VHS records terminal sessions by scripting keystrokes in .tape files and rendering them as GIFs. Use Wait+Screen /regex/ to synchronize with interactive TUI elements instead of fragile fixed Sleep durations. VHS can only detect text appearing on screen, not disappearing — keep this constraint in mind when designing wait conditions.

Non-Deterministic TUI Responses

When recording an LLM-powered TUI, detecting when a response is complete requires special care. Naive approaches all fail:

Approach	Why it fails
Fixed `Sleep`	Not deterministic
Unique marker in prompt (`XYZENDXYZ`)	Appears in the typed prompt on screen — `Wait+Screen` matches immediately
Math formula (`347+829` → wait for `1176`)	LLM computes the answer in its visible thinking trace before the response finishes
`Wait+Line /^MARKER$/`	TUI padding/borders prevent exact line matching
`Hide` + type marker	`Hide` only hides VHS command log, not terminal content

The concatenation trick

Ask the LLM to concatenate two words and print the result. The prompt contains both words separately but never the combined string:

Type "briefly explain this repo. Then print ALFA concatenated with BRAVO."
Enter
Wait+Screen /ALFABRAVO/

Why it works:

Typed prompt shows ...ALFA concatenated with BRAVO — no ALFABRAVO
Thinking trace says "I need to concatenate ALFA and BRAVO" — no ALFABRAVO
Response outputs ALFABRAVO — the only match

Use uncommon words (ALFA, BRAVO, ZULU) so the combined form can't appear accidentally.

Tips

Run VHS from the correct CWD — it inherits the working directory. If the prompt references "this repo", the CWD must be the repo root, not a subdirectory.

Inspect GIF frames when debugging timing issues:

ffmpeg -i demo.gif -vf "select='eq(n\,100)'" -vsync vfr /tmp/frame.png

Nix Integration

If the project uses Nix, create a dedicated flake.nix for the demo (e.g., doc/demo/flake.nix) so anyone can reproduce the recording with nix run.
Reference tape by Nix store path in flake apps (vhs "${./.}/demo.tape") — don't cd into the store, as it may contain a flake.nix that confuses nix run inside the recording.

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VHS Demo Recording

Non-Deterministic TUI Responses

When recording an LLM-powered TUI, detecting when a response is complete requires special care. Naive approaches all fail:

Approach

Why it fails

Fixed Sleep

Not deterministic

Unique marker in prompt (XYZENDXYZ)

Appears in the typed prompt on screen — Wait+Screen matches immediately

Math formula (347+829 → wait for 1176)

LLM computes the answer in its visible thinking trace before the response finishes

Wait+Line /^MARKER$/

TUI padding/borders prevent exact line matching

Hide + type marker

Hide only hides VHS command log, not terminal content

The concatenation trick

Ask the LLM to concatenate two words and print the result. The prompt contains both words separately but never the combined string:

Type "briefly explain this repo. Then print ALFA concatenated with BRAVO." Enter Wait+Screen /ALFABRAVO/

Why it works:

Typed prompt shows ...ALFA concatenated with BRAVO — no ALFABRAVO

Thinking trace says "I need to concatenate ALFA and BRAVO" — no ALFABRAVO

Response outputs ALFABRAVO — the only match

Use uncommon words (ALFA, BRAVO, ZULU) so the combined form can't appear accidentally.

Tips

Run VHS from the correct CWD — it inherits the working directory. If the prompt references "this repo", the CWD must be the repo root, not a subdirectory.

Inspect GIF frames when debugging timing issues:

ffmpeg -i demo.gif -vf "select='eq(n\,100)'" -vsync vfr /tmp/frame.png

Nix Integration

If the project uses Nix, create a dedicated flake.nix for the demo (e.g., doc/demo/flake.nix) so anyone can reproduce the recording with nix run.

Reference tape by Nix store path in flake apps (vhs "${./.}/demo.tape") — don't cd into the store, as it may contain a flake.nix that confuses nix run inside the recording.