Claude Opus 4.7 vs. GPT-5.5 Spud: What the Drift Evidence Actually Shows

The general drift evidence supports caution, not a winner

The broader research record supports the concern that LLM behavior can change and should be re-measured. One paper on nondeterministic drift says it quantifies baseline behavioral drift in two LLMs and notes that drift can manifest differently across models . A separate study of ChatGPT reports short-time drifts in the performance and behavior of GPT-3.5 and GPT-4 .

Those sources justify retesting after model or platform updates. They do not show that Claude Opus 4.7 or GPT-5.5 Spud has a specific drift rate, nor do they prove that one is more reproducible than the other.

What is documented for Claude Opus 4.7

Anthropic says developers can use claude-opus-4-7 through the Claude API . Anthropic’s model-specific update note says Claude Opus 4.7 introduces task budgets and a new tokenizer . The same note says the tokenizer may use roughly 1x to 1.35x as many tokens as previous models, up to about 35% more depending on content, and that /v1/messages/count_tokens will return a different token count for Claude Opus 4.7 than it did for Claude Opus 4.6 .

That supports a narrow but important conclusion: workflows that depend on token counts, budget thresholds, context limits, routing rules, or cost estimates may not behave identically after an Opus 4.7 migration, even when prompt text is unchanged .

It does not prove that Opus 4.7 has a measured quality regression. Tokenizer and task-budget changes can affect system-level reproducibility without showing that the model is worse.

What is verified for GPT-5.5 Spud

The source record is much weaker for GPT-5.5 Spud. The supplied OpenAI API page is a “Page not found” result for a GPT-3.5-turbo documentation URL, not an official GPT-5.5 Spud source . A secondary source discussing GPT-5.5 Spud says no official GPT-5.5 release date, model card, or API pricing has been announced .

That does not prove anything about Spud’s actual capabilities. It means this evidence set cannot support claims about Spud’s API behavior, update cadence, tokenizer, regression history, or reproducibility.

Evidence snapshot

How teams should test a model update

The practical takeaway is to treat a model update as a migration, not a drop-in swap. A reproducibility-focused evaluation should separate behavioral quality from infrastructure and measurement effects.

A minimum migration plan should include:

1. Record the exact test surface. Save the model identifier, prompt text, system instructions, tool schemas, retrieval context, input files, image inputs, and decoding settings. This aligns with the broader need to design LLM studies for reproducibility and replicability .
2. Run old and new setups on the same cases. Drift research supports measuring model behavior over time instead of assuming it remains stationary .
3. Add token- and budget-sensitive tests for Opus 4.7. Re-count tokens and rerun cases near limits because Anthropic says Opus 4.7 token counts can differ from Opus 4.6, and the model introduces task budgets .
4. Classify failures by likely cause. A budget cutoff, token-count shift, retrieval miss, or harness timeout is different from a reasoning-quality regression.
5. Avoid benchmarking an unverified model rumor against a documented API target. Until there is official documentation or a stable API reference for GPT-5.5 Spud in the source record, reproducibility claims about it remain unsupported here .
6. Retest after every material change. The literature supports the idea that LLM behavior can shift and that reproducibility has to be built into the evaluation process .

Bottom line

The defensible conclusion is limited but important: there is no verified head-to-head winner between Claude Opus 4.7 and GPT-5.5 Spud on regression drift or reproducibility after updates.

Claude Opus 4.7 has official Anthropic documentation and known operational changes that can affect repeatability in token- or budget-sensitive workflows . GPT-5.5 Spud does not have comparable official OpenAI evidence in the reviewed source set; the supplied OpenAI API page is a “Page not found” result, and a secondary source says no official release date, model card, or API pricing has been announced . The broader research record says LLM drift and reproducibility problems are real enough to measure carefully, not assume away .