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

7 files changed, 345 insertions, 16 deletions

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({