diff --git a/src/routes/(app)/matters/[id]/+page.server.ts b/src/routes/(app)/matters/[id]/+page.server.ts
index afe861d1..9edd6959 100644
--- a/src/routes/(app)/matters/[id]/+page.server.ts
+++ b/src/routes/(app)/matters/[id]/+page.server.ts
@@ -126,14 +126,17 @@ export const actions: Actions = {
 		return { success: true };
 	},
 
+	// Attach/detach go through the project-side endpoints (Wave D.1 T3). The legacy
+	// `PATCH /api/v1/knowledge-bases/{kb_id}` with {project_id} silently no-ops.
 	linkKb: async (event) => {
 		const data = await event.request.formData();
 		const kb_id = String(data.get('kb_id') ?? '');
 		if (!kb_id) return fail(400, { error: 'Missing kb_id.' });
-		const res = await lqFetch(event, `/api/v1/knowledge-bases/${kb_id}`, {
-			method: 'PATCH',
-			body: JSON.stringify({ project_id: event.params.id })
+		const res = await lqFetch(event, `/api/v1/projects/${event.params.id}/knowledge-bases`, {
+			method: 'POST',
+			body: JSON.stringify({ knowledge_base_id: kb_id })
 		});
+		// 200 = attached (idempotent — repeat attach returns 200, not 409).
 		if (!res.ok) {
 			if (res.status === 404) return fail(404, { error: 'Knowledge base no longer exists.' });
 			return fail(502, { error: 'Could not link the knowledge base.' });
@@ -145,11 +148,12 @@ export const actions: Actions = {
 		const data = await event.request.formData();
 		const kb_id = String(data.get('kb_id') ?? '');
 		if (!kb_id) return fail(400, { error: 'Missing kb_id.' });
-		const res = await lqFetch(event, `/api/v1/knowledge-bases/${kb_id}`, {
-			method: 'PATCH',
-			body: JSON.stringify({ project_id: null })
-		});
-		// 200 + 404 → success (already gone is fine for the UI).
+		const res = await lqFetch(
+			event,
+			`/api/v1/projects/${event.params.id}/knowledge-bases/${kb_id}`,
+			{ method: 'DELETE' }
+		);
+		// 204 (idempotent, incl. not-attached) + 404 (project gone) → success for the UI.
 		if (!res.ok && res.status !== 404) {
 			return fail(502, { error: 'Could not unlink the knowledge base.' });
 		}
diff --git a/src/routes/(app)/matters/[id]/page.server.test.ts b/src/routes/(app)/matters/[id]/page.server.test.ts
index 2ae44b94..b7a3796c 100644
--- a/src/routes/(app)/matters/[id]/page.server.test.ts
+++ b/src/routes/(app)/matters/[id]/page.server.test.ts
@@ -373,13 +373,14 @@ const kbEvent = (kb_id: string, id = 'p1') =>
 	}) as never;
 
 describe('/matters/[id] linkKb / unlinkKb actions', () => {
-	it('linkKb PATCHes the KB with the matter id', async () => {
+	it('linkKb POSTs { knowledge_base_id } to the project attach endpoint', async () => {
 		lqFetch.mockResolvedValue(new Response('{}', { status: 200 }));
 		const r = await actions.linkKb(kbEvent('k1'));
 		expect(r).toEqual({ success: true });
-		expect(lqFetch.mock.calls[0][1]).toBe('/api/v1/knowledge-bases/k1');
-		expect(lqFetch.mock.calls[0][2].method).toBe('PATCH');
-		expect(JSON.parse(lqFetch.mock.calls[0][2].body)).toEqual({ project_id: 'p1' });
+		expect(lqFetch).toHaveBeenCalledTimes(1);
+		expect(lqFetch.mock.calls[0][1]).toBe('/api/v1/projects/p1/knowledge-bases');
+		expect(lqFetch.mock.calls[0][2].method).toBe('POST');
+		expect(JSON.parse(lqFetch.mock.calls[0][2].body)).toEqual({ knowledge_base_id: 'k1' });
 	});
 
 	it('linkKb maps 404 to a friendly error', async () => {
@@ -394,11 +395,13 @@ describe('/matters/[id] linkKb / unlinkKb actions', () => {
 		expect(r).toMatchObject({ status: 502, data: { error: 'Could not link the knowledge base.' } });
 	});
 
-	it('unlinkKb PATCHes the KB with project_id: null', async () => {
-		lqFetch.mockResolvedValue(new Response('{}', { status: 200 }));
+	it('unlinkKb DELETEs the project-scoped KB join', async () => {
+		lqFetch.mockResolvedValue(new Response(null, { status: 204 }));
 		const r = await actions.unlinkKb(kbEvent('k1'));
 		expect(r).toEqual({ success: true });
-		expect(JSON.parse(lqFetch.mock.calls[0][2].body)).toEqual({ project_id: null });
+		expect(lqFetch).toHaveBeenCalledTimes(1);
+		expect(lqFetch.mock.calls[0][1]).toBe('/api/v1/projects/p1/knowledge-bases/k1');
+		expect(lqFetch.mock.calls[0][2].method).toBe('DELETE');
 	});
 
 	it('unlinkKb treats 404 as silent success', async () => {
@@ -407,6 +410,16 @@ describe('/matters/[id] linkKb / unlinkKb actions', () => {
 		expect(r).toEqual({ success: true });
 	});
 
+	it('never calls the legacy KB PATCH endpoint (silent no-op upstream)', async () => {
+		lqFetch.mockResolvedValue(new Response(null, { status: 204 }));
+		await actions.linkKb(kbEvent('k1'));
+		await actions.unlinkKb(kbEvent('k1'));
+		for (const call of lqFetch.mock.calls) {
+			expect(call[1]).not.toBe('/api/v1/knowledge-bases/k1');
+			expect(call[2].method).not.toBe('PATCH');
+		}
+	});
+
 	it('unlinkKb maps other failures to a 502', async () => {
 		lqFetch.mockResolvedValue(new Response('boom', { status: 500 }));
 		const r = await actions.unlinkKb(kbEvent('k1'));