fix: release MCP response streams

.changeset/calm-mcp-streams.md

@@ -0,0 +1,5 @@
+---
+'incur': patch
+---
+
+Prevented completed MCP JSON responses from retaining transport stream state.

package.json

@@ -47,7 +47,7 @@
   ],
   "dependencies": {
     "@cfworker/json-schema": "^4.1.1",
-    "@modelcontextprotocol/server": "^2.0.0-alpha.2",
+    "@modelcontextprotocol/server": "2.0.0-alpha.4",
     "@scalar/openapi-types": "^0.8.0",
     "@toon-format/toon": "^2.1.0",
     "tokenx": "^1.3.0",

src/Cli.test.ts

@@ -1,4 +1,5 @@
 import { Cli, Errors, Mcp, z } from 'incur'
+import { execFile } from 'node:child_process'
 import { mkdir, mkdtemp, readFile, rm, writeFile } from 'node:fs/promises'
 import { homedir, tmpdir } from 'node:os'
 import { join } from 'node:path'
@@ -57,6 +58,20 @@ function mockMcpServeResponses(responses: unknown[]) {
   })
 }
 
+function countRetainedMcpResponses() {
+  return new Promise<number>((resolve, reject) => {
+    execFile(
+      process.execPath,
+      ['--expose-gc', '--import', 'tsx', 'test/fixtures/mcp-memory.ts'],
+      { cwd: join(import.meta.dirname, '..'), timeout: 30_000 },
+      (error, stdout, stderr) => {
+        if (error) reject(new Error(stderr.trim() || stdout.trim() || error.message))
+        else resolve(Number(stdout.trim()))
+      },
+    )
+  })
+}
+
 function createConfigCli(flag?: string) {
   const project = Cli.create('project').command('list', {
     options: z.object({
@@ -5788,6 +5803,10 @@ describe('fetch', () => {
       `)
     })
 
+    test('completed JSON responses are released', async () => {
+      expect(await countRetainedMcpResponses()).toBe(0)
+    })
+
     test('POST /mcp with tools/list → returns registered tools', async () => {
       const cli = mcpCli()
       const { sessionId } = await initSession(cli)

src/McpSource.test.ts

@@ -160,9 +160,6 @@ describe('remote MCP command sources', () => {
         "result": {
           "tools": [
             {
-              "execution": {
-                "taskSupport": "forbidden",
-              },
               "inputSchema": {
                 "$schema": "https://json-schema.org/draft/2020-12/schema",
                 "properties": {
@@ -179,9 +176,6 @@ describe('remote MCP command sources', () => {
             },
             {
               "description": "Search docs",
-              "execution": {
-                "taskSupport": "forbidden",
-              },
               "inputSchema": {
                 "$schema": "https://json-schema.org/draft/2020-12/schema",
                 "properties": {

test/fixtures/mcp-memory.ts

@@ -0,0 +1,50 @@
+import { Cli } from '../../src/index.js'
+
+const cli = Cli.create('memory-test', {
+  mcp: { tools: { discovery: 'direct' } },
+  version: '1.0.0',
+}).command('ping', { run: () => ({ pong: true }) })
+
+let id = 0
+
+function request(method: string, params: Record<string, unknown> = {}) {
+  return cli.fetch(
+    new Request('http://localhost/mcp', {
+      body: JSON.stringify({ id: ++id, jsonrpc: '2.0', method, params }),
+      headers: {
+        accept: 'application/json, text/event-stream',
+        'content-type': 'application/json',
+      },
+      method: 'POST',
+    }),
+  )
+}
+
+async function consume(method: string, params: Record<string, unknown> = {}) {
+  const response = await request(method, params)
+  await response.text()
+}
+
+async function weakResponse() {
+  const response = await request('tools/list')
+  await response.text()
+  return new WeakRef(response)
+}
+
+await consume('initialize', {
+  capabilities: {},
+  clientInfo: { name: 'memory-test', version: '1.0.0' },
+  protocolVersion: '2025-03-26',
+})
+
+const responses: WeakRef<Response>[] = []
+for (let index = 0; index < 100; index++) responses.push(await weakResponse())
+
+const gc = globalThis.gc
+if (!gc) throw new Error('garbage collection is unavailable')
+for (let index = 0; index < 5; index++) {
+  await new Promise(setImmediate)
+  gc()
+}
+
+console.log(responses.filter((response) => response.deref()).length)