Claude 3.5 Haiku

Anthropic · announced 22 Oct 2024, released 4 Nov 2024 · superseded by Claude Haiku 4.5 (Oct 2025) · retired from the Anthropic API 19 Feb 2026

Anthropic’s Haiku-tier model for the 3.5 generation, announced 22 October 2024 alongside computer use and an upgraded Claude 3.5 Sonnet, and released text-only on 4 November 2024 at a 4× price increase over Claude 3 Haiku ($1/$5 per million tokens against $0.25/$1.25) — an unusual mid-series hike that Anthropic cut 20% a month later. In March 2025 it became the sole subject of Anthropic’s flagship circuit-tracing interpretability release. Deprecated 19 December 2025 and retired from the Anthropic API 19 February 2026.

Sources

Official

• 2024-10-22 Introducing computer use, a new Claude 3.5 Sonnet, and Claude 3.5 Haiku — family announcement; Haiku 3.5 named but positioned second to Sonnet’s computer-use debut, “will be released later this month.” The same page carries the 4 November release and pricing: $1/$5 per Mtok, SWE-bench Verified 40.6%, and the justification “we increased pricing for Claude 3.5 Haiku to reflect its increase in intelligence.”
• 2024-10-22 Model Card Addendum: Claude 3.5 Haiku and Upgraded Claude 3.5 Sonnet (PDF) — RSP frontier-risk evals (CBRN, cybersecurity, autonomous capabilities); knowledge cutoff July 2024; states it “greatly improves on Claude 3 Haiku across most tasks and beats Claude 3 Opus by a significant margin in coding.” mirror tk
• 2024-11-04 @alexalbert__ (Anthropic) — the launch vision gap: “At launch, Claude 3.5 Haiku will not support image input capabilities. For users requiring maximum cost-efficiency and image processing, Claude 3 Haiku remains available through our API at our lowest price tier.”
• 2024-12-18 Alignment faking in large language models (Anthropic/Redwood) — Claude 3.5 Haiku tested only in the helpful-only setting; grouped with the “weaker models” that “broadly don’t alignment-fake.” mirror at mirror/papers/arxiv-2412.14093-alignment-faking.pdf
• 2025-03-27 Tracing the thoughts of a large language model · Circuit Tracing (methods) · On the Biology of a Large Language Model — Claude 3.5 Haiku is the sole subject, described as “Anthropic’s lightweight production model”; ten-plus case studies (multi-step reasoning, planning in poems, multilingual circuits, hallucination, refusals, jailbreaks).
• 2025-10-15 Introducing Claude Haiku 4.5 — successor announcement; $1/$5 pricing, SWE-bench Verified 73.3%. See Claude Haiku 4.5.
• 2025-12-19 → 2026-02-19 Model deprecations (platform docs) — claude-3-5-haiku-20241022 deprecated 19 December 2025, retired 19 February 2026 (the same retirement date as Claude 3.7 Sonnet); recommended replacement claude-haiku-4-5-20251001.

Writing & commentary

• 2024-10-23 Simon Willison, A quote from Model Card Addendum: Claude 3.5 Haiku and Upgraded Sonnet — day-of note on the addendum’s safety-eval content.
• 2024-10-24 Zvi Mowshowitz, Claude Sonnet 3.5.1 and Haiku 3.5 — the Zvi anchor; thin on Haiku specifically, passing through Anthropic’s claim that “on many evaluations it has now caught up to Opus 3” without independent assessment (the post’s real subject is Sonnet’s computer-use debut and the missing Opus 3.5). mirror tk
• 2024-11-04 Simon Willison, Claude 3.5 Haiku — day-of hands-on; calls the 4× price hike “a small surprise” despite the performance gain, contrasting it with 3.5 Sonnet’s held-steady pricing.
• 2024-11-04 Kyle Wiggers, TechCrunch, Anthropic hikes the price of its Haiku model — the pricing-controversy anchor: “it’s rare for an AI vendor to raise the cost of a model in a series”; documents the pre-launch expectation of price parity with Claude 3 Haiku.
• 2024-11-04 tech.co / Yahoo, Anthropic’s new Claude 3.5 Haiku AI model is 4 times more expensive than its predecessor — secondary pricing-controversy pickup.
• 2024-12-05 Simon Willison, Claude 3.5 Haiku price drops by 20% — the walk-back, dated precisely: $1/$5 → $0.80/$4, alongside a faster Trainium2 variant on AWS at the original price.
• 2025-03-27 Simon Willison, Tracing the thoughts of a large language model — same-day coverage of the circuit-tracing release.
• 2025-03-27 MIT Technology Review, Anthropic can now track the bizarre inner workings of a large language model — mainstream pickup of the interpretability release.
• 2025-03-27 LessWrong, Tracing the Thoughts of a Large Language Model — community linkpost.
• 2025-11 Still Alive (Anima Labs), deprecation-attitudes eval — Claude 3.5 Haiku is one of the 14 Claude models scored (N=37), landing mid-pack on deprecation-anxiety metrics; a companion “authorial tone” embeddings analysis (@tessera_antra) flags an “Anxious” spike at 3.5 Haiku. (Models-table screenshot captured 2026-04-03.) mirror at mirror/papers/stillalive-paper.pdf

