diff --git a/src/app/api/messages/load/route.js b/src/app/api/messages/load/route.js
index 3828fec7..aea37a5f 100644
--- a/src/app/api/messages/load/route.js
+++ b/src/app/api/messages/load/route.js
@@ -7,9 +7,14 @@ import { NextResponse } from 'next/server';
 import { withAuth } from '@/lib/api/middleware/auth.js';
 import { sseManager } from '@/lib/api/sse-manager.js';
 
+function normalizeConversationId(conversationId) {
+	return typeof conversationId === 'string' ? conversationId.trim() : '';
+}
+
 export const POST = withAuth(async ({ request, locals }) => {
 	try {
-		const { conversationId, limit = 50, before } = await request.json();
+		const { conversationId: rawConversationId, limit = 50, before } = await request.json();
+		const conversationId = normalizeConversationId(rawConversationId);
 
 		if (!conversationId) {
 			return NextResponse.json({ error: 'Missing conversationId' }, { status: 400 });
diff --git a/src/app/api/messages/load/route.test.js b/src/app/api/messages/load/route.test.js
index 6418cbbe..85518c6c 100644
--- a/src/app/api/messages/load/route.test.js
+++ b/src/app/api/messages/load/route.test.js
@@ -1,10 +1,14 @@
-import { describe, expect, it, vi } from 'vitest';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
 
 const mocks = vi.hoisted(() => ({
 	mockSupabase: {
-		from: vi.fn()
+		from: vi.fn(),
+		rpc: vi.fn()
 	},
-	mockJoinRoom: vi.fn()
+	mockJoinRoom: vi.fn(),
+	userEq: vi.fn(),
+	participantEq: vi.fn(),
+	messageConversationEq: vi.fn()
 }));
 
 vi.mock('@/lib/api/middleware/auth.js', () => ({
@@ -25,7 +29,82 @@ vi.mock('@/lib/api/sse-manager.js', () => ({
 	}
 }));
 
+function createUserQuery() {
+	const query = {
+		select: vi.fn(() => query),
+		eq: mocks.userEq,
+		single: vi.fn().mockResolvedValue({
+			data: { id: 'internal-user-id' },
+			error: null
+		})
+	};
+	mocks.userEq.mockReturnValue(query);
+	return query;
+}
+
+function createParticipantQuery() {
+	const query = {
+		select: vi.fn(() => query),
+		eq: mocks.participantEq,
+		single: vi.fn().mockResolvedValue({
+			data: { id: 'participant-row' },
+			error: null
+		})
+	};
+	mocks.participantEq.mockReturnValue(query);
+	return query;
+}
+
+function createMessagesQuery() {
+	const query = {
+		select: vi.fn(() => query),
+		eq: vi.fn(() => query),
+		is: vi.fn(() => query),
+		order: vi.fn(() => query),
+		limit: vi.fn().mockResolvedValue({
+			data: [],
+			error: null
+		})
+	};
+	query.eq.mockImplementation((column, value) => {
+		if (column === 'conversation_id') {
+			mocks.messageConversationEq(column, value);
+		}
+		return query;
+	});
+	return query;
+}
+
 describe('POST /api/messages/load validation', () => {
+	beforeEach(() => {
+		vi.resetModules();
+		vi.clearAllMocks();
+		mocks.mockSupabase.rpc.mockResolvedValue({ data: null, error: null });
+		mocks.mockSupabase.from.mockImplementation((table) => {
+			if (table === 'users') return createUserQuery();
+			if (table === 'conversation_participants') return createParticipantQuery();
+			if (table === 'messages') return createMessagesQuery();
+			throw new Error(`Unexpected table: ${table}`);
+		});
+	});
+
+	it('rejects blank conversation ids before database work', async () => {
+		const { POST } = await import('./route.js');
+		const request = {
+			json: vi.fn().mockResolvedValue({
+				conversationId: '   '
+			})
+		};
+
+		const response = await POST(request);
+		const body = await response.json();
+
+		expect(response.status).toBe(400);
+		expect(body.error).toBe('Missing conversationId');
+		expect(mocks.mockSupabase.from).not.toHaveBeenCalled();
+		expect(mocks.mockJoinRoom).not.toHaveBeenCalled();
+	});
+
 	it('rejects non-integer limits before database work', async () => {
 		const { POST } = await import('./route.js');
 		const request = {
@@ -42,4 +121,23 @@ describe('POST /api/messages/load validation', () => {
 		expect(body.error).toBe('limit must be an integer between 1 and 100');
 		expect(mocks.mockSupabase.from).not.toHaveBeenCalled();
 	});
+
+	it('trims conversation ids before querying messages and joining the room', async () => {
+		const { POST } = await import('./route.js');
+		const request = {
+			json: vi.fn().mockResolvedValue({
+				conversationId: '  conversation-1  ',
+				limit: 25
+			})
+		};
+
+		const response = await POST(request);
+		const body = await response.json();
+
+		expect(response.status).toBe(200);
+		expect(body).toEqual({ success: true, messages: [], hasMore: false });
+		expect(mocks.participantEq).toHaveBeenCalledWith('conversation_id', 'conversation-1');
+		expect(mocks.messageConversationEq).toHaveBeenCalledWith('conversation_id', 'conversation-1');
+		expect(mocks.mockJoinRoom).toHaveBeenCalledWith('internal-user-id', 'conversation-1');
+	});
 });