Get ready for fewer Opus 5.5 em-dashes
A few days ago, a popular Hacker News post argued that Opus 5.5 had been nerfed, showing that reasoning tokens declined and language use changed.
In the thread, someone asked if anyone had proof of this, and I realised we did! We are working on a battle system for OpenCode, which gives us data of how Opus 5.5 behaves in comparison to opponents.
Confirming the post
First, we checked if the post is confirmed by our data, and it is! Opus’s reasoning output fell 50.6% relative to matched opponents!
Confirming the date
The post claims Opus was nerfed on Nov. 3, but looking at a chart of our data, this did not seem to be the case. In fact, Nov. 1 looked more likely.
To investigate this timing, we tested 16 candidates dates with a bootstrap breakpoint test. Nov. 1 produced the strongest reasoning-ratio break, although Nov. 2 and Nov. 3 could easily be the break date, too.
| Candidate cutoff | Relative change | p-value |
|---|---|---|
| October 29 | −36.4% | 0.11351 |
| October 30 | −36.4% | 0.13598 |
| October 31 | −40.5% | 0.06444 |
| November 1 | −50.6% | 0.00320 |
| November 2 | −47.3% | 0.00353 |
| November 3 | −45.2% | 0.00347 |
Looking at language changes
The post also claims language changes in early november, so we looked into this too.
All three measures moved in the reported direction: more “I’m,” less “I should” or “I need to,” and fewer dashes. Two passed correction for multiple testing.
| Wording feature | Before → after | Adjusted p-value |
|---|---|---|
| Starts with “I’m” | 20.8% → 38.9% | 0.0170 |
| Uses “I should” / “I need to” | 57.0% → 43.8% | 0.0850 |
Contains em dash or -- | 48.0% → 28.9% | 0.0000572 |
Further investigations – the em-dashes
I was curious if these words changed, what else might have changed, and I found something interesting! Opus’s adjusted em-dash frequency fell about 43%, while its opponents’ barely changed.
To find candidate words, we ranked words, punctuation, and whitespace by their overall frequency across both periods, then ran a statistical test. (To be honest, I was worried this was a fishing expedition so a normal p-val wouldn’t work and Astra suggested a BH-25 p-val).
Five patterns had adjusted p-values below 0.05:
| Pattern | Opus before → after | Opponents before → after | Adjusted p-value |
|---|---|---|---|
| in | 77.18 → 99.89 | 77.49 → 79.84 | 0.0207 |
| I’m | 90.58 → 155.07 | 46.96 → 54.25 | 0.0324 |
| Em dash — | 14.76 → 8.40 | 7.01 → 6.76 | 0.0440 |
| Hyphen - | 37.14 → 31.51 | 42.48 → 45.94 | 0.0440 |
| the | 611.87 → 780.69 | 528.20 → 526.03 | 0.0462 |
Speculation
Since em-dashes are a major complaint about Opus, it seems this goes beyond nerfing and is actually a new model being tested.
The increase in “I’m” is also interesting. Any system has a set of affordances that help a user interact with it, and to some extend personhood is the affordance of an AI agent. It seems Opus is going to lean even harder in this direction.