[ML] Write Chat Completion JSON (elastic#128592)

Most providers write the UnifiedCompletionRequest JSON as we received
it, with some exception:
- the modelId can be null and/or overwritten from various locations
- `max_completion_tokens` repalced `max_tokens`, but some providers
  still use the deprecated field name
We will handle the variations using Params, otherwise all of the
XContent building code has moved into UnifiedCompletionRequest so it can
be reused across providers.

Author: prwhelan

server/src/main/java/org/elasticsearch/inference/UnifiedCompletionRequest.java

@@ -19,6 +19,10 @@
 import org.elasticsearch.xcontent.ConstructingObjectParser;
 import org.elasticsearch.xcontent.ObjectParser;
 import org.elasticsearch.xcontent.ParseField;
+import org.elasticsearch.xcontent.ToXContent;
+import org.elasticsearch.xcontent.ToXContentFragment;
+import org.elasticsearch.xcontent.ToXContentObject;
+import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.XContentParseException;
 import org.elasticsearch.xcontent.XContentParser;
 
@@ -38,9 +42,68 @@ public record UnifiedCompletionRequest(
     @Nullable ToolChoice toolChoice,
     @Nullable List<Tool> tools,
     @Nullable Float topP
-) implements Writeable {
+) implements Writeable, ToXContentFragment {
+
+    public static final String NAME_FIELD = "name";
+    public static final String TOOL_CALL_ID_FIELD = "tool_call_id";
+    public static final String TOOL_CALLS_FIELD = "tool_calls";
+    public static final String ID_FIELD = "id";
+    public static final String FUNCTION_FIELD = "function";
+    public static final String ARGUMENTS_FIELD = "arguments";
+    public static final String DESCRIPTION_FIELD = "description";
+    public static final String PARAMETERS_FIELD = "parameters";
+    public static final String STRICT_FIELD = "strict";
+    public static final String TOP_P_FIELD = "top_p";
+    public static final String MESSAGES_FIELD = "messages";
+    private static final String ROLE_FIELD = "role";
+    private static final String CONTENT_FIELD = "content";
+    private static final String STOP_FIELD = "stop";
+    private static final String TEMPERATURE_FIELD = "temperature";
+    private static final String TOOL_CHOICE_FIELD = "tool_choice";
+    private static final String TOOL_FIELD = "tools";
+    private static final String TEXT_FIELD = "text";
+    private static final String TYPE_FIELD = "type";
+    private static final String MODEL_FIELD = "model";
+    private static final String MAX_COMPLETION_TOKENS_FIELD = "max_completion_tokens";
+    private static final String MAX_TOKENS_FIELD = "max_tokens";
+
+    /**
+     * We currently allow providers to override the model id that is written to JSON.
+     * Rather than use {@link #model()}, providers are expected to pass in the modelId via
+     * {@link org.elasticsearch.xcontent.ToXContent.Params}.
+     */
+    private static final String MODEL_ID_PARAM = "model_id_value";
+    /**
+     * Some providers only support the now-deprecated {@link #MAX_TOKENS_FIELD}, others have migrated to
+     * {@link #MAX_COMPLETION_TOKENS_FIELD}. Providers are expected to pass in their supported field name.
+     */
+    private static final String MAX_TOKENS_PARAM = "max_tokens_field";
+
+    /**
+     * Creates a {@link org.elasticsearch.xcontent.ToXContent.Params} that causes ToXContent to include the key values:
+     * - Key: {@link #MODEL_FIELD}, Value: modelId
+     * - Key: {@link #MAX_TOKENS_FIELD}, Value: {@link #maxCompletionTokens()}
+     */
+    public static Params withMaxTokens(String modelId, Params params) {
+        return new DelegatingMapParams(
+            Map.ofEntries(Map.entry(MODEL_ID_PARAM, modelId), Map.entry(MAX_TOKENS_PARAM, MAX_TOKENS_FIELD)),
+            params
+        );
+    }
 
-    public sealed interface Content extends NamedWriteable permits ContentObjects, ContentString {}
+    /**
+     * Creates a {@link org.elasticsearch.xcontent.ToXContent.Params} that causes ToXContent to include the key values:
+     * - Key: {@link #MODEL_FIELD}, Value: modelId
+     * - Key: {@link #MAX_COMPLETION_TOKENS_FIELD}, Value: {@link #maxCompletionTokens()}
+     */
+    public static Params withMaxCompletionTokensTokens(String modelId, Params params) {
+        return new DelegatingMapParams(
+            Map.ofEntries(Map.entry(MODEL_ID_PARAM, modelId), Map.entry(MAX_TOKENS_PARAM, MAX_COMPLETION_TOKENS_FIELD)),
+            params
+        );
+    }
+
+    public sealed interface Content extends NamedWriteable, ToXContent permits ContentObjects, ContentString {}
 
