Format guide

AAF files: structure and contents.

An AAF file is an Advanced Authoring Format package: an open interchange format, standardised by the Advanced Media Workflow Association, that carries an edited composition and its metadata between applications such as Avid Media Composer, Pro Tools, Adobe Premiere Pro and DaVinci Resolve. An AAF stores the edit — tracks, clips, timecode, transitions, effect parameters and source references — as a graph of mobs and slots inside a structured-storage container. It can either link to external media (a link AAF) or embed the essence itself (an embedded AAF), which is why AAF file sizes range from kilobytes to many gigabytes.

What does an AAF file contain?

AAF describes an edit as a hierarchy of mobs. Each mob is an object with a unique MobID, and mobs reference each other rather than duplicating data.

Composition mobs: the sequence itself. Each holds timeline slots for video and audio, and each slot holds components in order — source clips, filler, transitions and effects.
Master mobs: the logical clips the composition points at, one per piece of material.
Source mobs: the physical origin of that material, carrying the essence descriptor — codec, frame rate, raster, audio sample rate and channel count — plus reel name and start timecode.
Essence data: present only in an embedded AAF. A link AAF omits it and relies on the destination application relinking to media on disk.
Operation groups and parameters: effects and their keyframed values, which survive interchange only when both applications understand the same effect definitions.

AAF vs OMF

OMF (Open Media Framework) is AAF's predecessor and still appears in audio handoffs. AAF superseded it for good reasons.

	AAF	OMF
Status	Current AMWA standard	Legacy, effectively frozen
File size ceiling	No 2 GB limit	2 GB per file
Video handling	Video and audio	Audio in practice
Metadata depth	Rich: effects, descriptors, extensible	Limited
Typical use today	Picture and sound turnover	Older Pro Tools round trips

Why AAF conforms fail

Most failed conforms come from a small set of causes, and all of them are visible in the file before anyone opens it in the destination application.

Missing media: a link AAF whose source mobs point at essence that was never delivered alongside it.
Rate mismatch: an edit rate or audio sample rate that the destination timeline does not share, which silently shifts sync.
Unsupported effects: operation groups the destination cannot interpret, which arrive flattened, ignored or as offline gaps.
Truncated or malformed structure: a package cut short in transfer, which some applications open partially rather than rejecting.
Reel and timecode gaps: source mobs missing the reel names or start timecode a conform depends on.

How to inspect an AAF before you conform

Opening the AAF in the destination NLE is the slowest way to find a problem with it. Reading the structure directly is faster and is safe to automate.

pyaaf2 is a pure-Python implementation of the format that parses an AAF without Avid or Pro Tools installed. The Avid Media Composer MCP server's avid_analyze_aaf tool wraps it and returns mobs, slots, components, essence descriptors and detected structural risks as JSON, so an AI client can answer whether a package will conform from the file itself. The read is offline and never modifies the package.

MCP tools for this

The open-source Avid Media Composer MCP server exposes these tools to any MCP-compatible AI client. Every one of them is read-only.

Install the server All tools

Frequently asked questions

What is an AAF file?

An AAF file is an Advanced Authoring Format package: an open interchange format that carries an edited composition — tracks, clips, timecode, transitions and effect metadata — between Avid Media Composer, Pro Tools, Premiere Pro, Resolve and other applications.

What does an AAF file contain?

A graph of mobs: composition mobs holding the sequence's tracks and components, master mobs for each logical clip, and source mobs carrying essence descriptors, reel names and timecode. An embedded AAF also contains the media essence; a link AAF does not.

What is the difference between AAF and OMF?

AAF is the current AMWA standard with no 2 GB file size limit, full video support and rich effect metadata. OMF is its legacy predecessor, capped at 2 GB per file and used mainly for older Pro Tools audio round trips.

How do I open an AAF file to check it?

For structural inspection rather than editing, use the open-source pyaaf2 library, or the avid_analyze_aaf tool in the Avid Media Composer MCP server, which reports mobs, slots, components, descriptors and conform risks without opening an NLE.

Why is my AAF file so large?

Because it is an embedded AAF: the media essence is stored inside the package rather than linked externally. A link AAF describing the same sequence is usually only kilobytes.

Related guides

This page documents an independent open-source project. It is not affiliated with or endorsed by Avid Technology, Inc. Behaviour described for the MCP server reflects the published package; confirm format and application details against Avid's own documentation before relying on them in production.