ACP: `opencode acp` fails to parse Content-Length framing on stdio — empty initialize response, agents see empty turns

What happened

opencode acp (stdio, Agent Client Protocol) fails to parse the very first JSON-RPC message (initialize) when it is framed with the standard Content-Length header used by ACP/LSP over stdio. The server logs a parse error to stderr and never answers; the client sees an empty stream / hang, and ACP clients end up with "empty agent turns".

Environment

• opencode: 1.18.29
• installed via: standalone binary at /usr/bin/opencode (bun bundle)
• OS: CachyOS (Arch-based), Linux 7.2.3
• terminal: not relevant (stdio transport)

Repro

Minimal Python script that speaks the framing ACP specifies (Content-Length: N\r\n\r\n + JSON):

import subprocess, json, threading

p = subprocess.Popen(
    ["opencode", "acp", "--cwd", "/tmp"],
    stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE,
)

def send(obj):
    data = json.dumps(obj).encode()
    p.stdin.write(f"Content-Length: {len(data)}\r\n\r\n".encode() + data)
    p.stdin.flush()

send({
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
        "protocolVersion": 1,
        "clientCapabilities": {
            "fs": {"read_text_file": True, "write_text_file": True},
            "terminal": True,
        },
    },
})

out = []
def reader():
    while True:
        b = p.stdout.read1(65536)
        if not b:
            break
        out.append(b)

t = threading.Thread(target=reader, daemon=True)
t.start()
t.join(20)

print("BYTES RECEIVED:", sum(len(b) for b in out))
print("STDOUT RAW:", repr(b"".join(out)[:600]))
p.kill()
print("STDERR:", p.stderr.read().decode()[:800])

Actual result

BYTES RECEIVED: 0
STDOUT RAW: b''
STDERR: Failed to parse JSON message: Content-Length: 184 2 | // @bun
3 | `);o=ue.pop()||"";for(let We of ue){let N=We.trim();if(N)try{let E=JSON.parse(N);i.enqueue(E)}catch(E){console.error("Failed to parse JSON message:",N,E)}}}...
                                                                                 ^
SyntaxError: JSON Parse error: Unexpected identifier "Content"
      at start (/$bunfs/root/chunk-rat302vz.js:3:78)

Expected result

The server should consume the Content-Length: N\r\n\r\n framing header, parse the JSON body, and respond with a valid initialize result (protocolVersion + capabilities), then accept session/new / session/prompt normally.

Analysis

The error comes from the stdio JSON parser (bundled bun code): it appears to split the incoming buffer on newlines and JSON.parse each chunk, so the framing header line Content-Length: 184\r\n\r\n leaks into the parser as if it were JSON — hence Unexpected identifier "Content". When the body happens to be delivered in a way that survives this naive splitting, parsing sometimes succeeds; that matches the symptom we saw in production (via the opencode acp command run by an ACP client): sessions sometimes start and then return empty end_turn turns with zero tool calls, instead of failing loudly.

Context

This was diagnosed while running coding lanes through opencode acp on opencode 1.18.29: every lane returned end_turn with 0 chars and 0 tool calls in ~3-7s. The exact same model (opencode run --model <provider>/<model> "Responda exatamente: STATUS OK") answers correctly via the regular CLI, which rules out model/provider issues — the bug is in the ACP server's stdio framing parser.