Debugging Coding Agents with a Local Proxy

import http from "node:http";
import { appendFileSync } from "node:fs";
import { randomUUID } from "node:crypto";

const PORT = Number(process.env.PORT ?? 8787);
const UPSTREAM = process.env.VENICE_UPSTREAM ?? "https://api.venice.ai";
const LOG_FILE = process.env.LOG_FILE ?? "venice-debug.jsonl";

// Node's fetch sets these itself, and decompresses the response for us
const SKIP_REQUEST_HEADERS = new Set([
  "host", "connection", "content-length", "transfer-encoding", "accept-encoding",
]);
const SKIP_RESPONSE_HEADERS = new Set([
  "content-encoding", "content-length", "transfer-encoding", "connection", "set-cookie",
]);

function parseJson(text) {
  try {
    return JSON.parse(text);
  } catch {
    return text || undefined;
  }
}

function parseSse(text) {
  return text
    .split("\n")
    .filter((line) => line.startsWith("data: ") && line !== "data: [DONE]")
    .map((line) => parseJson(line.slice(6)));
}

function redact(headers) {
  const auth = headers.authorization;
  if (!auth) return headers;
  return { ...headers, authorization: `Bearer …${auth.slice(-4)}` };
}

function findUsage(body) {
  const items = Array.isArray(body) ? [...body].reverse() : [body];
  for (const item of items) {
    const usage = item?.usage ?? item?.response?.usage;
    if (usage) return usage;
  }
}

function veniceHeaders(headers) {
  const picked = {};
  headers.forEach((value, key) => {
    if (key.startsWith("x-venice-") || key.startsWith("x-ratelimit-")) picked[key] = value;
  });
  return picked;
}

const server = http.createServer(async (req, res) => {
  const id = randomUUID().slice(0, 8);
  const started = performance.now();

  const chunks = [];
  for await (const chunk of req) chunks.push(chunk);
  const requestBody = Buffer.concat(chunks);

  const headers = {};
  for (const [key, value] of Object.entries(req.headers)) {
    if (!SKIP_REQUEST_HEADERS.has(key)) headers[key] = value;
  }

  // Stop the upstream request if the agent hangs up mid-stream
  const abort = new AbortController();
  res.on("close", () => {
    if (!res.writableFinished) abort.abort();
  });

  const entry = {
    id,
    time: new Date().toISOString(),
    method: req.method,
    path: req.url,
    requestHeaders: redact(req.headers),
    request: parseJson(requestBody.toString()),
  };

  let upstream;
  try {
    upstream = await fetch(UPSTREAM + req.url, {
      method: req.method,
      headers,
      body: requestBody.length ? requestBody : undefined,
      signal: abort.signal,
    });
  } catch (err) {
    entry.error = `Could not reach Venice: ${err.message}`;
    res.writeHead(502, { "content-type": "application/json" });
    res.end(JSON.stringify({ error: entry.error }));
    return finish(entry, started);
  }

  const responseHeaders = {};
  upstream.headers.forEach((value, key) => {
    if (!SKIP_RESPONSE_HEADERS.has(key)) responseHeaders[key] = value;
  });
  res.writeHead(upstream.status, responseHeaders);

  const received = [];
  try {
    for await (const chunk of upstream.body ?? []) {
      entry.firstByteMs ??= Math.round(performance.now() - started);
      received.push(Buffer.from(chunk));
      res.write(chunk);
    }
  } catch (err) {
    entry.error = abort.signal.aborted
      ? "Agent disconnected before the response finished"
      : `Stream ended early: ${err.message}`;
  }
  res.end();

  const text = Buffer.concat(received).toString();
  const isStream = upstream.headers.get("content-type")?.includes("text/event-stream");
  entry.status = upstream.status;
  entry.stream = Boolean(isStream);
  entry.responseHeaders = veniceHeaders(upstream.headers);
  entry.response = isStream ? parseSse(text) : parseJson(text);
  entry.usage = findUsage(entry.response);
  finish(entry, started);
});

function finish(entry, started) {
  entry.durationMs = Math.round(performance.now() - started);
  appendFileSync(LOG_FILE, JSON.stringify(entry) + "\n");

  const model = entry.request?.model ?? "-";
  const tokens = entry.usage
    ? ` tokens=${entry.usage.prompt_tokens ?? entry.usage.input_tokens}→${entry.usage.completion_tokens ?? entry.usage.output_tokens}`
    : "";
  const ttfb = entry.firstByteMs === undefined ? "" : ` ttfb=${entry.firstByteMs}ms`;
  console.log(
    `[${entry.id}] ${entry.method} ${entry.path} model=${model} status=${entry.status ?? "ERR"}` +
      `${entry.stream ? " stream" : ""} ${entry.durationMs}ms${ttfb}${tokens}` +
      `${entry.error ? ` error="${entry.error}"` : ""}`,
  );
}

server.listen(PORT, "127.0.0.1", () => {
  console.log(`Venice debug proxy on http://127.0.0.1:${PORT}/api/v1 → ${UPSTREAM}`);
  console.log(`Logging full requests and responses to ${LOG_FILE}`);
});