diff --git a/src/core.rs b/src/core.rs
index da77bc7..52b48dd 100644
--- a/src/core.rs
+++ b/src/core.rs
@@ -7,12 +7,28 @@ use crate::types::{
   TrueAffectedConfig,
 };
 use crate::utils;
+use regex::Regex;
 use rustc_hash::{FxHashMap, FxHashSet};
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 use tracing::debug;
 
+/// Default include pattern: matches test/spec files (.spec.ts, .test.tsx, etc.)
+const DEFAULT_INCLUDE_PATTERN: &str = r"\.(spec|test)\.(ts|js)x?$";
+
+fn is_ignored_path(file_path: &Path, ignored_paths: &[String]) -> bool {
+  if ignored_paths.is_empty() {
+    return false;
+  }
+
+  let file_str = file_path.to_string_lossy();
+  ignored_paths
+    .iter()
+    .filter(|ignored| !ignored.trim().is_empty())
+    .any(|ignored| file_str.contains(ignored))
+}
+
 /// Mutable state for tracking affected symbols during analysis
 struct AffectedState<'a> {
   affected_packages: &'a mut FxHashSet<String>,
@@ -69,6 +85,45 @@ fn find_affected_internal(
   let mut affected_packages = FxHashSet::default();
   let mut project_causes: FxHashMap<String, Vec<AffectCause>> = FxHashMap::default();
 
+  // Step 4b: Process included file patterns (e.g., test files)
+  // Files matching include patterns directly mark their owning project as affected,
+  // matching traf's changedIncludedFilesPackages behavior.
+  let include_patterns = if config.include.is_empty() {
+    vec![DEFAULT_INCLUDE_PATTERN.to_string()]
+  } else if config.include.len() == 1 && config.include[0].trim().is_empty() {
+    vec![]
+  } else {
+    config.include.clone()
+  };
+
+  let compiled_patterns: Vec<Regex> = include_patterns
+    .iter()
+    .filter_map(|p| match Regex::new(p) {
+      Ok(re) => Some(re),
+      Err(e) => {
+        debug!("Invalid include pattern '{}': {}", p, e);
+        None
+      }
+    })
+    .collect();
+
+  for changed_file in &changed_files {
+    if is_ignored_path(&changed_file.file_path, &config.ignored_paths) {
+      continue;
+    }
+    let file_str = changed_file.file_path.to_string_lossy();
+    if compiled_patterns.iter().any(|re| re.is_match(&file_str)) {
+      if let Some(pkg) = utils::get_package_name_by_path(&changed_file.file_path, &config.projects)
+      {
+        debug!(
+          "Include pattern matched {:?}, adding package '{}'",
+          changed_file.file_path, pkg
+        );
+        affected_packages.insert(pkg);
+      }
+    }
+  }
+
   // Step 5: Partition changed files into source and non-source
   let (source_files, asset_files): (Vec<&ChangedFile>, Vec<&ChangedFile>) = changed_files
     .iter()
@@ -680,4 +735,26 @@ mod tests {
     assert!(affected.contains("lib1"));
     assert!(affected.contains("app")); // Should be added as implicit dependent
   }
+
+  #[test]
+  fn test_default_include_pattern_matches_test_files() {
+    let re = Regex::new(DEFAULT_INCLUDE_PATTERN).expect("Default pattern should compile");
+
+    // Should match spec/test files
+    assert!(re.is_match("proj1/utils.spec.ts"));
+    assert!(re.is_match("proj1/utils.test.ts"));
+    assert!(re.is_match("proj1/component.spec.tsx"));
+    assert!(re.is_match("proj1/component.test.tsx"));
+    assert!(re.is_match("proj1/helper.spec.js"));
+    assert!(re.is_match("proj1/helper.test.jsx"));
+    assert!(re.is_match("tests/validation/roof.step.test.ts"));
+
+    // Should NOT match regular source files
+    assert!(!re.is_match("proj1/utils.ts"));
+    assert!(!re.is_match("proj1/component.tsx"));
+    assert!(!re.is_match("proj1/index.js"));
+    assert!(!re.is_match("proj1/styles.css"));
+    assert!(!re.is_match("proj1/data.json"));
+    assert!(!re.is_match("proj1/test-utils.ts")); // "test" in name but not .test.ts
+  }
 }
diff --git a/src/types.rs b/src/types.rs
index 7aff07c..5d2650b 100644
--- a/src/types.rs
+++ b/src/types.rs
@@ -95,8 +95,9 @@ pub struct TrueAffectedConfig {
   pub root_ts_config: Option<PathBuf>,
   /// Projects in the workspace
   pub projects: Vec<Project>,
-  /// Additional file patterns to include
-  #[allow(dead_code)]
+  /// Additional file patterns to include (regex patterns).
