Which AI Engines Cite Reddit?

Reddit is the most-cited domain in AI answers and two of the six major engines never cite it. In the Machine Relations Index release of September 18, 2026 (release mri_score_v2.0+2026-09-18+8fa38e54dd0a, window May 10 to September 18, 15,782 observed answer runs), reddit.com holds the top citation rate of any domain at 12.75%, cited in 2,012 runs, and appears in the citations of Gemini, Google AI Mode, Google AI Overviews and Perplexity. It appears in zero ChatGPT runs and zero Claude runs. The same four-of-six pattern was measured on July 25 at 11.55% over 9,258 runs. Fifty-five days and 6,500 more runs later, the split has not moved.

That is the headline. The more useful finding is the map around it. Nineteen of the twenty-five most-cited domains are cited by all six engines. Four of the six exceptions line up with a documented robots.txt or licensing decision on one side or the other, and two, TechRadar and Substack, line up with nothing public. This brief reads the whole per-engine map, then checks it against what each platform and each engine has published about access.

The per-engine map of the top 25

The Index records, for every domain, which engines cited it at least once in the window. It does not publish a per-engine rate, so the table below says presence and nothing more. Engines: C = ChatGPT, A = Claude, G = Gemini, M = Google AI Mode, O = Google AI Overviews, P = Perplexity.

Rank	Domain	Source class	Citation rate	Runs cited	Grade	Engines present
1	reddit.com	Community	12.75%	2,012	A	G M O P
2	youtube.com	Platform	9.07%	1,431	A	all six
3	linkedin.com	Community	6.81%	1,075	A	all six
4	medium.com	Editorial	5.11%	807	A	all six
5	forbes.com	Editorial	4.11%	649	A	all six
6	nih.gov	Academic/government	3.07%	485	A	all six
7	gartner.com	Analyst	2.88%	454	A	all six
8	techradar.com	Editorial	2.84%	448	A	C A G P
9	g2.com	Market database	2.80%	442	A	all six
10	arxiv.org	Academic/government	2.60%	410	A	all six
11	yahoo.com	Editorial	1.94%	306	A	C A M O P
12	microsoft.com	Vendor	1.93%	305	A	all six
13	ibm.com	Vendor	1.93%	304	A	all six
14	landbase.com	Vendor	1.91%	301	A	all six
15	healthline.com	Unclassified	1.88%	297	B	all six
16	crunchbase.com	Market database	1.72%	271	B	all six
17	nerdwallet.com	Editorial	1.60%	253	B	C A M O P
18	substack.com	Community	1.58%	249	B	A G M O P
19	techtarget.com	Editorial	1.50%	237	B	all six
20	amazon.com	Editorial	1.50%	236	B	all six
21	paloaltonetworks.com	Vendor	1.37%	216	B	all six
22	dev.to	Community	1.32%	209	B	all six
23	facebook.com	Community	1.29%	203	B	G M O P
24	sentinelone.com	Vendor	1.27%	200	B	all six
25	deloitte.com	Analyst	1.24%	196	B	all six

Six exceptions, three shapes. Reddit and Facebook are missing from ChatGPT and Claude, and those two arms were collected without a gap, so those absences are exact. TechRadar was not recorded on either Google Search surface, Yahoo and NerdWallet were not recorded on Gemini, and Substack is missing from ChatGPT only. The Gemini and Google absences are ceilings, not measurements, for the reason in the September 26 correction below. Perplexity is present on all twenty-five.

Correction, September 26, 2026 — Gemini is a third gapped arm

The correction below named two engines with dated collection gaps. There are three. Gemini has 22 blind days inside the scoring window — May 11 to 15, then scattered single days from August 11 to September 16 — carrying 442 eligible successful runs that produced no citations. Intermittent single days rather than a continuous outage, which is why nine sweeps walked past it. Three arms collected without a gap — ChatGPT, Claude and Perplexity — and three are gapped: Gemini, Google AI Mode and Google AI Overviews. (An earlier form of this note, posted the same day, named only ChatGPT and Perplexity. Claude's single flagged date carried one answer run and was an artifact of a zero-citation scan with no run floor, not an outage.)

