`useH2c` with `connections` > 1 causes HTTPParserError

[rambo-panda]

Bug Description

When using undici's Agent with useH2c: true and connections > 1, it results in a HTTPParserError: Response does not match the HTTP/1.1 protocol error.

Reproducible By

Successful Case (connections=1)

const agent = new Agent({
useH2c: true,
connections: 1,// ✅ Works correctly
});

await request("http://localhost:3002", {
dispatcher: agent,
path: "/test",
});

Failure Case (connections>1)

const agent = new Agent({
useH2c: true,
connections: 2,// ❌ Fails
});

await Promise.all([
request("http://localhost:3002", { dispatcher: agent, path: "/test1" }),
request("http://localhost:3002", { dispatcher: agent, path: "/test2" }),
request("http://localhost:3002", { dispatcher: agent, path: "/test3" }),
]);

Expected Behavior

When useH2c: true is set, the Agent should properly handle HTTP/2 cleartext connections regardless of the connections parameter value, maintaining multiple HTTP/2 connections if specified.

Logs & Screenshots

Error Message

HTTPParserError: Response does not match the HTTP/1.1 protocol (Expected HTTP/, RTSP/ or ICE/)
at Parser.execute (/path/to/undici/lib/dispatcher/client-h1.js:361:17)
at Parser.readMore (/path/to/undici/lib/dispatcher/client-h1.js:301:12)
at Socket.onHttpSocketReadable (/path/to/undici/lib/dispatcher/client-h1.js:879:18)

Error data fragment: 0000000400000000000000080700000000000000 (HTTP/2 SETTINGS frame)

TCP Connection Monitoring

connections=1:

[TCP connection #1] Establishing connection
[TCP connection #1] ✅ Connection established
[TCP connection #1] 📥 Receiving HTTP/2 data
✅ Test passed

connections=2:

[TCP connection #2] Establishing connection
[TCP connection #3] Establishing connection
[TCP connection #2] ✅ Connection established
[TCP connection #3] ✅ Connection established
[TCP connection #2] 📥 Receiving data [HTTP/2 binary frame]: 0000000400000000...
[TCP connection #3] 📥 Receiving data [HTTP/2 binary frame]: 0000000400000000...
[TCP connection #3] 🔴 Connection closed
[TCP connection #4] Establishing connection ← Attempting reconnect with HTTP/1.1 parser
❌ HTTPParserError

Environment

• Node.js version: v23.6.1
• undici version: 7.18.2
• Test scenario: HTTP/2 Cleartext (h2c) connection

Additional context

Root cause analysis:

1. When connections > 1, Agent creates Pool instead of Client
2. Pool's constructor doesn't explicitly handle useH2c option
3. Pool doesn't pass useH2c to created Client instances
4. Subsequent connections use HTTP/1.1 parser but receive HTTP/2 frames

[rambo-panda]

If connections takes the default value of null, there will also be such issues