R6.2: Mutation side-effect assertions

Short answer: Assert persisted state or DOM changes, not just HTTP 200. A test that only checks a value exists passes when the behaviour is wrong.

Group: Testing and hermeticity. Status: stable. Generated from the rule definitions; do not edit by hand.

Playground: https://brevio.pro/tools/agent-guard-playground?rule=R6.2

What it catches

Mutation side-effect assertions: assert persisted state or DOM changes, not just HTTP 200.

Why

A test that only checks a value exists passes when the behaviour is wrong.

A bad example

expect(result).toBeDefined();
expect(row).toBeTruthy();

A good example

expect(res.status).toBe(200);
expect(row.name).toBe('Ada');

How it is checked

• The harness’s pre-flight scan of added lines (dev_rules.scan_added_lines); brevio-core scan does not run it.
• A one-line card in the prompt of a slice that touches matching files (dev_rules.slice_micro_rules).

How to silence it

A person can silence one finding with brevio-core: ignore R6.2 -- reason on the line or the line above. Without the reason it does not count. An agent cannot silence it: the agent hook and the MCP tools never honour the comment, and a pull request that adds one is reported until a reviewer allows it. Fixing the code is still the better answer.

A scan reads only the lines you add, so a line that was already in the file is not flagged.

To change what the rule flags, edit its definition in dev_rules.py or code_rules/__init__.py. That is a code change and is reviewed like one.

Related rules

• R6.3 Deterministic condition waits
• R6.4 Hermetic test state isolation
• R6.5 Post-incident regression crystallization
• R6.1 Real production import gate
• Every rule