diff --git a/.changeset/fix-platform-flat-anyof-union-payload.md b/.changeset/fix-platform-flat-anyof-union-payload.md
new file mode 100644
index 00000000000..7416144aee1
--- /dev/null
+++ b/.changeset/fix-platform-flat-anyof-union-payload.md
@@ -0,0 +1,5 @@
+---
+"@effect/platform": patch
+---
+
+`OpenApi.fromApi` now emits a flat `anyOf` array when multiple union members in `setPayload` share the same content type. Previously, each additional member was wrapped in a new `Union` node, producing deeply nested `anyOf: [ anyOf: [A, B], C ]` structures. The fix uses `HttpApiSchema.UnionUnifyAST` — already used by the response-map accumulator — to flatten unions as they are collected.
diff --git a/packages/platform/src/HttpApi.ts b/packages/platform/src/HttpApi.ts
index 2cbcf250648..c7f981d3d2d 100644
--- a/packages/platform/src/HttpApi.ts
+++ b/packages/platform/src/HttpApi.ts
@@ -419,7 +419,7 @@ const extractPayloads = (topAst: AST.AST): ReadonlyMap<string, {
         ast
       })
     } else {
-      current.ast = AST.Union.make([current.ast, ast])
+      current.ast = HttpApiSchema.UnionUnifyAST(current.ast, ast)
     }
   }
   if (topAst._tag === "Union") {
diff --git a/packages/platform/test/OpenApi.test.ts b/packages/platform/test/OpenApi.test.ts
index a067798a7b4..ff04514bbac 100644
--- a/packages/platform/test/OpenApi.test.ts
+++ b/packages/platform/test/OpenApi.test.ts
@@ -1951,6 +1951,78 @@ describe("OpenApi", () => {
           expectSpecPaths(api, expected)
         })
       })
+
+      it("setPayload with a flat JSON union produces flat anyOf (not nested)", () => {
+        const A = Schema.Struct({ _tag: Schema.Literal("A"), a: Schema.String })
+        const B = Schema.Struct({ _tag: Schema.Literal("B"), b: Schema.Number })
+        const C = Schema.Struct({ _tag: Schema.Literal("C"), c: Schema.Boolean })
+        const api = HttpApi.make("api").add(
+          HttpApiGroup.make("group").add(
+            HttpApiEndpoint.post("post", "/")
+              .addSuccess(Schema.String)
+              .setPayload(Schema.Union(A, B, C))
+          )
+        )
+        expectSpecPaths(api, {
+          "/": {
+            "post": {
+              "tags": ["group"],
+              "operationId": "group.post",
+              "parameters": [],
+              "security": [],
+              "requestBody": {
+                "content": {
+                  "application/json": {
+                    "schema": {
+                      "anyOf": [
+                        {
+                          "type": "object",
+                          "required": ["_tag", "a"],
+                          "properties": {
+                            "_tag": { "enum": ["A"], "type": "string" },
+                            "a": { "type": "string" }
+                          },
+                          "additionalProperties": false
+                        },
+                        {
+                          "type": "object",
+                          "required": ["_tag", "b"],
+                          "properties": {
+                            "_tag": { "enum": ["B"], "type": "string" },
+                            "b": { "type": "number" }
+                          },
+                          "additionalProperties": false
+                        },
+                        {
+                          "type": "object",
+                          "required": ["_tag", "c"],
+                          "properties": {
+                            "_tag": { "enum": ["C"], "type": "string" },
+                            "c": { "type": "boolean" }
+                          },
+                          "additionalProperties": false
+                        }
+                      ]
+                    }
+                  }
+                },
+                "required": true
+              },
+              "responses": {
+                "200": {
+                  "description": "a string",
+                  "content": {
+                    "application/json": {
+                      "schema": { "type": "string" }
+                    }
+                  }
+                },
+                "400": HttpApiDecodeError
+              }
+            }
+          }
+        })
+      })
     })
 
     describe("HttpApiEndpoint.del", () => {