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As ambiguous as the G-Man.
The canon of the Half-Life and Portal series has never been officially defined by Valve, and probably never will. Series' writer Marc Laidlaw stated that they do not "get involved in issues of canonicity", that "canon itself is non-canon", and that there is "no official stance", "just the games" as they are. "Canon itself is non-canon" emphasizes the fact that a few to no plot points are set in stone and that they can change anything during the development of further games, the aim being to have a good game before having a good story.
Valve also does not get involved in timeline issues. A complete official timeline of the Half-Life series has never been made and probably never will, since it could probably make the writers contradict themselves and limit their creativity for possible subsequent episodes. Several retcons have been made and others will likely be made in future games, confirming the "canon itself is non-canon" quote, meaning that any game can have its facts changed in a future release. In that case, if information contained in a new source contradicts facts laid out in an older source, the new source always prevails and overwrites any contradicting data.
Given this lack of reference and stance from Valve, the official source, the wiki had to define the canon itself to avoid confusion, and provide content as consistent as possible to what is set in the final, "real" Half-Life storyline, and what is not. These choices have been made as logically as possible and mirror the general consensus often viewed among fans.
The only requirement for a source to be canon is that it must be official, meaning it has to come directly from Valve, or, if not, be endorsed by Valve.
These endorsed sources are the Gearbox expansions, although their canonicity has been questioned for years, and the Prima Guides, although they sometimes contain goofs, inaccuracies and obsolete information, since they are always completed before the game's release. They are considered canon unless they contradict the certified canon material.
The cut material is endorsed by or directly comes from Valve, but is logically deemed non-canon by default. However, some cut content could be deemed canon if it does not contradict the canon material. For example, the Headcrab Shell launcher or the seagulls eating the Leeches featured in Lost Coast is likely canon, as it is reflected in the canon games.
Sometimes external sources, such as interviews of Valve employees, provide additional in-universe information. They are included in the wiki as canon as long as they do not contradict the previously established canon.
What follows is listed all the official material related to the Half-Life and Portal universe, with their canonicity level. Please note that games series from Valve such as the Counter-Strike, Team Fortress, Day of Defeat or Left 4 Dead series are obviously non-canon, being not set in the Half-Life and Portal universe.
The Gearbox expansions
- See also: Gearbox Software#Canonicity of the Half-Life expansions
The official guides
Even if they always contain a small amount of canonical information, most of what is found in these sources pertains to the game's development, and thus covers material that was altered or deleted, making them non-canon, though suitable for discussion regarding cut or altered content.