diff --git a/internal/api/doc.go b/internal/api/doc.go
index 5ed76df..7b4e78b 100644
--- a/internal/api/doc.go
+++ b/internal/api/doc.go
@@ -21,4 +21,4 @@
 //	@description				From GET /api/csrf; send its cookie too.
 package api
 
-//go:generate swag init --v3.1 --generalInfo doc.go --dir . --parseInternal --parseDependencyLevel 1 --outputTypes json --output openapi
+//go:generate swag init --v3.1 --generalInfo doc.go --dir . --parseInternal --parseDependencyLevel 1 --requiredByDefault --outputTypes json --output openapi
diff --git a/internal/httpx/problem.go b/internal/httpx/problem.go
index 5bdaf8b..8a77a02 100644
--- a/internal/httpx/problem.go
+++ b/internal/httpx/problem.go
@@ -8,12 +8,12 @@ import (
 
 // Problem is an RFC 9457 problem details body.
 type Problem struct {
-	Type      string `json:"type,omitempty"`
+	Type      string `json:"type,omitempty"      validate:"optional"`
 	Status    int    `json:"status"`
 	Title     string `json:"title"`
-	Detail    string `json:"detail,omitempty"`
-	Instance  string `json:"instance,omitempty"`
-	RequestID string `json:"requestId,omitempty"`
+	Detail    string `json:"detail,omitempty"    validate:"optional"`
+	Instance  string `json:"instance,omitempty"  validate:"optional"`
+	RequestID string `json:"requestId,omitempty" validate:"optional"`
 }
 
 // WriteProblem writes a problem+json error response.
diff --git a/internal/types/opportunities.go b/internal/types/opportunities.go
index 85bb0d6..c55dc8c 100644
--- a/internal/types/opportunities.go
+++ b/internal/types/opportunities.go
@@ -20,20 +20,20 @@ type Opportunity struct {
 	Status            string               `db:"status"              json:"status"`
 	Description       string               `db:"description"         json:"description"`
 	BuyerName         string               `db:"buyer_name"          json:"buyer_name"`
-	Classifications   []ClassificationCode `db:"-"                   json:"classifications"`
+	Classifications   []ClassificationCode `db:"-"                   json:"classifications"     extensions:"x-nullable"`
 	BuyerID           string               `db:"buyer_id"            json:"buyer_id"`
 	MainCategory      string               `db:"main_category"       json:"main_category"`
 	ProcurementMethod string               `db:"procurement_method"  json:"procurement_method"`
 	MultiLot          bool                 `db:"multi_lot"           json:"multi_lot"`
 	SMESuitable       bool                 `db:"sme_suitable"        json:"sme_suitable"`
 	VCSESuitable      bool                 `db:"vcse_suitable"       json:"vcse_suitable"`
-	StartDate         *time.Time           `db:"start_date"          json:"start_date"`
-	EndDate           *time.Time           `db:"end_date"            json:"end_date"`
+	StartDate         *time.Time           `db:"start_date"          json:"start_date"          extensions:"x-nullable"`
+	EndDate           *time.Time           `db:"end_date"            json:"end_date"            extensions:"x-nullable"`
 	GrossValue        float64              `db:"gross_value"         json:"gross_value"`
 	HasDocuments      bool                 `db:"has_documents"       json:"has_documents"`
 	Source            string               `db:"source"              json:"source"`
 	LatestReleaseDate time.Time            `db:"latest_release_date" json:"latest_release_date"`
-	Releases          []OpportunityRelease `db:"-"                   json:"releases"`
+	Releases          []OpportunityRelease `db:"-"                   json:"releases"            extensions:"x-nullable"`
 	Marks             OpportunityMarks     `db:"-"                   json:"marks"`
 }
 
@@ -43,7 +43,7 @@ type OpportunityRelease struct {
 	UUID        string                  `db:"uuid"         json:"uuid"`
 	ReleaseID   string                  `db:"release_id"   json:"release_id"`
 	ReleaseDate time.Time               `db:"release_date" json:"release_date"`
-	Tags        []OpportunityReleaseTag `db:"-"            json:"release_tags"`
+	Tags        []OpportunityReleaseTag `db:"-"            json:"release_tags" extensions:"x-nullable"`
 	JSON        string                  `db:"data"         json:"data"`
 }