Item IDs

New item ids are given every time an item is created (current_id) or modified (id). This graph shows how item IDs are increasing over time (red line). Also plotted are the number of new item IDs produced each day (peaks correspond to weekends).

To keep track of items in Team Fortress 2, each one is given a unique number called an <id>. No two items ever share the same id, which is why new items always have progressively higher id's as is evident from the graph above. Certain actions can update the id of an item. For example, if you trade an item to a friend, the item will be deleted from your backpack, and a new item with the same attributes will be 'created' in your friends backpack with a new id. Finally, with so many ways to 'destroy and recreate' items with new id's, Valve added an <original_id> attribute to all items in TF2. The original_id is the id given to an item when it is first created, for example as a Timed Drop, Purchase, or Craft.

A new original_id (brand new item) is created by the following:

Timed Drops
An item obtained through the drop system (or by using a Tag on a Stock Item)
Achievements
An item obtained by completing an achievement
Purchases
An item purchased from the Mann Co. Store or Mann Co. Online Store
Crafting
An item obtained through the crafting system
Store Promotions
An item obtained from the Mann Co. Store (e.g. Mann Co. Package)
Gifting
An item obtained from a Secret Saxton, Pile of Gifts, Pallet of Crates, etc. (Gift Wrap does not change an item's origin)
Support Granted
An item obtained from Steam Support
Opening a Crate
An item obtained from a crate
Earned Items
An item earned in-game (e.g. a reward for winning 10 duels)
MvM Rewards
An item awarded for completing a Mann Up mission/tour, or from a Squad Surplus Voucher

The following actions do not affect the original_id:

Trading
An item obtained through Steam Trade
Selling on Market
An item purchased from someone else via the Steam Marketplace
Using a Tool
E.g. Naming, Describing, Decaling, Painting, Strangifying, Killstreakifying, adding a Strange Part, Strange Filter, or Halloween Spell
Giftwrapping
Using giftwrap on an item
Restoring
Removing a modification
Adding Ingredients
Adding ingredients to a chemistry set or fabricator