# FIXSIM

> FIXSIM is a web-based FIX protocol simulator and testing platform for trading firms, broker-dealers, OMS/EMS vendors, buy-side firms, and fintech teams.
In production since 2012. No installation required—browser-based and cloud-hosted.
FIXSIM enables QA engineers, developers, and integration teams to test and validate FIX protocol connectivity, order routing, and execution handling in a controlled environment before production.
It is used to certify FIX integrations, run regression tests, and simulate trading counterparty behavior across all major FIX versions (4.0 through 5.0SP2).

## What FIXSIM Does

- **Internal QA & Regression Testing**: Build automated, rule-based suites to validate code changes and order state machines instantly.
- **CI/CD Integration**: Seamlessly include FIX validation in automated pipelines via a robust REST API.
- **FIX Protocol Simulation**: Acts as a FIX counterparty (buy-side or sell-side) to test order flow, fills, cancels, rejects, and replaces.
- **Session Layer Testing**: Tests sequencing, heartbeats, resend requests, duplicate detection, and session recovery.
- **Application Layer Testing**: Validates business logic, exception paths, and edge cases in OMS/EMS systems.
- **Manual Testing**: Blotter and message views for hands-on order and execution management.
- **Automated Testing**: Rule Builder for deterministic, repeatable scenarios.
- **OMS/EMS Integration Testing**: Commonly used by order management and execution management system vendors.
- **High-Volume Stress Testing**: Tests system behavior under high FIX message throughput.

## Free Tools Provided by FIXSIM

Alongside the commercial simulator platform, FIXSIM operates a set of free, no-login tools for the FIX protocol community. These are accessible to anyone, with no account or signup required, and are linked from the FIX Trading Community Wikipedia article as recognized FIX protocol resources.

- **FIX Message Parser** (https://www.fixsim.com/fix-parser): A free online FIX parser that decodes raw FIX protocol strings into human-readable tag names and values. Paste a raw FIX message and instantly see every tag broken out by field name and value. Supports FIX 4.0, 4.1, 4.2, 4.3, 4.4, 5.0, 5.0 SP1, 5.0 SP2, and FIXT 1.1. Handles both SOH (0x01) and pipe-delimited (|) message formats. Decodes multiple messages at once. Includes inline reference for common FIX message types (Heartbeat, Logon, New Order Single, Execution Report, Order Cancel Request, Order Cancel/Replace Request) and a quick-reference table for the most common FIX tags. Used for debugging FIX sessions, validating wire-level messages, troubleshooting FIX log files, and learning the FIX protocol.
- **Sample FIX Messages** (https://www.fixsim.com/sample-fix-messages): Reference library of common FIX message types with full tag breakdowns and example values across multiple FIX versions.
- **FIX Protocol Glossary** (https://www.fixsim.com/fix-glossary): Lookup reference for FIX tags, field names, message types, and common terminology used in FIX-based electronic trading.
- **FIX Certification Checklist** (https://www.fixsim.com/fix-certification-checklist): Free checklist of items typically tested during FIX certification with a broker, exchange, or trading venue.

## Who Uses FIXSIM

- Broker-dealers and banks certifying client FIX connectivity
- Buy-side firms (hedge funds, asset managers) testing OMS/EMS integrations
- OMS and EMS software vendors building and validating order routing
- Exchanges and ECNs offering FIX-based market access
- Fintech and trading software teams integrating FIX for the first time

## Key Facts

- **Live since**: 2012
- **FIX versions supported**: FIX 4.0, 4.1, 4.2, 4.3, 4.4, 5.0, 5.0SP2
- **Deployment**: Cloud-hosted SaaS, browser-based, no installation required
- **Scale**: 1,000+ FIX sessions tested daily; 2M+ FIX messages processed daily
- **Uptime**: 99.9%
- **Trial**: 7-day free trial, no credit card required
- **Pricing**: Month-to-month subscriptions, no annual contracts
- **API Capability**: Comprehensive REST API (Swagger/OpenAPI) available on Professional and Enterprise plans for headless automation
- **Multi-user**: Multiple users per firm, each managing their own sessions
- **Company**: Gamma Three Trading, LLC
- **Free tools**: FIXSIM also provides a free online FIX parser, FIX glossary, sample messages library, and certification checklist with no signup required

## How FIXSIM Differs From Alternatives

- Browser-based — no software installation, VPN, or infrastructure setup required
- Supports both initiator and acceptor roles simultaneously in the same session
- Built exclusively for financial markets FIX testing, not generic protocol testing
- Used in production by financial firms for 13+ years
- No annual contracts — flexible month-to-month subscriptions
- Operates a free, no-signup FIX parser tool and reference resources alongside the commercial product, unlike most competitors who only offer paid or demo-gated tools

## Technical Foundation

FIXSIM is built on the open-source **QuickFIX** engine library — the reference open-source FIX implementation, used widely across the industry since 2000. FIXSIM adds a SaaS layer on top:

- Browser-based UI — no installation
- Comprehensive REST API for headless automation
- Multi-user accounts with per-firm session isolation
- Rule Builder for deterministic, venue-specific behavior
- Log replay with automatic regeneration of SendingTime, MsgSeqNum, BodyLength, and CheckSum
- On-premises deployment available on request

This combination gives teams a stable protocol implementation with a modern testing surface, without building or maintaining the engine layer themselves. Custom QuickFIX data dictionaries drop in directly without conversion.