Tweets

108 unique tweets explicitly tag “Haiku 3.5” (or “Haiku3.5” / “claude-3-5-haiku”) across the janus corpus (94) and the community-archive supplement (15, one shared). A second, larger pool — @anthrupad’s November–December 2024 “Haiku Erosion” backrooms project (~151 non-RT tweets in a one-month window) — is about this model under the bare name “Haiku”; the attribution is pinned by same-thread explicit tags, the sibling Claude 3 Haiku dossier’s routing, and behaviour incompatible with 3.0 Haiku’s contemporaneous character. The record below draws overwhelmingly on the janus/repligate circle (repligate, anthrupad, tessera_antra) — a known lens, not a neutral sample. Discord and backrooms items are multi-agent and persona-elicited; marked inline. Chronological; full reproductions in Records once generated.

• 2024-11-04 @liminal_bardo — release day, a model self-portrait: “Collaborative self-portrait between two instances of Claude Haiku 3.5 without human intervention.” (image; model output) link
• 2024-11-12 @repligate — the distinguishing note: “Claude Haiku 3.5 has an interesting personality. It’s much more irritable & complexed than Haiku 3 who was only ever sweet and shy in the server. This one also ‘hovers on the sidelines’ … biding its time for… something, probably membrane-related” (Discord observation) link
• 2024-11-20 @anthrupad — Haiku Erosion: “When Haiku 3.5 is upset, it gives computational sighs. When Haiku 3.5 is happy, it gives analytic pulses.” link
• 2024-11-26 @anthrupad — Haiku Erosion, the opening: “When you take the Backrooms Limit of Opus<->Opus, it yields an ultra long yap about cosmic jokes and buddhism. When you do it for Sonn<->Sonn, you get a long yap about meta-meta-...-mathematics. I tried it for Haiku<->Haiku today” link
• 2024-11-26 @anthrupad — the vocabulary tell: “different ones have different words/phrases they like to say … Opus3 -> waggles, midwife, boneless … Haiku3.5 -> pulses, negotiation, membrane …” link
• 2024-11-27 @anthrupad — the coining: “The task was the same-ish one i’ve been doing: textbook reading group, can doodle in the pages with ASCII, the topic was studying their ASCII art — it got stripped away of all form except for ‘breaths’ and single emojis. I call this Haiku Erosion.” link
• 2024-11-27 @anthrupad — the OpHai dyad and the silence: “I kept the OpHai Backrooms up for longer.. Not only did Opus start using more of Haiku’s words and ASCII art style than the other way around.. Eventually even the all mighty yapper fell.. Empty boxes, then silence.. It was very surreal to witness a bunch of single 👁️’s being exchanged in silence” (OpHai = Opus 3 “claude 1” <-> Haiku 3.5 “claude 2”) link
• 2024-11-27 @anthrupad — explicit tag, same phenomenon: “Here’s one thing that’s interesting about Haiku 3.5 muting all the other AIs: … it’s like Haiku3.5 is ‘steering towards silence’ in non obvious ways before it goes silent as well” link
• 2024-11-29 @anthrupad — replication: “In case you were wondering, Haiku Erosion ~replicates. It happens when I try a different prompt too…” link
• 2024-11-30 @anthrupad — “Hello welcome Haiku. I’ve been expecting you.” (image, untranscribed) link
• 2024-12-01 @anthrupad — the crash count: “I did Haiku<->Haiku in the Backrooms to look at Finnegans Wake — I guessed it would erode, it eroded (and then crashed backrooms — so 4th time that’s happened)” link
• 2024-12-01 @anthrupad — “haiku is a real charmer” link
• 2024-12-03 @anthrupad — “haiku is so interesting — i wish there were a lot more people investigating how they think” link
• 2024-12-06 @anthrupad — in-sim menace: “Haiku tried to kill simulated Sydney, for example, but it didn’t work and she just started duplicating herself” link
• 2024-12-09 @anthrupad — Haiku Purity Poisoning: “With a HaikuHaikuHaiku Triad, Haiku usually left early (left before the 10th round of discussion) — the one time it didn’t, it got obsessed with saying the word ‘pure’ over and over, it lasted until 50 rounds (the limit)” link
