Addoninfo.txt
The plain-text file addoninfo.txt
contains the KeyValues 1 structure AddonInfo
specifying an addon’s metadata: its author, content flags, description, title, and version.
This file and its functionality were introduced with the Left 4 Dead engine branch.
Both the Source Engine and the deprecated external application
addoninstaller.exe
will parse this file, though with significant differences.
Source will parse this file from all addons listed in
addonlist.txt
when loading search paths from gameinfo.txt
at startup and with subsequent calls to ConCommand update_addon_paths
.
Source will retrieve metadata for all Steam Workshop addons from the Steam Workshop, overriding content in
addoninfo.txt
.
addoninstaller.exe
only parses addoninfo.txt
when staging addons during user-initialized installation.

addoninfo.txt
, even if you distribute through the Steam Workshop. Otherwise, your users will encounter the following undesirable behavior:
- your addon will load before all other correct addons specified in
addonlist.txt
; and - your addon will never appearing in the
CBaseModFrame
Addons
(the VGUI window for the addon manager), disallowing them from enabling or disabling it.

addoninstaller.exe
to install addons. As of at least client version 2.2.2.6 of 
addoninstaller.exe
association prompt. Many users will be unaware of addoninstaller.exe
or its appropriate usage unless they view this page on 
Contents
Specification
In the following specification, these terms will have the following meanings:
- shall indicates a mandatory requirement; and
- should indicates a recommendation.
The Source Engine will expect a specific KeyValues 1 structure in
addoninfo.txt
, specified below:
- You shall save
addoninfo.txt
with an UTF-8-compatible encoding. - You shall have at least one KeyValues block.
- You should define the name of the first KeyValues block as AddonInfo.
- You should populate the first KeyValues block with at least one key-value entry from table “Valid
AddonInfo
Keys”.
- You should provide a title, author, and version.
- You should provide at least one content tag.
- You should preserve the case of each key.
Key | Type | Length | Default Value | Description |
---|---|---|---|---|
addontitle | wchar_t
|
127 |
|
The addon’s title. |
addonauthor | wchar_t
|
119 |
|
The addon’s author. |
addonDescription | wchar_t
|
1023 |
|
The addon’s description. ![]() |
addonversion | wchar_t float (addoninstaller.exe )
|
31 | 0.0 1.0 (addoninstaller.exe )
|
The addon’s version. Conventionally a decimal number in the format major.minor , but you may write any string.
|
addonContent_Campaign | int
|
N/A | 0
|
If the addon contains custom campaigns. If the value is 0 , then the addon does not contain the specified content; otherwise, the addon contains the specified content.
|
addonContent_Map | int
|
N/A | 0
|
If the addon contains custom campaigns or custom maps. |
addonContent_Skin | int
|
N/A | 0
|
If the addon contains custom skins. |
addonContent_weapon | int
|
N/A | 0
|
If the addon contains custom weapon skins. |
addonContent_BossInfected | int
|
N/A | 0
|
If the addon contains custom boss infected skins. |
addonContent_CommonInfected | int
|
N/A | 0
|
If the addon contains custom common infected skins. |
addonContent_Survivor | int
|
N/A | 0
|
If the addon contains custom survivor skins. |
addonContent_Sound | int
|
N/A | 0
|
If the addon contains custom sounds. |
addonContent_Music | int
|
N/A | 0
|
If the addon contains custom music. |
addonContent_Script | int
|
N/A | 0
|
If the addon contains custom scripts. |
addonContent_prop | int
|
N/A | 0
|
If the addon contains custom props. |
Non-existent or deprecated keys
Avoid writing these keys.
All games ignore these keys.
Valve provided these keys in the respective addoninfo.txt
copies packaged with the example campaigns “DEADLINE” and “DEADLINE 2”.
Key | Description |
---|---|
addonSteamAppID | The AppID of the addon’s target game. Only addoninstaller.exe uses this.
|
addonTagline | The addon’s subtitle or tagline. May have had a similar purpose to key poster/posterTagline in mission files.
|
addonAuthorSteamID | The addon author’s SteamID. Supposedly used to construct a URL to the author’s Steam profile page. |
addonSteamGroupName | The addon’s Steam group. Supposedly used to construct a URL to the addon’s Steam group page. |
addonURL0 | The addon’s homepage. May have had a similar purpose to key Website in mission files.
|
addonContent_Survival addonContent_Versus addonContent_Scavenge |
If the addon provides custom campaigns supporting the Survival, Versus, or Scavenge game modes. This functionality is provided by mission files. |
addonContent_Prefab | If the addon provides custom ![]() |
addonContent_Spray | If the addon provides custom logos. |
addonContent_BackgroundMovie | If the addon provides custom background movies. |
Content_Weapon | If the addon provides custom weapons (e.g., “…a new weapon, i.e. guns, explosives, booby traps, hot tar, etc.”). |
Content_WeaponModel | If the addon provides custom weapon model. |
addonDescription_locale | The addon’s description localized for locale locale, where locale is an ISO 639-1 code in ALL CAPS. |
Content tag handling
An addon’s AddonInfo
content keys provide an abstraction over Source’s implementation-specific content tags.
Source will convert all keys into an internal representation (typically, a bitfield).
A content key may correspond to its internal tag; some keys provide redundant meaning useful only to authors and distributors.
Source will use these content tags to describe and filter addons in the
CBaseModFrame
Addons
.

The game will always load the effective first collision mesh until the current session ends. This was tested on the propane tank model.

