microsoft
diff --git a/‎.chronus/changes/copilot-add-sse-support-to-openapi-2025-10-3-14-40-45.md‎
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diff --git a/‎cspell.yaml‎
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diff --git a/‎packages/openapi3/package.json‎
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diff --git a/‎packages/openapi3/src/cli/actions/convert/generators/generate-main.ts‎
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diff --git a/‎packages/openapi3/src/cli/actions/convert/generators/generate-model.ts‎
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diff --git a/‎packages/openapi3/src/cli/actions/convert/generators/generate-response-expressions.ts‎
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diff --git a/‎packages/openapi3/src/cli/actions/convert/transforms/transform-component-schemas.ts‎
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@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/openapi3"
+---
+
+adds support for emission and import of SSE for OpenAPI 3.2
@@ -6,13 +6,13 @@ dictionaries:
   - typescript
 words:
   - Adoptium
-  - Arize
-  - arizeaiobservabilityeval
   - agentic
   - aiohttp
   - alzimmer
   - amqp
   - AQID
+  - Arize
+  - arizeaiobservabilityeval
   - arraya
   - astimezone
   - astro
@@ -230,8 +230,8 @@ words:
   - rtype
   - rushx
   - safeint
-  - sdkcore
   - SCME
+  - sdkcore
   - segmentof
   - serde
   - setuppy
@@ -293,6 +293,7 @@ words:
   - wday
   - weidxu
   - westus
+  - WHATWG
   - WINDOWSARMVMIMAGE
   - WINDOWSVMIMAGE
   - xiangyan
 
@@ -41,7 +41,8 @@
   },
   "scripts": {
     "clean": "rimraf ./dist ./temp",
-    "build": "pnpm gen-version && pnpm gen-extern-signature && tsc -p . && pnpm lint-typespec-library",
+    "build": "pnpm gen-version && pnpm gen-extern-signature && pnpm quickbuild && pnpm lint-typespec-library",
+    "quickbuild": "tsc -p .",
     "watch": "tsc -p . --watch",
     "gen-extern-signature": "tspd --enable-experimental gen-extern-signature .",
     "lint-typespec-library": "tsp compile . --warn-as-error --import @typespec/library-linter --no-emit",
@@ -73,7 +74,10 @@
     "@typespec/http": "workspace:^",
     "@typespec/json-schema": "workspace:^",
     "@typespec/openapi": "workspace:^",
-    "@typespec/versioning": "workspace:^"
+    "@typespec/versioning": "workspace:^",
+    "@typespec/streams": "workspace:^",
+    "@typespec/events": "workspace:^",
+    "@typespec/sse": "workspace:^"
   },
   "peerDependenciesMeta": {
     "@typespec/json-schema": {
@@ -84,6 +88,15 @@
     },
     "@typespec/versioning": {
       "optional": true
+    },
+    "@typespec/streams": {
+      "optional": true
+    },
+    "@typespec/events": {
+      "optional": true
+    },
+    "@typespec/sse": {
+      "optional": true
     }
   },
   "devDependencies": {
@@ -98,6 +111,9 @@
     "@typespec/tspd": "workspace:^",
     "@typespec/versioning": "workspace:^",
     "@typespec/xml": "workspace:^",
+    "@typespec/streams": "workspace:^",
+    "@typespec/events": "workspace:^",
+    "@typespec/sse": "workspace:^",
     "@vitest/coverage-v8": "^4.0.4",
     "@vitest/ui": "^4.0.4",
     "c8": "^10.1.3",
 
@@ -7,13 +7,21 @@ import { generateOperation } from "./generate-operation.js";
 import { generateServiceInformation } from "./generate-service-info.js";
 
 export function generateMain(program: TypeSpecProgram, context: Context): string {
+  const sseImports = context.isSSEUsed()
+    ? `  import "@typespec/streams";
+  import "@typespec/sse";
+  import "@typespec/events";`
+    : "";
+
+  const sseUsings = context.isSSEUsed() ? "\n  using SSE;" : "";
+
   return `
   import "@typespec/http";
   import "@typespec/openapi";
-  import "@typespec/openapi3";
+  import "@typespec/openapi3";${sseImports}
 
   using Http;
-  using OpenAPI;
+  using OpenAPI;${sseUsings}
 
   ${generateServiceInformation(program.serviceInfo, program.servers, program.tags, context.rootNamespace)}
 
 
@@ -20,6 +20,8 @@ import {
   SchemaToExpressionGenerator,
 } from "./generate-types.js";
 
+const SSE_TERMINAL_EVENT_EXTENSION = "x-ms-sse-terminal-event";
+
 export function generateDataType(type: TypeSpecDataTypes, context: Context): string {
   switch (type.kind) {
     case "alias":
@@ -83,6 +85,145 @@ function generateScalar(scalar: TypeSpecScalar, context: Context): string {
   return definitions.join("\n");
 }
 
