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## Thinking Path
> - Paperclip is the open source app people use to manage AI agents for
work
> - The PR verify workflow gates every pull request; its wall-clock time
sets the feedback loop for all contributors
> - In a recent successful PR run (actions run 32012408876), the slowest
check was "Verify serialized server suites (1/5)" at 337s, while its
four sibling shards finished in 212-238s
> - The serialized lane assigns suites to shards round-robin over an
alphabetical list, so the heavy heartbeat and issues suites cluster on
one runner
> - The general-server lane already solves this with a duration-aware
LPT partition backed by a recorded manifest
> - This pull request reuses that partitioner for the serialized lane
with a fresh per-suite duration manifest
> - The benefit is a balanced serialized matrix: the measured 968s suite
total levels to about 194s per shard, which removes about 80-100s from
the run's slowest check
## Linked Issues or Issue Description
**What existing behavior does this improve?**
The `Verify serialized server suites` shard matrix in
`.github/workflows/pr.yml` distributes route/authz test suites across
five runners.
**Subsystem affected**
CI / test infrastructure (`scripts/run-vitest-stable.mjs`).
**Current behavior**
`selectSerializedSuites` assigns suites round-robin (`index %
shardCount`) over the alphabetically sorted file list. The heavy suites
cluster on shard 1/5. In actions run 32012408876, shard 1/5 spent 291s
in its test step while the other shards spent 170-201s, which made that
job (337s total) the slowest check of the whole PR run.
**Proposed behavior**
Partition the serialized suites with the same duration-aware LPT
algorithm the general-server lane already uses
(`scripts/general-server-shard.mjs`), backed by a new per-suite duration
manifest. All five shards then carry about 194s of measured test time.
**Reason and benefit**
The slowest check bounds PR feedback time. Balancing the serialized
matrix removes about 80-100s from that bound without adding runners.
**Breaking changes**
None. The partition remains deterministic, complete, and
non-overlapping; suites missing from the manifest get the median weight.
## What Changed
- Added `scripts/serialized-shard-durations.json`: per-suite wall-clock
durations (ms) for all 134 serialized suites, sampled from actions run
32012408876 by diffing consecutive per-suite label timestamps in the
shard logs (captures vitest spawn overhead, not just reported test time)
- `scripts/run-vitest-stable.mjs`: `selectSerializedSuites` now uses the
existing LPT partitioner (`selectGeneralServerShard`) with the new
manifest instead of round-robin
- `scripts/__tests__/run-vitest-stable-shard.test.mjs`: added a
manifest-freshness test and a shard-balance test for the serialized
lane, mirroring the general-server ones
- `.github/workflows/pr.yml`: updated the serialized matrix comment with
the new measurement and mechanism
## Verification
- `node --test ./scripts/__tests__/run-vitest-stable-shard.test.mjs`
passes (13 tests), including the existing test that the serialized
shards form a complete, non-overlapping partition
- Dry-run of all five shards shows estimated totals of
194/194/194/194/193s (round-robin was 276/175/160/172/187s):
`node scripts/run-vitest-stable.mjs --mode serialized --shard-index N
--shard-count 5 --dry-run`
- The `Verify serialized server suites` jobs on this PR run the real
partition end to end
## Risks
- Low risk. Selection logic only; the vitest invocation per suite is
unchanged
- A stale manifest degrades gracefully: unknown suites get the median
weight, and a dedicated test fails if fewer than half the current suites
have recorded durations
## Model Used
- Claude (Anthropic), model ID `claude-fable-5`, agentic coding session
with tool use (Claude Code / Claude Agent SDK); no extended-thinking
mode
## Checklist
- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] All Paperclip CI gates are green
- [x] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge
Related prior work: #10923 (split serialized tests into five shards),
#10925 (general-server duration manifest), #11156 (workspaces-a native
shards).
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>
Devin Foley