diff --git a/packages/coding-agent/src/extensions/codemode/tool.ts b/packages/coding-agent/src/extensions/codemode/tool.ts
index b55a117a7..e28ea999f 100644
--- a/packages/coding-agent/src/extensions/codemode/tool.ts
+++ b/packages/coding-agent/src/extensions/codemode/tool.ts
@@ -271,8 +271,8 @@ export function createCodemodeDescription(
 
 /**
  * The field names of an object schema, `{ output, exit_code, full_output_path? }`, with the fields of
- * object-typed fields `depth` levels deep: `{ content, structuredContent?: { results } }`. Undefined
- * for other schemas.
+ * object-typed fields `depth` levels deep: `{ content, structuredContent?: { results } }`, and the
+ * value of `const` fields: `{ action: "start", id }`. Undefined for other schemas.
  */
 function describeFields(schema: unknown, depth: number): string | undefined {
 	if (typeof schema !== "object" || schema === null) return undefined;
@@ -280,7 +280,13 @@ function describeFields(schema: unknown, depth: number): string | undefined {
 	if (type !== "object" || typeof properties !== "object" || properties === null) return undefined;
 	const requiredNames = new Set(Array.isArray(required) ? required : []);
 	const fields = Object.entries(properties).map(([name, field]) => {
-		const nested = depth > 0 ? describeFields(field, depth - 1) : undefined;
+		const constValue = (field as Record<string, unknown> | null)?.const;
+		const nested =
+			constValue !== undefined
+				? JSON.stringify(constValue)
+				: depth > 0
+					? describeFields(field, depth - 1)
+					: undefined;
 		return `${name}${requiredNames.has(name) ? "" : "?"}${nested ? `: ${nested}` : ""}`;
 	});
 	return `{ ${fields.join(", ")} }`;
@@ -288,13 +294,30 @@ function describeFields(schema: unknown, depth: number): string | undefined {
 
 /**
  * What a script call resolves to, in one line: `a string`, the field names of an object (see
- * {@link describeFields}), or the rendered type for anything else.
+ * {@link describeFields}), the field names of each object in a union, the rendered type when it
+ * fits on one line, or a pointer to `describeTool()` for anything longer. The full type of a large
+ * schema would otherwise be sent with every request (#11501).
  */
-function describeOutput(schema: CodemodeJsonSchema | undefined): string {
+function describeOutput(name: string, schema: CodemodeJsonSchema | undefined): string {
 	const type = renderToolOutputType(schema);
 	if (type === "string") return "a string";
-	const fields = describeFields(schema, 1);
-	return `\`${fields ?? type.replace(/\s+/g, " ")}\``;
+	const fields = describeFields(schema, 1) ?? describeUnionFields(schema);
+	if (fields) return `\`${fields}\``;
+	if (!type.includes("\n")) return `\`${type}\``;
+	return `a value; \`await describeTool(${JSON.stringify(name)})\` shows its type`;
+}
+
+/**
+ * The field names of each variant of an `anyOf`/`oneOf` whose variants are all objects, joined by
+ * `|`.
+ */
+function describeUnionFields(schema: unknown): string | undefined {
+	if (typeof schema !== "object" || schema === null) return undefined;
+	const { anyOf, oneOf } = schema as Record<string, unknown>;
+	const variants = anyOf ?? oneOf;
+	if (!Array.isArray(variants) || variants.length === 0) return undefined;
+	const described = variants.map((variant) => describeFields(variant, 0));
+	return described.every((fields) => fields !== undefined) ? described.join(" | ") : undefined;
 }
 
 /**
@@ -302,7 +325,7 @@ function describeOutput(schema: CodemodeJsonSchema | undefined): string {
  * arguments are the tool's declared parameters, so they are not repeated.
  */
 function describeScriptCall(tool: AgentTool<any>): string {
-	return `${tool.description.trim()}\n\nCodemode: \`tools.${toCodemodeIdentifier(tool.name)}(args)\` resolves to ${describeOutput(toCodemodeDeclaration(tool).outputSchema)}.`;
+	return `${tool.description.trim()}\n\nCodemode: \`tools.${toCodemodeIdentifier(tool.name)}(args)\` resolves to ${describeOutput(tool.name, toCodemodeDeclaration(tool).outputSchema)}.`;
 }
 
 /**
diff --git a/packages/coding-agent/test/suite/agent-session-codemode.test.ts b/packages/coding-agent/test/suite/agent-session-codemode.test.ts
index 4aeeb9940..ff39f1cd6 100644
--- a/packages/coding-agent/test/suite/agent-session-codemode.test.ts
+++ b/packages/coding-agent/test/suite/agent-session-codemode.test.ts
@@ -184,6 +184,49 @@ describe("AgentSession codemode tool", () => {
 		expect(description("echo")).toBe("Echo text back.\n\nSecond paragraph.");
 	});
 
+	// #11501
+	it("summarizes union and multi-line output types instead of inlining them", async () => {
+		const outputTool = (name: string, outputSchema: AgentTool["outputSchema"]): AgentTool => ({
+			name,
+			label: name,
+			description: `${name} tool`,
+			parameters: Type.Object({}),
+			outputSchema,
+			execute: async () => ({ content: [], details: {} }),
+		});
+		const process = outputTool(
+			"process",
+			Type.Union([
+				Type.Object({
+					action: Type.Literal("start"),
+					process: Type.Object({ id: Type.String({ description: "Process id" }), pid: Type.Integer() }),
+				}),
+				Type.Object({
+					action: Type.Literal("list"),
+					processes: Type.Array(Type.Object({ id: Type.String() })),
+					truncated: Type.Optional(Type.Boolean()),
+				}),
+			]),
+		);
+		const entries = outputTool("entries", Type.Array(Type.Object({ id: Type.String(), size: Type.Integer() })));
+		const count = outputTool("count", Type.Number());
+		const harness = await setup([
+			(pi: ExtensionAPI) => {
+				for (const tool of [process, entries, count]) pi.registerTool(createToolDefinitionFromAgentTool(tool));
+			},
+		]);
+		harness.session.setActiveToolsByName(["process", "entries", "count", "codemode"]);
+		const description = (name: string) =>
+			harness.session.agent.state.tools.find((tool) => tool.name === name)?.description ?? "";
+		expect(description("process")).toBe(
+			'process tool\n\nCodemode: `tools.process(args)` resolves to `{ action: "start", process } | { action: "list", processes, truncated? }`.',
+		);
+		expect(description("entries")).toBe(
+			'entries tool\n\nCodemode: `tools.entries(args)` resolves to a value; `await describeTool("entries")` shows its type.',
+		);
+		expect(description("count")).toBe("count tool\n\nCodemode: `tools.count(args)` resolves to `number`.");
+	});
+
 	// #10343
 	it("shows the guidelines of tools that do not fit the inline budget through describeTool()", async () => {
 		const harness = await setup();