Halopedia:Halo: Reach is not canon
It is a long-disputed claim that "Halo: Reach is not canon", which has been postulated by websites such as haloreach.isnotcanon.net. Below are a number of commonly-used arguments regarding Halo: Reach's canonicity. Rather than continue debates that have been done to death many times over, please instead refer people to this page. Some of these examples are taken from "haloreach.isnotcanon.net" and, if the same quote has to be used multiple times, bold text will be used to specify exactly which part of the text is being brought up for criticism.
 Debunked claims
 The Autumn can't fly in-atmosphere
- "The Pillar of Autumn is now capable of flying in atmosphere; its size has also been significantly reduced."
At the risk of demonstrating an Argument from Ignorance: nowhere has it been said that the UNSC Pillar of Autumn, or any UNSC cruiser, is incapable of atmospheric flight. The only "evidence" supplied is the botched landing attempt on Installation 04. And even then it was coordinated manually on damaged warship while its bridge crew where being evacuated. Furthermore, we are aware that UNSC frigates use some manner of anti-gravity technology to keep them afloat in an Earth-like atmosphere - what aspect of the Halo universe denounces the possibility that other ships could employ such technology (it should be noted, however, that regardless of the Autum employing the technology or not, it was clearly dependent on peripheral-thrusters to nose itself up high enough for its actual engines to kick-in)?
Here's an interesting fact - we've seen cruisers on planets, before. No, not the decommissioning yard on Reach (take note, though, that they couldn't all have crashed on Reach - some are in the way of others' expected trajectory and haven't collided - that suggests that they all landed). The 2007 live-action Halo 3 trailer - the first one - which introduces us to the Sol Defense Group 3/A-5 shows us something very interesting early on. Check 00:04 - it's a Marathon-class cruiser on the planet Mars. Now, this next bit is going to get very mathematical.
- The gravity at Mars' equator is 0.376 g
- The gravity at Earth's equator is 0.99732 g
- From what we know of Reach, it is 1.08 g
As we can see, Mars' gravity is one-third the strength of Earth's, while Reach is not that further more. We also know that the Marathon-class cruiser is three-times the tonnage of the Halcyon-class. So, by demonstrating that a Marathon-class cruiser can be on the Martian surface to be launched, we can infer that a ship one-third the mass on a planet with three-times the gravity would be an approximate comparison. This supports the counter-argument that boosters are considerably-advanced in the 26th century - enough to launch cruisers. Considering how neat the decommissioning yard is on Reach, it is reasonable to assume that booster technology plays a major role in the job of engineering in both construction and dismantling. Perhaps cruisers are supposed to be launched from the ground due to their size, while frigates can be constructed in space - do you really want to see a Cruiser with no thrusters fall towards an inhabited planet? It'll be the National Holiday all over again.
 The Autumn is too small!
- "The Pillar of Autumn is now capable of flying in atmosphere; its size has also been significantly reduced."
The argument that "its size has also been significantly reduced" is based around the Autumn's point-defence turrets sharing the same design concept as those on UNSC frigates. While it's obvious that the in-game model is much smaller than its canon counterpart, this is for RAM purposes, and the turrets should be viewed as being larger-scale cousins.
 Two crystals? No - one
- "The crystal revealing Installation 04's location, found on Sigma Octanus IV, has been implicitly moved to Reach. The Covenant invasion of Sigma Octanus IV now makes no sense."
Based on what was said above, the crystal was not moved to Reach, although the Halsey epilogue does suggest that the Forerunner data contained its location (which is reasonable considering its importance - we don't know what the Forerunner artifact was intended for, after all). 2007's Halo: Contact Harvest established that the Covenant use "luminaries" - re-purposed Forerunner scanners which identify items or entities of notable nature (whether Forerunner objects of "reclaimers"). The crystal still showed up on the luminary due to its existence as a Forerunner artifact, explaining that the Covenant either just "found" the planet or were led there. Regardless, they still collected the data from the artifact.
