René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Client form bodies ignore `encoding` style/explode, so arrays are always sent as `name[0]=…`","articleBody":"### Summary\n\nFor `application/x-www-form-urlencoded` request bodies, the generated client encodes array properties as indexed keys (`topic[0]=news&topic[1]=releases`), even when the spec declares `style: form, explode: true` for the property. Per the OpenAPI 3.0 [Encoding Object](https://spec.openapis.org/oas/v3.0.3#encoding-object), that serializes as repeated keys: `topic=news&topic=releases`. A server that reads repeated keys sees no `topic` at all.\n\nThis looks like the same problem as #1575, which was closed as a JSON body question; the body there was form-encoded too.\n\n### Reproduction\n\n`openapi.yaml`:\n\n```yaml\nopenapi: 3.0.3\ninfo:\n  title: repro\n  version: 1.0.0\npaths:\n  /subscriptions:\n    post:\n      operationId: subscribe\n      requestBody:\n        required: true\n        content:\n          application/x-www-form-urlencoded:\n            schema:\n              type: object\n              required: [email, topic]\n              properties:\n                email:\n                  type: string\n                topic:\n                  type: array\n                  items:\n                    type: string\n            encoding:\n              topic:\n                style: form\n                explode: true\n      responses:\n        '204':\n          description: Subscribed\n```\n\n`cfg.yaml`:\n\n```yaml\npackage: api\ngenerate:\n  models: true\n  client: true\noutput: api/api.gen.go\n```\n\n`main.go`:\n\n```go\npackage main\n\nimport (\n\t\"context\"\n\t\"fmt\"\n\t\"io\"\n\t\"net/http\"\n\t\"net/http/httptest\"\n\n\t\"repro/api\"\n)\n\nfunc main() {\n\tserver := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {\n\t\tbody, _ := io.ReadAll(r.Body)\n\t\tfmt.Printf(\"body: %s\\n\", body)\n\t\tw.WriteHeader(http.StatusNoContent)\n\t}))\n\tdefer server.Close()\n\n\tclient, _ := api.NewClient(server.URL)\n\t_, _ = client.SubscribeWithFormdataBody(context.Background(), api.SubscribeFormdataRequestBody{\n\t\tEmail: \"jane@example.com\",\n\t\tTopic: []string{\"news\", \"releases\"},\n\t})\n}\n```\n\n```\n$ go mod init repro\n$ go run github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen@v2.8.0 -config cfg.yaml openapi.yaml\n$ go mod tidy\n$ go run .\nbody: email=jane%40example.com&topic%5B0%5D=news&topic%5B1%5D=releases\n```\n\nExpected `body: email=jane%40example.com&topic=news&topic=releases`.\n\n### Cause\n\n- The client template always calls `runtime.MarshalForm(body, nil)` (`pkg/codegen/templates/client.tmpl`), so the spec's `encoding` never reaches the runtime.\n- `runtime.MarshalForm` only reads `RequestBodyEncoding.ContentType`. `Style` and `Explode` are unused, and `marshalFormImpl` always writes slice elements as `name[i]`.\n\n`runtime.BindForm` accepts repeated keys, `name[]` and `name[i]`, so a generated client works against a generated server either way. The mismatch only shows up against other servers.\n\n### Proposal\n\nHonour `style`/`explode` when the spec declares them explicitly: the generator passes the operation's `encoding` to `MarshalForm` instead of `nil`, and `MarshalForm` writes repeated keys for arrays with `style: form, explode: true`. Bodies without an explicit `encoding` keep today's output, so the change isn't breaking.\n\nI can open PRs for both oapi-codegen and runtime if this direction sounds right.\n\noapi-codegen v2.8.0, runtime v1.7.0, Go 1.26.8\n","author":{"url":"https://github.com/kahlstrm","@type":"Person","name":"kahlstrm"},"datePublished":"2026-10-15T12:54:17Z","url":"https://github.com/oapi-codegen/oapi-codegen/issues/2604"}