diff --git a/Partner-Framework/src/main/java/work/slhaf/partner/framework/agent/model/provider/openai/JsonShapeInstructionBuilder.java b/Partner-Framework/src/main/java/work/slhaf/partner/framework/agent/model/provider/openai/JsonShapeInstructionBuilder.java
new file mode 100644
index 00000000..7e10b30e
--- /dev/null
+++ b/Partner-Framework/src/main/java/work/slhaf/partner/framework/agent/model/provider/openai/JsonShapeInstructionBuilder.java
@@ -0,0 +1,114 @@
+package work.slhaf.partner.framework.agent.model.provider.openai;
+
+import java.lang.reflect.*;
+import java.util.*;
+
+final class JsonShapeInstructionBuilder {
+
+    private static final int MAX_DEPTH = 4;
+
+    private JsonShapeInstructionBuilder() {
+    }
+
+    static String build(Class<?> responseType) {
+        return "Return only a valid JSON object.\n"
+                + "The JSON object must directly match this exact output shape for " + responseType.getSimpleName() + ":\n"
+                + buildJsonShape(responseType, 0, new HashSet<>()) + "\n\n"
+                + "Rules:\n"
+                + "- The top-level object must directly match the shape above.\n"
+                + "- Do not wrap it in \"" + responseType.getSimpleName() + "\" or any other class name.\n"
+                + "- Do not rename fields or invent alternative field names.\n"
+                + "- Do not output markdown, comments, explanations, or code fences.";
+    }
+
+    private static String buildJsonShape(Type type, int depth, Set<Type> visiting) {
+        if (depth > MAX_DEPTH) {
+            return "{}";
+        }
+        if (type instanceof ParameterizedType parameterizedType) {
+            Type rawType = parameterizedType.getRawType();
+            if (rawType instanceof Class<?> rawClass && Collection.class.isAssignableFrom(rawClass)) {
+                Type[] arguments = parameterizedType.getActualTypeArguments();
+                if (arguments.length == 0) {
+                    return "[]";
+                }
+                return arrayShape(buildJsonShape(arguments[0], depth + 1, visiting), depth);
+            }
+            if (rawType instanceof Class<?> rawClass && Map.class.isAssignableFrom(rawClass)) {
+                return "{}";
+            }
+            return buildJsonShape(rawType, depth, visiting);
+        }
+        if (type instanceof GenericArrayType genericArrayType) {
+            return arrayShape(buildJsonShape(genericArrayType.getGenericComponentType(), depth + 1, visiting), depth);
+        }
+        if (!(type instanceof Class<?> clazz)) {
+            return "null";
+        }
+        if (clazz.isArray()) {
+            return arrayShape(buildJsonShape(clazz.getComponentType(), depth + 1, visiting), depth);
+        }
+        if (clazz == String.class || clazz == Character.class || clazz == char.class) {
+            return "\"\"";
+        }
+        if (clazz == boolean.class || clazz == Boolean.class) {
+            return "false";
+        }
+        if (Number.class.isAssignableFrom(clazz) || clazz.isPrimitive()) {
+            return "0";
+        }
+        if (clazz.isEnum()) {
+            Object[] constants = clazz.getEnumConstants();
+            return constants == null || constants.length == 0 ? "\"\"" : "\"" + constants[0] + "\"";
+        }
+        if (Collection.class.isAssignableFrom(clazz)) {
+            return "[]";
+        }
+        if (Map.class.isAssignableFrom(clazz)) {
+            return "{}";
+        }
+        if (clazz.getName().startsWith("java.")) {
+            return "\"\"";
+        }
+        if (visiting.contains(clazz)) {
+            return "{}";
+        }
+
+        visiting.add(clazz);
+        List<Field> fields = Arrays.stream(clazz.getDeclaredFields())
+                .filter(field -> !field.isSynthetic())
+                .filter(field -> !Modifier.isStatic(field.getModifiers()))
+                .filter(field -> !Modifier.isTransient(field.getModifiers()))
+                .toList();
+        if (fields.isEmpty()) {
+            visiting.remove(clazz);
+            return "{}";
+        }
+
+        StringBuilder builder = new StringBuilder();
+        builder.append("{\n");
+        for (int i = 0; i < fields.size(); i++) {
+            Field field = fields.get(i);
+            builder.append(indent(depth + 1))
+                    .append("\"")
+                    .append(field.getName())
+                    .append("\": ")
+                    .append(buildJsonShape(field.getGenericType(), depth + 1, visiting));
+            if (i < fields.size() - 1) {
+                builder.append(",");
+            }
+            builder.append("\n");
+        }
+        builder.append(indent(depth)).append("}");
+        visiting.remove(clazz);
+        return builder.toString();
+    }
+
+    private static String arrayShape(String itemShape, int depth) {
+        return "[\n" + indent(depth + 1) + itemShape.replace("\n", "\n" + indent(depth + 1)) + "\n" + indent(depth) + "]";
+    }
+
+    private static String indent(int depth) {
+        return "  ".repeat(Math.max(0, depth));
+    }
+}
diff --git a/Partner-Framework/src/main/java/work/slhaf/partner/framework/agent/model/provider/openai/OpenAiCompatibleProvider.java b/Partner-Framework/src/main/java/work/slhaf/partner/framework/agent/model/provider/openai/OpenAiCompatibleProvider.java
index c3cd2c5e..02593dcd 100644
--- a/Partner-Framework/src/main/java/work/slhaf/partner/framework/agent/model/provider/openai/OpenAiCompatibleProvider.java
+++ b/Partner-Framework/src/main/java/work/slhaf/partner/framework/agent/model/provider/openai/OpenAiCompatibleProvider.java
@@ -100,7 +100,7 @@ public class OpenAiCompatibleProvider extends ModelProvider {
         return executeWithRetry(
                 "OpenAI-compatible provider failed to complete the structured chat request after 3 attempts.",
                 () -> {
-                    StructuredChatCompletionCreateParams<T> params = buildParams(ensureJsonInstruction(messages)).toBuilder()
+                    StructuredChatCompletionCreateParams<T> params = buildParams(ensureJsonInstruction(messages, responseType)).toBuilder()
                             .responseFormat(responseType)
                             .build();
                     return extractStructured(client.chat().completions().create(params));
@@ -108,23 +108,17 @@ public class OpenAiCompatibleProvider extends ModelProvider {
         );
     }
 
