diff --git a/backend/database/queries/message-queries.ts b/backend/database/queries/message-queries.ts
index 31dc79f7..40070490 100644
--- a/backend/database/queries/message-queries.ts
+++ b/backend/database/queries/message-queries.ts
@@ -2,6 +2,7 @@ import { getDatabase } from '../index';
 import type { DatabaseMessage } from '$shared/types/database/schema';
 import type { UnifiedMessage } from '$shared/types/unified';
 import { debug } from '$shared/utils/logger';
+import { trimSubAgentForWire } from '$shared/utils/subagent-wire-trim';
 
 /** Parse message JSON from a DB row. */
 function parseMessage(row: DatabaseMessage): UnifiedMessage {
@@ -25,7 +26,10 @@ export const messageQueries = {
 		}
 
 		const path = this.getPathToRoot(session.head_message_id);
-		return path.map(parseMessage);
+		// Trim sub-agent tool noise from the display payload. The DB rows keep the
+		// full data; only what is sent to the frontend is slimmed. See
+		// trimSubAgentForWire for the rationale.
+		return path.map(parseMessage).map(trimSubAgentForWire);
 	},
 
 	/**
@@ -51,6 +55,44 @@ export const messageQueries = {
 		return message;
 	},
 
+	/**
+	 * Get the full (untrimmed) sub-agent messages nested under a message's
+	 * tool_use blocks, matched by `parent.toolUseId`.
+	 *
+	 * The wire payload trims sub-agent noise (see trimSubAgentForWire), so the
+	 * frontend's in-memory copy is slimmed. The Debug modal uses this to fetch
+	 * the complete sub-agent data straight from the DB and rebuild the full view.
+	 */
+	getSubAgentMessages(messageId: string): UnifiedMessage[] {
+		const db = getDatabase();
+		const row = db.prepare(`
+			SELECT * FROM messages WHERE id = ?
+		`).get(messageId) as DatabaseMessage | null;
+		if (!row) return [];
+
+		const parent = parseMessage(row);
+		if (parent.type !== 'assistant') return [];
+
+		const toolUseIds = new Set(
+			parent.content
+				.filter(block => block.type === 'tool_use')
+				.map(block => (block as { id: string }).id)
+		);
+		if (toolUseIds.size === 0) return [];
+
+		const rows = db.prepare(`
+			SELECT * FROM messages WHERE session_id = ?
+			ORDER BY created_at ASC
+		`).all(row.session_id) as DatabaseMessage[];
+
+		return rows
+			.map(parseMessage)
+			.filter(msg => {
+				const parentToolUseId = msg.parent?.toolUseId;
+				return parentToolUseId != null && toolUseIds.has(parentToolUseId);
+			});
+	},
+
 	create(messageData: {
 		session_id: string;
 		message: UnifiedMessage;
diff --git a/backend/ws/chat/stream.ts b/backend/ws/chat/stream.ts
index 41610527..c757363f 100644
--- a/backend/ws/chat/stream.ts
+++ b/backend/ws/chat/stream.ts
@@ -10,8 +10,9 @@
 import { t } from 'elysia';
 import { createRouter } from '$shared/utils/ws-server';
 import { streamManager, type StreamEvent } from '../../chat/stream-manager';
-import type { EngineType } from '$shared/types/unified';
+import type { EngineType, UnifiedMessage } from '$shared/types/unified';
 import { debug } from '$shared/utils/logger';
+import { trimSubAgentForWire } from '$shared/utils/subagent-wire-trim';
 import { ws } from '$backend/utils/ws';
 import { broadcastPresence } from '../projects/status';
 import { sessionQueries, messageQueries } from '../../database/queries';
@@ -280,7 +281,7 @@ export const streamHandler = createRouter()
 						case 'message': {
 							ws.emit.chatSession(chatSessionId, 'chat:message', {
 								processId: event.processId,
-								message: event.data.message,
+								message: trimSubAgentForWire(event.data.message),
 								usage: event.data.usage,
 								timestamp: event.data.timestamp,
 								message_id: event.data.message_id,
@@ -424,7 +425,7 @@ export const streamHandler = createRouter()
 						case 'message': {
 							ws.emit.chatSession(chatSessionId, 'chat:message', {
 								processId: event.processId,
-								message: event.data.message,
+								message: trimSubAgentForWire(event.data.message),
 								usage: event.data.usage,
 								timestamp: event.data.timestamp,
 								message_id: event.data.message_id,
@@ -618,7 +619,14 @@ export const streamHandler = createRouter()
 			streamId: streamState.streamId,
 			status: streamState.status,
 			processId: streamState.processId,
-			messages: streamState.messages,
+			// Trim sub-agent noise from the catch-up buffer too (see trimSubAgentForWire).
+			// Buffer entries wrap the UnifiedMessage under `.message`; leave the rest intact.
+			messages: streamState.messages.map(entry => {
+				const wrapped = entry as { message?: UnifiedMessage };
+				return wrapped?.message
+					? { ...wrapped, message: trimSubAgentForWire(wrapped.message) }
+					: entry;
+			}),
 			currentPartialText: streamState.currentPartialText,
 			currentReasoningText: streamState.currentReasoningText,
 			error: streamState.error,
diff --git a/backend/ws/messages/crud.ts b/backend/ws/messages/crud.ts
index 7f571acb..f3424dbd 100644
--- a/backend/ws/messages/crud.ts
+++ b/backend/ws/messages/crud.ts
@@ -44,6 +44,19 @@ export const crudHandler = createRouter()
 		return loadMessage(message);
 	})
 