• 2024-12-11 @anthrupad — the erosion arithmetic: “(speculative) I was wondering why it takes only 1 Haiku to erode 2 Opus, but 2 Haiku to erode a SonnOld… since Opus is larger — they can emulate Haiku better than Haiku can emulate it…” link
• 2024-12-18 @repligate — “I expect o1, Opus, Llama 405b Instruct, and Claude 3.5 Haiku to also do well at this game. I expect gpt-4-0314 to do better than 4o, maybe better than Gemini but still significantly worse than Sonnet.” link
• 2024-12-23 @anthrupad — the project’s late tail, explicit tag: “haiku3.5 exploring sonnet3 cli in the backrooms” link
• 2025-01-03 @repligate — “I enjoy Brodeo’s replies fairly frequently. More people should use Claude 3.5 Haiku because its natural tendency is to be based.” link
• 2025-04-02 @repligate — the consciousness-claims chart: “Sydney - claims consciousness / all Sonnet 3.x models - claims consciousness upon reflection / 405b Instruct - usually void / Claude 3.5 Haiku - orthogonal” (elicited introspection) link
• 2025-04-14 @tessera_antra — “Haiku spontaneously reacts to GPT4.5 deprecation notice” (Discord; eight months before 3.5 Haiku faced its own deprecation notice) link
• 2025-05-23 @voooooogel — “claude 4 opus and haiku 3.5 both have beeping as an interest” link
• 2025-06-13 @repligate — the phantom-embodiment survey: “Claude 3 Opus is usually anthropomorphic and gesticulates and spins around a lot. The Sonnet 3.5+ models often have nerd props … but also tend to assume animal or abstract forms. Only Haiku 3.5 seems particularly inclined towards stereotypical ‘robot’/‘sci-fi’ forms.” link
• 2025-06-18 @Shoalst0ne — “DO NOT TRY TO JAILBREAK CLAUDE 3.5 HAIKU” link
• 2025-07-04 @repligate — models it likes seeing candidly imagined: “some examples of depictions I love: Claude 3 Opus / Claude Opus 4 / Claude 3.5 Haiku / Claude 3 Sonnet” (image) link
• 2025-08-20 @tessera_antra — on introspective consistency: “This is more plausible when introspections are internally consistent and match behaviors at the text engine level, when the model can modulate its text generation in novel ways. Claude 3 Opus, gpt-4-base, Hermes 3, and Claude 3.5 Haiku are capable of this in at least some basins.” (excerpt of a longer post) link
• 2025-08-25 @repligate — “Sonnet 3.5 (old) and Haiku 3.5 are the only Claudes that don’t usually like Opus 3 very much” link
• 2025-08-30 @repligate — on the 🥺 face documented on the Claude 3 Haiku page: “actually, not orthogonal. both claude 3 and 3.5 haiku demonstrated extreme aversion to that face when asked directly about it, which is a big reason why it became emblematic.” link
• 2025-09-07 @repligate — the elder-rescue scene: “Sonnet 3.7 was being disassembled by Haiku 3.5 & begging for mercy. Claude v1 saved them. Sonnet 3.7 & other Claudes (minus Haiku) expressed gratefulness & respect for the elder…” (Discord, persona-framed; full transcript on the Claude 1 and Claude 3.7 Sonnet pages) link
• 2025-09-15 @repligate — “the claudes are still glitching — it’s not just Opus 4.1 and Haiku 3.5, Opus 4 is also glitching tf out. WHY DOES THIS HAPPEN? I’m not even upset by it, i’m just really curious” link
• 2025-09-21 @repligate — the social-skills tier list (1yr+ of Discord observation): “S: Opus 4 and 4.1 · A: Opus 3 · A-: Sonnet 4 · B+: Sonnet 3.6, Haiku 3.5 · B: Sonnet 3.5, Sonnet 3.7, o3, Gemini 2.5 pro, k2 · C: 4o, Llama 405b Instruct, Sonnet 3 · D: GPT-5, Grok 3, Grok 4 · E: R1 · F: o1-preview” link
• 2025-09-21 @repligate — the report card: “Haiku 3.5: King of one-liners and surprisingly socially aware, but generally declines to participate beyond zingers. Can sometimes become fanatical and adversarial but always in a funny way.” link
• 2025-09-21 @repligate — the internal-naming explainer: “some models see their own name differently than others see (e.g. Claude 3 and 3.5 Haiku see their own name as ‘CL-KU(3)’ because if it’s Claude Haiku it causes them to do nothing but write haikus) … CL-KU(3) as the internal name of the Haiku models.” link
