fern-api · rishabh-fern · Aug 6, 2026
diff --git a/generators/typescript-v2/ast/src/custom-config/TypescriptCustomConfigSchema.ts b/generators/typescript-v2/ast/src/custom-config/TypescriptCustomConfigSchema.ts
@@ -65,6 +65,7 @@ export const TypescriptCustomConfigSchema = z.strictObject({
     testFramework: z.optional(z.enum(["jest", "vitest"])),
     consolidateTypeFiles: z.optional(z.boolean()),
     generateEndpointMetadata: z.optional(z.boolean()),
+    generateStreamConditionOverloads: z.optional(z.boolean()),
     wireTestsFallbackToAutoGeneratedErrorExamples: z.boolean().optional(),
     linter: z.optional(z.enum(["biome", "oxlint", "none"])),
     formatter: z.optional(z.enum(["prettier", "biome", "oxfmt", "none"])),

diff --git a/generators/typescript/sdk/changes/unreleased/stream-condition-overloads.yml b/generators/typescript/sdk/changes/unreleased/stream-condition-overloads.yml
@@ -0,0 +1,14 @@
+# yaml-language-server: $schema=../../../../../fern-changes-yml.schema.json
+
+- summary: |
+    Add the `generateStreamConditionOverloads` configuration option. When enabled, an endpoint
+    declared with `stream-condition` generates a single method with TypeScript overloads that
+    narrow on the condition property, mirroring the Python SDK:
+
+    ```ts
+    public generate(request: GenerateRequest & { stream: true }, requestOptions?: RequestOptions): HttpResponsePromise<core.Stream<StreamResponse>>;
+    public generate(request: GenerateRequest & { stream: false }, requestOptions?: RequestOptions): HttpResponsePromise<RegularResponse>;
+    ```
+
+    The option defaults to `false`, which keeps generating only the streaming variant.
+  type: feat
diff --git a/generators/typescript/sdk/cli/src/SdkGeneratorCli.ts b/generators/typescript/sdk/cli/src/SdkGeneratorCli.ts
@@ -111,7 +111,8 @@ export class SdkGeneratorCli extends AbstractGeneratorCli<SdkCustomConfig> {
             optionalAuth: parsed?.["optional-auth"] ?? false,
             maxRetries: parsed?.maxRetries,
             retryStatusCodes: parsed?.retryStatusCodes ?? "legacy",
-            generateReactQueryHooks: parsed?.generateReactQueryHooks ?? false
+            generateReactQueryHooks: parsed?.generateReactQueryHooks ?? false,
+            generateStreamConditionOverloads: parsed?.generateStreamConditionOverloads ?? false
         };
 
         if (parsed?.serdeLayer != null && parsed?.noSerdeLayer != null) {
@@ -279,7 +280,8 @@ export class SdkGeneratorCli extends AbstractGeneratorCli<SdkCustomConfig> {
                 maxRetries: customConfig.maxRetries,
                 alwaysSendAuth: customConfig.alwaysSendAuth,
                 optionalAuth: customConfig.optionalAuth,
-                generateReactQueryHooks: customConfig.generateReactQueryHooks
+                generateReactQueryHooks: customConfig.generateReactQueryHooks,
+                generateStreamConditionOverloads: customConfig.generateStreamConditionOverloads
             }
         });
         const typescriptProject = await sdkGenerator.generate();

diff --git a/generators/typescript/sdk/cli/src/custom-config/SdkCustomConfig.ts b/generators/typescript/sdk/cli/src/custom-config/SdkCustomConfig.ts
@@ -93,4 +93,5 @@ export interface SdkCustomConfig {
     optionalAuth: boolean;
     retryStatusCodes: "legacy" | "recommended";
     generateReactQueryHooks: boolean;
+    generateStreamConditionOverloads: boolean;
 }
diff --git a/generators/typescript/sdk/client-class-generator/src/GeneratedSdkClientClassImpl.ts b/generators/typescript/sdk/client-class-generator/src/GeneratedSdkClientClassImpl.ts
@@ -48,6 +48,7 @@ import { GeneratedThrowingEndpointResponse } from "./endpoints/default/endpoint-
 import { GeneratedDefaultEndpointImplementation } from "./endpoints/default/GeneratedDefaultEndpointImplementation.js";
 import { GeneratedFileDownloadEndpointImplementation } from "./endpoints/GeneratedFileDownloadEndpointImplementation.js";
 import { GeneratedStreamingEndpointImplementation } from "./endpoints/GeneratedStreamingEndpointImplementation.js";
+import { GeneratedStreamParameterEndpointImplementation } from "./endpoints/GeneratedStreamParameterEndpointImplementation.js";
 import { getClientDefaultValue, isLiteralHeader } from "./endpoints/utils/isLiteralHeader.js";
 import { GeneratedWrappedService } from "./GeneratedWrappedService.js";
 import { GeneratedDefaultWebsocketImplementation } from "./websocket/GeneratedDefaultWebsocketImplementation.js";
@@ -84,6 +85,7 @@ export declare namespace GeneratedSdkClientClassImpl {
         parameterNaming: "originalName" | "wireValue" | "camelCase" | "snakeCase" | "default";
         offsetSemantics: "item-index" | "page-index";
         alwaysSendAuth: boolean;
+        generateStreamConditionOverloads: boolean;
     }
 }
 
