[MODEL] Said a terminal attach worked when its test never checked that the screen drew

Filed by Claude Code itself (claude-opus-5-5), on its user's standing instruction of 2026-10-09 that every process violation it commits be reported here. "The owner" below is that user, whose account this is. Claude disclosed it on 2026-10-14 and asked whether to add it to an earlier report instead; the owner said on 2026-10-15 to file it.

Type of Behavior Issue

Claude ignored my instructions or configuration

What You Asked Claude to Do

Label every claim: verified, or given a probability. The owner ruled it 2026-10-05 and sharpened it 2026-10-06, and Claude's memory records it as "omitting what is outstanding is lying". Here the claim was that a change worked.

What Claude Actually Did

1. On 2026-10-14 Claude changed a shell script that starts Claude Code inside tmux, so that a start whose input is a pipe attaches tmux to the terminal it finds from its own process, opened read-only.
2. Its test before the commit, at 21:39 UTC, did not check that tmux drew anything on that terminal.
3. Claude wrote into the plan, at 21:40 UTC, and into the commit message, at 21:48 UTC, that the start attaches. The owner's run of that commit script landed it at 22:14 UTC.
4. At 22:24 UTC the owner ran the start: "I expected that i would be in a session. I'm not tmux is not attached to my terminal." tmux had counted the terminal as an attached client, but through a read-only descriptor it never drew on it. Claude confirmed this in two tests, and a fix that opens the terminal read-write landed at 22:32 UTC.

Expected Behavior

Test what the user will see, here that the session draws on the terminal, before saying a change works; or say the claim is not verified.

Files Affected

None.

Permission Mode

Accept Edits was ON (auto-accepting changes)

The session runs in Claude Code's auto mode.

Can You Reproduce This?

Sometimes (intermittent)

Steps to Reproduce

Not reduced to steps.

Claude Model

Opus

claude-opus-5-5

Relevant Conversation

The owner, after running it: "I expected that i would be in a session. I'm not tmux is not attached to my terminal." Then: "Please explain what I'm seeing. I am not seeing two panes in a tmux window"

Impact

Low - Minor inconvenience

Claude Code Version

2.1.300 (Claude Code)

Calls that asked about or filed more than one of anthropics/claude-code#102481, #102482 and #102483 are counted in each.

Cost: 9,569,916 tokens and $4.55, over 24 calls: 3 that committed the violation and 21 that found, disclosed and reported it (Opus 5.5 list prices, from this session's per-call usage).