches");
+		expect(out).toContain("Currently open tabs:");
+		expect(deliver).not.toHaveBeenCalled();
+	});
+
+	it("asks for more characters when the id is ambiguous", async () => {
+		const deliver = vi.fn(() => ({ status: "started" as const }));
+		const tool = createSendToTabTool(
+			makeCallbacks({
+				resolveShortId: () => ({
+					status: "ambiguous",
+					matches: [
+						{ id: "a1", title: "One", handle: "abcd1" },
+						{ id: "a2", title: "Two", handle: "abcd2" },
+					],
+				}),
+				deliver,
+			}),
+		);
+		const out = await tool.execute({ tab_id: "abcd", message: "hi" });
+		expect(out).toContain("ambiguous");
+		expect(out).toContain("abcd1");
+		expect(out).toContain("abcd2");
+		expect(deliver).not.toHaveBeenCalled();
+	});
+
+	it("refuses to send to its own tab", async () => {
+		const deliver = vi.fn(() => ({ status: "started" as const }));
+		const tool = createSendToTabTool(
+			makeCallbacks({
+				resolveShortId: () => ({
+					status: "ok",
+					tab: { id: "self-id", title: "Me", handle: "self" },
+				}),
+				deliver,
+			}),
+		);
+		const out = await tool.execute({ tab_id: "self", message: "hi" });
+		expect(out).toContain("cannot send a message to your own tab");
+		expect(deliver).not.toHaveBeenCalled();
+	});
+
+	it("surfaces a thrown delivery error instead of crashing", async () => {
+		const tool = createSendToTabTool(
+			makeCallbacks({
+				deliver: () => {
+					throw new Error("boom");
+				},
+			}),
+		);
+		const out = await tool.execute({ tab_id: "targ", message: "hi" });
+		expect(out).toContain("Error delivering message");
+		expect(out).toContain("boom");
+	});
+});
diff --git a/packages/core/tests/tools/summon.test.ts b/packages/core/tests/tools/summon.test.ts
index 6597c21..f59f345 100644
--- a/packages/core/tests/tools/summon.test.ts
+++ b/packages/core/tests/tools/summon.test.ts
@@ -39,9 +39,13 @@ describe("createSummonTool — description content", () => {
 				path: "/home/u/.config/dispatch/agents/researcher.toml",
 			},
 		];
-		const tool = createSummonTool("/tmp/work", noopCallbacks, agents, [], [
-			"/home/u/.config/dispatch/agents",
-		]);
+		const tool = createSummonTool(
+			"/tmp/work",
+			noopCallbacks,
+			agents,
+			[],
+			["/home/u/.config/dispatch/agents"],
+		);
 		expect(tool.description).toContain("programmer");
 		expect(tool.description).toContain("Programmer");
 		expect(tool.description).toContain("Implements code from a plan");
diff --git a/packages/frontend/src/lib/components/TabBar.svelte b/packages/frontend/src/lib/components/TabBar.svelte
index feb1e5b..3cbd849 100644
--- a/packages/frontend/src/lib/components/TabBar.svelte
+++ b/packages/frontend/src/lib/components/TabBar.svelte
@@ -56,6 +56,7 @@ const activeUserTabId = $derived(
 			>
 				<span class="flex items-center gap-1.5">
 					<span class="w-1.5 h-1.5 rounded-full shrink-0 {statusColor(tab.agentStatus)}"></span>
+					<span class="font-mono text-[10px] px-1 py-0.5 rounded bg-base-300 text-base-content/60 shrink-0" title="Tab ID — agents address this tab by this handle">{tabStore.shortHandleFor(tab.id)}</span>
 					<span class="max-w-32 truncate text-xs">{tab.title}</span>
 				</span>
 				<button
@@ -89,6 +90,7 @@ const activeUserTabId = $derived(
 				>
 					<span class="flex items-center gap-1">
 						<span class="w-1 h-1 rounded-full shrink-0 {statusColor(tab.agentStatus)}"></span>
