Claude Haiku 4.5 vs Sonnet 5

What changes from Haiku 4.5 to Sonnet 5

This is a comparison of two specific earlier models, not the latest Haiku and Sonnet generations. The checked documentation lists both as active legacy models. Availability is a lifecycle fact to review before committing a new workflow.

The Haiku 4.5 specification and Sonnet 5 specification describe model-level capabilities. A 1M context window does not establish perfect recall across a million tokens, and it is not an input allowance in HaikuChat.

The nominal price increase is twofold

For an uncached 20,000-input, 2,000-billable-output-token request, the published base rates give $0.03 for Haiku 4.5 and $0.06 for Sonnet 5. The second estimate is twice the first only because token counts are held equal.

That example excludes caching, discounted processing, additional tools and retries. Adaptive thinking and different answer lengths can change actual usage, so do not budget a migration by multiplying last month's bill by exactly two. Measure the new model's own tokens on representative work.

The larger context capacity may remove a need to split a document, but sending every available document can introduce irrelevant material. Compare a carefully selected input against a full-context request. Reducing the amount of irrelevant text can improve both the evaluation and the economics.

When a larger context or reasoning tier is useful

Several connected documents: if an existing Haiku 4.5 task exceeds its 200K context capacity, Sonnet 5 can accommodate a larger request at the model level. Test whether it correctly links definitions, revisions and exceptions across documents. Merely accepting the input is not the success criterion.

Conflicting project notes: require an answer that identifies which decision superseded which and lists unresolved questions. This exposes a different failure mode from ordinary summarization. A plausible summary that erases a conflict should fail your rubric.

Small repetitive extracts: if Haiku 4.5 already extracts a known set of fields accurately, greater context capacity may provide no benefit. Compare the current Haiku generation before paying more for a larger tier. The Haiku 5.5 vs Sonnet 5 guide covers that separate decision.

Very long outputs: Sonnet's higher documented maximum can matter in the right environment, but generating a longer answer introduces more content to verify. Ask whether a shorter answer with a source-backed table would meet the same need.

Reasoning behavior changes the evaluation

Haiku 4.5 uses a manually enabled extended-thinking budget. Sonnet 5 uses adaptive thinking with effort settings. A test that copies old settings without checking their meaning is not a controlled comparison.

For a user comparing chat tools, record the visible model name, selected reasoning mode and any tool permissions. For a workflow owner, check the specific model's supported controls and validation rules before moving production requests. This guide is about selection; HaikuChat does not provide a model integration endpoint.

Run an initial quality comparison under each model's intended settings, then test an alternative effort level if available. Track changes in correctness, complete-answer time and billed usage. Keep truncated answers in the failure count instead of quietly removing them from the results.

Include the current models in a new decision

Keeping a proven earlier workflow can be reasonable while you evaluate a replacement. It is different from choosing the best starting point today. The newer Haiku 5.5 has different rates, tokenizer behavior and context capacity; the newer Sonnet 5.5 also needs its own evaluation.

Use this article to understand an existing Haiku-4.5-to-Sonnet-5 choice, then read Haiku 5.5 vs Sonnet 5.5 before making a fresh shortlist. If your actual alternative is the earlier Sonnet 4.6, use the version-specific comparison. For short chats, summaries and tables, open the HaikuChat workspace and review its actual limits.

Questions about this comparison