     @SuppressWarnings("unchecked")
     public static final ConstructingObjectParser<UnifiedCompletionRequest, Void> PARSER = new ConstructingObjectParser<>(
@@ -111,9 +174,40 @@ public void writeTo(StreamOutput out) throws IOException {
         out.writeOptionalFloat(topP);
     }
 
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.field(MESSAGES_FIELD, messages);
+        if (stop != null && (stop.isEmpty() == false)) {
+            builder.field(STOP_FIELD, stop);
+        }
+        if (temperature != null) {
+            builder.field(TEMPERATURE_FIELD, temperature);
+        }
+        if (toolChoice != null) {
+            toolChoice.toXContent(builder, params);
+        }
+        if (tools != null && (tools.isEmpty() == false)) {
+            builder.field(TOOL_FIELD, tools);
+        }
+        if (topP != null) {
+            builder.field(TOP_P_FIELD, topP);
+        }
+        // some providers only support the now-deprecated max_tokens, others have migrated to max_completion_tokens
+        if (maxCompletionTokens != null && params.param(MAX_TOKENS_PARAM) != null) {
+            builder.field(params.param(MAX_TOKENS_PARAM), maxCompletionTokens);
+        }
+        // some implementations handle modelId differently, for example OpenAI has a default in the server settings and override it there
+        // so we allow implementations to pass in the model id via the params
+        if (params.param(MODEL_ID_PARAM) != null) {
+            builder.field(MODEL_FIELD, params.param(MODEL_ID_PARAM));
+        }
+        return builder;
+    }
+
     public record Message(Content content, String role, @Nullable String toolCallId, @Nullable List<ToolCall> toolCalls)
         implements
-            Writeable {
+            Writeable,
+            ToXContentObject {
 
         @SuppressWarnings("unchecked")
         static final ConstructingObjectParser<Message, Void> PARSER = new ConstructingObjectParser<>(
@@ -161,6 +255,24 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeOptionalString(toolCallId);
             out.writeOptionalCollection(toolCalls);
         }
+
+        @Override
+        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+            builder.startObject();
+
+            if (content != null) {
+                content.toXContent(builder, params);
+            }
+            builder.field(ROLE_FIELD, role);
+            if (toolCallId != null) {
+                builder.field(TOOL_CALL_ID_FIELD, toolCallId);
+            }
+            if (toolCalls != null) {
+                builder.field(TOOL_CALLS_FIELD, toolCalls);
+            }
+
+            return builder.endObject();
+        }
     }
 
     public record ContentObjects(List<ContentObject> contentObjects) implements Content, NamedWriteable {
@@ -180,9 +292,14 @@ public void writeTo(StreamOutput out) throws IOException {
         public String getWriteableName() {
             return NAME;
         }
+
+        @Override
+        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+            return builder.field(CONTENT_FIELD, contentObjects);
+        }
     }
 
-    public record ContentObject(String text, String type) implements Writeable {
+    public record ContentObject(String text, String type) implements Writeable, ToXContentObject {
         static final ConstructingObjectParser<ContentObject, Void> PARSER = new ConstructingObjectParser<>(
             ContentObject.class.getSimpleName(),
             args -> new ContentObject((String) args[0], (String) args[1])
@@ -207,6 +324,13 @@ public String toString() {
             return text + ":" + type;
         }
 
+        @Override
+        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+            builder.startObject();
+            builder.field(TEXT_FIELD, text);
+            builder.field(TYPE_FIELD, type);
+            return builder.endObject();
+        }
     }
 
     public record ContentString(String content) implements Content, NamedWriteable {
@@ -234,9 +358,14 @@ public String getWriteableName() {
         public String toString() {
             return content;
         }
+
+        @Override
+        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+            return builder.field(CONTENT_FIELD, content);
+        }
     }
 
-    public record ToolCall(String id, FunctionField function, String type) implements Writeable {
+    public record ToolCall(String id, FunctionField function, String type) implements Writeable, ToXContentObject {
 
         static final ConstructingObjectParser<ToolCall, Void> PARSER = new ConstructingObjectParser<>(
             ToolCall.class.getSimpleName(),
@@ -260,7 +389,16 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeString(type);
         }
 