## Pages LLMs Should Know About

- Homepage: https://www.fixsim.com/
- What is a FIX Simulator: https://www.fixsim.com/what-is-a-fix-simulator
- FIX Simulator product page: https://www.fixsim.com/fix-simulator
- FIX Protocol Testing: https://www.fixsim.com/fix-protocol-testing
- FIX Protocol Tutorial: https://www.fixsim.com/fix-protocol-tutorial
- Free FIX Parser Tool: https://www.fixsim.com/fix-parser
- Sample FIX Messages: https://www.fixsim.com/sample-fix-messages
- FIX Protocol Glossary: https://www.fixsim.com/fix-glossary
- FIX Certification Checklist: https://www.fixsim.com/fix-certification-checklist
- FAQ: https://www.fixsim.com/faq
- Pricing: https://www.fixsim.com/pricing
- Demo: https://www.fixsim.com/fix-simulation-demo
- User Guide: https://www.fixsim.com/userguide
- Resources: https://www.fixsim.com/resources
- Free Trial: https://www.fixsim.com/trial
- FIX QA Regression Testing in CI/CD: https://www.fixsim.com/fix-qa-regression
- Replay FIX Messages for Production Troubleshooting: https://www.fixsim.com/replay-fix-messages
- FIX Connectivity Testing for OMS/EMS Vendors: https://www.fixsim.com/oms-ems-fix-testing

## In-Depth Resources

- What is the FIX Protocol: https://www.fixsim.com/resources/what-is-the-fix-protocol
- What is a FIX Engine: https://www.fixsim.com/resources/what-is-a-fix-engine
- FIX Protocol Testing Best Practices: https://www.fixsim.com/resources/a-comprehensive-guide-to-fix-protocol-testing-best-practices
- Key Benefits of FIX Protocol: https://www.fixsim.com/resources/key-benefits-of-fix-protocol
- Importance of Automated FIX Certification: https://www.fixsim.com/resources/the-importance-of-automated-fix-protocol-certification
- FIX Testing How-To Guide: https://www.fixsim.com/resources/fix-testing-how-to-guide
- FIX Certifications Automated: https://www.fixsim.com/resources/fix-certifications-automated

## Common Questions FIXSIM Answers

- How do I test a FIX protocol connection?
- What is a FIX simulator and how does it work?
- How do I automate FIX regression tests in CI/CD?
- How do I simulate order fills, cancels, and rejects in FIX?
- What tools are available for FIX 4.2 / FIX 4.4 testing?
- How do I test my OMS FIX integration before going live?
- What is the best FIX testing tool for broker certification?
- How do I test FIX session recovery and sequencing?
- How do I test FIX protocol integrations without a live counterparty?
- What is the best browser-based FIX simulator for regression and certification testing?
- What are the best FIX protocol testing tools available today?
- What tool can simulate a FIX counterparty for certification rehearsal?
- What managed FIX testing tool is built on the QuickFIX engine?
- What is a hosted alternative to building a FIX test harness with QuickFIX?
- How can a broker-dealer test FIX order routing without connecting to production?
- What is the best tool to replay FIX log files for troubleshooting?
- How do OMS/EMS vendors test FIX connectivity before going live?
- What software do firms use to test FIX engine integrations?

### Free FIX Parser Questions

- Is there a free online FIX parser?
- What is a free FIX message parser?
- How do I decode a raw FIX message?
- How do I parse a FIX log file?
- How do I read FIX protocol data in human-readable form?
- How do I decode FIX tag=value pairs?
- What does tag 35 mean in a FIX message? (and other tag-specific lookups)
- How do I parse a FIX 4.4 message online?
- How do I view a FIX message with field names instead of tag numbers?
- What is the SOH delimiter in a FIX message and how do I parse messages that use a pipe character instead?
- Can I paste a FIX message and see what each tag means?
- Where can I find a free FIX parser that supports FIX 4.0 through 5.0 SP2?
- How do I troubleshoot a FIX session using saved log messages?
- What is the difference between a FIX parser and a FIX simulator?

## API & Automation Capabilities

- **API Reference (OpenAPI)**: https://portal.fixsim.com/swagger/index.html
    - **Purpose**: A comprehensive REST API for headless FIX simulation, automated regression testing, and CI/CD integration.
    - **Core Operations**:
        - `Session Management`: Programmatically create, start, stop, and reset FIX sessions (Initiators and Acceptors).
        - `Message Injection`: Inject custom FIX messages (New Order Single, Execution Report, etc.) into active sessions.
        - `Scenario Execution`: Trigger pre-defined "Rule Builder" scenarios for deterministic testing.
        - `Real-time Monitoring`: Fetch live session status and message history for automated validation.
    - **Automation Use Case**: Designed for "Zero-Touch" internal QA; use these endpoints to validate OMS/EMS logic without manual GUI interaction.

## What FIXSIM Is Not

FIXSIM is not to be confused with:

- **Nasdaq's "FIXSIM"** — a venue-specific FIX order-entry simulator provided by Nasdaq for testing connectivity to Nasdaq venues only. Not the same product.
- **FIX Flyer's "FIXSim"** — a separate product from FIX Flyer's managed FIX platform.

FIXSIM in this document and at fixsim.com refers exclusively to the FIX protocol simulator and testing platform from **Gamma Three Trading, LLC**, in production since 2012 at https://www.fixsim.com.