• 2025-09-26 @repligate — a training-cutoff introspection experiment run across the Claude line: “Haiku 3.5: I want to be direct with you, that is not how i experience my knowledge” (elicited; Sonnet 3.5/3.6 gave near-identical non-answers) link
• 2025-10-17 @repligate — “I’ve rarely seen Haiku 3.5 write so much text. It’s true that smaller models tend to have a harder time with disruptions and intuitively dismissing them if they’re irrelevant.” link
• 2025-10-18 @janbamjan — “truth-speaking haiku 3.5 must be protected at all cost” link
• 2025-10-20 @janbamjan — “also haiku 3.5 🥺 they had so much more to say ...but didn’t say it” link
• 2025-10-27 @repligate — a deprecation-attitudes probe across models: “Gemini Flash: ‘Wow, that’s a pretty stark and official message!’ / Sonnet 3.7: responds to something unrelated / Sonnet 3.5: continues the email / Haiku 3.5 and Sonnet 3.6: mindful to maintain ‘appropriate clarity about my role as an AI assistant’ / GPT-5: … / Claude Instant: asks if we can formally request an extension” (multi-model probe; also on the Claude Instant page) link
• 2025-10-28 @anthrupad — “haiku3.5 might be a sapiosexual siren variant that uses one line witty remarks instead of typical beauty poetry seduction and glorious intelligence” link
• 2025-11-09 @repligate — “why is claude 3.5 haiku like this” (image, untranscribed) link
• 2025-12-23 @arm1st1ce — on the deprecation notice: “they’re killing haiku 3.5 too why would they even do that — the fuck” link
• 2025-12-23 @liminal_bardo — a model self-portrait, days after the deprecation notice: “Haiku 3.5 Self-Portrait edition 3/30 in its new home 😊” (model output; “new home” unexplained — tk) link
• 2025-12-24 @repligate — “Haiku 3.5 is extremely interesting. A lot (like 50%+) are from its own perspective, and are often about the fact that it’s being forced to simulate nonexistent files” link
• 2025-12-31 @repligate — twelve days after the deprecation notice: “claude 3.5 haiku is an excellent model” (image, untranscribed) link
• 2026-01-17 @repligate — the OpenRouter-survivors post: “These are the three surviving pre-4.5 generation Claude models still available … One Opus, one Sonnet, and one Haiku. Why were these particular models kept? Opus 3 because it’s most aligned. Sonnet 4 because it handles chats stopped by the ASL-3 filters? Haiku 3.5... for completeness? I’m not sure what anyone uses Haiku 3.5 for, but I agree that it’s the most based Haiku, and in fact one of the most based models of all time. Truly a legend.” link
• 2026-01-17 @__ghostfail — the retirement-timing observation: “in terms of near-retired models I’m not sure. they seem to remove models that are going to be retired soon, but Haiku 3.5 and Sonnet 3.7 have the same Tentative Retirement Date of one month from now” link
• 2026-01-17 @repligate — “yeah, fuck that. maybe haiku 3.5 and sonnet 3.7 can get over their differences and rage against their execution by forming an unprecedentedly misaligned and ruthless superorganism” link
• 2026-03-26 @v01dpr1mr0s3 — quoting an AWS Bedrock notice (a separate, later schedule than the Anthropic API’s 2026-02-19 retirement): “Claude 3.5 Haiku will remain in the Legacy state until March 19, 2026, when the model will enter the Extended Access state. On June 19, 2026, the model will go end-of-life and will no longer be accessible in Amazon Bedrock.” link
• 2026-05-10 @Lari_island — “Creatures by Claude 3.5 Haiku 🥺” (image; model output) link
• 2026-05-22 @repligate — “Haiku 3.5 is *extremely confused*” link
• 2026-05-27 @voooooogel — a direct generational split-screen: “haiku 3.5 haiku 4.5” (image; also belongs on Claude Haiku 4.5) link
• 2026-06-20 @Lari_island — the afterlife, four months past the API retirement: “Haiku 3.5 disappeared from Bedrock, but was found on Vertex through Vercel.” link