Three things change on this page. Every Gemini count below is a floor and every Gemini absence is a ceiling. Any ranking that places another engine above Gemini is withdrawn — Perplexity's "widest source layer" and Claude's head coverage against Gemini's both go. And the Gemini-absent domain list in "Gemini is not Google Search" is an upper bound of fourteen rather than a measured fourteen; what that section can still show is set out in its own paragraph there.

What does not change: ChatGPT is the narrowest arm on every measure on this page, because its counts are exact and every other arm's floor already sits above it. Claude citing fewer distinct domains than Gemini also holds, for the same reason in reverse.

Correction, September 25, 2026

An earlier version of this section ranked engines by domain counts and absence figures computed across all six engines, and used Google AI Overviews' and Google AI Mode's low counts to call those two the narrowest engines at the head and by source class. Those rankings are withdrawn. The Index's own collection recorded zero citations from Google AI Overviews on any monitored query between August 15 and September 24, 2026 — 41 of the last 132 days — and zero from Google AI Mode between September 14 and September 24 — 11 days. The cause was a defect in the Index's Google collection path, fixed September 25, 2026: the prior collector graded a response as a success whether or not it carried content. A distinct-domain count only accumulates, so missing collection days can remove domains from a pool but never add them. Every Google AI Overviews and Google AI Mode figure below is a floor, not a measurement, and neither engine's position in a table is a ranking.

What survives, as further narrowed by the September 26 correction above: ChatGPT, Claude and Perplexity are the arms with gap-free collection — one Claude date was flagged by the same scan and carries exactly one answer run, a scan artifact rather than a gap — so only comparisons anchored on one of those three — or comparisons whose direction a floor cannot reverse — are stated as rankings below. Gemini, Google AI Overviews and Google AI Mode are reported with their blind days named, and are never ranked against each other.

How wide each engine's source layer is

The gap widens below the head. The four arms in the table that follows are ChatGPT, Claude, Gemini and Perplexity; only the first and last of those are exactly measured, and Gemini's row is a floor. On the current release (mri_score_v2.0+2026-09-25, window May 10 to September 25, 2026, 132 observed days, 131,670 source events, 16,788 monitored answer runs, 23,872 cited domains), 75 of the top 100 domains are cited by all four of ChatGPT, Claude, Gemini and Perplexity. Of the top 250, 181. Of the top 1,000, 474. Across the whole universe of 23,872 cited domains, 685 are cited by all four, and 14,009 (58.68%) were cited by exactly one of the four in the window — a figure that describes the tail rather than the engines, addressed below.

Counted per engine, the number of distinct domains each one cited at least once in the window, restricted to the four arms with enough collection to report at all (Gemini's row is a floor; the two Google surfaces are reported separately below):

Engine	Distinct domains cited	Domains only this engine cited (among the four)	Present among the top 1,000
Perplexity	10,667	5,597	974
Gemini	7,276	3,402	870
Claude	5,564	1,871	882
ChatGPT	5,355	3,139	612

Gemini's counts are floors: it has 22 blind days in this window, and a distinct-domain count only accumulates. Google AI Overviews and Google AI Mode are excluded from this table and reported separately below, with 41 and 11 blind days respectively.

How to read the exclusivity column. The "domains only this engine cited" column above and the 58.68% single-engine figure in the paragraph before it describe tail composition, not engine breadth. A domain cited once among the four is single-engine by arithmetic, so most of each engine's exclusive pool is the once-cited tail rather than a distinctive source list. Conditioned on evidence the picture narrows sharply: among the 2,290 domains cited ten or more times across the window, the range runs from ChatGPT at 1,094 to Perplexity at 2,150 — a 1.97x spread, against 1.99x unconditioned. The spread does not narrow by much. The distinct-domain counts, the top-1,000 presence counts and the source-class figures on this page are all frequency-ranked, over those four arms. The cross-engine agreement study applies the same conditioned reading; see the MRI methodology and update log.