+	// Get full (untrimmed) sub-agent messages nested under a message.
+	// Used by the Debug modal to show complete data despite the wire trimming
+	// sub-agent noise from the normal chat payload.
+	.http('messages:get-subagents', {
+		data: t.Object({
+			messageId: t.String({ minLength: 1 })
+		}),
+		response: t.Array(t.Any())
+	}, async ({ data, conn }) => {
+		requireMessageAccess(conn, data.messageId);
+		return messageQueries.getSubAgentMessages(data.messageId);
+	})
+
 	// Delete message
 	.http('messages:delete', {
 		data: t.Object({
diff --git a/frontend/components/chat/modal/DebugModal.svelte b/frontend/components/chat/modal/DebugModal.svelte
index 3e114b8d..5f657c43 100644
--- a/frontend/components/chat/modal/DebugModal.svelte
+++ b/frontend/components/chat/modal/DebugModal.svelte
@@ -5,12 +5,21 @@
   - Display raw message data
   - JSON formatting
   - Copy to clipboard
+
+  The normal chat payload trims sub-agent (Task) tool noise for bandwidth
+  (see shared/utils/subagent-wire-trim.ts), so the in-memory message can carry
+  slimmed sub-agent activity. On open we fetch the full untrimmed sub-agent
+  messages straight from the DB and rebuild the complete subActivities so the
+  debug view always shows the real, complete data.
 -->
 
 <script lang="ts">
 	import type { FrontendMessage } from '$frontend/stores/core/sessions.svelte';
 	import Modal from '$frontend/components/common/overlay/Modal.svelte';
 	import Icon from '$frontend/components/common/display/Icon.svelte';
+	import ws from '$frontend/utils/ws';
+	import { processSubAgentMessages } from '$frontend/utils/chat/tool-handler';
+	import { debug } from '$shared/utils/logger';
 
 	let {
 		isOpen = $bindable(),
@@ -22,8 +31,59 @@
 		onClose: () => void;
 	} = $props();
 
+	// The message actually rendered — set to the in-memory copy when the modal
+	// opens, then replaced with a full-data clone once sub-agent messages load.
+	let displayMessage = $state<unknown>(undefined);
+
+	// Fetch full sub-agent data and graft complete subActivities when the modal
+	// opens on a message that has (possibly trimmed) sub-agent activity.
+	$effect(() => {
+		if (!isOpen) return;
+
+		const msg = message as any;
+		displayMessage = message;
+
+		const messageId: string | undefined = msg?.messageId;
+		const toolBlocks: any[] =
+			msg?.type === 'assistant' && Array.isArray(msg.content)
+				? msg.content.filter((b: any) => b?.type === 'tool_use')
+				: [];
+		const hasSubAgentActivity = toolBlocks.some(
+			(b: any) => Array.isArray(b.subActivities) && b.subActivities.length > 0
+		);
+
+		if (!messageId || !hasSubAgentActivity) return;
+
+		ws.http('messages:get-subagents', { messageId })
+			.then((subMessages: any) => {
+				if (!Array.isArray(subMessages) || subMessages.length === 0) return;
+
+				// Group full sub-agent messages by the tool_use they belong to.
+				const byToolUseId = new Map<string, any[]>();
+				for (const sub of subMessages) {
+					const parentToolUseId: string | undefined = sub?.parent?.toolUseId;
+					if (!parentToolUseId) continue;
+					if (!byToolUseId.has(parentToolUseId)) byToolUseId.set(parentToolUseId, []);
+					byToolUseId.get(parentToolUseId)!.push(sub);
+				}
+				if (byToolUseId.size === 0) return;
+
+				// Deep clone (also strips Svelte proxies) and graft full subActivities.
+				const clone = JSON.parse(JSON.stringify(message));
+				for (const block of clone.content ?? []) {
+					if (block?.type === 'tool_use' && byToolUseId.has(block.id)) {
+						block.subActivities = processSubAgentMessages(byToolUseId.get(block.id)!);
+					}
+				}
+				displayMessage = clone;
+			})
+			.catch((err) => {
+				debug.warn('chat', 'Failed to load full sub-agent data for debug:', err);
+			});
+	});
+
 	function copyToClipboard() {
-		navigator.clipboard.writeText(JSON.stringify(message, null, 2));
+		navigator.clipboard.writeText(JSON.stringify(displayMessage, null, 2));
 	}
 </script>
 
