When writing poetry, Claude pre-activates candidate end-of-next-line rhyme words before composing the line

In the "Planning in Poems" section of Anthropic's On the Biology of a Large Language Model (Lindsey et al., 2025), attribution graphs show Claude 3.5 Haiku activating features for candidate end-of-next-line words before generating the line's text, then composing the wording so it lands on one of those pre-activated candidates. The report states: "The model often activates features corresponding to candidate end-of-next-line words prior to writing the line, and makes use of these features to decide how to compose the line." This is planning-like behavior — picking a target and back-filling toward it — rather than pure left-to-right generation with no lookahead.

The planning was validated causally with steering experiments: injecting an alternative planned-word feature changed the eventual rhyme word in most completions. A 2026-07-25 promotion resolved the quant flag this note carried since 2026-07-12: the report itself states, of injecting two planned-word features ("rabbit" and "green") across a random sample of 25 poems, that "the model ended its line with the injected planned word in 70% of cases." The report's own denominator is "a random sample of 25 poems" / "70% of cases," not "resamples" — worth noting because the figure had circulated in this vault as "~70% of resamples" before the verbatim line was captured. See claim-biology-llm-poetry-steering-swaps-rhyme-or-abandons-it for what happens qualitatively under two related interventions (ablating vs. injecting an unrelated concept) on the same planning feature. question-verify-biology-llm-poetry-planning-steering-figure is closed as answered.

This is one of four case-study "chains" traced in the same report. Unlike the sequential inference in claim-biology-llm-dallas-texas-austin-genuine-two-step-reasoning, poetry planning is lookahead-under-constraint-satisfaction: the model is not reasoning toward the rhyme so much as selecting a target and arranging the line to reach it. On whether such internal traces are faithfully reported, see cot-faithfulness-anthropic-biology. Compare the other two chains: claim-biology-llm-hallucination-is-known-entity-suppression-misfire and claim-biology-llm-jailbreak-assembles-bomb-by-parallel-letter-votes.