 Seraphs and Phantoms can't use Slipspace
- "The fighters in Long Night of Solace are using slipspace drives to reach Anchor 9. That's impossible as it was commented that a Spirit couldn't be used in slipspace-related missions in Halo: First Strike."
These aren't Spirits and there is nothing to say that it is impossible for a vacuum-sealed Banshee or a Seraph to use such technology to a small extent - both Covenant and human drives are designed well-enough to allow coordinated inter-system hops, with the superior Covenant drive capable of hopping around a planet's orbit with precision and the inferior human drive capable of hopping between planets at the shortest safe distance.
In fact we read in Halo: Ghosts of Onyx that the UNSC has designed small vehicles (modified HEV/SOEIVs, in fact) that can be launched out of a vessel in slipspace and exit into battle. Small satellites are also capable of going in and out of slipspace to detect any dangers nearby. Both of these facts confirm the plausibility of using Seraphs; Banshees and Phantoms to attack via Slipspace, and that they are not in fact 'too small'.
Continuing, the game refers to the technology in use as an 'impulse drive', the standard engine for Covenant vehicles previously described in Conversations from the Universe. Rather than using Slipspace to proceed to Anchor 9, the craft may instead have taken advantage of the forceless-nature of vacuum to reach speeds where they cannot be seen clearly by the human eye, though still below the speed of light. In fact, NOT using such speeds in a vacuum-based combat zone goes against the purpose of the dog fight - to stay out of your opponent's sights long enough for him to appear in your's.
 SPARTAN-IIIs can't use MJOLNIR Armor
- "NOBLE team is wearing the MJOLNIR Powered Assault Armor rather than SPI, and are not being injured by the weight of the armour. This is impossible, as only SPARTAN-IIs can wear them."
Both the S-IIs and S-IIIs have received extensive augmentation to their bones and muscles. While the average human will be crushed by the armour (through sheer weight or the force of impact as the armour moves) an S-III can endure more stresses. The reason for the armour change is the supremacy of MJOLNIR over the SPI in conventional warfare, and ONI could have considered the team as deserving enough of MJOLNIR, given their more S-II-esque missions (as opposed to the covert Headhunters, who used their SPI's active camouflage abilities to perform guerilla warfare tactics. It has also been put forward that the S-III complement of NOBLE team all shared one thing in common - they were pulled out of their respective companies because of they have a genetic match to the parameters initially set for the S-II candidates.
 Cortana's in the wrong place
- "Cortana is in SWORD base with Dr.Halsey instead of being on the UNSC Pillar of Autumn."
According to the journal written by Eric Nylund, Halsey had Cortana 'chip off' a fragment of her program to assist her while her main program remained on the Pillar of Autumn. We were not told that AIs could NOT perform this action (In fact, we have seen Melissa, an AI affected by a Covenant virus, brake-up into other AI programs in 'i love bees') and so is not an inaccuracy - it's simply a plot point we didn't know until recently.
 There can't be two Cortanas
- "Cortana has been split in two for unknown purposes... other than to let Noble Team feel important"
This is not made an example of retroactive continuity simply by not being mentioned before (is Halo 3: ODST non-canon for featuring the Spartan Laser, which wasn't seen until Halo 3's time?) I believe that, based on the writing here, you intentionally-neglected to mention the purpose of the split in order to give the appearance of a random, unnecessary plot change. It is shown that Dr. Halsey had Cortana shed-off a fraction of her programming (more of an independently-running subroutine than a duplicate AI, which is also shown to be possible in Halo: First Strike) to allow her to finish her research at the Forerunner site. Halsey believes the information Cortana has gathered will become very useful after the battle. While it is true that Cortana did not mention having such data when the subroutine's new information was integrated into her programming, First Strike shows that Cortana had hardly begun to analyse the data she had been collecting recently, allowing the possibility that she simply had not read it.
 No Brutes or Drones were on Reach
- "There were no Brutes and Drones on Reach. No one knew they existed! No one knew about the Elites either until just now."
