Add some dos and don'ts to behavioral evals README. (#20629)

evals/README.md

@@ -3,7 +3,8 @@
 Behavioral evaluations (evals) are tests designed to validate the agent's
 behavior in response to specific prompts. They serve as a critical feedback loop
 for changes to system prompts, tool definitions, and other model-steering
-mechanisms.
+mechanisms, and as a tool for assessing feature reliability by model, and
+preventing regressions.
 
 ## Why Behavioral Evals?
 
@@ -30,6 +31,48 @@ CLI's features.
   those that are generally reliable but might occasionally vary
   (`USUALLY_PASSES`).
 
+## Best Practices
+
+When designing behavioral evals, aim for scenarios that accurately reflect
+real-world usage while remaining small and maintainable.
+
+- **Realistic Complexity**: Evals should be complicated enough to be
+  "realistic." They should operate on actual files and a source directory,
+  mirroring how a real agent interacts with a workspace. Remember that the agent
+  may behave differently in a larger codebase, so we want to avoid scenarios
+  that are too simple to be realistic.
+  - _Good_: An eval that provides a small, functional React component and asks
+    the agent to add a specific feature, requiring it to read the file,
+    understand the context, and write the correct changes.
+  - _Bad_: An eval that simply asks the agent a trivia question or asks it to
+    write a generic script without providing any local workspace context.
+- **Maintainable Size**: Evals should be small enough to reason about and
+  maintain. We probably can't check in an entire repo as a test case, though
+  over time we will want these evals to mature into more and more realistic
+  scenarios.
+  - _Good_: A test setup with 2-3 files (e.g., a source file, a config file, and
+    a test file) that isolates the specific behavior being evaluated.
+  - _Bad_: A test setup containing dozens of files from a complex framework
+    where the setup logic itself is prone to breaking.
+- **Unambiguous and Reliable Assertions**: Assertions must be clear and specific
+  to ensure the test passes for the right reason.
+  - _Good_: Checking that a modified file contains a specific AST node or exact
+    string, or verifying that a tool was called with with the right parameters.
+  - _Bad_: Only checking for a tool call, which could happen for an unrelated
+    reason. Expecting specific LLM output.
+- **Fail First**: Have tests that failed before your prompt or tool change. We
+  want to be sure the test fails before your "fix". It's pretty easy to
+  accidentally create a passing test that asserts behaviors we get for free. In
+  general, every eval should be accompanied by prompt change, and most prompt
+  changes should be accompanied by an eval.
+  - _Good_: Observing a failure, writing an eval that reliably reproduces the
+    failure, modifying the prompt/tool, and then verifying the eval passes.
+  - _Bad_: Writing an eval that passes on the first run and assuming your new
+    prompt change was responsible.
+- **Less is More**: Prefer fewer, more realistic tests that assert the major
+  paths vs. more tests that are more unit-test like. These are evals, so the
+  value is in testing how the agent works in a semi-realistic scenario.
+
 ## Creating an Evaluation
 
 Evaluations are located in the `evals` directory. Each evaluation is a Vitest