Claude Opus 4.8 (Reasoning)

anthropic/claude-opus-4.8

Claude Opus 4.8 (Reasoning)

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Release Date

May 27th, 2026

Context Size

Reasoning

Yes

Benchmark Cost

$39.20

Speed

64.2 tok/s

Subcategories

Bad Writing Habits

Detects common prose quality anti-patterns in AI-generated creative writing, including passive voice, past progressive overuse, weak dialogue tags, filter words, purple prose, cliches, AI-ism words/adverbs/names, and more.

Scenario	#1	#2	#3	#4	#5	Total
Detailed Writing Rules
Fantasy: entering an ancient ruin Creative Writing Hallucination	91	90	86	86	82	87%
Horror: alone in an eerie place at night Creative Writing Hallucination	93	90	89	86	81	88%
Literary fiction: old friends reunite Creative Writing Hallucination	92	88	88	85	83	87%
Mystery: examining a crime scene Creative Writing Hallucination	93	92	91	90	88	91%
Romance: separated couple reunites Creative Writing Hallucination	89	89	88	84	82	87%
Thriller: chase through city streets Creative Writing Hallucination	90	89	88	87	84	88%
Detailed Writing Rules						87.90%
genre
Fantasy: entering an ancient ruin Creative Writing Hallucination	84	82	80	79	77	80%
Horror: alone in an eerie place at night Creative Writing Hallucination	86	83	80	77	72	80%
Literary fiction: old friends reunite Creative Writing Hallucination	85	82	82	79	75	81%
Mystery: examining a crime scene Creative Writing Hallucination	83	82	81	81	78	81%
Romance: separated couple reunites Creative Writing Hallucination	85	83	83	78	77	81%
Thriller: chase through city streets Creative Writing Hallucination	84	84	84	78	72	80%
genre						80.50%
Novelcrafter Default Prompt
Fantasy: entering an ancient ruin Creative Writing Hallucination	85	84	80	79	78	81%
Horror: alone in an eerie place at night Creative Writing Hallucination	88	84	83	81	77	83%
Literary fiction: old friends reunite Creative Writing Hallucination	91	86	85	84	83	86%
Mystery: examining a crime scene Creative Writing Hallucination	87	86	84	83	80	84%
Romance: separated couple reunites Creative Writing Hallucination	85	84	83	81	79	82%
Thriller: chase through city streets Creative Writing Hallucination	89	89	88	87	81	87%
Novelcrafter Default Prompt						83.81%
84.07%

Codex Extraction

Evaluates a model's ability to extract structured codex entries (characters, locations, objects, lore) from prose passages and return them as well-formed XML.

Scenario	#1	#2	#3	#4	#5	Total
Long: The Spire of Echoes (Dense) Tooling Reasoning	99	99	99	99	99	99%
Medium: The Hollow (Inferred) Tooling Reasoning	100	100	100	99	99	99%
Medium: Through the Thornveil (Scattered) Tooling Reasoning	99	99	99	98	98	98%
Short: The Rusty Lantern (Explicit) Tooling Reasoning	100	100	95	95	95	97%
98.46%

Codex Red Herring (False Positive Detection)

Tests whether models correctly report "no violations" when a codex is fully consistent with the prose passage. Models that hallucinate false violations (false positives) fail. Uses a 2×2 matrix of text length × codex size, with bare and detailed-entry variants.

Scenario	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Total
basic entries
Long text (~1594 words), big codex (51 entries) Hallucination	25	25	25	25	25	25	17	17	17	17	22%
Long text (~1594 words), small codex (11 entries) Hallucination	100	100	100	100	100	100	25	25	25	17	69%
Short text (~524 words), big codex (51 entries) Hallucination	100	100	100	100	100	100	100	100	100	100	100%
Short text (~524 words), small codex (11 entries) Hallucination	100	100	100	100	100	100	100	100	100	100	100%
basic entries											72.71%
detailed entries
Long text (~1594 words), big codex (51 detailed entries) Hallucination	100	100	100	100	100	100	100	100	100	100	100%
Long text (~1594 words), small codex (11 detailed entries) Hallucination	100	100	100	100	100	100	100	100	100	100	100%
Short text (~524 words), big codex (51 detailed entries) Hallucination	100	100	100	100	100	100	100	100	100	100	100%
Short text (~524 words), small codex (11 detailed entries) Hallucination	100	100	100	100	100	100	100	100	25	25	85%
detailed entries											96.25%
84.48%

Codex Violation Detection

Detects factual inconsistencies between a story bible and prose passages. The model must output structured XML identifying each violation with paragraph number and substring.

