diff --git a/packages/coding-agent/src/config/settings-schema.ts b/packages/coding-agent/src/config/settings-schema.ts
index 37c465faa..0b693f077 100644
--- a/packages/coding-agent/src/config/settings-schema.ts
+++ b/packages/coding-agent/src/config/settings-schema.ts
@@ -4036,6 +4036,28 @@ export const SETTINGS_SCHEMA = {
 		},
 	},
 
+	"tools.xdevDocs": {
+		type: "enum",
+		values: ["inline", "builtins", "catalog"] as const,
+		default: "builtins",
+		ui: {
+			tab: "tools",
+			group: "Discovery & MCP",
+			label: "xd:// Prompt Docs",
+			description:
+				"Choose which mounted-device docs and schemas are inlined in the system prompt. Built-ins keeps core tools inline while MCP and extension tools stay on-demand.",
+			options: [
+				{ value: "inline", label: "All Devices", description: "Inline docs and schemas for every mounted device." },
+				{
+					value: "builtins",
+					label: "Built-ins Only",
+					description: "Inline built-in docs; fetch MCP and extension docs on demand.",
+				},
+				{ value: "catalog", label: "Catalog Only", description: "List every device; fetch all docs on demand." },
+			],
+		},
+	},
+
 	// MCP
 	"mcp.enableProjectConfig": {
 		type: "boolean",
diff --git a/packages/coding-agent/src/sdk.ts b/packages/coding-agent/src/sdk.ts
index 3ed00eaff..80e6cd58e 100644
--- a/packages/coding-agent/src/sdk.ts
+++ b/packages/coding-agent/src/sdk.ts
@@ -2596,7 +2596,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 				cwd,
 				additionalWorkspaceRoots: sessionManager.getAdditionalDirectories(),
 				xdevTools: toolSession.xdevRegistry?.entries() ?? [],
-				xdevDocs: toolSession.xdevRegistry?.docsAll() ?? "",
+				xdevDocs: toolSession.xdevRegistry?.docsAll(settings.get("tools.xdevDocs")) ?? "",
 				autoQaEnabled: !restrictToolNames && isAutoQaEnabled(settings),
 				resolvedCustomPrompt: options.customSystemPrompt,
 				skills: session?.skills ?? skills,
diff --git a/packages/coding-agent/src/tools/xdev.ts b/packages/coding-agent/src/tools/xdev.ts
index 48ca0cd17..d85665d18 100644
--- a/packages/coding-agent/src/tools/xdev.ts
+++ b/packages/coding-agent/src/tools/xdev.ts
@@ -60,6 +60,9 @@ export const XDEV_KEEP_TOP_LEVEL: Record<string, true> = {
  */
 export const XDEV_TRANSPORT_TOOLS: Record<string, true> = { read: true, write: true };
 
