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prs/bolinfest/study/PR-1596-study.md

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+**DOs**
+- Put test-only crates under `[dev-dependencies]`: keeps runtime clean and avoids unnecessary transitive deps.
+```toml
+# core/Cargo.toml (right)
+[dependencies]
+uuid = { version = "1", features = ["serde", "v4"] }
+# … other runtime deps …
+
+[dev-dependencies]
+walkdir = "2.5.0"  # used only in tests
+```
+
+- Prefer `Command::cargo_bin` in tests: runs the built binary directly instead of spawning `cargo run`.
+```rust
+use assert_cmd::prelude::*;
+use assert_cmd::Command;
+use tempfile::TempDir;
+
+let home = TempDir::new().unwrap();
+
+Command::cargo_bin("codex-cli")
+    .unwrap()
+    .args([
+        "exec",
+        "--skip-git-repo-check",
+        "-C",
+        env!("CARGO_MANIFEST_DIR"),
+        "echo integration-test-marker",
+    ])
+    .env("CODEX_HOME", home.path())
+    .env("OPENAI_API_KEY", "dummy")
+    .env("OPENAI_BASE_URL", "http://unused.local")
+    .assert()
+    .success();
+```
+
+**DON’Ts**
+- Don’t add test-only crates to `[dependencies]`: this bloats the runtime dependency graph.
+```toml
+# core/Cargo.toml (wrong)
+[dependencies]
+walkdir = "2.5.0"  # ❌ test-only; should be under [dev-dependencies]
+```
+
+- Don’t shell out to `cargo run` from tests unless there’s a compelling reason.
+```rust
+// ❌ Avoid this in tests
+use assert_cmd::Command as AssertCommand;
+
+let mut cmd = AssertCommand::new("cargo");
+cmd.arg("run")
+   .arg("-p").arg("codex-cli")
+   .arg("--")
+   .arg("exec")
+   .arg("--skip-git-repo-check")
+   .arg("-C").arg(env!("CARGO_MANIFEST_DIR"))
+   .arg("echo integration-test-marker");
+
+// Prefer Command::cargo_bin(...) instead (see DOs).
+```