Two readings follow. ChatGPT cites the narrowest slice of the head of any arm here: absent from 388 of the top 1,000, against 26 for Perplexity, 118 for Claude and a ceiling of 130 for Gemini. That ranking survives every gap in the window, because a blind day can only push an absence count up, and ChatGPT's 388 is exact. Second, Claude cites fewer distinct domains than Gemini — 5,564 against a floor of 7,276 — while covering the head more thoroughly than ChatGPT, present on 882 of the top 1,000. Perplexity records the most distinct domains and the highest top-1,000 presence of the four, but it is not stated as the widest: Gemini's floor could in principle rise past it, and this page no longer publishes a rank it cannot defend.

By source class among the top 500 of the current release, ChatGPT is again the narrow one: absent from 25 of the 95 editorial publications, 16 of the 49 market databases and 20 of the 100 vendor-owned domains in that band, all exact counts. Gemini is absent from none of the 100 vendor-owned domains — a zero-absence finding a collection gap can only have made harder to reach, so it stands. Perplexity is absent from three of the 95 editorial publications and one of the 100 vendor-owned domains.

Google AI Overviews and Google AI Mode, reported as floors, never ranked. Google AI Overviews' pool on the current release is 2,304 domains and Google AI Mode's is 7,458; both are minima, depressed by the collection gap above, and neither is comparable to the four-engine figures above or to each other. By source class among the top 500, Google AI Overviews was not observed citing 24 of the 95 editorial publications, 17 of the 49 market databases and 17 of the 100 vendor-owned domains; Google AI Mode was not observed citing 8, 2 and 1 respectively. A fully collected window could only add citations to these counts, never remove them, so the true figures for both engines are at least this wide.

Where ChatGPT and Claude both go dark

Nine domains in the top 250 are cited by neither ChatGPT nor Claude: reddit.com (1), facebook.com (23), nytimes.com (42), pcmag.com (54), instagram.com (56), cnet.com (107), wsj.com (108), everydayhealth.com (152) and investopedia.com (175). Their robots.txt files, read on September 18, 2026, explain most of that list without any inference about ranking.

Reddit's robots.txt disallows every user agent from every path and points to its Public Content Policy. Reddit announced that change in June 2024 as an enforcement step: crawlers without an agreement should not be accessing Reddit data, and, as its chief legal officer told The Verge, "allow" in robots.txt never meant free use. Google has had access to Reddit's Data API since February 2024, which is the licensing layer on Google's side. OpenAI announced its own Reddit Data API partnership in May 2024, to "bring enhanced Reddit content to ChatGPT." In this panel, that agreement is not visible as a single ChatGPT citation. OpenAI's own crawler documentation says why that can be true at the same time: sites that opt out of OAI-SearchBot "will not be shown in ChatGPT search answers," and Reddit's file opts out every bot by name. Reddit sued Anthropic in June 2025, alleging that Anthropic accessed Reddit data without a licence. Claude's web search was reported in March 2025 to run on Brave's index, and Reddit's July 2024 robots change cut off search engines without an agreement: Microsoft confirmed at the time that Bing stopped crawling Reddit after the July 1 update.

Facebook's robots.txt is the opposite shape and lands in the same place. It names GPTBot, ClaudeBot, PerplexityBot and Google-Extended with path rules, and disallows everything for any agent it does not name. OAI-SearchBot and Claude-SearchBot, the two agents that decide search-answer inclusion for ChatGPT and Claude, are not named, so they fall under the wildcard block. Facebook is present on the three Google surfaces and Perplexity and absent from the other two.