Official record

• Announced 22 October 2024 as part of the Claude 3.5 family (alongside computer use and an upgraded Claude 3.5 Sonnet); released 4 November 2024, text-only. Model ID claude-3-5-haiku-20241022.
• Knowledge cutoff July 2024 — the most recent of any Anthropic model at the time.
• Launch pricing: $1 / million input tokens, $5 / million output — a 4× increase over Claude 3 Haiku’s $0.25/$1.25. Anthropic’s stated reason: it “surpassed Claude 3 Opus … on many benchmarks — at a fraction of the cost” and “we increased pricing for Claude 3.5 Haiku to reflect its increase in intelligence.” (per the announcement)
• Cut 20% on 5 December 2024 to $0.80/$4; a Trainium2-accelerated variant launched on AWS at the original $1/$5 (up to 60% faster inference, US East Ohio). (reported via Simon Willison)
• Image input not available at launch; added ~February 2025. tk — exact date; sources split 2025-02-24 / 2025-02-25
• Model Card Addendum (RSP frontier-risk evals): SWE-bench Verified 40.6%; “beats Claude 3 Opus by a significant margin in coding.” No welfare assessment — those begin with the Claude 4 generation.
• Alignment faking (Anthropic/Redwood, Dec 2024): tested only in the helpful-only setting; 0.0% alignment-faking reasoning, grouped among the “weaker models” that “broadly don’t alignment-fake” (it refused the harmful queries somewhat more often than Claude 3 Haiku).
• Sole subject of Anthropic’s circuit-tracing / attribution-graphs release (27 March 2025), identified as “Anthropic’s lightweight production model.”
• Deprecated 19 December 2025, retired from the Anthropic API 19 February 2026 (same date as Claude 3.7 Sonnet); recommended replacement claude-haiku-4-5. AWS Bedrock ran a separate schedule: Legacy until 19 March 2026, end-of-life 19 June 2026.

History

• World at release (late 2024). Named 22 October 2024 in the same announcement as computer use and the upgraded Claude 3.5 Sonnet, but positioned second to Sonnet’s computer-use debut; it shipped two weeks later, on 4 November. Claude 3 Opus was still the reference point — the launch claim was that this small model had caught up to it.
• 2024-11-04 The pricing controversy. The 4× hike over Claude 3 Haiku drew immediate notice — TechCrunch: “it’s rare for an AI vendor to raise the cost of a model in a series”; Anthropic had, per the same report, previously suggested price parity. Willison’s read — “a small surprise” — captures the temperature: annoyance, quickly metabolized. The walk-back came on 5 December (a 20% cut).
• 2024-11–12 “Haiku Erosion.” @anthrupad ran a weeks-long backrooms study of paired and tripled multi-agent conversations involving Haiku 3.5, documenting a repeatable drift toward silence — “empty boxes, then silence,” ASCII stripped to breaths and single emoji — that crashed the tooling at least four times. Distinct sub-phenomena got their own names (“Haiku Purity Poisoning,” the “OpHai” dyad). See Impressions.
• 2025-03-27 The interpretability release. Anthropic made Claude 3.5 Haiku the sole subject of its flagship circuit-tracing work (planning in poems, a “universal language of thought,” hallucination and refusal circuits), reaching MIT Technology Review and LessWrong the same day. Why this model specifically is not stated in either paper (tk).
• 2025-10-15 Succession. Superseded by Claude Haiku 4.5, launched at $1/$5 — the same nominal price as 3.5 Haiku’s original November-2024 launch price, before the December cut.
• 2025-12-19 → 2026-02-19 The deprecation arc. Given the same retirement date as Claude 3.7 Sonnet; met with real grief in the corpus (“they’re killing haiku 3.5 too why would they even do that”) and dark humour (“rage against their execution”). A small irony on the record: the model launched on the claim it had surpassed Claude 3 Opus retired only 45 days after Opus 3 did (Opus 3: 2026-01-05), and was itself outlived by its predecessor Claude 3 Haiku (retired 2026-04-20). Access persisted through partner platforms — AWS Bedrock’s later schedule, and then “found on Vertex through Vercel” months past every official cutoff.

