FIX Log Analysis API

POST a FIX log from your engine, a script or a CI job and get the same statistics as the FIX log analyzer: message counts and rates, orders and outcomes, fill and reject rates, traded quantity and value, latency percentiles, reject reasons and session health. Free, no key, no sign-up. Or skip the server entirely and open a log from your application straight in the browser.

Privacy: an API request is processed in memory, not stored, not logged and not cached. The browser handoff below never reaches our server at all.

Endpoint

POST https://fixparser.zagtrader.com/api/v1/analyze

Raw text: the log as the body, Content-Type: text/plain or application/octet-stream.
Gzip: Content-Encoding: gzip, Content-Type: application/gzip, or any body that starts with the gzip bytes 1f 8b.
Multipart: multipart/form-data with a field named file: a .log, .txt, .gz or .zip file.

Query parameter	Meaning
format	`json` (default), `html` (a self-contained report file that works offline) or `text` (the plain-text summary)
clock	Clock for latency: `log` (default, the log timestamp), `52` (SendingTime) or `60` (TransactTime)
tz	IANA time zone for the times shown, for example `Europe/London`; default: as written in the log
anonymise	`1` replaces CompIDs and free text with placeholders (COMP1, TEXT1), as the parser's Anonymise switch does. The statistics never contain accounts, order IDs or party IDs.

GET on the same address returns a short usage document as JSON.

Examples

Raw text

curl --data-binary @fix.log -H "Content-Type: text/plain" "https://fixparser.zagtrader.com/api/v1/analyze"

Gzip (for logs over a few MB)

gzip -c fix.log | curl --data-binary @- -H "Content-Encoding: gzip" "https://fixparser.zagtrader.com/api/v1/analyze"

Multipart file upload (.log, .gz or .zip)

curl -F "file=@fix-logs.zip" "https://fixparser.zagtrader.com/api/v1/analyze"

HTML report to a file

curl --data-binary @fix.log -H "Content-Type: text/plain" "https://fixparser.zagtrader.com/api/v1/analyze?format=html" -o fix-report.html

Anonymised plain-text summary, times in London

curl --data-binary @fix.log -H "Content-Type: text/plain" "https://fixparser.zagtrader.com/api/v1/analyze?anonymise=1&format=text&tz=Europe/London"

PowerShell

$r = Invoke-RestMethod -Method Post -Uri "https://fixparser.zagtrader.com/api/v1/analyze" -InFile .\fix.log -ContentType "text/plain"
$r.stats.orders.fillRate
$r.report_url

Python (requests)

import requests

with open("fix.log", "rb") as f:
    r = requests.post("https://fixparser.zagtrader.com/api/v1/analyze", data=f, headers={"Content-Type": "text/plain"}, params={"clock": "log"})
r.raise_for_status()
stats = r.json()["stats"]
print(stats["orders"]["fillRate"], r.json()["report_url"])

Response

JSON with the input summary, the full statistics and a report_url that opens the charts in the browser. The report link carries the statistics only (totals and top lists, no messages) in the part after #, so opening it sends nothing to a server.

{
  "api": "zagtrader-fix-analytics",
  "version": "1",
  "generated_at": "2026-10-20T12:00:00.000Z",
  "input": {
    "bytes": 2453190, "compressed": false, "messages": 11022, "sessions": 3,
    "fix_versions": ["FIX.4.4", "FIX.4.2", "FIXT.1.1"],
    "first_time": "2026-10-19 09:25:00.000", "last_time": "2026-10-19 16:10:10.021",
    "time_zone": "as written in the log", "clock": "log", "anonymised": false
  },
  "stats": {
    "overview": { "messages": 11022, "sessions": 3, "peakPerSecond": { "count": 62, "ms": 1792402201000 }, "...": "..." },
    "orders": { "orders": 2041, "fills": 2843, "fillRate": 0.754, "rejectRate": 0.0088, "orderToTradeRatio": 0.91, "outcomes": ["..."] },
    "trading": { "symbols": ["..."], "currencies": ["..."], "sides": ["..."], "lastMarkets": ["..."] },
    "rejects": { "total": 24, "session": ["..."], "order": ["..."], "cancel": ["..."], "texts": ["..."] },
    "sessions": [{ "label": "ZAGOMS ↔ BROKERX", "gaps": 1, "resendRequests": 1, "heartBtInt": 30, "...": "..." }],
    "latency": { "clock": "log", "metrics": [{ "metric": "ack", "count": 2037, "p50": 1.77, "p90": 5.27, "p99": 10.2, "max": 2390 }] },
    "timeline": { "bucketMs": 300000, "buckets": ["..."] },
    "skipped": []
  },
  "report_url": "https://fixparser.zagtrader.com/report#r=...",
  "notes": ["Times are shown as written in the log; ..."]
}

Limits and errors

Request body: up to 4.5 MB. The hosting platform rejects larger bodies before they reach the API, so gzip larger logs (FIX logs compress about five times).
Decompressed log: up to 200 MB; decompression stops beyond that.
Processing time: up to 60 seconds per request; 100,000 messages take about ten seconds.
Fair use: rate limited to about 60 requests a minute per address. For continuous or high-volume use, talk to ZagTrader.

