diff --git a/backend/src/test/kotlin/com/condado/newsletter/service/PromptBuilderServiceTest.kt b/backend/src/test/kotlin/com/condado/newsletter/service/PromptBuilderServiceTest.kt
new file mode 100644
index 0000000..6326007
--- /dev/null
+++ b/backend/src/test/kotlin/com/condado/newsletter/service/PromptBuilderServiceTest.kt
@@ -0,0 +1,75 @@
+package com.condado.newsletter.service
+
+import com.condado.newsletter.model.EmailContext
+import com.condado.newsletter.model.VirtualEntity
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import java.time.LocalDateTime
+
+class PromptBuilderServiceTest {
+
+    private val service = PromptBuilderService()
+
+    private val entity = VirtualEntity(
+        name = "Rodrigo Alves",
+        email = "rodrigo@condado.com",
+        jobTitle = "Head of Casual Affairs",
+        personality = "Extremely pedantic but only talks about cats",
+        scheduleCron = "0 9 * * 1",
+        contextWindowDays = 3
+    )
+
+    private val emailContext = listOf(
+        EmailContext(
+            from = "Maria Santos <maria@condado.com>",
+            subject = "Re: The weekly snack situation",
+            body = "I think we need more chips in the pantry.",
+            receivedAt = LocalDateTime.now().minusHours(2)
+        )
+    )
+
+    @Test
+    fun should_containEntityName_when_buildPromptCalled() {
+        val prompt = service.buildPrompt(entity, emailContext)
+        assertThat(prompt).contains("Rodrigo Alves")
+    }
+
+    @Test
+    fun should_containEntityJobTitle_when_buildPromptCalled() {
+        val prompt = service.buildPrompt(entity, emailContext)
+        assertThat(prompt).contains("Head of Casual Affairs")
+    }
+
+    @Test
+    fun should_containEntityPersonality_when_buildPromptCalled() {
+        val prompt = service.buildPrompt(entity, emailContext)
+        assertThat(prompt).contains("Extremely pedantic but only talks about cats")
+    }
+
+    @Test
+    fun should_containContextWindowDays_when_buildPromptCalled() {
+        val prompt = service.buildPrompt(entity, emailContext)
+        assertThat(prompt).contains("3")
+    }
+
+    @Test
+    fun should_containEachEmailSenderAndSubject_when_emailContextProvided() {
+        val prompt = service.buildPrompt(entity, emailContext)
+        assertThat(prompt).contains("Maria Santos <maria@condado.com>")
+        assertThat(prompt).contains("Re: The weekly snack situation")
+    }
+
+    @Test
+    fun should_containFormatInstruction_when_buildPromptCalled() {
+        val prompt = service.buildPrompt(entity, emailContext)
+        assertThat(prompt).contains("SUBJECT:")
+        assertThat(prompt).contains("BODY:")
+    }
+
+    @Test
+    fun should_returnPromptWithNoEmails_when_emailContextIsEmpty() {
+        val prompt = service.buildPrompt(entity, emptyList())
+        assertThat(prompt).contains("Rodrigo Alves")
+        assertThat(prompt).isNotEmpty()
+    }
+}