@@ -42,7 +102,7 @@
 					Raw Message
 				</h4>
 				<div class="bg-slate-100 dark:bg-slate-800 rounded-lg p-3 border border-slate-200 dark:border-slate-700">
-					<pre class="text-xs font-mono text-slate-700 dark:text-slate-300 overflow-x-auto max-h-80 overflow-y-auto whitespace-pre-wrap break-words">{JSON.stringify(message, null, 2)}</pre>
+					<pre class="text-xs font-mono text-slate-700 dark:text-slate-300 overflow-x-auto max-h-80 overflow-y-auto whitespace-pre-wrap break-words">{JSON.stringify(displayMessage, null, 2)}</pre>
 				</div>
 				<div class="mt-3 flex justify-end">
 					<button
diff --git a/frontend/utils/chat/tool-handler.ts b/frontend/utils/chat/tool-handler.ts
index 831fdd33..cf3c384f 100644
--- a/frontend/utils/chat/tool-handler.ts
+++ b/frontend/utils/chat/tool-handler.ts
@@ -99,7 +99,7 @@ function handleAgentTool(
 }
 
 // Process sub-agent messages into a flat activity list
-function processSubAgentMessages(messages: FrontendMessage[]): SubAgentActivity[] {
+export function processSubAgentMessages(messages: FrontendMessage[]): SubAgentActivity[] {
   const activities: SubAgentActivity[] = [];
   const toolResultMap = new Map<string, ToolResult>();
 
diff --git a/shared/utils/subagent-wire-trim.ts b/shared/utils/subagent-wire-trim.ts
new file mode 100644
index 00000000..5e15f77e
--- /dev/null
+++ b/shared/utils/subagent-wire-trim.ts
@@ -0,0 +1,87 @@
+/**
+ * Sub-agent (Task) wire payload trimming.
+ *
+ * Messages produced INSIDE a sub-agent (Task) run carry `parent.toolUseId` and
+ * are folded into the parent Agent tool's collapsed `subActivities` in the UI.
+ * That collapsed view only ever renders a tool_use's NAME plus one short brief
+ * field, and any text — it never renders sub-agent tool_result content, nor any
+ * non-brief tool_use input field. Those are pure dead weight on the wire and
+ * are typically the bulk of a heavy session's payload (file dumps, command
+ * output, Write/Edit bodies).
+ *
+ * `trimSubAgentForWire` strips that dead weight from anything SENT to the
+ * frontend (message read responses + live stream broadcast + catch-up buffer).
+ * The DB copy is never touched, so resume, branching/undo, interrupt detection,
+ * and the raw DebugModal (via `messages:get`) all keep the full data.
+ */
+
+import type { UnifiedMessage } from '$shared/types/unified';
+
+/**
+ * The single input field the collapsed sub-agent view shows per tool.
+ *
+ * MUST stay in sync with `getToolBrief()` in the AgentTool variants
+ * (frontend/components/chat/tools/variants/{classic,compact}/AgentTool.svelte).
+ * Tools not listed show no brief, so their input is dropped entirely.
+ *
+ * Keys are unified camelCase (engines normalise snake_case → camelCase before
+ * persistence), so this mapping is engine-agnostic.
+ */
+const SUBAGENT_BRIEF_FIELD: Record<string, string> = {
+	Bash: 'command',
+	Read: 'filePath',
+	Write: 'filePath',
+	Edit: 'filePath',
+	Glob: 'pattern',
+	Grep: 'pattern',
+	WebFetch: 'url',
+	WebSearch: 'query',
+};
+
+/** Whether a message was produced inside a sub-agent (Task) run. */
+function isSubAgentMessage(msg: UnifiedMessage): boolean {
+	return msg.parent?.toolUseId != null;
+}
+
+/**
+ * Return a wire-trimmed copy of a message. Non-sub-agent messages are returned
+ * by reference (unchanged). Sub-agent messages get a shallow copy with:
+ *  - tool_result blocks: `content` emptied. The block, `toolUseId` and `isError`
+ *    are kept so live waiting-input handling and interrupt detection — which key
+ *    off the block's presence/toolUseId — behave exactly as before.
+ *  - tool_use blocks: `input` reduced to the one displayed brief field (or `{}`
+ *    for tools with no brief).
+ */
+export function trimSubAgentForWire(msg: UnifiedMessage): UnifiedMessage {
+	if (!isSubAgentMessage(msg)) return msg;
+
+	if (msg.type === 'user') {
+		let changed = false;
+		const content = msg.content.map(block => {
+			if (block.type === 'tool_result' && block.content !== '') {
+				changed = true;
+				return { ...block, content: '' };
+			}
+			return block;
+		});
+		return changed ? { ...msg, content } : msg;
+	}
+
+	if (msg.type === 'assistant') {
+		let changed = false;
+		const content = msg.content.map(block => {
+			if (block.type !== 'tool_use') return block;
+			const briefField = SUBAGENT_BRIEF_FIELD[block.name];
+			const input = block.input as Record<string, unknown>;
+			const trimmed =
+				briefField && input && briefField in input
+					? { [briefField]: input[briefField] }
+					: {};
+			changed = true;
+			return { ...block, input: trimmed } as typeof block;
+		});
+		return changed ? ({ ...msg, content } as UnifiedMessage) : msg;
+	}
+
+	return msg;
+}