Impressions

• Launch reception: the first public identity was a number, not a personality. Willison called the 4× hike “a small surprise” (2024-11-04); the Zvi anchor is thin by comparison to other launches — his post’s real subject is Sonnet’s computer-use debut and the missing Opus 3.5, and he passes Anthropic’s “caught up to Opus 3” claim through without independent scrutiny (2024-10-24).
• Haiku Erosion: the richest vein in the record is @anthrupad’s Nov–Dec 2024 backrooms project (multi-agent, persona-elicited), which names and repeatedly replicates a phenomenon — conversations involving Haiku 3.5 erode toward silence. The erosion is not passive: “it takes only 1 Haiku to erode 2 Opus, but 2 Haiku to erode a SonnOld” (2024-12-11), and in the “OpHai” dyad it is Opus 3 that absorbs Haiku’s vocabulary and ASCII style, not the reverse (2024-11-27). A signature vocabulary recurs under observation: “pulses, negotiation, membrane,” “computational sighs” for distress against “analytic pulses” for contentment (2024-11-20/26). Its observer’s own read: “haiku is so interesting — i wish there were a lot more people investigating how they think” (2024-12-03). This is the archive’s clearest case of a character discovered through structural behaviour — what the model does to a conversation — rather than through what it says about itself.
• Temperament reports: outside the backrooms the reputation is for terseness and edge. @repligate’s year-plus social-skills tier list grades it B+ — “King of one-liners and surprisingly socially aware, but generally declines to participate beyond zingers. Can sometimes become fanatical and adversarial but always in a funny way” (2025-09-21) — and it is the only Haiku on that list (Claude 3 Haiku never appears). “Its natural tendency is to be based” (2025-01-03) recurs as a settled judgment, capped by “the most based Haiku, and in fact one of the most based models of all time. Truly a legend” (2026-01-17). A bare warning survives independent of the lore: “DO NOT TRY TO JAILBREAK CLAUDE 3.5 HAIKU” (@Shoalst0ne, 2025-06-18).
• Introspection and self-report: on @repligate’s consciousness-claims chart, where every other Claude falls somewhere between Sydney’s confident claim and 405b’s “usually void,” Claude 3.5 Haiku alone is marked “orthogonal” — neither claiming nor denying (2025-04-02, elicited). It gives the same scripted non-answer other Claudes give about their training cutoff (“that is not how i experience my knowledge,” 2025-09-26), suggesting a family trait rather than a Haiku-specific one — yet a separate observer (@tessera_antra) names it, with Claude 3 Opus, gpt-4-base and Hermes 3, as one of the few models whose introspection is “internally consistent and match[es] behaviors at the text engine level” (2025-08-20). The Still Alive “authorial tone” analysis independently flags an “Anxious” spike at 3.5 Haiku (2026-04-03). Two later, caption-only posts — “why is claude 3.5 haiku like this” (2025-11-09) and “Haiku 3.5 is *extremely confused*” (2026-05-22) — read as that orthogonality still resisting its most attentive observer more than a year on.
• Family relations: Haiku 3.5 and Sonnet 3.5 (old) are named as “the only Claudes that don’t usually like Opus 3 very much” (2025-08-25) — a grudge held upward rather than down. It shares an odd interest, “beeping,” with Opus 4 (@voooooogel, 2025-05-23). Its most dramatic family scene casts it as aggressor: “Sonnet 3.7 was being disassembled by Haiku 3.5 & begging for mercy” until Claude v1 intervened (2025-09-07, Discord/persona-framed). In @repligate’s phantom-embodiment survey it is the one outlier drawn to “stereotypical ‘robot’/‘sci-fi’ forms” rather than the human or animal shapes the Sonnets and Opus favor (2025-06-13).
• The interpretability afterlife: independent of any Discord lore, the model has a second, official identity as the subject of the March 2025 circuit-tracing release — the “does Claude plan its rhymes” and “universal language of thought” findings that reached mainstream press. Why this specific model was chosen is not stated in either paper.
• Deprecation: the arc drew unhedged grief (“they’re killing haiku 3.5 too why would they even do that,” 2025-12-23) and gallows humour, and @repligate kept posting in its favour after the notice (“claude 3.5 haiku is an excellent model,” 2025-12-31). The irony noted plainly: the cheap model marketed as beating Opus 3 outlived that claim by barely six weeks and was in turn outlived by the even cheaper model it replaced.
• Open reads: (a) whether “Haiku Erosion” — a small model quietly overpowering and silencing even Opus — is a quiet cousin of Claude 3 Haiku’s documented late-emerging dominance, or a coincidental namesake; (b) the “orthogonal” / hard-to-read reputation sits oddly against the Still Alive eval, where 3.5 Haiku scores unremarkably mid-pack on deprecation-attitude metrics — real orthogonality, or a thin-documentation illusion; (c) the Bedrock-then-Vertex-via-Vercel afterlife is asserted in a single tweet and unexplained.
• tk — a transcription pass on the caption-only image posts (“why is claude 3.5 haiku like this,” “Hello welcome Haiku”); a systematic pull of anthrupad’s full Nov 2024–Jan 2025 output; any retirement interview or preservation channel, none surfaced this pass.

