diff --git a/src/main/java/com/kntro/reqsai/discovery/domain/exception/DiscoveryExceptions.java b/src/main/java/com/kntro/reqsai/discovery/domain/exception/DiscoveryExceptions.java
index 9b0c028f..74c72ded 100644
--- a/src/main/java/com/kntro/reqsai/discovery/domain/exception/DiscoveryExceptions.java
+++ b/src/main/java/com/kntro/reqsai/discovery/domain/exception/DiscoveryExceptions.java
@@ -4,6 +4,8 @@
 import com.kntro.reqsai.shared.domain.exception.DomainException;
 import com.kntro.reqsai.shared.domain.exception.EntityNotFoundException;
 
+import java.util.Locale;
+
 /**
  * Factory for Requirement Discovery domain exceptions — the context-specific counterpart of the shared
  * {@code Exceptions} and a mirror of {@code WorkspaceExceptions}.
@@ -15,8 +17,11 @@ private DiscoveryExceptions() {
     }
 
     public static DomainException duplicateUserStory(double similarity) {
+        // Format with Locale.ROOT so the decimal separator is always a dot: the client parses this
+        // similarity out of the ProblemDetail `detail` expecting "0.87", and a comma-decimal JVM locale
+        // would otherwise make it read the score as 0 (0%) on the duplicate-story surface.
         return new DomainException(DiscoveryError.DUPLICATE_USER_STORY,
-                "A near-duplicate user story already exists (similarity %.2f)".formatted(similarity));
+                String.format(Locale.ROOT, "A near-duplicate user story already exists (similarity %.2f)", similarity));
     }
 
     public static EntityNotFoundException sessionNotFound(java.util.UUID id) {
diff --git a/src/test/java/com/kntro/reqsai/discovery/domain/exception/DiscoveryExceptionsTest.java b/src/test/java/com/kntro/reqsai/discovery/domain/exception/DiscoveryExceptionsTest.java
new file mode 100644
index 00000000..e7f23b6f
--- /dev/null
+++ b/src/test/java/com/kntro/reqsai/discovery/domain/exception/DiscoveryExceptionsTest.java
@@ -0,0 +1,34 @@
+package com.kntro.reqsai.discovery.domain.exception;
+
+import com.kntro.reqsai.shared.domain.exception.DomainException;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.Locale;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@DisplayName("Discovery: duplicate-story exception")
+class DiscoveryExceptionsTest {
+
+    private final Locale original = Locale.getDefault();
+
+    @AfterEach
+    void restoreLocale() {
+        Locale.setDefault(original);
+    }
+
+    @Test
+    @DisplayName("similarity is dot-formatted even under a comma-decimal default locale")
+    void similarity_dot_formatted_regardless_of_locale() {
+        // A comma-decimal locale (es-PE/Germany) would otherwise render "0,87", which the client parses
+        // as 0 (0%) off the ProblemDetail detail. Locale.ROOT keeps the machine-parsed value a dot.
+        Locale.setDefault(Locale.GERMANY);
+
+        DomainException ex = DiscoveryExceptions.duplicateUserStory(0.87);
+
+        assertThat(ex.getMessage()).contains("similarity 0.87");
+        assertThat(ex.getMessage()).doesNotContain("0,87");
+    }
+}