I think I may have found a reproducible issue in Claude Code, and I'm curious whether anyone else has seen this.



I wanted to verify that Claude was actually seeing the same user messages that I sent.



So I added a simple instruction:



"When you receive a message, first repeat the last user message verbatim, prefixed with 'Repeat:'"



Then I sent ONLY ONE user message:



\[MSG 01\]

I found a solution. From now on my messages will have increasing IDs...



I did NOT send any \[MSG 02\].



However, Claude immediately responded with:



Repeat:

\[MSG 02\] Let's get back to the main topic. Could you try connecting to this proxy...



It even continued with:



"The numbering continues correctly after MSG 01, so I trust it."



The strange part is:



\- The JSONL transcript only contains my MSG 01.

\- There is no user message containing MSG 02.

\- Claude behaved as if MSG 02 had just been received.



Later in the conversation I changed my numbering scheme and stopped explaining the numbering rules. The behavior disappeared and Claude started repeating my messages correctly again.



I am NOT claiming Anthropic is injecting fake user messages.



At this point I only have these observable facts:



1. Claude was instructed to repeat the last user message verbatim.

2. The JSONL transcript contains only MSG 01.

3. Claude repeated a non-existent MSG 02 instead.

4. Later the issue disappeared.



Has anyone else observed something similar in Claude Code?



Could this be:

\- prompt reconstruction,

\- summary/handoff contamination,

\- tool/context desynchronization,

\- or simply a rare model hallucination?


Note:
This documented case occurred on Claude Opus 4.8 after a fallback. I believe I noticed similar behavior earlier in the session, but I only started logging and numbering messages later, so this screenshot is the first recorded instance.

Claude Code repeated a user message that does not exist in the JSONL transcript (possible prompt/context desynchronization?)