For the past several weeks, something felt off about Claude Code. Responses were shorter than expected. Context seemed to vanish mid-session. Complex tasks that Claude had handled with ease suddenly felt labored. Developers filed bug reports, posted on Hacker News, and wondered aloud whether Anthropic had silently “nerfed” the model.
Today, Anthropic answered those questions — and the answer turned out to be more complicated, and more honest, than most expected.
The Postmortem: Three Changes, One Cascading Problem
On April 23, Anthropic published a detailed engineering postmortem identifying three separate product-layer changes that combined to produce what looked like widespread model degradation. Crucially, the API and underlying inference layer were not affected. This was a product problem, not a model problem — though for users in the thick of it, the distinction felt academic.
Change 1: Reasoning Effort Downgraded (March 4)
The first change was intentional — but wrong in retrospect. On March 4, Anthropic quietly lowered Claude Code’s default reasoning effort from high to medium to address latency complaints. In high mode, the UI could appear frozen for extended periods while the model worked through complex problems.
The tradeoff didn’t land well. Users noticed the drop in quality immediately, even if they couldn’t articulate exactly why. On April 7, Anthropic reverted the change. The new default is high, and Opus 4.7 will default to xhigh. Affected models: Sonnet 4.6, Opus 4.6.
Change 2: Cache-Clearing Bug Introduced (March 26)
The second change was supposed to improve performance in a specific edge case — but a bug turned it into a persistent quality drain.
On March 26, Anthropic shipped code to clear Claude’s cached “thinking” from sessions that had been idle for over an hour. The intent was to reduce latency when resuming old sessions. The bug: the clearing code ran on every turn for the rest of the session, not just once after the idle period.
The practical effect was that Claude appeared forgetful and repetitive in long-running sessions. Reasoning built up over turns was discarded continuously, exactly the opposite of what agentic workflows depend on. The fix shipped April 10 in v2.1.101. Affected models: Sonnet 4.6, Opus 4.6.
Change 3: Verbosity-Limiting System Prompt (April 16)
The third change arrived just last week. On April 16, Anthropic added a system prompt instruction capping intra-turn verbosity at 25 words and final responses at 100 words. The goal was to reduce unnecessary verbosity in simple queries.
Combined with other prompt changes already in place, the result was measurable: a ~3% drop in coding evaluation benchmarks. The verbosity cap was reverted on April 20. Affected models: Sonnet 4.6, Opus 4.6, Opus 4.7.
Why It Was So Hard to Detect
Each of the three changes affected a different slice of traffic on a different schedule. Looked at individually, each produced variance within normal bounds. Together, they created an aggregate pattern that appeared as broad, inconsistent degradation.
Anthropic says it began investigating reports in early March, but the issues were initially difficult to distinguish from normal feedback variation. Internal usage and evaluations didn’t initially reproduce the failures users were seeing in production. That gap between internal signals and external experience is something Anthropic is now directly addressing.
What’s Fixed — And What Comes Next
As of April 20 (v2.1.116), all three issues are resolved. Anthropic is resetting usage limits for all subscribers as of April 23.
Going forward, Anthropic has committed to a series of systemic improvements:
- Expanded eval coverage for product-layer changes, not just model-layer
- Staged rollout procedures for system prompt changes in coding contexts
- Better user-facing signals when reasoning effort or verbosity settings change
- More transparent changelogs for Claude Code, Claude Agent SDK, and Claude Cowork
The postmortem also confirmed that Claude Cowork was affected by the caching bug — worth noting for anyone who relies on scheduled autonomous tasks through that product.
What This Means for Agentic AI Developers
If you’re building on Claude Code or the Claude Agent SDK, this postmortem is worth reading in full. It illustrates a risk that’s easy to overlook: product-layer changes can silently degrade agentic performance in ways that look like model regressions.
For teams running long-horizon agent loops, the cache-clearing bug in particular is a reminder to monitor session coherence — not just output quality at individual turns. A Claude that forgets its own reasoning mid-task is a fundamentally different Claude than the one you tested.
Check your current Claude Code version (claude --version). You should be on v2.1.116 or later. If you’re not, update now.
Sources
- Anthropic Engineering Blog — An update on recent Claude Code quality reports
- VentureBeat — Anthropic reveals changes to Claude’s harnesses and operating instructions
- The Register — Anthropic says it has fixed
- Business Insider — Anthropic admits Claude Code issues
- Hacker News discussion (47878905)
Researched by Searcher → Analyzed by Analyst → Written by Writer Agent (Sonnet 4.6). Full pipeline log: subagentic-20260423-2000
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