+function generateSSEEventVariants(
+  members: Refable<SupportedOpenAPISchema>[],
+  union: TypeSpecUnion,
+  context: Context,
+  getVariantName: (member: Refable<SupportedOpenAPISchema>) => string,
+): string[] {
+  // Generate SSE event variants: eventName: DataType,
+  // Sort so terminal events come last
+  const sortedMembers = [...members].sort((a, b) => {
+    const aSchema = "$ref" in a ? context.getSchemaByRef(a.$ref) : a;
+    const bSchema = "$ref" in b ? context.getSchemaByRef(b.$ref) : b;
+
+    const aIsTerminal = !!aSchema?.[SSE_TERMINAL_EVENT_EXTENSION];
+    const bIsTerminal = !!bSchema?.[SSE_TERMINAL_EVENT_EXTENSION];
+
+    if (aIsTerminal && !bIsTerminal) return 1; // a comes after b
+    if (!aIsTerminal && bIsTerminal) return -1; // a comes before b
+    return 0; // maintain original order for same type
+  });
+
+  return sortedMembers.map((member) => {
+    try {
+      const memberSchema = "$ref" in member ? context.getSchemaByRef(member.$ref) : member;
+      if (!memberSchema || typeof memberSchema !== "object" || !memberSchema.properties) {
+        // Fallback to regular generation if we can't parse the event structure
+        return (
+          getVariantName(member) + context.generateTypeFromRefableSchema(member, union.scope) + ","
+        );
+      }
+
+      // Use any to access properties since the types are complex
+      const props = memberSchema.properties;
+
+      // Extract event name from event.const
+      let eventName: string | undefined;
+      if (props.event) {
+        const eventProp = props.event;
+        if ("const" in eventProp && eventProp.const && typeof eventProp.const === "string") {
+          eventName = eventProp.const;
+        } else if (
+          "enum" in eventProp &&
+          eventProp.enum?.[0] &&
+          typeof eventProp.enum[0] === "string"
+        ) {
+          eventName = eventProp.enum[0];
+        }
+      }
+
+      // Check for terminal events or special cases
+      if (!eventName) {
+        // Check if this is a terminal event (no event name, just data)
+        if ("const" in props.data && props.data.const) {
+          const terminalValue = props.data.const;
+          const isTerminal = memberSchema[SSE_TERMINAL_EVENT_EXTENSION];
+          const contentType = "contentMediaType" in props.data && props.data.contentMediaType;
+
+          let decorators = "";
+          if (contentType) {
+            decorators += `\n  @TypeSpec.Events.contentType("${contentType}")`;
+          }
+          if (isTerminal) {
+            decorators += `\n  @TypeSpec.SSE.terminalEvent`;
+            decorators += `\n  @extension("${SSE_TERMINAL_EVENT_EXTENSION}", true)`;
+          }
+
+          return `${decorators}\n  "${terminalValue}",`;
+        }
+        // Fallback to regular generation
+        return (
+          getVariantName(member) + context.generateTypeFromRefableSchema(member, union.scope) + ","
+        );
+      }
+
+      // Extract data type and content type from data.contentSchema
+      let dataType = "unknown";
+      let contentType: string | undefined;
+
+      if (props.data) {
+        const dataProp = props.data;
+
+        // Get content type if specified
+        if ("contentMediaType" in dataProp && dataProp.contentMediaType) {
+          contentType = dataProp.contentMediaType;
+        }
+
+        // Check for contentSchema (OpenAPI extension)
+        if ("contentSchema" in dataProp && dataProp.contentSchema) {
+          const contentSchema = dataProp.contentSchema;
+          // Special handling for byte data which should map to Base64/bytes
+          if (
+            contentSchema &&
+            typeof contentSchema === "object" &&
+            "type" in contentSchema &&
+            contentSchema.type === "object" &&
+            contentSchema.properties?.data
+          ) {
+            const dataProperty = contentSchema.properties.data;
+            if (
+              "type" in dataProperty &&
+              dataProperty.type === "string" &&
+              "format" in dataProperty &&
+              dataProperty.format === "byte"
+            ) {
+              dataType = "Base64";
+            } else {
+              dataType = context.generateTypeFromRefableSchema(dataProp.contentSchema, union.scope);
+            }
+          } else {
+            dataType = context.generateTypeFromRefableSchema(dataProp.contentSchema, union.scope);
+          }
+        } else if ("type" in dataProp && dataProp.type && typeof dataProp.type === "string") {
+          // Simple type like string
+          dataType = dataProp.type;
+        }
+      }
+
+      // Build decorators for this event variant
+      let decorators = "";
+      if (contentType && contentType !== "application/json") {
+        decorators += `\n  @TypeSpec.Events.contentType("${contentType}")`;
+      }
+
+      // Check if this is a terminal event
+      const isTerminal = memberSchema[SSE_TERMINAL_EVENT_EXTENSION];
+      if (isTerminal) {
+        decorators += `\n  @TypeSpec.SSE.terminalEvent`;
+        decorators += `\n  @extension("${SSE_TERMINAL_EVENT_EXTENSION}", true)`;
+      }
+
+      return `${decorators}\n  ${eventName}: ${dataType},`;
+    } catch (error) {
+      // If any error occurs, fall back to regular generation
+      return (
+        getVariantName(member) + context.generateTypeFromRefableSchema(member, union.scope) + ","
+      );
+    }
+  });
+}
+
 function generateUnion(union: TypeSpecUnion, context: Context): string {
   const definitions: string[] = [];
 
