diff --git a/packages/openapi-ts-tests/sdks/__snapshots__/opencode/flat/sdk.gen.ts b/packages/openapi-ts-tests/sdks/__snapshots__/opencode/flat/sdk.gen.ts
index ee09385a4..281dbfd24 100644
--- a/packages/openapi-ts-tests/sdks/__snapshots__/opencode/flat/sdk.gen.ts
+++ b/packages/openapi-ts-tests/sdks/__snapshots__/opencode/flat/sdk.gen.ts
@@ -50,7 +50,7 @@ export const sessionCreate = <ThrowOnError extends boolean = false>(parameters?:
     parentID?: string;
     title?: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "body", key: "parentID" }, { in: "body", key: "title" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "body", key: "parentID" }, { in: "body", key: "title" }] }]);
     return (options?.client ?? client).post<SessionCreateResponses, SessionCreateErrors, ThrowOnError>({
         url: "/session",
         ...options,
@@ -69,7 +69,7 @@ export const sessionCreate = <ThrowOnError extends boolean = false>(parameters?:
 export const sessionDelete = <ThrowOnError extends boolean = false>(parameters: {
     id: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }] }]);
     return (options?.client ?? client).delete<SessionDeleteResponses, unknown, ThrowOnError>({
         url: "/session/{id}",
         ...options,
@@ -83,7 +83,7 @@ export const sessionDelete = <ThrowOnError extends boolean = false>(parameters:
 export const sessionGet = <ThrowOnError extends boolean = false>(parameters: {
     id: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }] }]);
     return (options?.client ?? client).get<SessionGetResponses, unknown, ThrowOnError>({
         url: "/session/{id}",
         ...options,
@@ -98,7 +98,7 @@ export const sessionUpdate = <ThrowOnError extends boolean = false>(parameters:
     id: string;
     title?: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }, { in: "body", key: "title" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }, { in: "body", key: "title" }] }]);
     return (options?.client ?? client).patch<SessionUpdateResponses, unknown, ThrowOnError>({
         url: "/session/{id}",
         ...options,
@@ -117,7 +117,7 @@ export const sessionUpdate = <ThrowOnError extends boolean = false>(parameters:
 export const sessionChildren = <ThrowOnError extends boolean = false>(parameters: {
     id: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }] }]);
     return (options?.client ?? client).get<SessionChildrenResponses, unknown, ThrowOnError>({
         url: "/session/{id}/children",
         ...options,
@@ -134,12 +134,12 @@ export const sessionInit = <ThrowOnError extends boolean = false>(parameters: {
     providerID?: string;
     modelID?: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [
-        { in: "path", key: "id" },
-        { in: "body", key: "messageID" },
-        { in: "body", key: "providerID" },
-        { in: "body", key: "modelID" }
-    ]);
+    const params = buildClientParams([parameters], [{ args: [
+                { in: "path", key: "id" },
+                { in: "body", key: "messageID" },
+                { in: "body", key: "providerID" },
+                { in: "body", key: "modelID" }
+            ] }]);
     return (options?.client ?? client).post<SessionInitResponses, unknown, ThrowOnError>({
         url: "/session/{id}/init",
         ...options,
@@ -158,7 +158,7 @@ export const sessionInit = <ThrowOnError extends boolean = false>(parameters: {
 export const sessionAbort = <ThrowOnError extends boolean = false>(parameters: {
     id: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }] }]);
     return (options?.client ?? client).post<SessionAbortResponses, unknown, ThrowOnError>({
         url: "/session/{id}/abort",
         ...options,
@@ -172,7 +172,7 @@ export const sessionAbort = <ThrowOnError extends boolean = false>(parameters: {
 export const sessionUnshare = <ThrowOnError extends boolean = false>(parameters: {
     id: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }] }]);
     return (options?.client ?? client).delete<SessionUnshareResponses, unknown, ThrowOnError>({
         url: "/session/{id}/share",
         ...options,
@@ -186,7 +186,7 @@ export const sessionUnshare = <ThrowOnError extends boolean = false>(parameters:
 export const sessionShare = <ThrowOnError extends boolean = false>(parameters: {