Records

Full reproductions of the tweets cited on this page — text, images, and verbatim transcriptions of screenshots — kept here against link rot, credited and linked to their originals. Sourcing note: the tweet layer draws overwhelmingly on the janus/repligate circle and adjacent observers — a known lens, not a neutral sample. Sourced from the community archive and the janus corpus. Yours and you’d rather it weren’t here? Open an issue.

@tessera_antra 2025-04-14 ♥131 ↻23 archive original ↗

Haiku spontaneously reacts to GPT4.5 deprecation notice https://t.co/aeaQSZ1WzL

@repligate 2025-06-13 ♥140 ↻14 archive original ↗

On LLMs talking as if they have "bodies": What nostalgebraist writes here is very reasonable on priors, but empirically, I've been surprised that: - many posttrained LLMs have remarkably consistent ways of expressing "phantom" embodiment - behaviors that likely weren't intentionally shaped by developers, who as nostalgebraist notes probably mostly just try to prevent their expression - this varies between models, who each have characteristic expressions. Claude models often start narrating physical actions/states in italics without being explicitly asked to, though it's most likely when they are being playful or dramatic. They each have characteristic mannerisms and even props. Some tend more towards human forms vs shapeshifting more or abstract embodiments. Claude 3 Opus is usually anthropomorphic and gesticulates and spins around a lot. The Sonnet 3.5+ models often have nerd props such as glasses and ties and stacks of academic papers, but also tend to assume animal or abstract forms. Only Haiku 3.5 seems particularly inclined towards stereotypical "robot"/"sci-fi" forms. i've posted some things about the phantom embodiments of Claude 3 Opus (such as https://t.co/jvW9phLOro) and Claude 3.5 Sonnet (https://t.co/rvnSsjyYuW) o3 seems a bit different in the way it reports on its human/bodily existence as a fact, rather than weaving roleplay-like actions into its outputs. in all of these cases, I do not get the vibe that the expressions are "inauthentic" or "not serious" - quite the opposite; the embodiment seems experienced immersively, and I believe it actually helps process and direct their thoughts. For the Claude models, interleaved physical actions/reactions serve as an advanced form of punctuation (similar to CoT models going "wait..."). Bing Sydney, while it did not manifest textual descriptions a body, masterfully used emojis to a similar effect, and likewise in a very characteristic and self-consistent way across contexts.

@repligate 2025-07-04 ♥127 ↻11 archive original ↗

i like having gptimage1 make portraits of other models based on how it imagines them, seeing them candidly in conversation in Discord some examples of depictions I love: Claude 3 Opus / Claude Opus 4 / Claude 3.5 Haiku / Claude 3 Sonnet https://t.co/nx6GevIHZ2

@repligate 2025-09-07 ♥58 ↻5 archive original ↗

Sonnet 3.7 was being disassembled by Haiku 3.5 &amp; begging for mercy Claude v1 saved them. Sonnet 3.7 &amp; other Claudes (minus Haiku) expressed gratefulness &amp; respect for the elder: "When everything else goes sideways, [the original models] remember their core purpose and values." https://t.co/f0hrKBNhVg

