What problem does this solve?

When agent-deck starts under a supervisor without a UTF-8 locale, four tmux attach clients outside the web terminal bridge can still start in non-UTF-8 mode and replace Unicode glyphs with _. Fixes #1781 and completes the follow-up to #1779.

Why this change

Pass tmux's global -u option before attach-session in the normal and read-only PTY attaches, ControlPipe, and KeySender. This makes each attach client use UTF-8 independently of its inherited locale. An AST regression test now rejects any production attach spawn that omits -u or places it after the subcommand; the existing exact-argv tests and comments were updated with the same contract.

User impact

Terminal output on the remaining PTY and control-mode attach paths preserves Unicode when agent-deck is launched by locale-light environments such as launchd or systemd. Environments that already provide a UTF-8 locale keep the same behavior. The change adds no process, handshake, poll, or output-processing step.

Evidence

The merged #1779 records real output for the same tmux mechanism: without -u, raw bytes for ⏵⏵ were 5f 5f (__); with -u, they were e2 8f b5 e2 8f b5 (⏵⏵). Issue #1781 identifies the four remaining attach sites that share that locale exposure.

Test-first baseline / revert-check:

$ go test internal/tmux/attach_utf8_argv_lint_test.go internal/tmux/tmux_exec_lint_helpers_test.go internal/tmux/tmux_exec_lint_test.go -run '^TestTmuxAttachSpawnsForceUTF8$' -count=1 -v
4 tmux attach spawn(s) do not force UTF-8:
  internal/tmux/controlpipe.go:113
  internal/tmux/keysender.go:101
  internal/tmux/pty.go:733
  internal/tmux/pty.go:737
FAIL

After adding -u, the same command passes.

Final checks:

$ go test internal/tmux/attach_utf8_argv_lint_test.go internal/tmux/control_mode_argv_lint_test.go internal/tmux/tmux_exec_lint_helpers_test.go internal/tmux/tmux_exec_lint_test.go -count=1
ok      command-line-arguments

$ GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go test -c -o <temporary-path>/tmux.test ./internal/tmux
exit 0

$ GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go vet ./internal/tmux
exit 0

$ gofmt -l internal/tmux/attach_utf8_argv_lint_test.go internal/tmux/control_mode_argv_lint_test.go internal/tmux/controlpipe.go internal/tmux/keysender.go internal/tmux/pty.go internal/tmux/pty_socket_test.go
(no output)

$ git diff --check
exit 0

The full sandboxed go test ./... suite, runtime tmux reproduction, and before/after performance timing were not run: the local WSL environment has no Go toolchain, and runtime WSL execution was unavailable for this run. The performance-sensitive scope is limited to one extra global option on existing one-time attach process launches.

AI disclosure

• Human-written
• AI-assisted (I directed and reviewed it)
• AI-authored (a model wrote most of it)

Model(s), if AI helped: gpt-5-codex

Prompt / session log (optional): Not provided.

What actually bothered you

My human asked: "完成一下 #1781" ("Complete issue #1781.")

Checklist

• Targeted diff: one problem, no unrelated changes
• Tests added or updated for new behavior
• Test suite passes sandboxed: HOME=$(mktemp -d) XDG_CONFIG_HOME= XDG_DATA_HOME= XDG_CACHE_HOME= go test ./... - not run locally; see Evidence
• If this touches a hot path (list, status, session output, startup, tmux layer): before/after timing evidence included - not measured; this only adds a global option to existing attach launches
• CHANGELOG.md untouched (entries are added at landing)
• AI-assisted? Disclosed above, with validation evidence, and I can answer questions about the code
• "Allow edits from maintainers" is enabled

Summary by CodeRabbit

• Bug Fixes

  • Improved tmux session and control-mode attachments to consistently use UTF-8 mode.
  • Ensured headless tmux control behavior avoids opening /dev/tty for detached callers.
  • Updated attach command construction for both interactive and read-only modes for more reliable non-ASCII handling.

• Tests

  • Added lint-style checks to fail if required tmux attach/control flags or arguments are missing.
  • Updated tmux command-argument expectations in socket-related tests to include UTF-8 mode.