     id: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }] }]);
     return (options?.client ?? client).post<SessionShareResponses, unknown, ThrowOnError>({
         url: "/session/{id}/share",
         ...options,
@@ -202,11 +202,11 @@ export const sessionSummarize = <ThrowOnError extends boolean = false>(parameter
     providerID?: string;
     modelID?: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [
-        { in: "path", key: "id" },
-        { in: "body", key: "providerID" },
-        { in: "body", key: "modelID" }
-    ]);
+    const params = buildClientParams([parameters], [{ args: [
+                { in: "path", key: "id" },
+                { in: "body", key: "providerID" },
+                { in: "body", key: "modelID" }
+            ] }]);
     return (options?.client ?? client).post<SessionSummarizeResponses, unknown, ThrowOnError>({
         url: "/session/{id}/summarize",
         ...options,
@@ -225,7 +225,7 @@ export const sessionSummarize = <ThrowOnError extends boolean = false>(parameter
 export const sessionMessages = <ThrowOnError extends boolean = false>(parameters: {
     id: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }] }]);
     return (options?.client ?? client).get<SessionMessagesResponses, unknown, ThrowOnError>({
         url: "/session/{id}/message",
         ...options,
@@ -254,16 +254,16 @@ export const sessionChat = <ThrowOnError extends boolean = false>(parameters: {
         type: "agent";
     } & AgentPartInput)>;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [
-        { in: "path", key: "id" },
-        { in: "body", key: "messageID" },
-        { in: "body", key: "providerID" },
-        { in: "body", key: "modelID" },
-        { in: "body", key: "agent" },
-        { in: "body", key: "system" },
-        { in: "body", key: "tools" },
-        { in: "body", key: "parts" }
-    ]);
+    const params = buildClientParams([parameters], [{ args: [
+                { in: "path", key: "id" },
+                { in: "body", key: "messageID" },
+                { in: "body", key: "providerID" },
+                { in: "body", key: "modelID" },
+                { in: "body", key: "agent" },
+                { in: "body", key: "system" },
+                { in: "body", key: "tools" },
+                { in: "body", key: "parts" }
+            ] }]);
     return (options?.client ?? client).post<SessionChatResponses, unknown, ThrowOnError>({
         url: "/session/{id}/message",
         ...options,
@@ -283,7 +283,7 @@ export const sessionMessage = <ThrowOnError extends boolean = false>(parameters:
     id: string;
     messageID: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }, { in: "path", key: "messageID" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }, { in: "path", key: "messageID" }] }]);
     return (options?.client ?? client).get<SessionMessageResponses, unknown, ThrowOnError>({
         url: "/session/{id}/message/{messageID}",
         ...options,
@@ -299,11 +299,11 @@ export const sessionShell = <ThrowOnError extends boolean = false>(parameters: {
     agent?: string;
     command?: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [
-        { in: "path", key: "id" },
-        { in: "body", key: "agent" },
-        { in: "body", key: "command" }
-    ]);
+    const params = buildClientParams([parameters], [{ args: [
+                { in: "path", key: "id" },
+                { in: "body", key: "agent" },
+                { in: "body", key: "command" }
+            ] }]);
     return (options?.client ?? client).post<SessionShellResponses, unknown, ThrowOnError>({
         url: "/session/{id}/shell",
         ...options,
@@ -324,11 +324,11 @@ export const sessionRevert = <ThrowOnError extends boolean = false>(parameters:
     messageID?: string;
     partID?: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [
-        { in: "path", key: "id" },
-        { in: "body", key: "messageID" },
-        { in: "body", key: "partID" }
-    ]);
+    const params = buildClientParams([parameters], [{ args: [
+                { in: "path", key: "id" },
+                { in: "body", key: "messageID" },
+                { in: "body", key: "partID" }
+            ] }]);
