fix: do not match dollar-brace or escaped patterns in instructions

Author: Laurianti

core/src/main/java/com/google/adk/utils/InstructionUtils.java

@@ -32,8 +32,10 @@
 /** Utility methods for handling instruction templates. */
 public final class InstructionUtils {
 
+  // Braces preceded by '$', '{' or '\' are left alone, so literal ${expression},
+  // ${{expression}} and \{expression} in instructions are not treated as placeholders.
   private static final Pattern INSTRUCTION_PLACEHOLDER_PATTERN =
-      Pattern.compile("\\{+[^\\{\\}]*\\}+");
+      Pattern.compile("(?<![\\$\\{\\\\])\\{+[^\\{\\}]*\\}+");
 
   private InstructionUtils() {}
 

core/src/test/java/com/google/adk/utils/InstructionUtilsTest.java

@@ -220,4 +220,45 @@ public void injectSessionState_stateVariableWithInvalidPrefix_returnsPlaceholder
 
     assertThat(result).isEqualTo(template);
   }
+
+  @Test
+  public void injectSessionState_dollarBracePattern_returnsTemplateAsIs() {
+    String template = "The formatString supports interpolation via ${expression} syntax.";
+
+    String result = InstructionUtils.injectSessionState(templateContext, template).blockingGet();
+
+    assertThat(result).isEqualTo(template);
+  }
+
+  @Test
+  public void injectSessionState_dollarBracePatternWithStateVariable_replacesOnlyStateVariable() {
+    InvocationContext testContext = templateContext.toBuilder().build();
+    testContext.session().state().put("user_name", "Foo");
+    String template = "Hello {user_name}! Interpolation via ${expression} syntax.";
+
+    String result = InstructionUtils.injectSessionState(testContext, template).blockingGet();
+
+    assertThat(result).isEqualTo("Hello Foo! Interpolation via ${expression} syntax.");
+  }
+
+  @Test
+  public void injectSessionState_escapedBraces_returnsTemplateAsIs() {
+    String template = "Literal \\{expression\\} syntax.";
+
+    String result = InstructionUtils.injectSessionState(templateContext, template).blockingGet();
+
+    assertThat(result).isEqualTo(template);
+  }
+
+  @Test
+  public void injectSessionState_dollarDoubleBracePattern_returnsPatternAsIs() {
+    InvocationContext testContext = templateContext.toBuilder().build();
+    testContext.session().state().put("expression", "foo");
+    testContext.session().state().put("user_name", "bar");
+    String template = "Workflow syntax: ${{expression}} and {user_name}.";
+
+    String result = InstructionUtils.injectSessionState(testContext, template).blockingGet();
+
+    assertThat(result).isEqualTo("Workflow syntax: ${{expression}} and bar.");
+  }
 }