Nothing has been said that neither of these species - or the Huragok for that matter - were not present on Reach. What we see in the novel Halo: The Fall of Reach is a brief-clash with Covenant borders on Gamma Station and fighting with an undefined number of Covenant soldiers across a small portion of Reach's surface. We know in other entries to the series that Sangheili and Jiralhanae forces do not work well together and, in fact, serve in largely-distinct units and run their own ships (perhaps to avoid mutiny) by design. The relaunch of Halo: The Fall of Reach brought up the fact that there were multiple fleets present over the course of the campaign; any one of these could include the Jiralhanae corvette(s) that attacked New Alexandria. We'd also like the point out that Halo: The Fall of Reach was written and published in 2001, and that the Jiralhanae and Yanme'e first appeared in 2004's Halo 2; while the gap in real-world development explains why they were not present in Nylund's 2001 work, it does not prove that they weren't there canonically, and simply not mentioned. Nylund later departed from his concept of the Lekgolo; Jiralhanae Sangheili being newly-discovered species, retconning it in 2006's Halo: Ghosts of Onyx, which was later followed by 2007's Halo: Contact Harvest; 2008's Halo: The Cole Protocol and 2009's Halo Wars. Evidently, the plot point has been long abandoned.
It should be noted that the Yanme'e are left out of mention in both early Nylund works, altogether, and so can't be used in the argument - 'absence of evidence is not evidence of absence.' That is an Appeal to ignorance - a common logical fallacy based on the "Argument from ignorance" that asserts that just because a statement hasn't been disproven, it logically must be true (eg. "there's no proof that aliens aren't visiting Earth, therefore they are).
 Which Company? None
- "Carter; Emile; Jun and Kat can't possibly be SPARTAN-IIIs as Gamma company are still on Reach while Alpha and Beta were annihilated."
- "If all of Alpha company died in Operation: TORPEDO and all but two of Beta in PROMETHEUS, then how are the NOBLE team SPARTANs alive?"
- "They can't be SPARTAN-IIIs. Alpha and Beta companies were eradicated, so they must be Gamma company. But that doesn't exist yet"
The novella Headhunters has has already proven that members of Alpha and Beta companies were taken out of the main group and into specialist units. Halo: Ghosts of Onyx also alluded to a specialist unit outside of the main companies.
While we know that 300 S-IIIs took part in Operation: TORPEDO we do not know how much of Alpha company this unit consisted of - there were 497 recruits, after all. Beta Company yielded another 418, of whom 298 died during Operation: PROMETHEUS. Like the Headhunters, we can assume that there were a number of S-IIIs active outside of their companies' deployments, allowing for their survival. Otherwise, you're expecting ONI to be babysitting 315 kids to be kept outside the project (and yet know about the project's existence). In fact Tom-B292 was one of Col. Ackerson's interests, and wanted him to serve as his own private soldier - this role of course would be filled by B312.
There is another claim that the only remotely-logical company they can all be in is Gamma company, which they use as a strawman, improperly. Proponents of this argument seem to believe that not only was Gamma Company not activated in 2552 (which Ghosts of Onyx states otherwise), but that it did not exist in August, 2552 at all. This is clearly ridiculous, as an elite-company would not go through conscription; training and active deployment in a two-month timespan.
 They're too old
- "NOBLE team's S-IIIs are too old to be part of their respective companies."
The general assumption is that Alpha and Beta companies consisted only of 4-6 year olds, putting the oldest at the time of Alpha's founding as 2525. Jun is over a year older as 2524; Emile two at 2523 and Carter four and five at 2520 (his birthday occurs during the invasion).
While this appears highly inconsistent with Halo: Ghosts of Onyx, we can justify Jun's and Emile's ages. Kurt stated that some recruits were Harvest refugees; these refugees could be 6-8 years old at the time of induction, unless they were newborns when evacuated. They are still within the pre-pubescent recommendations for the project, though. It could be that Kurt's line is now an age average rather than a specific definition of the age groups in the company. A similar argument explains Kat being in Beta company. Carter, at eleven, is too close to puberty and so could not have been selected unless of course he had considerable time in cryo, allowing him to remain an eligible candidate despite his official age.
