LGTM. Verified against the source: the diff is exactly the intended one-line change at test-engine/client/fetching.mjs:26, swapping init.mode = reqConfig.credentials for init.credentials = reqConfig.credentials.

• Correctness: init.credentials is the right fetch() init key, and the schema (test-engine/lib/testsuite-schema.json:109-113) constrains the value to the valid enum omit / same-origin / include, so the value passed through maps directly onto Fetch's credentials option.
• No regression to mode: line 25 (if ('mode' in reqConfig) init.mode = reqConfig.mode) is untouched, and since the old line overwrote init.mode, this fix actually restores mode handling for any request that also set credentials. The surrounding option assembly (cache, redirect, headers) is unaffected and remains consistent.
• Latent-fix characterization is accurate: grep -rc credentials tests/ returns no matches, so no current test exercises this path — nothing changes for the existing suite, and the first test to set credentials will now behave correctly. The explain-test.liquid template already documents credentials separately, so docs and behavior are now aligned.

No side effects spotted. Ship it.

This is an AI-generated review (Claude Code), posted as part of a maintainer-directed review of the test suite.