Claude Code Was Broken for 6 Weeks. AMD Caught It in 6,852 Sessions Before Anthropic Did.

For six weeks, you thought you were writing your prompts wrong.

You could feel Claude Code messing up. Refactors going sideways, files edited without being read, thinking cut mid-sentence. You re-read your CLAUDE.md, tweaked your instructions, blamed your harness. The Anthropic dashboard said everything was fine.

You had your feeling against their telemetry.

Guess who lost.

TLDR: On April 23, 2026, the day GPT-5.5 dropped, Anthropic published a postmortem validating six weeks of user complaints. Twenty-one days earlier, an AI director at AMD had already filed a forensic audit of 6,852 sessions on GitHub. The bugs are documented, the timing is worse, and the lesson isn't the one most coverage is selling.

For most press coverage, the event is the postmortem. Not for this article. The event is the 21 days between the AMD audit and the Anthropic confirmation, the word a tech publication put in its headline without drawing the operational consequence, and the reason thousands of paying devs spent six weeks doubting themselves while the truth was sitting on GitHub.

The Postmortem Dropped on GPT-5.5 Day. The Audit Dropped Three Weeks Earlier.

Timeline of Six-Week AI Performance Degradation Incident

April 23, 2026. Anthropic published its postmortem.

The same day, OpenAI shipped GPT-5.5. The timing wasn't lost on anyone reading the dev forums that morning.

The postmortem documented three changes that silently degraded Claude Code over six weeks. Default reasoning effort dropped from "high" to "medium" between March 4 and April 7, thirty-three days. A caching bug (clear_thinking_20251015 with keep:1) ran on every turn instead of once, between March 26 and April 10, fifteen days. A system prompt verbosity limit capped responses at 25 words between tool calls and 100 words for the final response, between April 16 and April 20, four days.

Anthropic called the first one "the wrong tradeoff." That phrase is rare. Vendors usually say "we have identified an issue" or "an unexpected interaction." Not "the wrong tradeoff."

For most coverage, that was the event. The bugs catalogued, the fixes shipped in v2.1.116, the usage limits reset, the API unaffected. Roll credits.

Not for this article.

Twenty-one days before the postmortem, on April 2, Stella Laurenzo, Senior Director of AI at AMD and former lead of Google's OpenXLA project, filed GitHub issue #42796 against the Claude Code repo. She attached 6,852 sessions of telemetry, named the regressions, documented the dates, and quoted Anthropic's own behavior back to itself.

She knew. Reddit and Twitter had been logging the same symptoms for weeks.

Anthropic took three weeks to confirm.

Every vendor ships bugs. The story is the timeline. Six weeks of degraded code stayed invisible to thousands of paying customers until somebody outside the building built her own forensic infrastructure and dropped the receipts on GitHub. The bugs are documented. The timeline is what nobody wants to talk about.

The Audit That Forced the Confession

Stella Laurenzo doesn't tweet vibes.

She runs AI infrastructure at AMD. Before that, she led the OpenXLA project at Google. Her audit reads like a court filing.

GitHub issue #42796. 6,852 Claude Code sessions captured between January and early April. 234,760 tool calls. 17,871 thinking blocks.

The behavioral metrics were the part nobody could argue with. Median thinking length went from 2,200 characters in January to 600 characters in March, a 73% collapse. Files-read-before-edit dropped from 6.6 to 2.0. Stop-hook violations climbed from zero to roughly ten a day after March 8.

These aren't perceptual claims. Nobody's saying "it feels worse." She measured what the agent did, and the agent did less. Less reading, less thinking, more premature stops.

The conclusion landed at the top of the issue: "Claude cannot be trusted to perform complex engineering tasks."

Read that sentence again with the source attached. AI infrastructure director of one of the largest chip makers on the planet. 234,760 tool calls behind it.

Then a detail that should have ended the news cycle right there. AMD switched providers during the incident. The Register reported it on April 6. Laurenzo wrote that her team had moved to another vendor producing superior quality work, with the implication that they kept the Claude option open in hope it would get fixed. She didn't say which provider.

A few caveats, because honesty matters. Anthropic disputed some interpretations on the issue thread itself. And a separate viral benchmark claim from a different group, circulating in parallel at the time, was independently debunked for methodology issues. Worth not mixing up with the Laurenzo audit, which stands on its own numbers.

Six thousand eight hundred and fifty-two sessions don't unhappen.

It read like an indictment with footnotes. Anthropic took three weeks to confirm any of it.

Why You Were Right and Couldn't Prove It

Six weeks before the audit, the dev forums were already on fire.

Catalin Pit on Twitter, March 20: "Lately, Claude makes some shocking mistakes." On Reddit r/ClaudeCode, April 7, u/marcin_dev posted: "has Claude Code become significantly dumber over the past few days?" The replies all said yes. On Twitter, April 13, @safetyth1rd: "It's taking 2-3x longer to do stuff."

None of it moved a needle.