From an entertainment standpoint, this is more likely to have been to limit the impact on having Child Soldiers in the story (even ones who have grown up) - remember that, as a Beta Company member, Kat is only twenty-two years old.
 No Halo 3 weapons existed then
- "The soldiers on Reach are using battle rifles and Halo 3-style assault rifles. These couldn't have existed back then."
The UNSC Army utilizes their own weapons arsenal distinct from the UNSC Marine Corps. This is only three to four months before the start of Halo 3 - it is highly plausible for such weapons to be available in July. As such, any member of the MA5-series Assault Rifle could have been used on Reach. The Army troopers we see use an Army-specific variant of the MA5 designed in a similar way to the MA5C. The BR55 went through a number of design alterations between 2524 and 2552 and saw little use before the Battle of Earth as it was mostly a prototype. However, as the DMR is Army, there is no conflict at all between it and the BR55's production - the two military branches may have simply chosen to have similarly-designed weapons mass-produced.
 'Not seen before?' Sorry - made-up!
- "The Covenant invasion of Reach begins much earlier than before; they use new [and] never-before-seen technology to make this work: massive cloaking, and teleportation for example."
Mentioning "new never-before-seen technology" is used to construct a strawman. Already on the premise that Halo: Reach isn't canon, some people may look for any excuse to back up their claims, with the teleportation and massive-cloaking serving as such concepts.
Teleporting is a canonical concept explained by Forerunner technology in the first game and Halo Wars, and is not - as some may describe - a mere gameplay mechanic intended solely for multiplayer. Evidently the Covenant managed to reverse-engineer this technology for their own purposes. The Active camouflage is not shown in the series to have specific size-restrictions. If there's a big enough power source, it probably could cover the Long Night of Solace - nothing says that can't happen.
 Valid claims
This is the section which describes claims and accusations (both words have been used by non-canon proponents) that cannot be debunked and can only be explained by there being a genuine retcon specifically made by Bungie (and not, like the "Brutes not introduced yet", a concept already retconned by earlier works).
 Halsey's in the wrong place
- "Dr. Halsey is supposed to be in the Forerunner caves under CASTLE base. The original story is basically cast aside and repurposed using Noble team and SWORD base."
In the game Halsey has stayed behind with the few remaining researchers to get as much information from the artefact as they can before its inevitable destruction in order to prevent the Covenant from gaining knowledge from it, should it turn out to be a weapon. After the artefact's destruction Halsey and Jun travel to CASTLE base, though Jun's status afterwards is unknown. Halsey proceeds to CASTLE base and is forced to stay there to destroy the facility. The Menachite Mountain artefact still exists, and only the time Halsey and RED team meet at CASTLE base has been changed - this can also be explained away by the possibility that the two bases are in different time zones.
 It happened too early
"The Covenant invasion of Reach begins much earlier than before; they use new never-before-seen technology to make this work: massive cloaking, and teleportation for example."
The relaunch of Halo: Fall of Reach gave the impression from Keyes that he was aware of the Covenant threat. A series of advertisements for Halo: Combat Evolved Anniversary established that the Reach Admiralty intentionally surpressed information regarding the invasion to stop the majority of its defence fleet (who they had parked in-orbit away from Viery) from finding out and launching a daring attack. It was all part of a trap to trick the shipmaster of a high-priority ship - a carrier - into descending into the Reach atmosphere on a relic-collecting operation. The -intact- UNSC fleet would then swarm the unsuspecting Covenant ships and a SPARTAN team would then capture the carrier to fulfil "RED FLAG". The arrival of the Fleet of Particular Justice on August 29th ruins the plan, as the information-suppressed UNSC fleet soon realise that the Covenant is here.