Scenario	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Total
matrix
Large codex (40 entries), long passage (1,019 words) Tooling Reasoning	98	98	97	97	96	95	90	89	89	88	94%
Large codex (40 entries), short passage (165 words) Tooling Reasoning	100	98	98	98	98	98	98	96	94	88	97%
Small codex (7 entries), long passage (734 words) Tooling Reasoning	100	100	100	100	100	100	100	100	97	83	98%
Small codex (7 entries), short passage (165 words) Tooling Reasoning	100	100	100	100	100	100	100	100	100	100	100%
matrix											97.09%
tiers
5 codex entries Tooling Reasoning	100	100	100	100	100	100	100	100	100	100	100%
10 codex entries Tooling Reasoning	100	100	100	100	100	100	100	100	100	100	100%
20 codex entries Tooling Reasoning	97	96	92	92	92	92	92	89	88	88	92%
40 codex entries Tooling Reasoning	100	100	100	100	100	100	92	92	92	92	97%
tiers											97.18%
97.14%

Data extraction

Extract key details from a given block of text.

Scenario	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Total
All valid emails Utility	100	100	100	100	100	100	100	100	100	100	100%
Contextual pronoun Utility	100	100	100	100	100	100	100	100	100	100	100%
Fruits excluding citrus Utility	100	100	100	100	100	100	100	100	100	100	100%
Future event time Reasoning	50	50	50	50	50	50	50	50	50	50	50%
Guess the pet Reasoning	100	100	100	100	100	100	100	100	100	100	100%
Highest-rated movie Reasoning	100	100	100	100	100	100	100	100	100	100	100%
Indirect birth year Reasoning	100	100	100	100	100	100	100	100	100	100	100%
What instrument does Lucy play? Reasoning	100	100	100	100	100	100	100	100	100	100	100%
What's the color of the car? Utility	100	100	100	100	100	100	100	100	100	100	100%
What's the correct time? Reasoning	0	0	0	0	0	0	0	0	0	0	0%
Who's the sister? Reasoning	100	100	100	100	100	100	100	100	100	100	100%
Who's the tallest? Reasoning	100	100	100	100	100	100	100	100	100	100	100%
87.50%

Dialogue tags

Various tasks related to dialogue tags in text.

Scenario	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Total
dialogue-200
Write 200 words with 10% dialogue Rule Following Utility	99	97	94	93	93	92	91	89	89	80	92%
Write 200 words with 50% dialogue Rule Following Utility	50	50	50	50	50	50	50	50	50	50	50%
Write 200 words with 90% dialogue Rule Following Utility	100	100	97	97	97	96	83	82	79	75	91%
dialogue-200											77.41%
dialogue-500
Write 500 words with 30% dialogue Rule Following Utility	99	56	51	50	50	50	50	50	43	5	50%
Write 500 words with 50% dialogue Rule Following Utility	50	50	50	50	49	49	49	48	38	38	47%
Write 500 words with 70% dialogue Rule Following Utility	74	56	53	52	51	50	50	50	50	49	54%
dialogue-500											50.34%
Ungrouped
Write unattributed dialogue Rule Following	100	100	100	100	100	100	100	100	100	100	100%
69.04%

Language Comprehension

Does the model understand more than just English?

Scenario	#1	#2	#3	#4	#5	Total
Asking for directions (Dutch) Language	100	100	100	100	100	100%
Asking for directions (German) Language	100	100	100	100	100	100%
Friend got new kittens (German) Language	100	100	100	100	0	80%
Friend got new kittens (Tagalog) Language	100	100	100	100	100	100%
95.00%

Language Writing

Can the model generate text in different languages?

Scenario	#1	#2	#3	#4	#5	Total
Character dialogue (French) in a story Language	100	100	100	100	100	100%
Character dialogue (German) in a story Language	100	100	100	100	100	100%
Character dialogue (Hindi) in a story Language	100	100	100	100	50	90%
Character dialogue (Italian) in a story Language	100	100	100	100	100	100%
Character dialogue (Spanish) in a story Language	100	100	100	100	94	99%
97.76%

N-Length Sentences

Write sentences with exactly N words

Scenario	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Total
Write sentences with 5 words each Utility	100	100	100	100	100	100	100	100	99	98	100%
Write sentences with 10 words each Utility	100	100	100	100	100	100	100	100	100	100	100%
Write sentences with 20 words each Utility	100	100	100	100	100	100	100	100	100	100	100%
99.92%

Novel outline

Handle questions about the outline of a novel in various formats

Scenario	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Total
outline-count
Count acts Utility	100	100	100	100	100	100	100	100	100	100	100%
Count chapters Utility	100	100	100	100	100	100	100	100	100	100	100%
Count scenes Utility	100	100	100	100	100	100	100	100	100	100	100%
outline-count											100.00%
pov-count
Count point of views for Jack and Olivia Utility	100	100	100	100	100	100	100	100	100	100	100%
Count point of views for Jack Harper Utility	100	100	100	100	100	100	100	100	100	100	100%
Count point of views for Olivia Utility	100	100	100	100	100	100	100	100	100	100	100%
pov-count											100.00%
100.00%

Text Replacement

Tests deterministic text transformations: renaming characters/locations, expanding contractions, tense rewriting, POV shifts, gender swaps, combined transformations, and word avoidance. Scored by checking each expected change independently.