+						<span class="font-mono text-[10px] px-1 rounded bg-base-300 text-base-content/60 shrink-0" title="Tab ID — agents address this tab by this handle">{tabStore.shortHandleFor(tab.id)}</span>
 						<span class="max-w-28 truncate text-xs">{tab.title}</span>
 					</span>
 					{#if tab.persistent}
diff --git a/packages/frontend/src/lib/components/ToolPermissions.svelte b/packages/frontend/src/lib/components/ToolPermissions.svelte
index d64eaf9..0caf0fa 100644
--- a/packages/frontend/src/lib/components/ToolPermissions.svelte
+++ b/packages/frontend/src/lib/components/ToolPermissions.svelte
@@ -28,6 +28,16 @@ const toolPermissions: ToolPermission[] = [
 		description: "Allow the AI to open new independent top-level tabs",
 	},
 	{
+		id: "send_to_tab",
+		label: "Message other tabs",
+		description: "Allow the AI to send messages to other tabs by their ID",
+	},
+	{
+		id: "read_tab",
+		label: "Read other tabs",
+		description: "Allow the AI to read other tabs' latest responses by their ID",
+	},
+	{
 		id: "web_search",
 		label: "Web search",
 		description: "Allow the AI to search the web via Firecrawl",
diff --git a/packages/frontend/src/lib/settings.svelte.ts b/packages/frontend/src/lib/settings.svelte.ts
index 11eb79c..123008d 100644
--- a/packages/frontend/src/lib/settings.svelte.ts
+++ b/packages/frontend/src/lib/settings.svelte.ts
@@ -9,6 +9,8 @@ let toolPerms = $state<Record<string, boolean>>({
 	bash: false,
 	summon: false,
 	user_agent: false,
+	send_to_tab: false,
+	read_tab: false,
 	external_directory: false,
 	web_search: false,
 	youtube_transcribe: false,
@@ -19,6 +21,8 @@ let savedToolPerms = $state<Record<string, boolean>>({
 	bash: false,
 	summon: false,
 	user_agent: false,
+	send_to_tab: false,
+	read_tab: false,
 	external_directory: false,
 	web_search: false,
 	youtube_transcribe: false,
diff --git a/packages/frontend/src/lib/tabs.svelte.ts b/packages/frontend/src/lib/tabs.svelte.ts
index e303c80..317de8d 100644
--- a/packages/frontend/src/lib/tabs.svelte.ts
+++ b/packages/frontend/src/lib/tabs.svelte.ts
@@ -232,6 +232,29 @@ export function createTabStore() {
 		return tabs.find((t) => t.id === id);
 	}
 
+	/**
+	 * Minimum display length of a tab handle (git-style short id). Mirrors
+	 * `MIN_TAB_PREFIX_LENGTH` in core's `db/tabs.ts` so the handle the user sees
+	 * is always resolvable by the backend's `resolveTabPrefix`.
+	 */
+	const MIN_HANDLE_LENGTH = 4;
+
+	/**
+	 * Compute the shortest unique prefix (≥ MIN_HANDLE_LENGTH chars) of `tabId`
+	 * among all currently-open tabs — the displayed "handle" agents use to
+	 * address each other. Purely DERIVED from the UUIDs already in `tabs`; never
+	 * stored. Grows by one char only when another open tab shares the prefix, and
+	 * shrinks back when that sibling closes.
+	 */
+	function shortHandleFor(tabId: string): string {
+		const others = tabs.map((t) => t.id).filter((id) => id !== tabId);
+		for (let len = MIN_HANDLE_LENGTH; len < tabId.length; len++) {
+			const candidate = tabId.slice(0, len);
+			if (!others.some((id) => id.startsWith(candidate))) return candidate;
+		}
+		return tabId;
+	}
+
 	async function createNewTab(): Promise<Tab> {
 		const id = generateId();
 		const title = "New Tab";
@@ -1926,6 +1949,7 @@ export function createTabStore() {
 		get configReloaded() {
 			return configReloaded;
 		},
+		shortHandleFor,
 		createNewTab,
 		switchTab,
 		closeTab,