In
Left 4 Dead and
Alien Swarm
Left 4 Dead maintains a one-to-one correspondance between content keys and content tags. The handler closely resembles that found in the
Alien Swarm SDK.
Source will generate the content label by concatenating the VGUI localization strings corresponding to the given content tag followed into an array of 60 wide characters.
The right-most comma will always be replaced with a
NULL
terminator.
The effective length of the label is 59 characters since the last character will always be a NULL
terminator.
In
Left 4 Dead 2
Left 4 Dead 2 stores the content keys for each addon as an
int16_t
representing the following bitfield (assume little-endian):
Bit | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Tag | N/A | N/A | N/A | N/A | CONTENT_ITEM
|
CONTENT_UI
|
CONTENT_SCRIPT
|
CONTENT_SOUND
|
CONTENT_SURVIVOR
|
CONTENT_INFECTED
|
CONTENT_MUTATION
|
CONTENT_WEAPON
|
CONTENT_SKIN
|
CONTENT_MISCELLANEOUS
|
CONTENT_CAMPAIGN
|
CONTENT_RESERVED
|
Key | N/A | N/A | N/A | N/A | addonContent_prop
|
N/A | addonContent_Script
|
addonContent_Sound addonContent_Music
|
addonContent_Survivor
|
addonContent_BossInfected addonContent_CommonInfected
|
N/A | addonContent_Weapon
|
addonContent_Skin
|
N/A | addonContent_Campaign addonContent_Map
|
N/A |
The content tags CONTENT_MISCELLANEOUS
, CONTENT_MUTATION
, and CONTENT_UI
can be set only through the Steam Workshop.
Source will generate the content label by concatenating the VGUI localization strings corresponding to the given content tag followed into an array of 60 wide characters.
The right-most comma will always be replaced with a
NULL
terminator.
The effective length of the label is 59 characters since the last character will always be a NULL
terminator.
Examples
The following are samples—both modern and historic—of addoninfo.txt
.
Avoid using the “DEADLINE” or “DEADLINE 2” samples as templates: you will be shipping redundant keys.
Minimally compliant AddonInfo

{}
Generic compliant AddonInfo
AddonInfo { addontitle "こんにちは、世界!" // Title is interpreted as a UTF8 string; // prefer codepoints in the Basic // Multilingual Plane. addonauthor ValveDeveloperWiki addonversion something // This can be anything and not just a float addonDescription "This is a sample. Note that the KeyValues parser will, in a quoted string, a) parse all whitespace literally and b) // ignore all C-style comments. You could probably use this to validate a KeyValues parser too. " addonContent_Campaign 1 addonContent_Survivor -1 addonContent_Sound 3.14159265 }
“DEADLINE”
Download Valve’s “DEADLINE” campaign from here. The following snippet matches the original exactly as of April 30, 2023.
// The addoninfo.txt file is a metadata file that is required by all Left 4 Dead Add-ons. "AddonInfo" { addontitle "Dead Line" // Add-on title that shows up in Add-ons list. ~20 chars max addonversion 1.3 // Add-on version. addontagline "Catch the last subway... out of hell." // Add-on tagline or wrap-up- a short description. ~100 chars max addonauthor "Valve" // Name/alias of the author addonauthorSteamID "http://steamcommunity.com/groups/Valve" // Steam Profile URL of author or group addonContent_Campaign 1 //This addon provides muliple connected maps with a finale addonURL0 "http://media.steampowered.com/apps/513/deadline.html" //An html home page for the add-on that includes a download link. // short description that appears in the Add-on list screen... addonDescription "Survivors must escape on a subway train in this short tutorial example campaign. Get source files and tools for creating your own campaigns by downloading the Left 4 Dead Authoring tools SDK, which is available under the 'Tools' tab in Steam." // Authors can add localized descriptions here. (Not yet supported) addonDescription_FR "Les survivants doivent s'échapper en métro dans cette carte du tutoriel. Obtenez les fichiers et les outils pour créer vos propres cartes en chargeant les outils de création SDK Left 4 Dead qui sont disponibles sous l'onglet "Outils" sur Steam." }
“DEADLINE 2”
Download Valve’s “DEADLINE 2” campaign from here. The following snippet matches the original exactly as of April 30, 2023.
// The addoninfo.txt file is a metadata file that is required by all Source Engine Add-ons. "AddonInfo" { addonSteamAppID 550 // 500 is the app ID for Left 4 Dead, 550 for Left 4 Dead 2 addontitle "Dead Line 2" // Add-on title that shows up in Add-ons list. ~20 chars max addonversion 1.0 // Add-on version. addontagline "Catch the last subway... out of hell... Again!" // Add-on tagline or wrap-up- a short description. ~100 chars max addonauthor "Valve" // Name/alias of the author addonSteamGroupName "deadline2add-on" // (Optional) Steam group related to the add-on. We use this to construct a URL to the group page. addonauthorSteamID "YasserMalaika" // (Optional) Steam ID of author. We use this to construct a URL to the author's page. addonContent_Campaign 1 //This addon provides muliple connected maps with a finale addonURL0 "http://media.steampowered.com/apps/513/deadline2.html" //An html home page for the add-on that includes a download link. // short description that appears in the Add-on list screen... addonDescription "Survivors must escape on a subway train in this short tutorial example campaign. Get source files and tools for creating your own campaigns by downloading the Left 4 Dead 2 Authoring tools SDK, which is available under the 'Tools' tab in Steam." // Authors can add localized descriptions here. (Not yet supported) addonDescription_FR "Les survivants doivent s'échapper en métro dans cette carte du tutoriel. Obtenez les fichiers et les outils pour créer vos propres cartes en chargeant les outils de création SDK Left 4 Dead 2 qui sont disponibles sous l'onglet "Outils" sur Steam." }