The New York Times disallows GPTBot, OAI-SearchBot, ChatGPT-User, ClaudeBot, Claude-SearchBot, Claude-User, Google-Extended, PerplexityBot and Perplexity-User outright while allowing Googlebot with path rules; it sued OpenAI and Microsoft in December 2023. In the Index it is present on Google AI Mode, Google AI Overviews and Perplexity, and absent from ChatGPT, Claude and Gemini. PCMag and cnet.com carry the same full OpenAI, Anthropic and Perplexity blocks; PCMag's parent Ziff Davis sued OpenAI in April 2025. Instagram blocks GPTBot, ClaudeBot, PerplexityBot and Google-Extended and disallows all other agents. Everydayhealth.com blocks every OpenAI, Anthropic and Perplexity agent by name.

Seven of the nine are explained by the file alone. The two that are not are the interesting ones. wsj.com blocks all agents by default but allows GPTBot, OAI-SearchBot and ChatGPT-User by name, consistent with the News Corp agreement that gives OpenAI permission to display News Corp content in answers. Investopedia allows the three OpenAI agents and blocks Anthropic's. Both are absent from ChatGPT in this panel anyway. A permission is a necessary condition for a citation, not a sufficient one.

The file is not a law in the other direction either. CNBC disallows GPTBot, OAI-SearchBot, ChatGPT-User, ClaudeBot and Claude-User by name and is cited by both ChatGPT and Claude, and quora.com blocks every OpenAI and Anthropic agent by name and is cited by Claude. User-triggered fetchers such as ChatGPT-User and Perplexity-User are documented by their vendors as not always bound by robots.txt, and answers can cite a page the engine reached through a search index, a licensed feed or a cached copy rather than a fresh crawl. Presence in the Index is a measurement of citations, not of crawling.

Gemini is not Google Search

Google runs three of the six engines, and they do not cite the same sources. Within the top 250, at most fourteen domains are cited by both Google AI Mode and Google AI Overviews and never recorded on Gemini: yahoo.com, nerdwallet.com, techcrunch.com, nytimes.com, sciencedirect.com, cnbc.com, instagram.com, tech-insider.org, projectstartups.com, usnews.com, zoominfo.com, stackmatix.com, investopedia.com and hrexecutive.com. At most twelve run the other way, cited by Gemini and recorded on neither Search surface: techradar.com, allaboutcookies.org, useboomerang.com, gitnux.org, safetydetectives.com, safewise.com, unbiased.com, time.com, zipdo.co, goodhousekeeping.com, security.org and medicalbillersandcoders.com. Even the two Search surfaces diverge: at most 29 top-250 domains are cited by AI Mode and never by AI Overviews, and at most seven the reverse. Every count in this paragraph is a ceiling, because all three Google-family arms carry blind days in this window and an absence in our record cannot be separated from a day we did not see. What a ceiling still supports is the shape: the three Google engines do not cite one identical pool, and the divergence between them is somewhere between substantial and zero-but-for-our-gaps.

Google's documentation describes the control that separates the first list from the second. Google-Extended is a robots.txt token that governs whether crawled content may be used for training Gemini models and "for grounding" in Gemini Apps; it "does not impact a site's inclusion in Google Search." Of the fourteen domains not recorded on Gemini, twelve had a readable robots.txt on September 18, and eight of those twelve disallow Google-Extended outright: Yahoo, NerdWallet (which blocks nothing else), TechCrunch, The New York Times, CNBC, Instagram, Investopedia and U.S. News. Of the twelve domains on the reverse list, eleven had a readable file and none of them disallows Google-Extended; one, security.org, allows it explicitly. For the Search surfaces, Google's AI features guidance sets one condition, that a page be indexed and eligible for a snippet in Search, and notes that "AI Mode and AI Overviews may use different models and techniques, so the set of responses and links they show will vary."

TechRadar is the case that no file explains. It carries no AI-specific robots rules, is cited by ChatGPT, Claude, Gemini and Perplexity at an A-grade 2.84%, and appeared in neither Google Search surface across 15,782 runs. That last clause is a non-observation rather than a measured absence, since both Search lanes were blind for part of the window — so what the Index records is that nothing in TechRadar's own file accounts for a pattern we may or may not have fully seen.