     return (options?.client ?? client).post<SessionRevertResponses, unknown, ThrowOnError>({
         url: "/session/{id}/revert",
         ...options,
@@ -347,7 +347,7 @@ export const sessionRevert = <ThrowOnError extends boolean = false>(parameters:
 export const sessionUnrevert = <ThrowOnError extends boolean = false>(parameters: {
     id: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }] }]);
     return (options?.client ?? client).post<SessionUnrevertResponses, unknown, ThrowOnError>({
         url: "/session/{id}/unrevert",
         ...options,
@@ -363,11 +363,11 @@ export const postSessionByIdPermissionsByPermissionId = <ThrowOnError extends bo
     permissionID: string;
     response?: "once" | "always" | "reject";
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [
-        { in: "path", key: "id" },
-        { in: "path", key: "permissionID" },
-        { in: "body", key: "response" }
-    ]);
+    const params = buildClientParams([parameters], [{ args: [
+                { in: "path", key: "id" },
+                { in: "path", key: "permissionID" },
+                { in: "body", key: "response" }
+            ] }]);
     return (options?.client ?? client).post<PostSessionByIdPermissionsByPermissionIdResponses, unknown, ThrowOnError>({
         url: "/session/{id}/permissions/{permissionID}",
         ...options,
@@ -391,7 +391,7 @@ export const configProviders = <ThrowOnError extends boolean = false>(options?:
 export const findText = <ThrowOnError extends boolean = false>(parameters: {
     pattern: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "query", key: "pattern" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "query", key: "pattern" }] }]);
     return (options?.client ?? client).get<FindTextResponses, unknown, ThrowOnError>({
         url: "/find",
         ...options,
@@ -405,7 +405,7 @@ export const findText = <ThrowOnError extends boolean = false>(parameters: {
 export const findFiles = <ThrowOnError extends boolean = false>(parameters: {
     query: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "query", key: "query" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "query", key: "query" }] }]);
     return (options?.client ?? client).get<FindFilesResponses, unknown, ThrowOnError>({
         url: "/find/file",
         ...options,
@@ -419,7 +419,7 @@ export const findFiles = <ThrowOnError extends boolean = false>(parameters: {
 export const findSymbols = <ThrowOnError extends boolean = false>(parameters: {
     query: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "query", key: "query" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "query", key: "query" }] }]);
     return (options?.client ?? client).get<FindSymbolsResponses, unknown, ThrowOnError>({
         url: "/find/symbol",
         ...options,
@@ -433,7 +433,7 @@ export const findSymbols = <ThrowOnError extends boolean = false>(parameters: {
 export const fileRead = <ThrowOnError extends boolean = false>(parameters: {
     path: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "query", key: "path" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "query", key: "path" }] }]);
     return (options?.client ?? client).get<FileReadResponses, unknown, ThrowOnError>({
         url: "/file",
         ...options,
@@ -457,12 +457,12 @@ export const appLog = <ThrowOnError extends boolean = false>(parameters?: {
         [key: string]: unknown;
     };
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [
-        { in: "body", key: "service" },
-        { in: "body", key: "level" },
-        { in: "body", key: "message" },
-        { in: "body", key: "extra" }
-    ]);
+    const params = buildClientParams([parameters], [{ args: [
+                { in: "body", key: "service" },
+                { in: "body", key: "level" },
+                { in: "body", key: "message" },
+                { in: "body", key: "extra" }
+            ] }]);
     return (options?.client ?? client).post<AppLogResponses, unknown, ThrowOnError>({
         url: "/log",
         ...options,
@@ -486,7 +486,7 @@ export const appAgents = <ThrowOnError extends boolean = false>(options?: Option
 export const tuiAppendPrompt = <ThrowOnError extends boolean = false>(parameters?: {
