diff --git a/packages/coding-agent/src/core/agent-session.ts b/packages/coding-agent/src/core/agent-session.ts
index 17b4a5da6..4e35a524d 100644
--- a/packages/coding-agent/src/core/agent-session.ts
+++ b/packages/coding-agent/src/core/agent-session.ts
@@ -1997,6 +1997,11 @@ export class AgentSession {
 		if (this._goalModeEnabled) {
 			systemPrompt +=
 				"\n\nPersisted plans and checklist snapshots in conversation history are historical evidence, not an authoritative active checklist in goal mode.";
+			systemPrompt +=
+				"\n\nGoal mode subagents:\n" +
+				"Do not spawn subagents on your own initiative. Only call spawn_agent when the user explicitly instructs you to use subagents or delegate work to them.\n" +
+				"Enabling /goal, task complexity, or opportunities for parallel work do not authorize subagent spawning.\n" +
+				"Follow the user's specified subagent count and purpose for each subagent. If the user requests subagents but omits the count or any purposes, infer the missing details from the conversation context and proceed within that request.";
 		}
 		// A custom prompt or per-turn extension override still needs the active
 		// context tools' contract, without replacing any of the supplied instructions.
diff --git a/packages/coding-agent/test/suite/interactive-goal.test.ts b/packages/coding-agent/test/suite/interactive-goal.test.ts
index 5d2c87807..729fdc2c9 100644
--- a/packages/coding-agent/test/suite/interactive-goal.test.ts
+++ b/packages/coding-agent/test/suite/interactive-goal.test.ts
@@ -1,12 +1,15 @@
+import { type Context, fauxAssistantMessage, fauxToolCall } from "@diffexai/diffex-ai";
 import type { Container, EditorComponent } from "@diffexai/diffex-tui";
 import { visibleWidth } from "@diffexai/diffex-tui";
 import { afterEach, beforeAll, describe, expect, it } from "vitest";
 import type { AgentSessionRuntime } from "../../src/core/agent-session-runtime.ts";
+import { createSpawnAgentTool } from "../../src/core/tools/subagents.ts";
 import { InteractiveMode } from "../../src/modes/interactive/interactive-mode.ts";
 import { initTheme } from "../../src/modes/interactive/theme/theme.ts";
 import { stripAnsi } from "../../src/utils/ansi.ts";
+import { createTestExtensionsResult, createTestResourceLoader } from "../extension-test-utils.ts";
 import { goalControls } from "./goal-harness.ts";
-import { createHarness, type Harness } from "./harness.ts";
+import { createHarness, type Harness, type HarnessOptions } from "./harness.ts";
 
 type InteractiveState = {
 	defaultEditor: { onSubmit?: (text: string) => Promise<void> };
@@ -28,8 +31,8 @@ describe("interactive /goal", () => {
 		while (harnesses.length) await harnesses.pop()?.cleanup();
 	});
 
-	async function setup(tuiMode: "regular" | "fullscreen") {
-		const harness = await createHarness();
+	async function setup(tuiMode: "regular" | "fullscreen", options: HarnessOptions = {}) {
+		const harness = await createHarness(options);
 		harnesses.push(harness);
 		const runtime = {
 			session: harness.session,
@@ -74,6 +77,71 @@ describe("interactive /goal", () => {
 		},
 	);
 
+	it.each(["default", "custom", "extension"] as const)(
+		"restricts subagent spawning only while /goal is enabled with %s prompts",
+		async (promptSource) => {
+			const spawned: string[] = [];
+			const extensionsResult = await createTestExtensionsResult([
+				(extension) => {
+					extension.registerTool(createSpawnAgentTool((name) => spawned.push(name)));
+					if (promptSource === "extension") {
+						extension.on("before_agent_start", () => ({ systemPrompt: "Extension prompt." }));
+					}
+				},
+			]);
+			const resourceLoader = createTestResourceLoader({ extensionsResult });
+			if (promptSource === "custom") resourceLoader.getSystemPrompt = () => "Custom prompt.";
+			const { harness, state } = await setup("regular", { resourceLoader });
+			const prompts: Array<string | undefined> = [];
+			const schemas: unknown[] = [];
+			const capture = (context: Context) => {
+				prompts.push(context.systemPrompt);
+				schemas.push(structuredClone(context.tools?.find((tool) => tool.name === "spawn_agent")?.parameters));
+				return fauxAssistantMessage("Done.");
+			};
+			harness.setResponses([
+				capture,
+				(context) => {
+					capture(context);
+					return fauxAssistantMessage(
+						[
+							fauxToolCall("spawn_agent", { name: "frontend", task: "Inspect the frontend." }),
+							fauxToolCall("spawn_agent", { name: "backend", task: "Inspect the backend." }),
+						],
+						{ stopReason: "toolUse" },
+					);
+				},
+				capture,
+				capture,
+			]);
+			await harness.session.prompt("Review the frontend and backend.");
+			await state.defaultEditor.onSubmit?.("/goal");
+			await harness.session.prompt("Use two subagents: one to inspect the frontend and one to inspect the backend.");
+			await state.defaultEditor.onSubmit?.("/goal");
+			await harness.session.prompt("Continue the review.");
+
+			expect(prompts).toHaveLength(4);
+			for (const prompt of [prompts[0], prompts[3]]) {
+				expect(prompt).not.toContain("Do not spawn subagents on your own initiative.");
+			}
+			for (const prompt of [prompts[1], prompts[2]]) {
+				expect(prompt).toContain("Do not spawn subagents on your own initiative.");
+				expect(prompt).toContain("Only call spawn_agent when the user explicitly instructs you");
+				expect(prompt).toContain("Follow the user's specified subagent count and purpose for each subagent.");
+				expect(prompt).toContain("infer the missing details from the conversation context");
+			}
+			if (promptSource !== "default") {
+				for (const prompt of prompts) {
+					expect(prompt).toContain(promptSource === "custom" ? "Custom prompt." : "Extension prompt.");
+				}
+			}
+			expect(spawned).toEqual(["frontend", "backend"]);
+			expect(schemas[0]).toBeDefined();
+			for (const schema of schemas) expect(schema).toEqual(schemas[0]);
+			expect(harness.getPendingResponseCount()).toBe(0);
+		},
+	);
+
 	it("preserves the command in the editor while direct shell handling is busy", async () => {
 		const { harness, state } = await setup("regular");
 		const goal = goalControls(harness);