diff --git a/src/builtins/strings.rs b/src/builtins/strings.rs
index fbc8bd9..0204554 100644
--- a/src/builtins/strings.rs
+++ b/src/builtins/strings.rs
@@ -13,6 +13,7 @@ use anyhow::Result;
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
     m.insert("concat", concat);
+    m.insert("contains", contains);
 }
 
 fn concat(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
@@ -22,3 +23,11 @@ fn concat(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
     let collection = ensure_string_collection(name, &params[1], &args[1])?;
     Ok(Value::String(collection.join(&delimiter)))
 }
+
+fn contains(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "contains";
+    ensure_args_count(span, name, params, args, 2)?;
+    let s1 = ensure_string(name, &params[0], &args[0])?;
+    let s2 = ensure_string(name, &params[1], &args[1])?;
+    Ok(Value::Bool(s1.contains(&s2)))
+}
diff --git a/tests/interpreter/cases/builtins/strings/tests.yaml b/tests/interpreter/cases/builtins/strings/concat.yaml
similarity index 83%
rename from tests/interpreter/cases/builtins/strings/tests.yaml
rename to tests/interpreter/cases/builtins/strings/concat.yaml
index 6341b0a..67d6f2d 100644
--- a/tests/interpreter/cases/builtins/strings/tests.yaml
+++ b/tests/interpreter/cases/builtins/strings/concat.yaml
@@ -31,6 +31,26 @@ cases:
       s3: ""
       s4: ""
 
+  - note: undefined-delimiter
+    data: {}
+    modules:
+      - |
+        package test
+        x { false }
+        y = concat(x, [])
+    query: data.test
+    want_result: {}
+    
+  - note: undefined-collection
+    data: {}
+    modules:
+      - |
+        package test
+        x { false }
+        y = concat(",", x)
+    query: data.test
+    want_result: {}
+    
   - note: invalid-null-delimiter
     data: {}
     modules: ["package test\nx=concat(null, [])"]
diff --git a/tests/interpreter/cases/builtins/strings/contains.yaml b/tests/interpreter/cases/builtins/strings/contains.yaml
new file mode 100644
index 0000000..8f00ec0
--- /dev/null
+++ b/tests/interpreter/cases/builtins/strings/contains.yaml
@@ -0,0 +1,116 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+cases:
+  - note: all
+    data: {}
+    modules:
+      - |
+        package test
+
+        v1 = contains("Hello, world", ", ")
+        v2 = contains("Hello world", "") # empty substring
+        v3 = contains("", ",") # empty string
+        v4 = contains("", "") # empty substring and string
+
+    query: data.test
+    want_result:
+      v1: true
+      v2: true
+      v3: false
+      v4: true
+
+  - note: undefined-string
+    data: {}
+    modules:
+      - |
+        package test
+        x { false }
+        y = contains(x, "")
+    query: data.test
+    want_result: {}
+
+  - note: undefined-substring
+    data: {}
+    modules:
+      - |
+        package test
+        x { false }
+        y = contains(",", x)
+    query: data.test
+    want_result: {}
+
+  - note: invalid-null-string
+    data: {}
+    modules: ["package test\nx=contains(null, ``)"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-bool-string
+    data: {}
+    modules: ["package test\nx=contains(true, ``)"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-number-string
+    data: {}
+    modules: ["package test\nx=contains(1, ``)"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-array-string
+    data: {}
+    modules: ["package test\nx=contains([], ``)"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-set-string
+    data: {}
+    modules: ["package test\nx=contains(set(), ``)"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-object-string
+    data: {}
+    modules: ["package test\nx=contains({}, ``)"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+
+
+
+  - note: invalid-null-substring
+    data: {}
+    modules: ["package test\nx=contains(``, null)"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-bool-substring
+    data: {}
+    modules: ["package test\nx=contains(``, true)"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-number-substring
+    data: {}
+    modules: ["package test\nx=contains(``, 1)"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-array-substring
+    data: {}
+    modules: ["package test\nx=contains(``, [])"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-set-substring
+    data: {}
+    modules: ["package test\nx=contains(``, set())"]
+    query: data.test
+    error: "`contains` expects string argument."
+
+  - note: invalid-object-substring
+    data: {}
+    modules: ["package test\nx=contains(``, {})"]
+    query: data.test
+    error: "`contains` expects string argument."