What this section can and cannot claim after the September 26 correction. The eight-of-twelve match between a Google-Extended block and a domain never recorded on Gemini is a correlation with an independently documented mechanism — Google's own documentation says the token governs Gemini grounding and does not affect Search inclusion — and that mechanism does not depend on our collection at all. What our data can no longer do is measure the size of the effect. Fourteen is a ceiling, the true Gemini-absent set may be smaller, and no percentage computed from these lists is published here.

Perplexity: the widest engine, and where it gets Reddit

Perplexity cites Reddit and Facebook, whose files disallow every agent they do not name, and The New York Times and PCMag, which disallow PerplexityBot and Perplexity-User by name. Perplexity's crawler documentation distinguishes PerplexityBot, which builds search results and honours robots.txt, from Perplexity-User, which fetches a page when a user asks a question and "generally ignores robots.txt rules." Reddit's October 2025 complaint alleges a third route: that Perplexity and its co-defendants obtained Reddit content by scraping Google's search results, which Reddit tested by posting content visible only in Google's results and watching it surface in Perplexity's answers within hours. Perplexity denied wrongdoing. The Index does not measure which route produced a citation; it records that Perplexity is the one engine of six with Reddit present and the widest source layer at every tier.

What other panels see, and why they differ

Two other measurements are worth setting beside this one, because they disagree with it in an informative way. Semrush and Kevin Indig ran 100 buyer-journey prompts through GPT-5.2 in minimal and high reasoning and found Reddit cited in both modes, with Reddit and other user-generated sources losing "roughly half their share of citations" when reasoning is on. Loudmink's vendor panel of 20 B2B SaaS queries across five engines put Reddit at 3 to 5% of ChatGPT's citation URLs, zero for Claude, and about 2% for Perplexity through May 2026 before what it describes as a reversal to about 90% of answers in June.

The Index's ChatGPT runs show zero Reddit citations over 125 days. The difference is the panel. The Index asks 912 buyer questions across 24 subject categories in six question shapes, from the same fixed basket every day, and reports a domain present only if it was cited at least once in a run. A panel of twenty queries with 25 named brands, or 100 prompts in consumer categories, samples a different slice of what an engine will cite. Both can be right about their own panels. The claim that survives all three is the one on Claude: no measurement reviewed here has recorded Claude citing Reddit.

What this changes for publishers and brands

The engine is the unit of measurement. A source that ranks first across the market can still be absent from individual engines — reddit.com holds rank 1 by citation rate and is absent from ChatGPT and Claude, both exactly measured — and a robots.txt written for training opt-outs is now a per-engine distribution decision: a Google-Extended block removes a site from Gemini's grounding while leaving it eligible for AI Overviews, and a wildcard block that does not name OAI-SearchBot or Claude-SearchBot removes it from two engines' search answers even if GPTBot and ClaudeBot are allowed. A licensing agreement, on the evidence of Reddit and The Wall Street Journal in this panel, guarantees access, not citation. Anyone measuring their own presence should read it engine by engine, on a fixed panel, over a window long enough to see whether a split holds. This one has held for two months.

