fix: remove omitempty from required ChatMessagePart fields (#23250)

ChatMessagePart uses a flat struct with omitempty on all fields,
but some fields are required in their TypeScript variant (no ?
suffix in the variants struct tag). When Go omits a zero-valued
required field, the frontend receives undefined where it expects
a concrete value.

Remove omitempty from fields that are required in at least one
variant: Text, URL, MediaType, FileName, StartLine, EndLine,
Content. Fields where all variants use ? keep omitempty.

Add a sub-test to TestChatMessagePartVariantTags that enforces
this invariant via reflection so future additions cannot
reintroduce the mismatch.

Supersedes #23249

codersdk/chats.go

@@ -125,9 +125,18 @@ func AllChatMessagePartTypes() []ChatMessagePartType {
 // name = required, ? suffix = optional. Fields without a variants
 // tag are excluded from the generated union. See
 // scripts/apitypings/main.go for the codegen that reads these.
+//
+// omitempty rules (enforced by TestChatMessagePartVariantTags):
+//   - If a field is required (no ? suffix) in ANY variant, it
+//     must NOT use omitempty. Go would silently drop zero values
+//     that TypeScript expects to always be present.
+//   - If a field is optional (? suffix) in ALL of its variants,
+//     it MUST use omitempty. Sending zero values for fields that
+//     the frontend does not expect adds noise to the wire format
+//     and wastes space in persisted chat_messages rows.
 type ChatMessagePart struct {
 	Type        ChatMessagePartType `json:"type"`
-	Text        string              `json:"text,omitempty" variants:"text,reasoning?"`
+	Text        string              `json:"text" variants:"text,reasoning"`
 	Signature   string              `json:"signature,omitempty"`
 	ToolCallID  string              `json:"tool_call_id,omitempty" variants:"tool-call?,tool-result?"`
 	ToolName    string              `json:"tool_name,omitempty" variants:"tool-call?,tool-result?"`
@@ -137,16 +146,16 @@ type ChatMessagePart struct {
 	ResultDelta string              `json:"result_delta,omitempty"`
 	IsError     bool                `json:"is_error,omitempty" variants:"tool-result?"`
 	SourceID    string              `json:"source_id,omitempty" variants:"source?"`
-	URL         string              `json:"url,omitempty" variants:"source"`
+	URL         string              `json:"url" variants:"source"`
 	Title       string              `json:"title,omitempty" variants:"source?"`
-	MediaType   string              `json:"media_type,omitempty" variants:"file"`
+	MediaType   string              `json:"media_type" variants:"file"`
 	Data        []byte              `json:"data,omitempty" variants:"file?"`
 	FileID      uuid.NullUUID       `json:"file_id,omitempty" format:"uuid" variants:"file?"`
-	FileName    string              `json:"file_name,omitempty" variants:"file-reference"`
-	StartLine   int                 `json:"start_line,omitempty" variants:"file-reference"`
-	EndLine     int                 `json:"end_line,omitempty" variants:"file-reference"`
+	FileName    string              `json:"file_name" variants:"file-reference"`
+	StartLine   int                 `json:"start_line" variants:"file-reference"`
+	EndLine     int                 `json:"end_line" variants:"file-reference"`
 	// The code content from the diff that was commented on.
-	Content string `json:"content,omitempty" variants:"file-reference"`
+	Content string `json:"content" variants:"file-reference"`
 	// ProviderMetadata holds provider-specific response metadata
 	// (e.g. Anthropic cache control hints) as raw JSON. Internal
 	// only: stripped by db2sdk before API responses.

codersdk/chats_test.go

@@ -259,6 +259,46 @@ func TestChatMessagePartVariantTags(t *testing.T) {
 		assert.True(t, coveredTypes[pt],
 			"ChatMessagePartType %q is not referenced by any variants tag; %s", pt, editHint)
 	}
+
+	// Enforce the omitempty <-> variants invariant:
+	//   required in any variant  => must NOT have omitempty
+	//   optional in all variants => MUST have omitempty
+	// See the struct comment on ChatMessagePart for rationale.
+	t.Run("omitempty must match variant optionality", func(t *testing.T) {
+		t.Parallel()
+
+		typ := reflect.TypeOf(codersdk.ChatMessagePart{})
+		for i := range typ.NumField() {
+			f := typ.Field(i)
+			varTag := f.Tag.Get("variants")
+			if varTag == "" {
+				continue
+			}
+
+			allOptional := true
+			for _, entry := range strings.Split(varTag, ",") {
+				if !strings.HasSuffix(entry, "?") {
+					allOptional = false
+					break
+				}
+			}
+
+			jsonTag := f.Tag.Get("json")
+			hasOmitEmpty := strings.Contains(jsonTag, "omitempty")
+
+			if !allOptional {
+				assert.False(t, hasOmitEmpty,
+					"field %s is required in at least one variant but has omitempty in its json tag; "+
+						"remove omitempty so Go does not silently drop the zero value that TypeScript expects to always be present",
+					f.Name)
+			} else {
+				assert.True(t, hasOmitEmpty,
+					"field %s is optional in all variants but is missing omitempty in its json tag; "+
+						"add omitempty to avoid sending zero values for fields the frontend does not expect",
+					f.Name)
+			}
+		}
+	})
 }
 
 func TestModelCostConfig_LegacyNumericJSON(t *testing.T) {

site/src/api/typesGenerated.ts

@@ -1320,6 +1320,15 @@ export interface ChatMessage {
  * name = required, ? suffix = optional. Fields without a variants
  * tag are excluded from the generated union. See
  * scripts/apitypings/main.go for the codegen that reads these.
+ *
+ * omitempty rules (enforced by TestChatMessagePartVariantTags):
+ *   - If a field is required (no ? suffix) in ANY variant, it
+ *     must NOT use omitempty. Go would silently drop zero values
+ *     that TypeScript expects to always be present.
+ *   - If a field is optional (? suffix) in ALL of its variants,
+ *     it MUST use omitempty. Sending zero values for fields that
+ *     the frontend does not expect adds noise to the wire format
+ *     and wastes space in persisted chat_messages rows.
  */
 export type ChatMessagePart =
 	| ChatTextPart
@@ -1682,7 +1691,7 @@ export interface ChatQueuedMessage {
 // From codersdk/chats.go
 export interface ChatReasoningPart {
 	readonly type: "reasoning";
-	readonly text?: string;
+	readonly text: string;
 }
 
 // From codersdk/chats.go

site/src/pages/AgentsPage/AgentDetail/messageParsing.ts

@@ -131,9 +131,8 @@ export const parseMessageContent = (
 				break;
 			}
 			case "reasoning": {
-				const text = part.text ?? "";
-				parsed.reasoning = appendText(parsed.reasoning, text);
-				parsed.blocks = appendTextBlock(parsed.blocks, "thinking", text);
+				parsed.reasoning = appendText(parsed.reasoning, part.text);
+				parsed.blocks = appendTextBlock(parsed.blocks, "thinking", part.text);
 				break;
 			}
 			case "tool-call": {