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Control Percy parallel snapshots by coordinating CI shards into one build: use the same run-unique PERCY_PARALLEL_NONCE on every shard, set PERCY_PARALLEL_TOTAL to the number of shards Percy should receive when that number is fixed, and finalize once the expected work is complete. For a variable shard count, Percy documents total -1 with an explicit finalize step after all test jobs finish. First check whether your CI integration already detects parallel metadata; many supported setups do.
What Percy concurrency settings coordinate
These settings coordinate parallel Percy builds or shards; they are not a single knob that limits how many snapshots can run simultaneously. The nonce groups shards from one CI run into the same Percy build. The total tells Percy how many shard finalizations to expect when you know the count. See BrowserStack Percy’s parallel test suites guide and environment-variable reference.
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Count the Percy shard invocations that report to the build, not the test cases running inside them. The official guidance does not establish one universal maximum concurrency limit across plans; check current plan documentation or support for any account-specific cap.
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Choose the coordination mode
| CI situation | Configuration | How completion works | Risk to avoid |
|---|---|---|---|
| Fixed, known shard count | One shared, unique nonce and an exact PERCY_PARALLEL_TOTAL |
Percy waits for that many shard finalizations. | An incorrect total can leave the build waiting for a missing shard. |
| Variable or unknown shard count | Parallel mode with total -1 |
Run an explicit finalize operation after every shard has finished. | A missing finalizer or a different nonce can leave the build open. |
Percy documents these modes in its parallel suite guide. Choose based on whether the shard count is known and whether your CI workflow can run a dependent finalizer job.
Configure a fixed shard count
- Determine the count. Set the total to the number of Percy shard invocations expected for this build.
- Choose a run-unique nonce. Use a CI run identifier that all shards share, but that differs between separate runs. Confirm reruns do not reuse an identifier that could point at an already finalized build.
- Pass both values to every shard. Example for four shards, assuming
CI_RUN_IDis shared across those four invocations and unique for each run:PERCY_PARALLEL_NONCE="$CI_RUN_ID" PERCY_PARALLEL_TOTAL=4 npx percy exec --parallel -- npm test - Wait for all expected shard finalizations. Keep the values consistent across jobs and verify your installed CLI and CI integration’s completion behavior.
Percy documents percy exec --parallel -- [test command] for tests distributed across machines or containers. The command above is illustrative; adapt the shell and CI variable name to your environment. A fixed total is useful only if every expected shard can report.
Configure a variable shard count
- Run the test shards in parallel mode with the shared nonce and total
-1. - Add one dependent CI job that waits until all test shards have finished.
- In that job, run
percy build:finalizeusing the same nonce/environment context as the shards. Run it once, after—not alongside—the shard jobs.
Percy identifies percy build:finalize in its command reference. The parallel-suite guide also describes finalizing after all tests complete. Follow the syntax supported by the CLI version installed in your project.
Check automatic CI detection before overriding values
Percy says most supported CI configurations detect parallel variables automatically. Inspect the environment detection and job logs before adding manual overrides; conflicting values across shards can prevent them from joining correctly. For a custom or unsupported provider, map the Percy token and parallel metadata explicitly. Percy’s guide to other CI/CD tools says PERCY_PARALLEL_NONCE is required for custom providers and should be shared within a run but unique between runs.
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- The build remains in “receiving.” Compare the configured fixed total with the number of shards that actually finalized. If the total is four and only three shards completed, Percy can continue waiting.
- You use total
-1. Check that the finalize job ran after every test shard, ran only once, and used the same nonce as those shards. - A rerun attaches to an old build or fails after finalization. Make the nonce unique across runs, including reruns. A provider’s workflow identifier may be reused, so verify its behavior rather than assuming it is unique.
- Only some jobs join the parallel build. Compare the nonce and total visible to every shard, and inspect whether supported CI detection is already setting values that your configuration overrides inconsistently.
- You use a custom CI provider. Map the Percy token and shared parallel metadata into each job, and ensure the finalizer receives the same nonce when using total
-1.
These failure modes and remedies follow Percy’s parallel suite troubleshooting guidance and its environment-variable reference.
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