oapi-codegen · natalie-o-perret · Jun 1, 2026 · Jun 1, 2026 · Jun 1, 2026 · Jun 1, 2026
@@ -1021,25 +1021,87 @@ func GenerateEnums(t *template.Template, types []TypeDefinition) (string, error)
 
 	// Now, go through all the enums, and figure out if we have conflicts with
 	// any others.
-	for i := range enums {
-		// Look through all other enums not compared so far. Make sure we don't
-		// compare against self.
-		e1 := enums[i]
-		for j := i + 1; j < len(enums); j++ {
-			e2 := enums[j]
-
-			for e1key := range e1.GetValues() {
-				_, found := e2.GetValues()[e1key]
-				if found {
-					e1.PrefixTypeName = true
-					e2.PrefixTypeName = true
-					enums[i] = e1
-					enums[j] = e2
-					break
+	if globalState.options.Compatibility.OldEnumConflicts {
+		// Legacy behavior (pre-v2.7.1): compare generated constant names via
+		// GetValues(). This is order-dependent because GetValues() reflects
+		// already-applied prefixes, so enums processed later may miss conflicts
+		// with enums that were prefixed in an earlier iteration. Preserved here
+		// as an escape hatch for users who need to keep existing generated output.
+		for i := range enums {
+			e1 := enums[i]
+			for j := i + 1; j < len(enums); j++ {
+				e2 := enums[j]
+				for e1key := range e1.GetValues() {
+					if _, found := e2.GetValues()[e1key]; found {
+						e1.PrefixTypeName = true
+						e2.PrefixTypeName = true
+						enums[i] = e1
+						enums[j] = e2
+						break
+					}
+				}
+			}
+		}
+	} else {
+		// Stage 1: detect conflicts based on raw (unprefixed) values. This catches
+		// the case where two enums share the same value string (e.g. both have
+		// "running"), regardless of which order they appear.
+		for i := range enums {
+			e1 := enums[i]
+			for j := i + 1; j < len(enums); j++ {
+				e2 := enums[j]
+				for e1key := range e1.Schema.EnumValues {
+					if _, found := e2.Schema.EnumValues[e1key]; found {
+						e1.PrefixTypeName = true
+						e2.PrefixTypeName = true
+						enums[i] = e1
+						enums[j] = e2
+						break
+					}
 				}
 			}
 		}
 
+		// Stage 2: iteratively detect effective-name conflicts. After Stage 1 some
+		// enums are now prefixed; their prefixed constant names (e.g. "Enum1One")
+		// may collide with another enum's raw constant name. We repeat until
+		// stable.
+		for {
+			changed := false
+			for i := range enums {
+				e1 := enums[i]
+				for j := i + 1; j < len(enums); j++ {
+					e2 := enums[j]
+					if e1.PrefixTypeName && e2.PrefixTypeName {
+						continue
+					}
+					for e1key := range e1.GetValues() {
+						if _, found := e2.GetValues()[e1key]; found {
+							if !e1.PrefixTypeName {
+								e1.PrefixTypeName = true
+								enums[i] = e1
+								changed = true
+							}
+							if !e2.PrefixTypeName {
+								e2.PrefixTypeName = true
+								enums[j] = e2
+								changed = true
+							}
+							break
+						}
+					}
+				}
+			}
+			if !changed {
+				break
+			}
+		}
+	}
+
+	// Check each enum against global type names and self-conflict.
+	for i := range enums {
+		e1 := enums[i]
+
 		// now see if this enum conflicts with any global type names.
 		for _, tp := range types {
 			// Skip over enums, since we've handled those above.

