Originally published at trustboundarystudio.com. The video version, with diagrams, is on YouTube.
Key facts
- When: Between August and early September 2025, according to Anthropic's post-mortem of 17 September 2025. The first bug was introduced on 5 August, and two more arose from deployments on 25 and 26 August.
- Peak: At the worst impacted hour, on 31 August 2025, 16% of Sonnet 4 requests were affected, after a routine load balancing change on 29 August sent more short-context requests to servers configured for a 1M token context window.
- Claude Code: Approximately 30% of Claude Code users who made requests during the period had at least one message routed to the wrong server type. That is a share of users with at least one bad message, not a share of requests.
- Other platforms: On Amazon Bedrock, misrouted traffic peaked at 0.18% of all Sonnet 4 requests; on Google Cloud's Vertex AI it affected less than 0.0004% of requests between 27 August and 16 September. The other two bugs did not affect third-party platforms.
- Detection: In Anthropic's words, the evaluations it ran simply did not capture the degradation users were reporting, and its privacy controls limited engineers' access to the conversations that showed it.
- Primary source: Anthropic, "A postmortem of three recent issues", 17 September 2025. The post-mortem gives no overall total of affected users or requests.
In early August 2025, people using Claude started saying it had got worse.
Answers that were off. Code with obvious syntax errors in it. In one example
Anthropic gives itself, Thai characters in the middle of an English reply.
They were right. Between August and early September, three separate
infrastructure bugs degraded Claude's responses. Nothing went down. Nothing was
attacked. At the worst hour, on 31 August, 16% of requests to Sonnet 4 were
affected, and the checks that are meant to catch exactly this had passed.
Anthropic's own post-mortem, published 17 September 2025, is almost entirely
about routing, a server misconfiguration and a compiler. It is a clean example
of the difference between a service being up and a service being right.
How Claude is served
Anthropic serves Claude through its own API, Amazon Bedrock and Google Cloud's
Vertex AI, on three kinds of hardware: AWS Trainium, NVIDIA GPUs and Google
TPUs. Each needs its own optimisations. The rule across all of them, in the
post-mortem's words: "Despite these variations, we have strict equivalence
standards for model implementations."
So every change to the plumbing has to hold everywhere. In August 2025, three
did not.
Bug one: requests sent to the wrong servers
On 5 August, some Sonnet 4 requests began to be "misrouted to servers
configured for the upcoming 1M token context window." That affected
approximately 0.8% of requests to Sonnet 4, which is small enough to look like
noise.
Then, on 29 August, "a routine load balancing change unintentionally increased
the number of short-context requests routed to the 1M context servers." At the
worst impacted hour, on 31 August, 16% of Sonnet 4 requests were affected.
It was not spread evenly. Routing was sticky, so "once a request was served by
the incorrect server, subsequent follow-ups were likely to be served by the
same incorrect server." Some people got degraded answers again and again.
The figure most likely to be misquoted: "Approximately 30% of Claude Code users
who made requests during this period had at least one message routed to the
wrong server type, resulting in degraded responses." That is a share of users,
not 30% of requests, and it does not add to the 16%, which is a share of Sonnet
4 requests in one hour. The post-mortem gives no overall total, and neither
does this article.
On Amazon Bedrock, misrouted traffic peaked at 0.18% of all Sonnet 4 requests;
on Vertex AI it affected less than 0.0004% of requests between 27 August and
16 September. The fix was deployed on 4 September and, per the post-mortem,
fully rolled out by 18 September.
Bug two: the wrong token
The second bug was "a misconfiguration to the Claude API TPU servers that
caused an error during token generation." A model writes its answer one token
at a time, and a runtime performance optimisation "occasionally assigned a high
probability to tokens that should rarely be produced given the context."
The result was Thai or Chinese characters in answers to English prompts, and
obvious syntax errors in code. It affected Opus 4.1 and Opus 4 from 25 to 28
August, and Sonnet 4 until 2 September, when the change was rolled back.
"Third-party platforms were not affected by this issue." Anthropic has since
"added detection tests for unexpected character outputs to our deployment
process."
Bug three: the compiler
To choose its next token, the model ranks candidates by probability, and on
TPUs that used approximate top-k, a faster shortcut.
In December 2024, Anthropic found that its "TPU implementation would
occasionally drop the most probable token when temperature was zero," and
deployed a workaround. In August it deployed code to improve how Claude selects
tokens. In the post-mortem's words: "Our fix removed the December workaround
because we believed we'd solved the root cause."
The change "inadvertently triggered a latent bug in the XLA:TPU compiler." The
approximation "sometimes returned completely wrong results, but only for
certain batch sizes and model configurations." "The December workaround had
been inadvertently masking this problem." And "The same prompt might work perfectly on one request and
fail on the next."
