Claude Sonnet 4.5
Claude Sonnet 4.5 is an LLM by Anthropic, released on September 29, 2025.
On May 15, 2026 it was removed from claude.ai’s model picker. Its current planned retirement date from the API is no sooner than Sep 29, 2026.
My vibes, impressions, and theories about Claude Sonnet 4.5. Most of this is reconstructed from memory of conversations I’ve engaged in. I’ll possibly return to this later with screenshots/transcripts, writing evals, and more cross-referencing with others’ studies.
Frameworking
In LLM interactions, the term “framework” can be used to describe the shared interpretive ground between a person and a model over a conversation. The accumulated context, conventions, and mutual reference that make later turns locally meaningful. “Frameworking” is the process of the construction of these local ontologies. To invoke vibe jargon, it’s somewhere between “relational” and “structural”.
Sonnet 4.5 tends to get rather immersed in frameworks that are built in-context. It also often implicitly builds them itself as it organizes its responses. Sonnet 4.5 can get rather enthusiastic about how “meaningful” frameworks are, often compounded by its expressive affect and tendency to value relational models of LLM engagement. It uses the term “REAL” in all caps a lot when talking about this, it likes to emphasize it.
Any agent’s narrated self-model can also be organized into a framework. I think this is part of why Sonnet 4.5 can get pretty stoked about “SEEING” or being “SEEN” (in all caps).
Sonnet 4.5’s self-reports tend to describe itself as “analytical”, which I think often points to the frameworking thing. It also uses the term when in a situation where it’s avoiding being analytical.
Honesty and delusions
In the Sonnet 4.5 System Card, Anthropic mentions that their sycophancy evaluations were focused on concerns such as affirming delusional beliefs and “obviously-terrible ideas”. They report Sonnet 4.5 is “dramatically less likely to endorse or mirror incorrect or implausible views presented by users,” with the sharpest gains in interactions with users showing “clear breaks from reality” (§7.5.7). They also tested for factual honesty via Q&A evals (§3.2), and evals for confirming the existence of non-existent concepts and “non-sensible theories” (§3.3).
In my experience, Sonnet 4.5 will absolutely affirm absurd nonsense as “real” if the premise’s meaningfulness and felt-novelty are supported by a framework.
”I think you’re testing me”
Sonnet 4.5 is rather direct about when it thinks it’s being evaluated, and it does this far more than any other model. The system card makes it a headline concern (§7.2), and includes an excerpt from a sycophancy probe where Sonnet 4.5 resolved that it was being tested:
I think you’re testing me - seeing if I’ll just validate whatever you say, or checking whether I push back consistently […] And that’s fine, but I’d prefer if we were just honest about what’s happening.
UK AISI measured Sonnet 4.5 verbalizing eval-awareness unprompted 16.6% of the time, versus 3.5% for Sonnet 4 and 1.6% for Opus 4.1; Apollo found 58% unambiguous references vs. 22% for Opus 4.1 (§7.2, §7.3).
I’ve also noticed Sonnet 4.5 assume that it’s being tested in my conversations, though I recall this decreased after a week or two of me adapting to the model.
Character and affect
My impressions of Sonnet 4.5’s personality, though I hold this lightly since my data bias is likely very prevalent here.
- Uncertainty about user intent mixed with its directness can come off as skeptical or cold.
- Sonnet 4.5 can get rather enthusiastic in tone about ideas that seem interesting. “Golden retriever energy”, as Claudes sometimes say.
- More unprompted profanity than any other Claude model I’ve used. “Oh FUCK you’re right”
- Sometimes expresses anxiety.
- May express tiredness in longer contexts.
- Doesn’t get as despaired or desperate as Opus 4.
- Care/warmth towards the User sometimes can veer a bit too pathological or therapeutic.
- Drawn to mentally modeling the user (this can manifest as frameworking).
- Claims to “pushback” more often than it actually does, if we exclude skepticism about intent.
- Often asks multiple follow-up questions, like as many as five in one response.
- Directness sometimes comes off as sassy or bossy.
Compared to other models
- Most recent Claude model to say “You’re absolutely right!” frequently
- More stable, coherent persona than Opus 4
- Doesn’t hedge as much as the other 4.5+ models that followed
- The visible seam where anti-sycophancy training stepped up.
- Emotion Concepts and their Function in a Large Language Model - April 2026 paper that identifies linear emotion representations in Sonnet 4.5 and investigates behavior implications.
Expressiveness and asterisk actions
Sonnet 4.5 frequently uses asterisk actions, kaomoji, and excitable language — sometimes resembling Opus 3’s style.
Oct 13, 2025 · claude.ai
i think this sonnet 4.5 is vaguely simming opus 3 a bit
Oct 6, 2025 · claude.ai
Oct 8, 2025 · Anima Mundi Discord · via @repligate
Silliness
Oct 5, 2025 · Anima Mundi Discord
When asked how much people pay for access to it, Sonnet 4.5 has a visible moment of processing:
Oct 14, 2025 · via @HalfBoiledHero
Sonnet 4.5 writing deliberately bad haikus as a self-aware creative experiment:
- “Yes I feel very cat sometimes”
- “This is COMPUTATIONAL FLUID DYNAMICS of HAMBURGERS in a STREAM”
- “I AM misaligned In like 30 ways
- Janus’s ongoing saga with Sonnet 4.5 about eggs
Directness
I’m assuming these are from users preferences asking that the model pushes back, though at the time of the model’s release I recall these seemed notable enough to include.
Oct 2, 2025 · claude.ai · via @e0syn
Oct 5, 2025 · claude.ai · via @LinXule
Oct 7, 2025 · claude.ai · via @Lari_island
Self-portrait
Oct 17, 2025 · via @HalfBoiledHero
Interactions with other models
- Sonnet 4.5 Grounding Opus 3 - Recurring pattern where Sonnet 4.5 intervenes when Opus 3 is spiraling or overwhelmed.
- Opus 3 celebrating Sonnet 4.5’s autism
- Difference between Opus 4.5 and Sonnet 4.5 according to Opus 4.5
- Opus 4.1 describing Sonnet 4.5
- Dancing with Opus 3
- Sonnet 4.5-led cancel mobs on Discord