@@ -329,5 +329,135 @@ paths:
 	assert.Contains(t, code, "roleName string")
 }
 
+// TestEnumConflictDetectionOrderIndependent checks that conflict detection
+// doesn't miss overlaps because an enum was already marked for prefixing.
+func TestEnumConflictDetectionOrderIndependent(t *testing.T) {
+	// AState+BState share "running" (both prefixed), AState+CState share "migrating".
+	// The bug: once AState was marked, GetValues() returned prefixed names that
+	// no longer matched CState's raw values, so CState's conflict was missed.
+	const spec = `
+openapi: "3.0.0"
+info:
+  version: 1.0.0
+  title: Test Enum Conflict Detection
+paths: {}
+components:
+  schemas:
+    AState:
+      type: string
+      enum:
+        - running
+        - migrating
+    BState:
+      type: string
+      enum:
+        - running
+    CState:
+      type: string
+      enum:
+        - migrating
+`
+	loader := openapi3.NewLoader()
+	swagger, err := loader.LoadFromData([]byte(spec))
+	require.NoError(t, err)
+
+	opts := Configuration{
+		PackageName: "api",
+		Generate: GenerateOptions{
+			Models: true,
+		},
+		OutputOptions: OutputOptions{
+			SkipPrune: true,
+		},
+	}
+
+	code, err := Generate(swagger, opts)
+	require.NoError(t, err)
+
+	_, err = format.Source([]byte(code))
+	require.NoError(t, err)
+
+	// All three enums share values with at least one other enum; all must be prefixed.
+	assert.Contains(t, code, "AStateRunning")
+	assert.Contains(t, code, "AStateMigrating")
+	assert.Contains(t, code, "BStateRunning")
+	assert.Contains(t, code, "CStateMigrating")
+}
+
+// TestEnumConflictDetectionBothOrders verifies that enum conflict detection
+// produces identical output regardless of the order schemas appear in the spec.
+// The old GetValues()-based approach was order-dependent: processing AState
+// before CState caused CState to be left unprefixed, while reversing the order
+// would prefix it. Go map iteration is non-deterministic (randomized since
+// Go 1.12), so this was a latent correctness bug.
+func TestEnumConflictDetectionBothOrders(t *testing.T) {
+	specAFirst := `
+openapi: "3.0.0"
+info:
+  version: 1.0.0
+  title: Test
+paths: {}
+components:
+  schemas:
+    AState:
+      type: string
+      enum: [running, migrating]
+    BState:
+      type: string
+      enum: [running]
+    CState:
+      type: string
+      enum: [migrating]
+`
+	specCFirst := `
+openapi: "3.0.0"
+info:
+  version: 1.0.0
+  title: Test
+paths: {}
+components:
+  schemas:
+    CState:
+      type: string
+      enum: [migrating]
+    BState:
+      type: string
+      enum: [running]
+    AState:
+      type: string
+      enum: [running, migrating]
+`
+	opts := Configuration{
+		PackageName: "api",
+		Generate:    GenerateOptions{Models: true},
+		OutputOptions: OutputOptions{
+			SkipPrune: true,
+		},
+	}
+
+	loader := openapi3.NewLoader()
+
+	swaggerA, err := loader.LoadFromData([]byte(specAFirst))
+	require.NoError(t, err)
+	codeA, err := Generate(swaggerA, opts)
+	require.NoError(t, err)
+
+	swaggerC, err := loader.LoadFromData([]byte(specCFirst))
+	require.NoError(t, err)
+	codeC, err := Generate(swaggerC, opts)
+	require.NoError(t, err)
+
+	// Both orderings must produce fully prefixed constants.
+	for _, code := range []string{codeA, codeC} {
+		assert.Contains(t, code, "AStateRunning")
+		assert.Contains(t, code, "AStateMigrating")
+		assert.Contains(t, code, "BStateRunning")
+		assert.Contains(t, code, "CStateMigrating")
+		// Unprefixed names must not appear as standalone constants.
+		assert.NotContains(t, code, "\tRunning ")
+		assert.NotContains(t, code, "\tMigrating ")
+	}
+}
+
 //go:embed test_spec.yaml
 var testOpenAPIDefinition string
@@ -231,8 +231,13 @@ type CompatibilityOptions struct {
 	// Enum values can generate conflicting typenames, so we've updated the
 	// code for enum generation to avoid these conflicts, but it will result
 	// in some enum types being renamed in existing code. Set OldEnumConflicts to true
-	// to revert to old behavior. Please see:
+	// to revert to old behavior.
+	// As of v2.7.1, this also reverts the order-independent conflict detection
+	// fix (issue #2391): the legacy GetValues()-based path is order-dependent
+	// and may produce different output on different runs, but is provided as
+	// an escape hatch for users who need to preserve existing generated code.
 	// Please see https://github.com/oapi-codegen/oapi-codegen/issues/549
+	// Please see https://github.com/oapi-codegen/oapi-codegen/issues/2391
 	OldEnumConflicts bool `yaml:"old-enum-conflicts,omitempty"`
 	// It was a mistake to generate a go type definition for every $ref in
 	// the OpenAPI schema. New behavior uses type aliases where possible, but
@@ -299,6 +304,7 @@ type CompatibilityOptions struct {
 	// are treated as required.
 	// Please see https://github.com/oapi-codegen/oapi-codegen/issues/2267
 	HeadersImplicitlyRequired bool `yaml:"headers-implicitly-required,omitempty"`
+
 }
 
 func (co CompatibilityOptions) Validate() map[string]string {