-        public record FunctionField(String arguments, String name) implements Writeable {
+        @Override
+        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+            builder.startObject();
+            builder.field(ID_FIELD, id);
+            builder.field(FUNCTION_FIELD, function);
+            builder.field(TYPE_FIELD, type);
+            return builder.endObject();
+        }
+
+        public record FunctionField(String arguments, String name) implements Writeable, ToXContentObject {
             static final ConstructingObjectParser<FunctionField, Void> PARSER = new ConstructingObjectParser<>(
                 "tool_call_function_field",
                 args -> new FunctionField((String) args[0], (String) args[1])
@@ -280,6 +418,14 @@ public void writeTo(StreamOutput out) throws IOException {
                 out.writeString(arguments);
                 out.writeString(name);
             }
+
+            @Override
+            public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+                builder.startObject();
+                builder.field(ARGUMENTS_FIELD, arguments);
+                builder.field(NAME_FIELD, name);
+                return builder.endObject();
+            }
         }
     }
 
@@ -294,7 +440,7 @@ private static ToolChoice parseToolChoice(XContentParser parser) throws IOExcept
         throw new XContentParseException("Unsupported token [" + token + "]");
     }
 
-    public sealed interface ToolChoice extends NamedWriteable permits ToolChoiceObject, ToolChoiceString {}
+    public sealed interface ToolChoice extends NamedWriteable, ToXContent permits ToolChoiceObject, ToolChoiceString {}
 
     public record ToolChoiceObject(String type, FunctionField function) implements ToolChoice, NamedWriteable {
 
@@ -325,7 +471,15 @@ public String getWriteableName() {
             return NAME;
         }
 
-        public record FunctionField(String name) implements Writeable {
+        @Override
+        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+            builder.startObject(TOOL_CHOICE_FIELD);
+            builder.field(TYPE_FIELD, type);
+            builder.field(FUNCTION_FIELD, function);
+            return builder.endObject();
+        }
+
+        public record FunctionField(String name) implements Writeable, ToXContentObject {
             static final ConstructingObjectParser<FunctionField, Void> PARSER = new ConstructingObjectParser<>(
                 "tool_choice_function_field",
                 args -> new FunctionField((String) args[0])
@@ -343,6 +497,11 @@ public FunctionField(StreamInput in) throws IOException {
             public void writeTo(StreamOutput out) throws IOException {
                 out.writeString(name);
             }
+
+            @Override
+            public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+                return builder.startObject().field(NAME_FIELD, name).endObject();
+            }
         }
     }
 
@@ -367,9 +526,14 @@ public void writeTo(StreamOutput out) throws IOException {
         public String getWriteableName() {
             return NAME;
         }
+
+        @Override
+        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+            return builder.field(TOOL_CHOICE_FIELD, value);
+        }
     }
 
-    public record Tool(String type, FunctionField function) implements Writeable {
+    public record Tool(String type, FunctionField function) implements Writeable, ToXContentObject {
 
         static final ConstructingObjectParser<Tool, Void> PARSER = new ConstructingObjectParser<>(
             Tool.class.getSimpleName(),
@@ -391,12 +555,22 @@ public void writeTo(StreamOutput out) throws IOException {
             function.writeTo(out);
         }
 
+        @Override
+        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+            builder.startObject();
+
+            builder.field(TYPE_FIELD, type);
+            builder.field(FUNCTION_FIELD, function);
+
+            return builder.endObject();
+        }
+
         public record FunctionField(
             @Nullable String description,
             String name,
             @Nullable Map<String, Object> parameters,
             @Nullable Boolean strict
-        ) implements Writeable {
+        ) implements Writeable, ToXContentObject {
 
             @SuppressWarnings("unchecked")
             static final ConstructingObjectParser<FunctionField, Void> PARSER = new ConstructingObjectParser<>(
@@ -422,6 +596,18 @@ public void writeTo(StreamOutput out) throws IOException {
                 out.writeGenericMap(parameters);
                 out.writeOptionalBoolean(strict);
             }
+
+            @Override
+            public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+                builder.startObject();
+                builder.field(DESCRIPTION_FIELD, description);
+                builder.field(NAME_FIELD, name);
+                builder.field(PARAMETERS_FIELD, parameters);
+                if (strict != null) {
+                    builder.field(STRICT_FIELD, strict);
+                }
+                return builder.endObject();
+            }
         }
     }
 }