Best possible solution:

Keep the fixture cleanup paired with #77010 and land it after or with the parent if the managed-proxy policy change is accepted.

Do we have a high-confidence way to reproduce the issue?

Not applicable as a PR cleanup rather than a current-main bug. Source inspection shows the debug proxy CA helper writes generated cert material under settings.certDir, so the parent branch's repo-relative test cert fixture is the relevant behavior being cleaned up.

Is this the best way to solve the issue?

Yes, the proposed direction is the narrowest maintainable fix for the test-fixture concern: allocate a per-test temp root under os.tmpdir() and remove it during cleanup. The merge path should remain tied to the parent PR rather than handled as an independent main-branch change.

What I checked:

• Current main state: The checkout is on main at 11c600c, and src/proxy-capture/proxy-server.managed-proxy.test.ts is not present on current main. (src/proxy-capture, 11c600cf1993)
• Generated cert location: startDebugProxyServer calls ensureDebugProxyCa(params.settings.certDir), so a test-supplied cert directory controls where the generated CA files are written. (src/proxy-capture/proxy-server.ts:64, 11c600cf1993)
• Generated key filename: ensureDebugProxyCa creates root-ca.pem and root-ca-key.pem under the supplied cert directory, which makes the PR's move away from repo-relative cert fixtures relevant. (src/proxy-capture/ca.ts:13, 11c600cf1993)
• PR diff scope: The provided PR diff modifies only src/proxy-capture/proxy-server.managed-proxy.test.ts, replacing repo-relative .tmp-debug-proxy-test-* paths with an mkdtemp(join(tmpdir(), ...)) root that is removed by cleanup. (src/proxy-capture/proxy-server.managed-proxy.test.ts:1, 9152c89ad32d)
• Stacked parent: The PR body and timeline state that this branch is stacked on open draft PR fix: guard debug proxy CONNECT under managed proxy #77010, whose head is debug-proxy-managed-proxy; the target test file belongs to that parent branch, not current main. (f1f7ffce2c7a)
• Worktree cleanliness: Read-only inspection left the checkout clean on main...origin/main. (11c600cf1993)

Likely related people:

• Vincent Koc: Current-main blame and git log -S for the debug proxy server and CA helper point to commit d5ecee2 for the relevant implementation path. (role: current debug proxy history owner; confidence: medium; commits: d5ecee2cf332; files: src/proxy-capture/proxy-server.ts, src/proxy-capture/ca.ts, src/proxy-capture/proxy-server.test.ts)
• jesse-merhi: The current PR is explicitly stacked on fix: guard debug proxy CONNECT under managed proxy #77010, which introduces the managed-proxy CONNECT policy and the test fixture this PR adjusts. (role: parent PR owner; confidence: medium; commits: f1f7ffce2c7a; files: src/proxy-capture/proxy-server.managed-proxy.test.ts, src/proxy-capture/proxy-server.ts, docs/cli/proxy.md)

Remaining risk / open question:

• The changed test file is not on current main, so final mergeability depends on the open draft parent PR fix: guard debug proxy CONNECT under managed proxy #77010 staying aligned.
• No tests were executed because this review was constrained to read-only inspection.

Codex review notes: model gpt-5.5, reasoning high; reviewed against 11c600cf1993.