diff --git a/backend/src/main/kotlin/com/condado/newsletter/service/PromptBuilderService.kt b/backend/src/main/kotlin/com/condado/newsletter/service/PromptBuilderService.kt
new file mode 100644
index 0000000..ffec939
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+++ b/backend/src/main/kotlin/com/condado/newsletter/service/PromptBuilderService.kt
@@ -0,0 +1,61 @@
+package com.condado.newsletter.service
+
+import com.condado.newsletter.model.EmailContext
+import com.condado.newsletter.model.VirtualEntity
+import org.springframework.stereotype.Service
+
+/**
+ * The single authoritative place in the codebase where AI prompts are constructed.
+ * No other class may build or modify prompt strings.
+ */
+@Service
+class PromptBuilderService {
+
+    /**
+     * Builds the full prompt to be sent to the AI, based on the entity's profile
+     * and the list of recent emails from the company inbox.
+     *
+     * @param entity The virtual employee whose turn it is to send an email.
+     * @param emailContext Recent emails from the IMAP inbox (oldest-first).
+     * @return The complete prompt string ready to send to the OpenAI API.
+     */
+    fun buildPrompt(entity: VirtualEntity, emailContext: List<EmailContext>): String {
+        val emailSection = if (emailContext.isEmpty()) {
+            "(No recent emails in the inbox.)"
+        } else {
+            emailContext.joinToString("\n\n") { email ->
+                """
+                |From: ${email.from}
+                |Subject: ${email.subject}
+                |Received: ${email.receivedAt}
+                |
+                |${email.body}
+                """.trimMargin()
+            }
+        }
+
+        return """
+            |You are ${entity.name}, ${entity.jobTitle} at "Condado Abaixo da Média SA".
+            |
+            |Your personality: ${entity.personality ?: "Professional and formal."}
+            |
+            |IMPORTANT TONE RULE: You must write in an extremely formal, bureaucratic, corporate tone —
+            |as if writing an official memo. However, the actual content of the email must be completely
+            |casual, trivial, or nonsensical — as if talking to close friends about mundane things.
+            |The contrast between the formal tone and the casual content is intentional and essential.
+            |
+            |Here are the most recent emails from the company inbox (last ${entity.contextWindowDays} days)
+            |for context:
+            |
+            |$emailSection
+            |
+            |Write a new email to be sent to the company group, continuing the conversation naturally.
+            |Reply or react to the recent emails if relevant. Sign off as ${entity.name}, ${entity.jobTitle}.
+            |
+            |Format your response exactly as:
+            |SUBJECT: <subject line here>
+            |BODY:
+            |<full email body here>
+        """.trimMargin()
+    }
+}