Then, post-postmortem, u/Enthu-Cutlet-1337 wrote the line that everyone in the thread recognized. The 25-word cap explained so much, they had been seeing Opus truncate mid-reasoning on refactors for weeks and "thought my prompts were off."

Four words doing the heaviest lifting in the whole story.

Thought my prompts were off.

That's the cognitive trap. When the user perceives degradation and the vendor dashboard says everything is fine, the user doubts themselves first. Not because they're naive. Because of the asymmetry of evidence.

The vendor has the telemetry, the eval suites, the regression tests, the dashboards. The user has a feeling. When the feeling and the dashboard disagree, the dashboard wins. It looks more like evidence.

A vibe is easy to dismiss. "Maybe you wrote the prompt wrong. Maybe your CLAUDE.md drifted. Or the task was just harder this time."

A 6,852-session audit isn't easy to dismiss.

That's why nobody confirmed anything until Laurenzo.

Post-postmortem, u/Sufficient-Farmer243 closed the loop on r/ClaudeCode. They wrote that every single issue the community had been "gaslit" about for weeks turned out to be exactly what people had been describing. (Their wording, in quotes for a reason. Whether you agree with the verb or not, it was the dominant register in the thread.)

Once the postmortem dropped, the thread filled with confirmation replies. Not new bugs. Old bugs people had been logging silently into private diaries for five weeks straight.

You weren't wrong. You just didn't have AMD-grade telemetry sitting on your laptop.

The Word Anthropic Picked, the Connection Most Coverage Missed

VentureBeat put one word in its headline: "harnesses."

"Mystery solved: Anthropic reveals changes to Claude's harnesses and operating instructions likely caused degradation."

That's the framing Anthropic itself confirmed. The model didn't get worse. The harness around the model got worse. Default reasoning effort. Caching behavior. System prompt verbosity. Three knobs in the wrapper, not the weights.

Most coverage noted the word and moved on. Few drew the consequence.

If the harness matters more than the model, and the harness can be silently modified by the vendor for six weeks at a time, then the harness isn't really yours.

It's their territory.

Your CLAUDE.md is one layer. The default reasoning effort, the caching behavior, the verbosity prompt, those are layers in their codebase you'll never see. I've written before about the layer most developers treat as a readme, arguing CLAUDE.md was the new .env. I still think that. The piece nobody talks about is what sits underneath.

You write 47 lines of CLAUDE.md. The vendor's harness loads dozens of instructions before yours even runs. You control the top of the stack. They control everything below.

When the bottom of the stack changes, your top is decoration.

What is striking about this postmortem, it's not the harness mattering. Most senior devs already suspected it did. The new piece is the published, vendor-confirmed admission that yes, the wrapper is doing more work than the model in many tasks, and yes, the wrapper can be modified mid-month without you knowing.

Extended thinking is load-bearing for senior engineering workflows. The user-facing layer most paying customers tune (CLAUDE.md, slash commands, custom prompts) sits on top of vendor-controlled defaults that decide how much the model thinks before acting. When those defaults shift, every workflow built on top shifts too. Silently.

Read your CLAUDE.md tonight. Still useful, still load-bearing in the part you control. But you're tuning the steering wheel.

Somebody else is changing the gearbox.

AMD Switched. Reddit Knew. Anthropic Confirmed Last.

Three facts in a line.

AMD's AI director switched to another provider during the incident. The Register reported it on April 6. Reddit had been documenting symptoms since early March. Anthropic confirmed the bugs on April 23, twenty-one days after the audit landed in their own GitHub repo.

Pattern: operational truth bubbled up from the user base before the vendor validated it.

That's not a fluke. That's the structural shape of any hosted AI degradation. The vendor has eval suites and dashboards optimized for the metrics they care about. The user base runs the real workload, in real codebases, with real consequences. When the two diverge, the user base notices first. The vendor confirms last.

If the gap is twenty-one days, the user base eats twenty-one days of degraded output.

If your AI workflow can be silently degraded for six weeks, you don't have a workflow. You have a single point of failure with autocomplete.

I wrote the pricing-side version of this argument last month. Same vendor, different leverage. The reliability-side version is worse, because it's invisible. A pricing change shows up on your invoice. A harness change shows up six weeks later in a forensic audit you didn't run.

Yes, multi-stack costs more to set up. Routing logic, eval glue, redundant API keys, two flavors of CLAUDE.md to maintain. It's annoying. The cost of not doing it is six weeks of degraded code you shipped without knowing it, plus a 6,852-session audit run by somebody else to find out. You can't observe what the vendor changed, so you hope.

Anyway, point is this: you spent six weeks re-reading your prompts while an AI director at AMD was logging 6,852 sessions to prove you weren't crazy.

Your AI workflow doesn't rest on your harness. It rests on a vendor's patience to maybe ship a postmortem. That's not a workflow, that's a bet.

Next time something feels off, don't ask if your prompts suck.

Ask if you have your own telemetry.