Percentage sign fix

think/think-consumer/src/main/java/io/flightdeck/think/consumer/ThinkConsumer.java

@@ -47,7 +47,7 @@ public class ThinkConsumer implements AutoCloseable {
 
             Be concise and helpful. When using tools, explain what you're doing and why.""";
 
-    private static final String SYSTEM_PROMPT_TEMPLATE = loadSystemPromptTemplate() + "\n\n%s";
+    private static final String SYSTEM_PROMPT_BASE = loadSystemPromptTemplate();
 
     private static String loadSystemPromptTemplate() {
         return loadSystemPromptFromFile(AppConfig.SYSTEM_PROMPT_FILE);
@@ -353,15 +353,19 @@ private void produceResponse(String sessionId, ThinkResponse response) throws Ex
      * Builds the system prompt, injecting memoir context if available.
      */
     static String buildSystemPrompt(String memoirContext) {
-        StringBuilder extra = new StringBuilder();
+        return buildSystemPrompt(SYSTEM_PROMPT_BASE, memoirContext);
+    }
+
+    static String buildSystemPrompt(String basePrompt, String memoirContext) {
+        StringBuilder out = new StringBuilder(basePrompt).append("\n\n");
 
         if (memoirContext != null && !memoirContext.isBlank()) {
-            extra.append("\n\nUser memoir (known facts about this user from previous sessions):\n");
-            extra.append(memoirContext);
-            extra.append("\n\nUse the memoir to personalize your responses.");
+            out.append("\n\nUser memoir (known facts about this user from previous sessions):\n");
+            out.append(memoirContext);
+            out.append("\n\nUse the memoir to personalize your responses.");
         }
 
-        return String.format(SYSTEM_PROMPT_TEMPLATE, extra.toString());
+        return out.toString();
     }
 
     /**

think/think-consumer/src/test/java/io/flightdeck/think/consumer/SystemPromptTest.java

@@ -36,4 +36,34 @@ void missingFile_throws() {
                 .isInstanceOf(IllegalStateException.class)
                 .hasMessageContaining("SYSTEM_PROMPT_FILE not found");
     }
+
+    @Test
+    @DisplayName("System prompt containing special characters is preserved literally (issue #23)")
+    void specialChars_inPrompt_doesNotThrow() {
+        String basePrompt = "Apply a 10% discount when asked.";
+
+        String withoutMemoir = ThinkConsumer.buildSystemPrompt(basePrompt, null);
+        assertThat(withoutMemoir).contains("10% discount");
+
+        String withMemoir = ThinkConsumer.buildSystemPrompt(basePrompt, "user likes 50% off coupons");
+        assertThat(withMemoir)
+                .contains("10% discount")
+                .contains("50% off coupons");
+
+        // Sequences that String.format would interpret as conversions/specifiers
+        // must survive unchanged now that the prompt is concatenated, not formatted.
+        assertThat(ThinkConsumer.buildSystemPrompt("Print %s and %d and %n literally.", null))
+                .contains("%s and %d and %n literally.");
+        assertThat(ThinkConsumer.buildSystemPrompt("A 100%% bonus", "memoir with %s token"))
+                .contains("100%% bonus")
+                .contains("memoir with %s token");
+
+        // Other characters that are easy to mishandle in string plumbing.
+        assertThat(ThinkConsumer.buildSystemPrompt("Use a backslash \\ and a dollar $ sign.", null))
+                .contains("backslash \\ and a dollar $ sign.");
+        assertThat(ThinkConsumer.buildSystemPrompt("Braces {0} {1} and brackets [x] stay.", null))
+                .contains("Braces {0} {1} and brackets [x] stay.");
+        assertThat(ThinkConsumer.buildSystemPrompt("Quotes \"q\" 'a' and emoji 🚀 stay.", null))
+                .contains("Quotes \"q\" 'a' and emoji 🚀 stay.");
+    }
 }