| name | update-vt |
| description | Update the libghostty-vt (ghostty) dependency to a newer upstream commit. Use when the user asks to bump/update libghostty-vt or the ghostty dependency, compare the pinned commit against upstream, assess impact on mostty, and rebuild. Covers the whole runbook — diff analysis, impact gating against mostty's vt API surface, build.zig.zon update, build + smoke test. |
Update libghostty-vt
mostty consumes only the ghostty-vt module of the ghostty dependency. That
module is the upstream repo's src/terminal/ tree. Everything outside
src/terminal/ (macOS, GTK, libghostty C-ABI, renderer, etc.) is irrelevant to
mostty even though it lives in the same repo and shows up in the commit range.
The dependency is pinned in build.zig.zon under .ghostty (url = git commit,
hash = Zig package hash).
Goal
Bump .ghostty from the current pinned commit to a newer one (default: upstream
main HEAD), only after confirming the change set doesn't break mostty's
actual vt API usage. Produce a PLAN.md, update the dep, build, smoke test.
Environment gotchas (this repo / this harness)
cmd.exe is not on PATH in the bash tool. Ignore AGENTS.md's
cmd.exe /c "..." build wrapper here and call the zig exe directly:
"D:/zig-x86_64-windows-0.15.2/zig.exe" build --global-cache-dir D:/zig-cache- Setting
.hash = "" in build.zig.zon makes zig build panic ("Bad
@dependencies source") instead of printing the expected hash. Do not blank
the hash. Use zig fetch --save (below), which computes and writes it.
gh api ... --jq chokes on \. in regexes ("invalid escape sequence"). Use a
character class [.] instead, e.g. test("terminal/(Terminal|Screen)[.]zig").- Use
${project_root}/tmp for any scratch files, never /tmp.
Steps
1. Identify current and target commits
gh api repos/ghostty-org/ghostty/commits/main --jq '.sha + " " + .commit.committer.date'
gh api repos/ghostty-org/ghostty/commits/<CURRENT_SHA> --jq '.commit.committer.date'
If the user named a specific target commit, use that instead of main HEAD.
2. Diff, scoped to the vt module
Only src/terminal/ matters. Get per-file stats:
gh api repos/ghostty-org/ghostty/compare/<CURRENT>...<TARGET> \
--jq '.files[] | select(.filename | test("src/terminal/")) | "\(.status)\t+\(.additions)\t-\(.deletions)\t\(.filename)"'
Pull patches for the files that define mostty's API surface (see step 3):
gh api repos/ghostty-org/ghostty/compare/<CURRENT>...<TARGET> \
--jq '.files[] | select(.filename | test("terminal/(Terminal|Screen|PageList|stream_terminal|main|lib)[.]zig")) | "=== \(.filename) ===\n" + .patch'
Also confirm the build wiring didn't change in a way that affects the module:
gh api repos/ghostty-org/ghostty/compare/<CURRENT>...<TARGET> \
--jq '.files[] | select(.filename | test("^build[.]zig$|^src/simd")) | .filename'
3. Impact-gate against mostty's real vt usage
Enumerate what mostty actually touches, then check each changed export against it:
Grep src/**/*.zig for \bvt\.[A-Za-z_][A-Za-z0-9_.]*.
As of the last update mostty used: vt.Terminal (.init, .flags),
vt.TerminalStream.Handler + .Effects, vt.Cell, vt.Selection (.init),
vt.Pin, vt.Coordinate, vt.Style.Color, vt.color.*, vt.Stream /
vt.ReadonlyHandler. Re-derive this list each run — don't trust this snapshot.
Classify every non-trivial change as one of:
- Additive — new field with a default, new method, new export. Safe.
- Critical pattern: mostty builds
Effects as
var e: vt.TerminalStream.Handler.Effects = .readonly; e.foo = ...;
(see src/win32/tab_mgmt.zig). A new Effects field is safe iff the
.readonly preset gives it a default. Verify that in the diff.
Terminal.init takes named options (.cols/.rows/.default_modes); a new
Terminal struct field with a default does not break it.
- Bug fix — e.g. resize/saturation fixes in
PageList.zig, pin-leak fixes
in Screen.select. Usually a net win for mostty (resize, selection).
- Potentially breaking — signature change, removed/renamed export, an enum
tag that gained a payload (e.g.
SemanticClick.click_events). For each, rg
the mostty tree for the symbol. If unreferenced, it's a non-issue; record that.
Stop and surface to the user if any breaking change is actually referenced by
mostty — don't paper over it.
4. Write PLAN.md
Record: version delta (commit+date), scoped file-stat table, per-item impact
verdict tied to the API list, build/smoke steps, success criteria. PLAN.md at
repo root is an untracked scratch doc — overwriting a stale/DONE plan is fine,
but say so.
5. Update the dependency
Do not hand-edit the hash. Let zig fetch compute it:
"D:/zig-x86_64-windows-0.15.2/zig.exe" fetch --global-cache-dir D:/zig-cache \
--save=ghostty "git+https://github.com/ghostty-org/ghostty#<TARGET>"
This rewrites .ghostty.url and .ghostty.hash in build.zig.zon. A
overwriting existing dependency named 'ghostty' warning is expected.
6. Build + smoke test
"D:/zig-x86_64-windows-0.15.2/zig.exe" build --global-cache-dir D:/zig-cache
Clean build (no output) = pass. Then verify the binary is fresh and starts:
ls -la --time-style=+%H:%M:%S zig-out/bin/Mostty.exe
( ./zig-out/bin/Mostty.exe & MPID=$!; sleep 4; \
if kill -0 $MPID 2>/dev/null; then echo "RUNNING ok"; kill $MPID; \
else wait $MPID; echo "EXITED early code=$?"; fi )
Mostty is a GUI Win32 app: a headless launch only proves it reaches the message
loop (D3D11 device created, config loaded, no early crash). Pre-existing
warnings like missing background-image or unimplemented mode: 9001 are not
regressions. Tell the user that full interactive regression (resize, multi-tab,
selection) needs a manual pass.
Success criteria
build.zig.zon .ghostty points at the target commit with a valid hash.- Every changed vt export is classified; nothing breaking is referenced by
mostty (or the user has been told about it).
zig build is clean and Mostty.exe starts without crashing.PLAN.md reflects the actual delta and verdict.
Rollback
git checkout build.zig.zon restores the previous pin; rebuild to revert.