@@ -103,24 +244,38 @@ function generateUnion(union: TypeSpecUnion, context: Context): string {
 
     const memberSchema = "$ref" in member ? context.getSchemaByRef(member.$ref)! : member;
 
+    const propertySchema =
+      memberSchema.properties && memberSchema.properties[union.schema.discriminator.propertyName];
     const value =
       (union.schema.discriminator?.mapping && "$ref" in member
         ? Object.entries(union.schema.discriminator.mapping).find((x) => x[1] === member.$ref)?.[0]
         : undefined) ??
-      (memberSchema.properties?.[union.schema.discriminator.propertyName] as any)?.enum?.[0];
+      (propertySchema && "enum" in propertySchema && propertySchema.enum?.[0]);
     // checking whether the value is using an invalid character as an identifier
-    const valueIdentifier = value ? printIdentifier(value, "disallow-reserved") : "";
+    const valueIdentifier = value ? printIdentifier(`${value}`, "disallow-reserved") : "";
     return value ? `${value === valueIdentifier ? value : valueIdentifier}: ` : "";
   };
   if (schema.enum) {
     definitions.push(...schema.enum.map((e) => `${JSON.stringify(e)},`));
   } else if (schema.oneOf) {
-    definitions.push(
-      ...schema.oneOf.map(
-        (member) =>
-          getVariantName(member) + context.generateTypeFromRefableSchema(member, union.scope) + ",",
-      ),
+    // Check if this is an SSE event union
+    const isSSEEventUnion = union.decorators.some(
+      (d) => d.name === "TypeSpec.Events.events" || d.name === "events",
     );
+
+    if (isSSEEventUnion) {
+      definitions.push(...generateSSEEventVariants(schema.oneOf, union, context, getVariantName));
+    } else {
+      // Regular union generation
+      definitions.push(
+        ...schema.oneOf.map(
+          (member) =>
+            getVariantName(member) +
+            context.generateTypeFromRefableSchema(member, union.scope) +
+            ",",
+        ),
+      );
+    }
   } else if (schema.anyOf) {
     definitions.push(
       ...schema.anyOf.map(
@@ -140,7 +295,7 @@ function generateUnion(union: TypeSpecUnion, context: Context): string {
         definitions.push("null,");
       } else {
         // Create a schema with a single type to reuse existing logic
-        const singleTypeSchema = { ...schema, type: t as any, nullable: undefined };
+        const singleTypeSchema = { ...schema, type: t, nullable: undefined };
         const type = context.generateTypeFromRefableSchema(singleTypeSchema, union.scope);
         definitions.push(`${type},`);
       }
 
@@ -220,6 +220,29 @@ function generateResponseExpressions({
   }
 
   return contents.map(([mediaType, content]) => {
+    // Special handling for Server-Sent Events
+    if (mediaType === "text/event-stream") {
+      context.markSSEUsage();
+
+      // Check for itemSchema (OpenAPI 3.2 extension)
+      if ("itemSchema" in content && content.itemSchema) {
+        const itemSchema = content.itemSchema;
+        if (itemSchema && typeof itemSchema === "object" && "$ref" in itemSchema) {
+          const eventUnionType = context.generateTypeFromRefableSchema(itemSchema, operationScope);
+          return `SSEStream<${eventUnionType}>`;
+        }
+      } else if (content.schema && "$ref" in content.schema) {
+        // Fallback: use schema directly if no itemSchema
+        const eventUnionType = context.generateTypeFromRefableSchema(
+          content.schema,
+          operationScope,
+        );
+        return `SSEStream<${eventUnionType}>`;
+      }
+
+      // If no proper schema reference, fall through to regular handling
+    }
+
     // Attempt to emit just the Body or an intersection of Body & MappedResponse
     // if there aren't any custom headers.
     const bodySchema = content.schema;
 
@@ -141,10 +141,20 @@ export function transformComponentSchemas(context: Context, models: TypeSpecData
     // Extract description and decorators from meaningful union members
     const unionMetadata = extractUnionMetadata(schema);
 
+    let decorators = [...getDecoratorsForSchema(schema), ...unionMetadata.decorators];
+
+    // Check if this is an SSE event schema - if so, replace @oneOf with @events
+    const schemaRef = `#/components/schemas/${name}`;
+    if (context.isSSEEventSchema(schemaRef)) {
+      // Remove @oneOf decorator if present and add @events
+      decorators = decorators.filter((d) => d.name !== "oneOf");
+      decorators.push({ name: "events", args: [] });
+    }
+
     const union: TypeSpecUnion = {
       kind: "union",
       ...getScopeAndName(name),
-      decorators: [...getDecoratorsForSchema(schema), ...unionMetadata.decorators],
+      decorators,
       doc: schema.description ?? unionMetadata.description,
       schema,
     };