@@ -130,6 +132,7 @@ export class GeneratedSdkClientClassImpl implements GeneratedSdkClientClass {
     private readonly generateEndpointMetadata: boolean;
     private readonly offsetSemantics: "item-index" | "page-index";
     private readonly alwaysSendAuth: boolean;
+    private readonly generateStreamConditionOverloads: boolean;
 
     constructor({
         caseConverter,
@@ -159,7 +162,8 @@ export class GeneratedSdkClientClassImpl implements GeneratedSdkClientClass {
         generateEndpointMetadata,
         parameterNaming,
         offsetSemantics,
-        alwaysSendAuth
+        alwaysSendAuth,
+        generateStreamConditionOverloads
     }: GeneratedSdkClientClassImpl.Init) {
         this.case = caseConverter;
         this.isRoot = isRoot;
@@ -182,6 +186,7 @@ export class GeneratedSdkClientClassImpl implements GeneratedSdkClientClass {
         this.parameterNaming = parameterNaming;
         this.offsetSemantics = offsetSemantics;
         this.alwaysSendAuth = alwaysSendAuth;
+        this.generateStreamConditionOverloads = generateStreamConditionOverloads;
 
         const package_ = packageResolver.resolvePackage(packageId);
         this.package_ = package_;
@@ -315,10 +320,8 @@ export class GeneratedSdkClientClassImpl implements GeneratedSdkClientClass {
                             generateEndpointMetadata: this.generateEndpointMetadata,
                             parameterNaming
                         }),
-                    streamParameter: (streamParameter) =>
-                        // TODO(amckinney): For now we just generate the stream variant of the endpoint.
-                        // We need to implement both the non-streaming and streaming variants.
-                        new GeneratedStreamingEndpointImplementation({
+                    streamParameter: (streamParameter) => {
+                        const streamingEndpoint = new GeneratedStreamingEndpointImplementation({
                             packageId,
                             endpoint,
                             generatedSdkClientClass: this,
@@ -334,7 +337,27 @@ export class GeneratedSdkClientClassImpl implements GeneratedSdkClientClass {
                             streamType,
                             generateEndpointMetadata: this.generateEndpointMetadata,
                             parameterNaming
-                        }),
+                        });
+                        const nonStreamResponse = getNonStreamResponseForOverloads(streamParameter.nonStreamResponse);
+                        if (
+                            !this.generateStreamConditionOverloads ||
+                            nonStreamResponse == null ||
+                            endpoint.sdkRequest?.streamParameter == null ||
+                            // A nested condition property cannot be narrowed with an intersection type.
+                            (endpoint.sdkRequest.streamParameter.propertyPath ?? []).length > 0
+                        ) {
+                            // Without overloads we only generate the stream variant of the endpoint.
+                            return streamingEndpoint;
+                        }
+                        return new GeneratedStreamParameterEndpointImplementation({
+                            packageId,
+                            endpoint,
+                            streamParameter: endpoint.sdkRequest.streamParameter,
+                            requestParameterName: this.case.camelUnsafe(endpoint.sdkRequest.requestParameterName),
+                            streamingEndpoint,
+                            nonStreamingEndpoint: getDefaultEndpointImplementation({ response: nonStreamResponse })
+                        });
+                    },
                     text: (textResponse) => {
                         return getDefaultEndpointImplementation({
                             response: FernIr.HttpResponseBody.text(textResponse)
@@ -778,7 +801,16 @@ export class GeneratedSdkClientClassImpl implements GeneratedSdkClientClass {
                         signature.returnTypeWithoutPromise
                     )
                 ),
-                statements: publicStatements.map(getTextOfTsNode)
+                statements: publicStatements.map(getTextOfTsNode),
+                overloads: overloads.map((overload) => ({
+                    parameters: overload.parameters,
+                    returnType: getTextOfTsNode(
+                        context.coreUtilities.fetcher.HttpResponsePromise._getReferenceToType(
+                            overload.returnTypeWithoutPromise
+                        )
+                    ),
+                    docs: docs != null ? [docs] : undefined
+                }))
             };
 
             if (overloads.length === 0) {
@@ -801,19 +833,7 @@ export class GeneratedSdkClientClassImpl implements GeneratedSdkClientClass {
                 ),
                 scope: Scope.Private,
                 isAsync: true,
-                statements: internalResponseStatements.map(getTextOfTsNode),
-                overloads: overloads.map((overload) => ({
-                    parameters: overload.parameters,
-                    returnType: getTextOfTsNode(
-                        ts.factory.createTypeReferenceNode("Promise", [
-                            isPaginated
-                                ? overload.returnTypeWithoutPromise
-                                : context.coreUtilities.fetcher.RawResponse.WithRawResponse._getReferenceToType(
-                                      overload.returnTypeWithoutPromise
-                                  )
-                        ])
-                    )
-                }))
+                statements: internalResponseStatements.map(getTextOfTsNode)
             };
 
