Use a loopback address for local mock servers
Point tests at 127.0.0.1 where resolving localhost is slow.
How Stripe used this technique
Read what Stripe changed and follow the original sources.
Use a loopback address for local mock servers
Stripe Python points its tests at 127.0.0.1 for the local mock server on Windows runners, where localhost resolution is slow, and keeps localhost elsewhere.
- Use it when
- Local integration tests repeatedly resolve localhost on a slow platform.
- Before applying it
- Bind the mock server and its clients to the same address, and keep dedicated hostname, IPv6 and DNS tests where the product needs them.
Company Playbooks
- Read Anthropic’s playbookRun only affected tests
- Read DoorDash’s playbookRun build and test in parallel, sharded across CI agents
- Read Linear’s playbookBenchmark the same job on faster hardware
- Read Mastra’s playbookStarSling runners for the slowest test workflow
- Read OpenAI’s playbookRank CI fixes by the minutes they save
- Read Ramp’s playbookMove a CPU-bound test suite to a faster CPU
- Read Shopify’s playbookLet an agent iterate against one CI metric
- Read Spotify’s playbookBuild arm64 on native arm64 runners
- Read Stripe’s playbookRun only the tests a change can reach
- Read Uber’s playbookBuild the merge-queue speculations most likely to be needed