MM-67336: Request structured JSON output for AI message rewrites (#37581)

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

server/channels/app/post.go

@@ -3447,6 +3447,20 @@ func (a *App) SendTestMessage(rctx request.CTX, userID string) (*model.Post, *mo
 	return post, nil
 }
 
+// rewriteResponseJSONSchema is the structured output schema for the LLM rewrite response.
+// Defined at package level to avoid re-allocating on every call.
+var rewriteResponseJSONSchema = map[string]any{
+	"type": "object",
+	"properties": map[string]any{
+		"rewritten_text": map[string]any{
+			"type":        "string",
+			"description": "The rewritten version of the message",
+		},
+	},
+	"required":             []any{"rewritten_text"},
+	"additionalProperties": false,
+}
+
 // RewriteMessage rewrites a message using AI based on the specified action
 func (a *App) RewriteMessage(
 	rctx request.CTX,
@@ -3495,6 +3509,7 @@ func (a *App) RewriteMessage(
 			{Role: "system", Message: systemPrompt},
 			{Role: "user", Message: userPrompt},
 		},
+		JSONOutputFormat: rewriteResponseJSONSchema,
 		OperationSubType: normalizeRewriteAction(action),
 		UserID:           sessionUserID,
 	}

server/channels/app/post_rewrite_test.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	"github.com/mattermost/mattermost/server/public/model"
@@ -26,3 +27,25 @@ func TestBuildRewriteSystemPrompt(t *testing.T) {
 		require.Equal(t, basePrompt, prompt)
 	})
 }
+
+func TestRewriteMessage(t *testing.T) {
+	t.Run("sets_structured_output_schema_on_bridge_request", func(t *testing.T) {
+		bridge := &testAgentsBridge{
+			completeFn: func(sessionUserID, agentID string, req BridgeCompletionRequest) (string, error) {
+				return `{"rewritten_text":"Rewritten message"}`, nil
+			},
+		}
+
+		th := Setup(t, WithAgentsBridge(bridge)).InitBasic(t)
+		ctx := th.Context.WithSession(&model.Session{UserId: th.BasicUser.Id})
+
+		response, appErr := th.App.RewriteMessage(ctx, model.NewId(), "original message", model.RewriteActionImproveWriting, "", "")
+		require.Nil(t, appErr)
+		require.NotNil(t, response)
+		assert.Equal(t, "Rewritten message", response.RewrittenText)
+		require.Len(t, bridge.completeCalls, 1)
+		assert.Equal(t, BridgeOperationRewrite, bridge.completeCalls[0].request.Operation)
+		assert.Equal(t, string(model.RewriteActionImproveWriting), bridge.completeCalls[0].request.OperationSubType)
+		assert.Equal(t, rewriteResponseJSONSchema, bridge.completeCalls[0].request.JSONOutputFormat)
+	})
+}