diff --git a/electron/ai-edition/agent-tools.test.ts b/electron/ai-edition/agent-tools.test.ts
index c105d2e4b..55a9c17bd 100644
--- a/electron/ai-edition/agent-tools.test.ts
+++ b/electron/ai-edition/agent-tools.test.ts
@@ -10,6 +10,7 @@ import { ZOOM_DEPTH_SCALES } from "../../src/lib/ai-edition/timeline/zoom-scale"
 import { zoomScaleLimit } from "../../src/native/sceneDescription";
 import {
 	type AgentToolOptions,
+	addZoomArgs,
 	executeAgentTool,
 	isMutatingTool,
 	MUTATING_TOOL_NAMES,
@@ -3342,6 +3343,23 @@ describe("region tools reach the inspector's settings (#1180)", () => {
 		expect(both.ok).toBe(false);
 	});
 
+	it("takes customScale alone wherever the arguments were parsed first", () => {
+		// The MCP server and the in-app agent hand the tool its arguments through the same schema,
+		// and addZooms hands each region to addZoom parsed. A default depth filled in there read as
+		// "depth and customScale both passed", so a free level could never be set in one call.
+		const zoomOnly = { startSec: 2, endSec: 4, customScale: 1.6 };
+		const parsed = run(fixtureDocument(), "addZoom", addZoomArgs.parse(zoomOnly));
+		expect(parsed.ok).toBe(true);
+		expect(parsed.document?.zoomRanges[0]).toMatchObject({ customScale: 1.6 });
+		const batch = run(fixtureDocument(), "addZooms", { regions: [zoomOnly] });
+		expect(batch.ok).toBe(true);
+		expect(batch.document?.zoomRanges[0]).toMatchObject({ customScale: 1.6 });
+		// Neither level given: the default depth still applies.
+		const plain = run(fixtureDocument(), "addZoom", addZoomArgs.parse({ startSec: 2, endSec: 4 }));
+		expect(plain.document?.zoomRanges[0]).toMatchObject({ depth: 3 });
+		expect(plain.document?.zoomRanges[0]).not.toHaveProperty("customScale");
+	});
+
 	it("reads an annotation back in the terms its writes take", () => {
 		const added = run(fixtureDocument(), "addAnnotation", {
 			startSec: 1,
diff --git a/electron/ai-edition/agent-tools.ts b/electron/ai-edition/agent-tools.ts
index 50bb5de41..c9b64ecb6 100644
--- a/electron/ai-edition/agent-tools.ts
+++ b/electron/ai-edition/agent-tools.ts
@@ -791,7 +791,12 @@ const customScaleSchema = z
 export const addZoomArgs = z.object({
 	startSec: secondsSchema,
 	endSec: secondsSchema,
-	depth: depthSchema.default(3),
+	// No schema default: the MCP server, the in-app agent and addZooms all parse the arguments
+	// before the tool runs, and a default filled in there reads as a depth passed next to
+	// customScale. The handler applies 3 when neither level is given.
+	depth: depthSchema
+		.optional()
+		.describe("Defaults to 3 when neither depth nor customScale is given."),
 	focus: focusSchema.default({ cx: 0.5, cy: 0.5 }),
 	rotationPreset: rotationPresetSchema.default("none"),
 	focusMode: focusModeSchema.default("manual"),
@@ -2897,7 +2902,7 @@ export function executeAgentTool(
 					id: createId("zoom"),
 					startMs,
 					endMs,
-					depth: parsed.data.depth as 1 | 2 | 3 | 4 | 5 | 6,
+					depth: (parsed.data.depth ?? 3) as 1 | 2 | 3 | 4 | 5 | 6,
 					...level,
 					focus: parsed.data.focus,
 					focusMode: parsed.data.focusMode,