+/** Controls which mounted-device docs are inlined into the system prompt. */
+export type XdevDocsMode = "inline" | "builtins" | "catalog";
+
 /**
  * Whether an enabled tool is presented under `xd://` (rather than top-level)
  * while the `xd://` transport is active. Discoverable tools mount unless they
@@ -273,11 +276,16 @@ export class XdevRegistry {
 	 * Dynamic mounts embed at most {@link EXTERNAL_DESCRIPTION_CAP} description
 	 * chars (schema always intact); `read xd://<tool>` returns the full text.
 	 */
-	docsAll(): string {
+	docsAll(mode: XdevDocsMode = "inline"): string {
 		const sections: string[] = [];
 		const overflow: Tool[] = [];
 		let used = 0;
 		for (const tool of this.list()) {
+			const isBuiltin = this.#builtins.has(tool.name);
+			if (mode === "catalog" || (mode === "builtins" && !isBuiltin)) {
+				overflow.push(tool);
+				continue;
+			}
 			const descriptionCap = this.#dynamic.has(tool.name) ? XdevRegistry.EXTERNAL_DESCRIPTION_CAP : undefined;
 			const docs = renderDocs(tool, "##", descriptionCap);
 			if (docs.length > XdevRegistry.DOCS_PER_DEVICE_CAP || used + docs.length > XdevRegistry.DOCS_TOTAL_BUDGET) {
diff --git a/packages/coding-agent/test/write-xdev-dispatch.test.ts b/packages/coding-agent/test/write-xdev-dispatch.test.ts
index 23f717cfd..cd87faf6d 100644
--- a/packages/coding-agent/test/write-xdev-dispatch.test.ts
+++ b/packages/coding-agent/test/write-xdev-dispatch.test.ts
@@ -186,6 +186,7 @@ describe("read and write route xd:// device URLs", () => {
 		const tempDir = await fs.mkdtemp(path.join(os.tmpdir(), "write-xdev-docs-"));
 		try {
 			const session = xdevSession(tempDir);
+			expect(session.settings.get("tools.xdevDocs")).toBe("builtins");
 			await createTools(session);
 			const mounted = session.xdevRegistry?.list() ?? [];
 			expect(mounted.length).toBeGreaterThan(0);
@@ -208,30 +209,37 @@ describe("read and write route xd:// device URLs", () => {
 		}
 	});
 
-	it("docsAll truncates external (dynamic-mount) descriptions to the cap; built-ins and read xd:// stay full", async () => {
+	it("docsAll supports inline, builtins, and catalog prompt modes", async () => {
 		const tempDir = await fs.mkdtemp(path.join(os.tmpdir(), "write-xdev-external-"));
 		try {
 			const session = xdevSession(tempDir);
+			expect(session.settings.get("tools.xdevDocs")).toBe("builtins");
 			await createTools(session);
 			const registry = session.xdevRegistry;
 			if (!registry) throw new Error("expected xdev registry");
 			const mounted = registry.list();
 
-			const longDescription = `LEDE ${"y".repeat(XdevRegistry.EXTERNAL_DESCRIPTION_CAP * 3)} TAIL`;
+			const longDescription = "LEDE " + "y".repeat(XdevRegistry.EXTERNAL_DESCRIPTION_CAP * 3) + " TAIL";
 			const external = Object.create(mounted[0]!) as (typeof mounted)[number];
 			Object.defineProperty(external, "name", { value: "mcp_external_tool" });
 			Object.defineProperty(external, "description", { value: longDescription });
 			registry.reconcile([external]);
 
-			const docs = registry.docsAll();
-			// External device: schema section present, description cut at the cap.
-			expect(docs).toContain("## mcp_external_tool");
-			expect(docs).toContain("LEDE ");
-			expect(docs).not.toContain("TAIL");
-			expect(docs).toContain("… (full docs: read xd://mcp_external_tool)");
-			// Built-in devices keep their full curated description.
-			expect(docs).toContain(mounted[0]!.description ?? "");
-			// On-demand docs return the untruncated text.
+			const inlineDocs = registry.docsAll("inline");
+			expect(inlineDocs).toContain("## mcp_external_tool");
+			expect(inlineDocs).toContain("LEDE ");
+			expect(inlineDocs).not.toContain("TAIL");
+			expect(inlineDocs).toContain("… (full docs: read xd://mcp_external_tool)");
+
+			const builtinsDocs = registry.docsAll("builtins");
+			expect(builtinsDocs).toContain("## ");
+			expect(builtinsDocs).not.toContain("## mcp_external_tool");
+			expect(builtinsDocs).toContain("- xd://mcp_external_tool —");
+
+			const catalogDocs = registry.docsAll("catalog");
+			expect(catalogDocs).not.toContain(`## ${mounted[0]!.name}`);
+			expect(catalogDocs).toContain("- xd://");
+			expect(catalogDocs).toContain("- xd://mcp_external_tool —");
 			expect(registry.docs("mcp_external_tool")).toContain("TAIL");
 		} finally {
 			await removeWithRetries(tempDir);