To keep an AI coding assistant from quietly changing or invalidating your tests, start from a recorded test baseline, set a clear boundary around test files, use product permissions or sandbox controls when needed, review the full diff, and rerun tests before accepting the work. A prompt can communicate expectations, but it cannot guarantee that every assistant or harness will obey them.
1. Record the baseline before asking for a code change
Begin with a recoverable worktree and run the test command your project treats as canonical. Record the exact command and its result, including failures and skipped checks. That starting point helps distinguish a regression from a failure that was already present.
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Microsoft’s VS Code repository customization guidance recommends tracking which files change, which tests run, and any failures, skipped checks, or corrections during a representative task. Keep that record specific enough to compare with the assistant’s final report.
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2. State what may change—and what should not
Give the assistant a concrete application-code task and name the protected boundary: for example, the test directory, fixtures, snapshots, and test configuration. Ask it to identify any proposed test changes and explain why they are needed before making them. Treat this as project guidance, not as a technical lock: an instruction alone does not prevent a tool from writing to a file.
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A prompt you can adapt:
First inspect the repository instructions and identify the canonical test command. Before editing, report the files you expect to change and confirm whether any tests, fixtures, snapshots, or test configuration would need modification. Preserve the existing tests unless I approve a specific test change. Make only the requested application-code change. Afterward, show the full diff, run the canonical test command, and report the exact command and result, including failures or skipped tests.
Make the rule durable only after it solves a recurring problem. Put the project structure, test command, conventions, and test-edit review rule in the instruction file the selected harness actually discovers. VS Code notes that discovery differs by harness; verify that the instruction applies rather than assuming one file or role description works everywhere. Keep the customization as small as possible and tied to an observed need.
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3. Use permissions and isolation for stronger boundaries
When a task carries higher risk, constrain the assistant’s available tools or access rather than relying only on wording. The available controls differ by product, and official documentation describes product behavior—not a head-to-head security evaluation.
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| Assistant | Documented control or behavior | Practical implication |
|---|---|---|
| GitHub Copilot CLI | --available-tools limits the available set; --allow-tool and --deny-tool permit or block specific tools and subcommands. GitHub says deny rules take precedence over allow rules. GitHub Docs |
Use narrow tool and subcommand rules where appropriate. CLI syntax can change by version; consult the documentation for the version in use. GitHub cautions that broad all-tools options belong only in an isolated environment. |
| Cursor | Its security documentation says workspace changes can save immediately without approval, except configuration files; terminal commands require approval by default. Run modes are described as best-effort guardrails, not a hard security boundary. Cursor security documentation | Do not assume edits will wait for approval. Keep version control available for recovery and inspect changes promptly. |
| Claude Code | Anthropic describes a permission-based model that is read-only by default for modifications and most commands, and documents filesystem and network boundaries in its sandboxing guidance. Anthropic’s sandboxing article | Filesystem and network limits address different kinds of access; a sandbox is a security control, not proof that generated tests or code are correct. Check current product availability and defaults. |
In its October 20, 2025 sandboxing article, Anthropic reported an internal 84% reduction in permission prompts. That vendor-reported result measures prompts, not test preservation, defect rates, or independently verified safety efficacy.
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4. Preview and inspect the complete diff
Before accepting the change, review everything—not just the application source file. Pay particular attention to tests, fixtures, snapshots, test configuration, and generated files. Compare the final worktree with the baseline so an unexpected test edit or unrelated change is visible.
For a multi-file change, use a plan or preview if the harness offers one. Anthropic’s Claude Code help describes a plan mode in which the assistant lists intended files and changes before editing, then waits for plan approval. A plan clarifies intent; it does not replace reviewing the resulting diff.
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- Check whether each changed file is within the requested scope.
- For any test-related change, confirm it was explicitly approved and understand its purpose.
- Look for deleted assertions, weakened expectations, altered fixtures or snapshots, and configuration changes that could reduce coverage.
5. Rerun tests and report the evidence
After the edits, run the relevant tests again—ideally the same canonical command used for the baseline—and record the exact command and outcome. Report failures and skipped checks plainly. If a test fails, do not present the task as verified; investigate whether the failure existed at baseline or was introduced by the change.
Anthropic’s documented workflow for approved fixes includes rerunning tests, while Microsoft’s guidance recommends tracking commands, failures, and skipped tests. The final test run is evidence about the current worktree, not a guarantee that every behavior is correct. If a proposed test edit was approved, review that edit separately and confirm the intended behavior remains covered.
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A practical acceptance checklist
- Did you record the initial worktree state, canonical test command, and baseline result?
- Did the assistant identify the files it expected to change and acknowledge the protected test boundary?
- Were permissions or sandbox controls appropriate to the task’s risk?
- Have you inspected the full diff, including tests and supporting files?
- Did you rerun tests and report the exact result, including skips or failures?
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