-    private List<Message> ensureJsonInstruction(List<Message> messages) {
-        boolean containsJsonInstruction = messages.stream()
-                .map(Message::getContent)
-                .filter(content -> !content.isBlank())
-                .anyMatch(content -> content.toLowerCase(Locale.ROOT).contains("json"));
-        if (containsJsonInstruction) {
-            return messages;
-        }
+    private List<Message> ensureJsonInstruction(List<Message> messages, Class<?> responseType) {
+        String jsonInstruction = JsonShapeInstructionBuilder.build(responseType);
 
-        String jsonInstruction = "Return only a valid JSON object.";
         List<Message> patched = new ArrayList<>(messages.size() + 1);
         boolean merged = false;
         for (Message message : messages) {
             if (!merged && message.getRole() == Message.Character.SYSTEM) {
+                String separator = message.getContent().isBlank() ? "" : "\n\n";
                 patched.add(new Message(
                         Message.Character.SYSTEM,
-                        message.getContent() + "\n\n" + jsonInstruction
+                        message.getContent() + separator + jsonInstruction
                 ));
                 merged = true;
                 continue;
@@ -137,6 +131,7 @@ public class OpenAiCompatibleProvider extends ModelProvider {
         return patched;
     }
 
+
     private ChatCompletionCreateParams buildParams(List<Message> messages) {
         ChatCompletionCreateParams.Builder paramsBuilder = ChatCompletionCreateParams.builder()
                 .model(model)
diff --git a/Partner-Framework/src/test/java/work/slhaf/partner/framework/agent/model/provider/openai/JsonShapeInstructionBuilderTest.java b/Partner-Framework/src/test/java/work/slhaf/partner/framework/agent/model/provider/openai/JsonShapeInstructionBuilderTest.java
new file mode 100644
index 00000000..ee391357
--- /dev/null
+++ b/Partner-Framework/src/test/java/work/slhaf/partner/framework/agent/model/provider/openai/JsonShapeInstructionBuilderTest.java
@@ -0,0 +1,79 @@
+package work.slhaf.partner.framework.agent.model.provider.openai;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class JsonShapeInstructionBuilderTest {
+
+    @Test
+    void shouldBuildShapeInstructionForComplexPojo() {
+        String instruction = JsonShapeInstructionBuilder.build(ComplexResponse.class);
+
+        assertTrue(instruction.startsWith("Return only a valid JSON object."));
+        assertTrue(instruction.contains("The JSON object must directly match this exact output shape for ComplexResponse:"));
+        assertTrue(instruction.contains("Do not wrap it in \"ComplexResponse\" or any other class name."));
+        assertTrue(instruction.contains("Do not rename fields or invent alternative field names."));
+
+        assertTrue(instruction.contains("\"id\": \"\""));
+        assertTrue(instruction.contains("\"success\": false"));
+        assertTrue(instruction.contains("\"retryCount\": 0"));
+        assertTrue(instruction.contains("\"score\": 0"));
+        assertTrue(instruction.contains("\"status\": \"READY\""));
+        assertTrue(instruction.contains("\"tags\": ["));
+        assertTrue(instruction.contains("\"items\": ["));
+        assertTrue(instruction.contains("\"metadata\": {}"));
+        assertTrue(instruction.contains("\"matrix\": ["));
+        assertTrue(instruction.contains("\"nested\": {"));
+        assertTrue(instruction.contains("\"name\": \"\""));
+        assertTrue(instruction.contains("\"enabled\": false"));
+        assertTrue(instruction.contains("\"notes\": ["));
+
+        assertFalse(instruction.contains("staticValue"));
+        assertFalse(instruction.contains("transientValue"));
+    }
+
+    @Test
+    void shouldPreventRecursiveExpansion() {
+        String instruction = JsonShapeInstructionBuilder.build(RecursiveResponse.class);
+
+        assertTrue(instruction.contains("\"name\": \"\""));
+        assertTrue(instruction.contains("\"next\": {}"));
+    }
+
+    private enum Status {
+        READY,
+        DONE
+    }
+
+    private static class ComplexResponse {
+        private static String staticValue;
+        private transient String transientValue;
+
+        private String id;
+        private boolean success;
+        private int retryCount;
+        private Double score;
+        private Status status;
+        private List<String> tags;
+        private List<NestedItem> items;
+        private Map<String, String> metadata;
+        private int[] matrix;
+        private NestedItem nested;
+    }
+
+    private static class NestedItem {
+        private String name;
+        private Boolean enabled;
+        private List<String> notes;
+    }
+
+    private static class RecursiveResponse {
+        private String name;
+        private RecursiveResponse next;
+    }
+}