Return compact JSON from MCP tool and resource responses

AGENTS.md

@@ -81,6 +81,10 @@ bun run format
 
 Prettier formats the following file types: `*.ts`, `*.js`, `*.json`, `*.md`
 
+### MCP responses
+
+Model-facing JSON (tool results and resources) is compact: use `jsonResponse` or `JSON.stringify(value)` without indentation.
+
 ## Project Structure
 
 - `src/` - Source code directory

src/lib/mcp/resource-templates.ts

@@ -85,7 +85,7 @@ export function registerJsonResourceCollection(
             mimeType: "application/json",
             text:
               resources.length > 0
-                ? JSON.stringify(resources, null, 2)
+                ? JSON.stringify(resources)
                 : options.emptyText,
           },
         ],
@@ -127,7 +127,7 @@ export function registerJsonResourceTemplate(
           {
             uri: uri.toString(),
             mimeType: "application/json",
-            text: JSON.stringify(resource, null, 2),
+            text: JSON.stringify(resource),
           },
         ],
       };

src/lib/mcp/responses.test.ts

@@ -10,7 +10,11 @@ import {
 } from "@onkernel/sdk";
 import { describe, expect, test } from "bun:test";
 
-import { errorResponse, throwToolError } from "@/lib/mcp/responses";
+import {
+  errorResponse,
+  jsonResponse,
+  throwToolError,
+} from "@/lib/mcp/responses";
 import { z } from "zod";
 
 function apiError(status: number, message: string) {
@@ -30,6 +34,14 @@ function caught(error: unknown) {
   throw new Error("throwToolError did not throw");
 }
 
+describe("jsonResponse", () => {
+  test("emits compact JSON", () => {
+    expect(jsonResponse({ a: 1, b: [2] }).content[0].text).toBe(
+      '{"a":1,"b":[2]}',
+    );
+  });
+});
+
 describe("throwToolError classification", () => {
   test("names Kernel API failures after their status", () => {
     expect(caught(apiError(404, "not found")).name).toBe("KernelApiError404");

src/lib/mcp/responses.ts

@@ -31,7 +31,7 @@ export function textResponse(text: string) {
 }
 
 export function jsonResponse(value: unknown) {
-  return textResponse(JSON.stringify(value, null, 2) ?? String(value));
+  return textResponse(JSON.stringify(value) ?? String(value));
 }
 
 export function itemsJsonResponse<T, U = T>(

src/lib/mcp/tools/auth-connections.ts

@@ -31,7 +31,7 @@ function safeJsonResponse(value: unknown) {
     content: [
       {
         type: "text" as const,
-        text: JSON.stringify(value, null, 2),
+        text: JSON.stringify(value),
       },
     ],
     structuredContent: value as Record<string, unknown>,

src/lib/mcp/tools/browser-repl.ts

@@ -137,7 +137,7 @@ function replToolContent(result: BrowserReplResult): ReplToolContent[] {
   };
 
   const content: ReplToolContent[] = [
-    { type: "text", text: JSON.stringify(summary, null, 2) },
+    { type: "text", text: JSON.stringify(summary) },
   ];
   for (const item of result.content ?? []) {
     if (item.type === "image") {

src/lib/mcp/tools/browsers.ts

@@ -346,8 +346,6 @@ async function readBrowserTelemetry(
     if (rawError) return errorResponse(`error: ${rawError}`);
   }
 
-  // Single-line JSON rather than the pretty-printed house helpers: a page
-  // carries up to 100 events and indentation would inflate the token cost.
   const serializedResponse = JSON.stringify(response);
   if (
     params.compact === false &&

src/lib/mcp/tools/durable-contracts.test.ts

@@ -271,7 +271,7 @@ describe("durable profile contracts", () => {
         expect(result.content).toEqual([
           {
             type: "text",
-            text: JSON.stringify({ id: "profile-1", name: "Renamed" }, null, 2),
+            text: JSON.stringify({ id: "profile-1", name: "Renamed" }),
           },
         ]);
       }
@@ -426,7 +426,7 @@ describe("durable proxy contracts", () => {
       expect(result.content).toEqual([
         {
           type: "text",
-          text: JSON.stringify({ id: "proxy-1", name: "Renamed" }, null, 2),
+          text: JSON.stringify({ id: "proxy-1", name: "Renamed" }),
         },
       ]);
     } finally {

src/lib/mcp/tools/shell.ts

@@ -5,7 +5,7 @@ import {
   type McpDependencies,
 } from "@/lib/mcp/dependencies";
 import { longOperationOptions } from "@/lib/mcp/request-options";
-import { throwToolError } from "@/lib/mcp/responses";
+import { jsonResponse, throwToolError } from "@/lib/mcp/responses";
 import {
   projectForOperation,
   projectSelectionInputSchema,
@@ -102,23 +102,12 @@ export function registerShellTool(
           ? Buffer.from(result.stderr_b64, "base64").toString("utf-8")
           : "";
 
-        return {
-          content: [
-            {
-              type: "text",
-              text: JSON.stringify(
-                {
-                  exit_code: result.exit_code,
-                  duration_ms: result.duration_ms,
-                  stdout,
-                  stderr,
-                },
-                null,
-                2,
-              ),
-            },
-          ],
-        };
+        return jsonResponse({
+          exit_code: result.exit_code,
+          duration_ms: result.duration_ms,
+          stdout,
+          stderr,
+        });
       } catch (error) {
         throwToolError("exec_command", "exec", error);
       }