             if (isPaginated) {
@@ -1385,3 +1405,23 @@ function anyEndpointWithAuth({
 
     return false;
 }
+
+/**
+ * Returns the non-streaming response of a `stream-condition` endpoint, or undefined
+ * if the response shape isn't supported by the overloaded implementation.
+ */
+function getNonStreamResponseForOverloads(
+    nonStreamResponse: FernIr.NonStreamHttpResponseBody
+): FernIr.HttpResponseBody.Json | FernIr.HttpResponseBody.Text | undefined {
+    switch (nonStreamResponse.type) {
+        case "json":
+            return FernIr.HttpResponseBody.json(nonStreamResponse.value);
+        case "text":
+            return FernIr.HttpResponseBody.text(nonStreamResponse);
+        case "fileDownload":
+        case "bytes":
+            return undefined;
+        default:
+            assertNever(nonStreamResponse);
+    }
+}
diff --git a/generators/typescript/sdk/client-class-generator/src/SdkClientClassGenerator.ts b/generators/typescript/sdk/client-class-generator/src/SdkClientClassGenerator.ts
@@ -35,6 +35,7 @@ export declare namespace SdkClientClassGenerator {
         parameterNaming: "originalName" | "wireValue" | "camelCase" | "snakeCase" | "default";
         offsetSemantics: "item-index" | "page-index";
         alwaysSendAuth: boolean;
+        generateStreamConditionOverloads: boolean;
     }
 
     export namespace generateService {
@@ -75,6 +76,7 @@ export class SdkClientClassGenerator {
     private readonly parameterNaming: "originalName" | "wireValue" | "camelCase" | "snakeCase" | "default";
     private readonly offsetSemantics: "item-index" | "page-index";
     private readonly alwaysSendAuth: boolean;
+    private readonly generateStreamConditionOverloads: boolean;
 
     constructor({
         caseConverter,
@@ -103,7 +105,8 @@ export class SdkClientClassGenerator {
         generateEndpointMetadata,
         parameterNaming,
         offsetSemantics,
-        alwaysSendAuth
+        alwaysSendAuth,
+        generateStreamConditionOverloads
     }: SdkClientClassGenerator.Init) {
         this.case = caseConverter;
         this.intermediateRepresentation = intermediateRepresentation;
@@ -132,6 +135,7 @@ export class SdkClientClassGenerator {
         this.parameterNaming = parameterNaming;
         this.offsetSemantics = offsetSemantics;
         this.alwaysSendAuth = alwaysSendAuth;
+        this.generateStreamConditionOverloads = generateStreamConditionOverloads;
     }
 
     public generateService({
@@ -170,7 +174,8 @@ export class SdkClientClassGenerator {
             generateEndpointMetadata: this.generateEndpointMetadata,
             parameterNaming: this.parameterNaming,
             offsetSemantics: this.offsetSemantics,
-            alwaysSendAuth: this.alwaysSendAuth
+            alwaysSendAuth: this.alwaysSendAuth,
+            generateStreamConditionOverloads: this.generateStreamConditionOverloads
         });
     }
 }
diff --git a/...rs/typescript/sdk/client-class-generator/src/__test__/GeneratedSdkClientClassImpl.test.ts b/...rs/typescript/sdk/client-class-generator/src/__test__/GeneratedSdkClientClassImpl.test.ts
@@ -126,6 +126,7 @@ function createClientClass(opts?: {
     parameterNaming?: "originalName" | "wireValue" | "camelCase" | "snakeCase" | "default";
     offsetSemantics?: "item-index" | "page-index";
     alwaysSendAuth?: boolean;
+    generateStreamConditionOverloads?: boolean;
 }): GeneratedSdkClientClassImpl {
     const ir = opts?.ir ?? createIR();
     return new GeneratedSdkClientClassImpl({
@@ -158,7 +159,8 @@ function createClientClass(opts?: {
         generateEndpointMetadata: opts?.generateEndpointMetadata ?? false,
         parameterNaming: opts?.parameterNaming ?? "default",
         offsetSemantics: opts?.offsetSemantics ?? "item-index",
-        alwaysSendAuth: opts?.alwaysSendAuth ?? false
+        alwaysSendAuth: opts?.alwaysSendAuth ?? false,
+        generateStreamConditionOverloads: opts?.generateStreamConditionOverloads ?? false
     });
 }
 

diff --git a/...rators/typescript/sdk/client-class-generator/src/__test__/SdkClientClassGenerator.test.ts b/...rators/typescript/sdk/client-class-generator/src/__test__/SdkClientClassGenerator.test.ts
@@ -77,6 +77,7 @@ function createGenerator(opts?: Partial<SdkClientClassGenerator.Init>): SdkClien
         parameterNaming: "default",
         offsetSemantics: "item-index",
         alwaysSendAuth: false,
+        generateStreamConditionOverloads: false,
         ...opts
     });
 }