feat(mcp): cap MCP tool output fed back to the model

A runaway MCP tool response could blow the context window unbounded. Truncate
each MCP tool's text output at 50k chars (capMcpOutput) with a visible
"[… N characters truncated]" notice. +2 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: oratis

packages/core/src/mcp/client.test.ts

@@ -11,6 +11,7 @@ import { mkdtemp, rm } from 'node:fs/promises';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import {
+  capMcpOutput,
   connectAllMcpServers,
   connectMcpServer,
   expandMcpResourceRefs,
@@ -442,6 +443,19 @@ describe('parseHelperOutput', () => {
   });
 });
 
+describe('capMcpOutput', () => {
+  it('passes through output under the cap', () => {
+    expect(capMcpOutput('short', 100)).toBe('short');
+  });
+  it('truncates over-long output with a notice', () => {
+    const big = 'x'.repeat(120);
+    const out = capMcpOutput(big, 100);
+    expect(out.startsWith('x'.repeat(100))).toBe(true);
+    expect(out).toMatch(/20 characters truncated/);
+    expect(out).toMatch(/100-char cap/);
+  });
+});
+
 describe('parseResourceRefs', () => {
   it('finds @server:scheme://path references', () => {
     const refs = parseResourceRefs(

packages/core/src/mcp/client.ts

@@ -179,6 +179,20 @@ interface FinishableTransport extends Transport {
   finishAuth(code: string): Promise<void>;
 }
 
+/** Max characters of MCP tool output fed back to the model (keeps a runaway
+ *  server response from blowing the context window). */
+export const MCP_OUTPUT_CAP = 50_000;
+
+/** Truncate over-long MCP output with a visible notice. Exported for testing. */
+export function capMcpOutput(text: string, cap = MCP_OUTPUT_CAP): string {
+  if (text.length <= cap) return text;
+  const omitted = text.length - cap;
+  return (
+    text.slice(0, cap) +
+    `\n\n[… ${omitted} characters truncated — MCP output exceeded the ${cap}-char cap]`
+  );
+}
+
 /**
  * Connect to one MCP server (stdio / http / sse). Returns a handle containing
  * the registered tools (qualified as `mcp__<server>__<tool>`).
@@ -264,7 +278,7 @@ export async function connectMcpServer(
               .map((c) => c.text ?? '')
               .join('\n') || '';
           return {
-            content: textParts || '(MCP tool returned no text content)',
+            content: textParts ? capMcpOutput(textParts) : '(MCP tool returned no text content)',
             isError: result.isError === true,
             data: { serverName, serverToolName: t.name },
           };