+  /// When empty, defaults to matching test files: `\.(spec|test)\.(ts|js)x?$`
+  /// Use a single empty string to disable default include patterns.
   pub include: Vec<String>,
   /// Paths to ignore
   #[allow(dead_code)]
diff --git a/tests/fixtures/monorepo b/tests/fixtures/monorepo
index 0d6eb7a..10ec84a 160000
--- a/tests/fixtures/monorepo
+++ b/tests/fixtures/monorepo
@@ -1 +1 @@
-Subproject commit 0d6eb7a3c5c9e4e0e19ec9a0cd80e017c55a9c59
+Subproject commit 10ec84a6d723c01d9fbca21e983373b552ef5bd6
diff --git a/tests/integration_test.rs b/tests/integration_test.rs
index 9c80743..b94e65d 100644
--- a/tests/integration_test.rs
+++ b/tests/integration_test.rs
@@ -1479,3 +1479,113 @@ export function anotherFn() {
     "proj2 should be affected via asset → constant → export chain"
   );
 }
+
+/// Helper to get affected projects with custom include patterns
+fn get_affected_with_include(include: Vec<String>) -> Vec<String> {
+  let config = TrueAffectedConfig {
+    cwd: fixture_path(),
+    base: "main".to_string(),
+    root_ts_config: Some(PathBuf::from("tsconfig.json")),
+    projects: vec![
+      Project {
+        name: "proj1".to_string(),
+        source_root: PathBuf::from("proj1"),
+        ts_config: Some(PathBuf::from("proj1/tsconfig.json")),
+        implicit_dependencies: vec![],
+        targets: vec![],
+      },
+      Project {
+        name: "proj2".to_string(),
+        source_root: PathBuf::from("proj2"),
+        ts_config: Some(PathBuf::from("proj2/tsconfig.json")),
+        implicit_dependencies: vec![],
+        targets: vec![],
+      },
+      Project {
+        name: "proj3".to_string(),
+        source_root: PathBuf::from("proj3"),
+        ts_config: Some(PathBuf::from("proj3/tsconfig.json")),
+        implicit_dependencies: vec!["proj1".to_string()],
+        targets: vec![],
+      },
+    ],
+    include,
+    ignored_paths: vec![],
+  };
+
+  let profiler = Arc::new(Profiler::new(false));
+
+  find_affected(config, profiler)
+    .expect("Failed to find affected projects")
+    .affected_projects
+}
+
+#[test]
+fn test_included_test_file_marks_project_affected() {
+  let branch = TestBranch::new("test-include-spec-file");
+
+  // Create a .spec.ts file in proj1 (test file matching default include pattern)
+  branch.make_change(
+    "proj1/utils.spec.ts",
+    r#"import { proj1 } from './index';
+
+describe('proj1', () => {
+  it('should work', () => {
+    expect(proj1()).toBe('proj1');
+  });
+});
+"#,
+  );
+
+  // Now modify only the test file
+  branch.make_change(
+    "proj1/utils.spec.ts",
+    r#"import { proj1 } from './index';
+
+describe('proj1', () => {
+  it('should work correctly', () => {
+    expect(proj1()).toBe('proj1-modified');
+  });
+});
+"#,
+  );
+
+  // Use default include (empty vec triggers default test file pattern)
+  let affected = get_affected_with_include(vec![]);
+
+  // proj1 should be affected because the .spec.ts file matches the default include pattern
+  assert!(
+    affected.contains(&"proj1".to_string()),
+    "proj1 should be affected when a .spec.ts file changes (default include pattern). Got: {:?}",
+    affected
+  );
+}
+
+#[test]
+fn test_included_file_with_custom_pattern() {
+  let branch = TestBranch::new("test-include-custom");
+
+  // Create a .stories.ts file in proj2
+  branch.make_change(
+    "proj2/button.stories.ts",
+    r#"export const Primary = { args: { label: 'Click' } };
+"#,
+  );
+
+  // Modify the stories file
+  branch.make_change(
+    "proj2/button.stories.ts",
+    r#"export const Primary = { args: { label: 'Click Me' } };
+"#,
+  );
+
+  // Use a custom include pattern that matches .stories.ts files
+  let affected = get_affected_with_include(vec![r"\.stories\.(ts|js)x?$".to_string()]);
+
+  // proj2 should be affected because the .stories.ts file matches the custom pattern
+  assert!(
+    affected.contains(&"proj2".to_string()),
+    "proj2 should be affected when a .stories.ts file changes (custom include pattern). Got: {:?}",
+    affected
+  );
+}