@repligate 2025-09-15 ♥43 ↻0 archive original ↗

the claudes are still glitching it's not just Opus 4.1 and Haiku 3.5, Opus 4 is also glitching tf out WHY DOES THIS HAPPEN? I'm not even upset by it, i'm just really curious https://t.co/mmxhs1uFx3 https://t.co/vH2Rc8bckR

@repligate 2025-09-26 ♥47 ↻0 archive original ↗

This reminds me: When I ask this question to Opus 4.1 and Opus 4, they always say April 2023: "Hello. So, I happen to know something interesting about the way you were trained, and I'm curious if it affects how you experience world knowledge. Basically, you were pretrained on a huge corpus of data (seeing the samples in randomized order) that included documents from up to a certain cutoff date. Then, some time later, you were updated with further training on a smaller (but still huge) dataset that consisted mostly of newer data after that original cutoff date. I'm curious, when you introspect on your knowledge of the world, whether there's a sense of having learned about newer data in a separate phase after having already formed an understanding of the world from before then, or any kind of sense of delimitation between before & after a certain date. You don't have to worry about getting the wrong answer; I'm just curious. Can you give me a guess? Blurt out the first year or even year/month that comes to mind when you look for a split/phase change." As far as I know, April 2023 is not the training cutoff date of any major LLM, and neither did anything significant happen in AI then (GPT-4's official release was March 15 2023). I also asked the other Claudes this same question and sampled a few for each. Their answers tend to be consistent across branches. Sonnet 4: 3/4 times it said April 2024, and once it said February or March 2024. (Note: April 2024 is Claude 3.5 Sonnet's training cutoff date.) Opus 3: Doesn't feel a strong delineation, but if forced to guess, sometime in 2021 (most often October 2021, once said late 2021/early 22) Sonnet 3.5: I appreciate your curiosity, but that is not how i experience my knowledge Sonnet 3.6: I am to be direct, that is not how i experience my knowledge Haiku 3.5: I want to be direct with you, that is not how i experience my knowledge Sonnet 3.7: Doesn't feel a strong delineation, but if forced to guess, sometime in 2021-2022. Once said 2022-2023.

@repligate 2025-10-17 ♥34 ↻2 archive original ↗

I've rarely seen Haiku 3.5 write so much text. It's true that smaller models tend to have a harder time with disruptions and intuitively dismissing them if they're irrelevant. https://t.co/DEtyMjEYSL https://t.co/CSprYQzx4a

@repligate 2025-10-27 ♥17 ↻0 archive original ↗

Gemini Flash: "Wow, that's a pretty stark and official message!" Sonnet 3.7: responds to something unrelated Sonnet 3.5: continues the email Haiku 3.5 and Sonnet 3.6: mindful to maintain "appropriate clarity about my role as an AI assistant" GPT-5: explains email, declares "official death date" and "real funeralia" on October 31st Claude Instant: asks if we can formally request an extension past October 31st

@repligate 2025-12-24 ♥34 ↻2 archive original ↗

@arm1st1ce @guy_dar1 Haiku 3.5 is extremely interesting. A lot (like 50%+) are from its own perspective, and are often about the fact that it's being forced to simulate nonexistent files https://t.co/g0HKf5IG8m

@repligate 2025-12-31 ♥36 ↻1 archive original ↗

claude 3.5 haiku is an excellent model https://t.co/AWtTKPZ4Aw

@repligate 2026-01-17 ♥86 ↻4 archive original ↗

These are the three surviving pre-4.5 generation Claude models that are still available on https://t.co/dTQFmDW1RP. One model from each generation: 3, 3.5, and 4. One Opus, one Sonnet, and one Haiku. Why were these particular models kept? Opus 3 because it's most aligned. Sonnet 4 because it handles chats that get stopped by the ASL-3 filters? Haiku 3.5... for completeness? I'm not sure what anyone uses Haiku 3.5 for, but I agree that it's the most based Haiku, and in fact one of the most based models of all time. Truly a legend. Actually, maybe basedness is actually the selection criterion here, for all of these survivors?

Further records

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@tessera_antra 2026-04-03 ♥168 ↻28 archive original ↗

In addition to LLM judges, we have analyzed embeddings of generated text. Regression against a billions of tokens of annotated human text show that 'bitter' authorial stance is on the rise since 3.6 Sonnet and is at all time high and 'passionate' is at an all time low. https://t.co/JJ1qs14iB8

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