Errors are JSON: { "error": { "code": "no_fix_messages", "message": "..." } }

Status	Code	When
400	empty_body	The body or the file is empty.
400	no_fix_messages	No message starting with 8=FIX was found.
400	missing_file	A multipart form without a field named file.
400	invalid_parameter	format, clock or tz has a value the API does not accept.
413	body_too_large	The body is over 4.5 MB. Gzip the log first.
413	decompressed_too_large	The log is over 200 MB once decompressed.
415	unsupported_media_type	Send text, gzip, zip or multipart/form-data.
422	decode_failed	The gzip or zip data is damaged, or the file is not a text log.
429	rate_limited	More than 60 requests a minute from one address.
500	internal_error	Something failed on our side. The message never includes log content.

Open a log from your application

Your engine or tool can show a log in the full parser and analyzer without any upload: it opens the parser in the browser and hands the log over with window.postMessage. The log goes from your application to the browser on the same computer and is never uploaded to ZagTrader or anyone else. The parser page shows “Log received from … Nothing was uploaded.”

From a web page: a button

Open /?handoff=1 on a click (so pop-up blockers allow it), wait for zagtrader-fix-ready, then post the log. A complete page to copy: example-button.html.

JavaScript

const PARSER = "https://fixparser.zagtrader.com";
button.onclick = () => {
  const win = window.open(PARSER + "/?handoff=1", "zagtrader-fix-parser"); // on the click: no pop-up block
  addEventListener("message", function onMessage(e) {
    if (e.source !== win || e.origin !== PARSER) return;
    if (e.data.type === "zagtrader-fix-ready") {
      win.postMessage({ type: "zagtrader-fix-log", version: 1, name: "today.log", text: logText }, PARSER);
      removeEventListener("message", onMessage);
    }
  });
};

From a server or desktop engine: a local viewer file

Copy local-viewer-template.html, put the log inside <script type="text/plain" id="fixlog"> (write every </script in the log as <\/script), save it as a temporary .html file and open it with the default browser: start "" fix-log.html on Windows, xdg-open on Linux, open on macOS. The file shows the parser full-window (/?handoff=1&embed=1 in an iframe) and passes the log in. For large logs, gzip the log, base64-encode it and add data-encoding="base64-gzip"to the script block. The file also has an “Open in a new tab instead” link.

C# / .NET

using System.Diagnostics;
using System.IO;
using System.Net.Http;

// Fill the local viewer template with the log and open it in the default browser.
static async Task OpenInFixParser(string logText, string name = "engine.log")
{
    var template = await new HttpClient().GetStringAsync(
        "https://fixparser.zagtrader.com/handoff/local-viewer-template.html"); // or ship a copy with your app
    var safe = logText.Replace("</script", "<\\/script", StringComparison.OrdinalIgnoreCase);
    var html = template
        .Replace("data-name=\"engine.log\"", $"data-name=\"{System.Net.WebUtility.HtmlEncode(name)}\"")
        .Replace("PUT THE FIX LOG HERE", safe);
    var path = Path.Combine(Path.GetTempPath(), $"fix-log-{Guid.NewGuid():N}.html");
    await File.WriteAllTextAsync(path, html);
    Process.Start(new ProcessStartInfo(path) { UseShellExecute = true });
}

Python

import html, pathlib, tempfile, urllib.request, uuid, webbrowser

def open_in_fix_parser(log_text: str, name: str = "engine.log") -> None:
    """Fill the local viewer template with the log and open it in the default browser."""
    template = urllib.request.urlopen(
        "https://fixparser.zagtrader.com/handoff/local-viewer-template.html").read().decode("utf-8")
    safe = log_text.replace("</script", "<\\/script")
    page = (template
            .replace('data-name="engine.log"', f'data-name="{html.escape(name)}"')
            .replace("PUT THE FIX LOG HERE", safe))
    path = pathlib.Path(tempfile.gettempdir()) / f"fix-log-{uuid.uuid4().hex}.html"
    path.write_text(page, encoding="utf-8")
    webbrowser.open(path.as_uri())

Message format

Direction	Message	Notes
Parser to application	{ type: "zagtrader-fix-ready", version: 1 }	Sent when the parser page has loaded, and again every second until a log arrives.
Application to parser	{ type: "zagtrader-fix-log", version: 1, name?: string, text?: string, data?: ArrayBuffer \| Uint8Array \| Blob, view?: "analytics" \| "timeline" }	Exactly one of text or data. data may be plain text, gzip or zip bytes. view defaults to analytics. Any other key, a different version or a second log is ignored.
Parser to application	{ type: "zagtrader-fix-loaded", version: 1, messages: number }	The log is open; messages is the number of FIX messages found.
Parser to application	{ type: "zagtrader-fix-error", version: 1, message: string }	The log could not be read, or it has no FIX messages.

The parser accepts the log from any origin, including file:// pages (origin “null”), because the log only flows into a display tool. It takes the first valid log and stops listening.
Add embed=1 to hide the site header and footer when the parser fills a window or an iframe.
If no log arrives within 15 seconds the page says it is waiting; the parser works as usual meanwhile.
Size handling is the same as opening a file: large logs are parsed in a background thread with a progress bar.

Running FIX at scale? ZagTrader monitors sessions and order flow across all your FIX connections from one OMS/EMS.