     text?: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "body", key: "text" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "body", key: "text" }] }]);
     return (options?.client ?? client).post<TuiAppendPromptResponses, unknown, ThrowOnError>({
         url: "/tui/append-prompt",
         ...options,
@@ -535,7 +535,7 @@ export const tuiClearPrompt = <ThrowOnError extends boolean = false>(options?: O
 export const tuiExecuteCommand = <ThrowOnError extends boolean = false>(parameters?: {
     command?: string;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "body", key: "command" }]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "body", key: "command" }] }]);
     return (options?.client ?? client).post<TuiExecuteCommandResponses, unknown, ThrowOnError>({
         url: "/tui/execute-command",
         ...options,
@@ -556,11 +556,11 @@ export const tuiShowToast = <ThrowOnError extends boolean = false>(parameters?:
     message?: string;
     variant?: "info" | "success" | "warning" | "error";
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [
-        { in: "body", key: "title" },
-        { in: "body", key: "message" },
-        { in: "body", key: "variant" }
-    ]);
+    const params = buildClientParams([parameters], [{ args: [
+                { in: "body", key: "title" },
+                { in: "body", key: "message" },
+                { in: "body", key: "variant" }
+            ] }]);
     return (options?.client ?? client).post<TuiShowToastResponses, unknown, ThrowOnError>({
         url: "/tui/show-toast",
         ...options,
@@ -580,7 +580,7 @@ export const authSet = <ThrowOnError extends boolean = false>(parameters: {
     id: string;
     auth?: Auth;
 }, options?: Options<never, ThrowOnError>) => {
-    const params = buildClientParams([parameters], [{ in: "path", key: "id" }, {}]);
+    const params = buildClientParams([parameters], [{ args: [{ in: "path", key: "id" }, {}] }]);
     return (options?.client ?? client).put<AuthSetResponses, AuthSetErrors, ThrowOnError>({
         url: "/auth/{id}",
         ...options,
diff --git a/packages/openapi-ts/src/plugins/@hey-api/sdk/shared/operation.ts b/packages/openapi-ts/src/plugins/@hey-api/sdk/shared/operation.ts
index 3193f3cb0..e23477466 100644
--- a/packages/openapi-ts/src/plugins/@hey-api/sdk/shared/operation.ts
+++ b/packages/openapi-ts/src/plugins/@hey-api/sdk/shared/operation.ts
@@ -544,19 +544,44 @@ export const operationStatements = ({
     for (const argName of opParameters.argNames) {
       args.push($(argName));
     }
-    for (const field of opParameters.fields) {
-      const shape = $.object();
-      if ('in' in field) {
-        shape.prop('in', $.literal(field.in));
-        if (field.key) {
-          shape.prop('key', $.literal(field.key));
+
+    // When using flat params, fields need to be wrapped in an args array
+    if (plugin.config.paramsStructure === 'flat') {
+      const fieldShapes: Array<ReturnType<typeof $.object>> = [];
+      for (const field of opParameters.fields) {
+        const shape = $.object();
+        if ('in' in field) {
+          shape.prop('in', $.literal(field.in));
+          if (field.key) {
+            shape.prop('key', $.literal(field.key));
+          }
+          if (field.map) {
+            shape.prop('map', $.literal(field.map));
+          }
         }
-        if (field.map) {
-          shape.prop('map', $.literal(field.map));
+        fieldShapes.push(shape);
+      }
+      // Wrap all fields in an args array for flat params
+      const argsWrapper = $.object();
+      argsWrapper.prop('args', $.array(...fieldShapes));
+      config.push(argsWrapper);
+    } else {
+      // For grouped params, generate fields as before
+      for (const field of opParameters.fields) {
+        const shape = $.object();
+        if ('in' in field) {
+          shape.prop('in', $.literal(field.in));
+          if (field.key) {
+            shape.prop('key', $.literal(field.key));
+          }
+          if (field.map) {
+            shape.prop('map', $.literal(field.map));
+          }
         }
+        config.push(shape);
       }
-      config.push(shape);
     }
+
     const symbol = plugin.referenceSymbol({
       category: 'external',
       resource: 'client.buildClientParams',