It was confirmed on Haiku 3.5, rolled back on 4 September. Anthropic believes
it "could have impacted a subset of Sonnet 4 and Opus 3 on the Claude API."
Opus 3 was rolled back on 12 September, after user reports "compatible with
this bug." On Sonnet 4, in Anthropic's words, "we were unable to reproduce this
bug on Sonnet 4 but decided to also roll it back out of an abundance of
caution." Third-party platforms were not affected.
The fix was to drop the shortcut: exact top-k, and some additional operations
standardised on fp32. "Model quality is non-negotiable, so we accepted the
minor efficiency impact."
Why it took weeks to see
Anthropic gives four reasons.
The checks passed. "Our validation process ordinarily relies on benchmarks
alongside safety evaluations and performance metrics. Engineering teams perform
spot checks and deploy to small 'canary' groups first." And the evaluations
"simply didn't capture the degradation users were reporting, in part because
Claude often recovers well from isolated mistakes."
The symptoms overlapped. "Each bug produced different symptoms on different
platforms at different rates." Taken together, "It looked like random,
inconsistent degradation."
The timing hid the cause. "When negative reports spiked on August 29, we
didn't immediately make the connection to an otherwise standard load balancing
change."
The door was shut. "Our internal privacy and security controls limit how
and when engineers can access user interactions with Claude, in particular
when those interactions are not reported to us as feedback. This protects user
privacy but prevents engineers from examining the problematic interactions
needed to identify or reproduce bugs."
That control is correct to have. It is also why the people holding the
evidence were the users, whose reports, Anthropic says, "all helped us isolate
the issues."
The plain statement
The post-mortem opens with this:
"To state it plainly: We never reduce model quality due to demand, time of
day, or server load. The problems our users reported were due to
infrastructure bugs alone."
That is Anthropic's statement, and the document is the evidence it offers. It
is linked below for anyone who wants to read it and decide.
What Anthropic changed
Three commitments, under the post-mortem's own headings. More sensitive
evaluations, able to tell a working implementation from a broken one.
Quality evaluations in more places: "we will run them continuously on true
production systems." Faster debugging tooling, to "better debug
community-sourced feedback without sacrificing user privacy."
It also asked users to keep reporting, through the /bug command in Claude
Code or the thumbs down in the apps.
Four lessons for infrastructure teams
Up is not right. Every one of these bugs left the service running. If your
monitoring measures whether the system answers, and not whether the answer is
correct, a quality failure is invisible to it.
Small changes stack. A routine load balancing change took a bug touching
approximately 0.8% of Sonnet 4 requests to one touching 16% in the worst hour.
Before you call a new change clean, find out what was already broken.
Workarounds hide things. Removing one can uncover whatever it was quietly
covering. Treat its removal as a change in its own right.
Decide in advance how you will investigate failures you are not allowed to
look at. The privacy door should stay. The way through it, with consent and
with tooling, has to exist before the day you need it.
Sources
- Anthropic, A postmortem of three recent issues, 17 September 2025. The company's own account of its own incident, and the only source for every claim above.
Corrections are welcome, and any made are listed, dated, at the end of this
article.
The video version
Questions this answers
Why was Claude giving worse answers in August 2025?
Anthropic's post-mortem attributes it to three infrastructure bugs: some Sonnet 4 requests routed to servers configured for a 1M token context window, a misconfiguration on the Claude API's TPU servers that corrupted output, and a latent bug in the XLA:TPU compiler triggered by a change to how Claude selects tokens.
Did Anthropic reduce Claude's quality to save money?
Anthropic's post-mortem says: "To state it plainly: We never reduce model quality due to demand, time of day, or server load. The problems our users reported were due to infrastructure bugs alone." The document sets out the three bugs as its evidence.
What does 30% of Claude Code users mean in the Anthropic post-mortem?
Approximately 30% of Claude Code users who made requests during the period had at least one message routed to the wrong server type. It is a share of users, not of requests, and it does not combine with the 16% figure, which is of Sonnet 4 requests in the single worst hour.
Why did it take Anthropic weeks to find the Claude bugs?
The post-mortem says its evaluations did not capture the degradation users reported, each bug produced different symptoms on different platforms at different rates, the 29 August spike was not immediately linked to a standard load balancing change, and privacy controls limited engineers' access to the affected conversations.
Was Claude on Amazon Bedrock or Google Vertex AI affected?
Only by the routing bug, and far less: misrouted traffic peaked at 0.18% of all Sonnet 4 requests on Bedrock, and affected less than 0.0004% of requests on Vertex AI between 27 August and 16 September. The other two bugs did not affect third-party platforms.