Change log

Measured change since July 25, 2026. Reddit's overall citation rate rose from 11.55% (1,069 of 9,258 runs) to 12.75% (2,012 of 15,782 runs). Engine presence unchanged at four of six; ChatGPT and Claude absent in both reads.
New in this read. The Gemini-versus-Search split and its match to Google-Extended; the nine head domains absent from both ChatGPT and Claude. This bullet originally also announced "Per-engine width across the universe (Perplexity 9,497 distinct domains to Google AI Overviews 2,279 on the September 18 release)"; that width ranking is withdrawn for the reason given in the correction below, and Google AI Overviews' pool is reported as a floor rather than as the bottom of a range.
Correction, September 26, 2026. Gemini was found to be a third arm with collection gaps — 22 intermittent blind days — so its counts here are floors, its absences ceilings, and no engine is ranked above it. The Gemini-absent domain list in "Gemini is not Google Search" became an upper bound of fourteen. See the correction note above.
Correction, September 25, 2026. The per-engine width and source-class tables above were recomputed over four engines (ChatGPT, Claude, Gemini, Perplexity) on the current release (mri_score_v2.0+2026-09-25). Google AI Overviews and Google AI Mode are reported separately as floors, with their collection blackouts named, and are no longer ranked against the four or against each other. See the correction note above.
Watch next. Whether the OpenAI Reddit partnership becomes visible in ChatGPT citations on this panel; the Reddit v. Perplexity docket; TechRadar's absence from both Google Search surfaces at the next release.

Method note

Source: the public Machine Relations Index JSON, release mri_score_v2.0+2026-09-18+8fa38e54dd0a, window 2026-05-10 to 2026-09-18, 125 observed days, 15,782 answer runs, 124,397 citation events, 22,179 cited domains, six engines, evidence floor of 10 runs across 7 dates for any published segment rate. Rank here is the overall citation-rate order across all measured runs. Per-engine figures are presence (cited at least once in the window) and never a rate; the public view carries no per-engine rate. Domain profiles for reddit.com, techradar.com, facebook.com and nytimes.com are published by the Index; the week's full read is the Index Report for September 18. Robots.txt files were read on September 18, 2026, and change without notice. Correlation between a robots rule, an agreement or a lawsuit and an engine's citation behaviour is reported as correlation. Paralax's earlier note on Reddit's search-referral warning covers the traffic side of the same relationship.

The per-engine width and source-class tables in the September 25, 2026 correction above use the more recent release mri_score_v2.0+2026-09-25, window 2026-05-10 to 2026-09-25, 132 observed days, 131,670 source events, 16,788 monitored answer runs, 23,872 cited domains, restricted to four arms (ChatGPT, Claude, Gemini, Perplexity), of which ChatGPT, Claude and Perplexity were collected without a gap; Gemini, Google AI Overviews and Google AI Mode figures in that section are floors, named as such. All other figures on this page — the top-25 map, the robots.txt findings and the FAQ's reddit.com citation counts — use the September 18 release named above.

FAQ

Which AI engines cite Reddit? In the Index's September 18 release, Gemini, Google AI Mode, Google AI Overviews and Perplexity. ChatGPT and Claude cited reddit.com in zero of 15,782 runs.

Why would ChatGPT not cite Reddit when OpenAI has a Reddit agreement? The Index cannot say why. What is documented is that Reddit's robots.txt disallows every agent including OAI-SearchBot, and OpenAI states that sites opted out of OAI-SearchBot are not shown in ChatGPT search answers. Other panels, run on different prompts, do record ChatGPT citing Reddit.

Does blocking Google-Extended remove a site from AI Overviews? Not according to Google's documentation, which says the token controls Gemini training and grounding and does not affect Search inclusion. The Index's data is consistent with that: eight top-250 domains that block Google-Extended are cited by both Search surfaces and were never recorded on Gemini. That is a correlation with a documented mechanism, not a measured effect size — Gemini has blind days in this window, so the absent set is an upper bound.

Is a per-engine citation rate available? No. The public Index records which engines cited a domain at least once in the window. Rates are published per domain and per segment across all engines.

Which engine cites the most sources? This release cannot settle it. Perplexity records the most — 10,667 distinct domains and presence on 974 of the top 1,000 — and it is one of the three arms collected without a gap, but Gemini's 7,276 is a floor that could in principle rise past it. What the release does settle is the bottom: ChatGPT is narrowest on every measure here. Its 5,597 domains cited by no other of the four are kept in the table above but should not be added to that list — exclusive counts are substantially a composition of the once-cited tail, per the note above. Google AI Overviews and Google AI Mode are excluded from this ranking; their collection had gaps of 41 and 11 days respectively in this window.