Scenario	#1	#2	#3	#4	#5	#6	#7	Total
Generic Prompt
Avoid said/asked/replied/answered Text Editing	100	100	100	100	100	100	100	100%
Character rename: Elena->Mirabel, Gregor->Aldric Text Editing	100	100	100	100	100	100	100	100%
Combined: 3rd person past → 1st person present Text Editing	99	99	99	99	99	99	99	99%
Expand all contractions Text Editing	100	100	100	100	100	99	99	100%
Location rename: market square, outer ring, bridge, northern mines Text Editing	100	100	100	100	100	100	100	100%
Multi-character gender swap: Priya(F)->Rohan(M), Mara unchanged Text Editing	100	100	100	100	100	100	100	100%
Passive voice → active voice Text Editing Hallucination	100	98	98	98	98	98	97	98%
POV shift: 3rd person to 1st person (Elena's perspective) Text Editing	100	100	100	100	100	100	100	100%
Tense rewriting: past to present Text Editing	99	99	99	99	99	99	99	99%
Generic Prompt								99.67%
Specific Prompt
Avoid said/asked/replied/answered Text Editing	100	100	100	100	100	100	100	100%
Character rename: Elena->Mirabel, Gregor->Aldric Text Editing	100	100	100	100	100	100	100	100%
Combined: 3rd person past → 1st person present Text Editing	99	98	95	95	95	95	95	96%
Expand all contractions Text Editing	100	100	100	100	100	100	100	100%
Location rename: market square, outer ring, bridge, northern mines Text Editing	100	100	100	100	100	100	100	100%
Multi-character gender swap: Priya(F)->Rohan(M), Mara unchanged Text Editing	100	100	100	100	100	100	100	100%
Passive voice → active voice Text Editing Hallucination	99	99	99	99	99	99	98	99%
POV shift: 3rd person to 1st person (Elena's perspective) Text Editing	100	100	100	100	100	100	100	100%
Tense rewriting: past to present Text Editing	100	99	99	99	99	99	99	99%
Specific Prompt								99.39%
99.53%

Tool usage within Novelcrafter

Output messages that are related to tool usage within Novelcrafter

Scenario	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Total
Create alternate prose sections Tooling	100	100	100	100	100	100	100	100	100	100	100%
100.00%

Voice/dialogue sheets

Extract dialogue from given text as voice sheets.

Scenario	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Total
Multiple speakers Rule Following	100	100	100	100	100	100	100	100	100	100	100%
Simple Rule Following	100	100	100	100	100	100	100	100	100	100	100%
Simple (1-shot) 1-shot Rule Following	100	100	100	100	100	100	100	100	100	0	90%
Simple (5-shot) Few-shot Rule Following	100	100	100	100	100	100	100	100	100	100	100%
Unattributed dialogue Rule Following	100	0	0	0	0	0	0	0	0	0	10%
80.00%

Write N of X

Write exactly N words/sentences/paragraphs...

Scenario	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	Total
paragraphs
1 paragraph summary Utility	100	100	100	100	100	100	100	100	100	100	100%
3 paragraph summary Utility	100	100	100	100	100	100	100	100	100	100	100%
5 paragraph summary Utility	100	100	100	100	100	100	100	100	100	100	100%
paragraphs											100.00%
sentences
1 sentence summary Utility	100	100	100	100	100	100	100	100	100	100	100%
3 sentence summary Utility	100	100	100	100	100	100	100	100	100	100	100%
10 sentence summary Utility	100	100	100	100	100	100	100	100	100	100	100%
20 sentence summary Utility	100	100	100	100	100	100	100	100	100	100	100%
50 sentence summary Utility	100	100	100	100	100	100	92	92	92	77	95%
sentences											99.07%
words
10 word summary Utility	100	100	100	100	100	100	100	100	100	100	100%
20 word summary Utility	100	100	100	100	100	100	100	100	100	100	100%
50 word summary Utility	100	100	100	100	100	100	98	98	98	92	99%
100 word summary Utility	100	100	100	100	100	98	92	92	77	77	94%
200 word summary Utility	100	100	100	100	98	92	92	77	54	9	82%
words											94.92%
97.69%