diff --git a/.chronus/changes/sramsey-csharp-server-union-failure-2026-5-17-14-5-13.md b/.chronus/changes/sramsey-csharp-server-union-failure-2026-5-17-14-5-13.md
new file mode 100644
index 00000000000..9619c70081f
--- /dev/null
+++ b/.chronus/changes/sramsey-csharp-server-union-failure-2026-5-17-14-5-13.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-csharp"
+---
+
+Update the union definition to include unnamed string literals and null
\ No newline at end of file
diff --git a/packages/http-server-csharp/src/components/enums/enums.test.tsx b/packages/http-server-csharp/src/components/enums/enums.test.tsx
index e0583855c93..811d2cb2c2e 100644
--- a/packages/http-server-csharp/src/components/enums/enums.test.tsx
+++ b/packages/http-server-csharp/src/components/enums/enums.test.tsx
@@ -1,10 +1,18 @@
 import { Tester } from "#test/tester.js";
 import { type Children } from "@alloy-js/core";
-import { createCSharpNamePolicy, SourceFile } from "@alloy-js/csharp";
+import {
+  Attribute,
+  createCSharpNamePolicy,
+  EnumDeclaration as CsEnumDeclaration,
+  EnumMember,
+  SourceFile,
+} from "@alloy-js/csharp";
+import { type Union } from "@typespec/compiler";
 import { t, type TesterInstance } from "@typespec/compiler/testing";
 import { Output } from "@typespec/emitter-framework";
 import { EnumDeclaration } from "@typespec/emitter-framework/csharp";
 import { beforeEach, describe, expect, it } from "vitest";
+import { getUnionEnumMembers, isUnionEnum } from "./enums.jsx";
 
 let runner: TesterInstance;
 
@@ -21,6 +29,26 @@ function Wrapper(props: { children: Children }) {
   );
 }
 
+/**
+ * Renders a union-as-enum using the same logic as the Enums component,
+ * but without the file/namespace/useTsp wrapping.
+ */
+function UnionEnumDecl(props: { union: Union }) {
+  const members = getUnionEnumMembers(props.union);
+  return (
+    <CsEnumDeclaration name={props.union.name!} public>
+      {members.map((member, i) => (
+        <>
+          <Attribute name="JsonStringEnumMemberName" args={[`"${member.value}"`]} />
+          {"\n"}
+          <EnumMember name={member.name} />
+          {i < members.length - 1 ? ",\n" : ""}
+        </>
+      ))}
+    </CsEnumDeclaration>
+  );
+}
+
 describe("EnumDeclaration", () => {
   it("renders a simple enum", async () => {
     const { Color } = await runner.compile(t.code`
@@ -97,3 +125,188 @@ describe("EnumDeclaration", () => {
     `);
   });
 });
+
+describe("isUnionEnum", () => {
+  it("returns true for extensible union with string base and named variants", async () => {
+    const { ReasoningEffort } = await runner.compile(t.code`
+      union ${t.union("ReasoningEffort")} {
+        string,
+        none: "none",
+        low: "low",
+        medium: "medium",
+        high: "high",
+      }
+    `);
+
+    expect(isUnionEnum(ReasoningEffort)).toBe(true);
+  });
+
+  it("returns true for fixed union with named variants only", async () => {
+    const { Priority } = await runner.compile(t.code`
+      union ${t.union("Priority")} {
+        low: "low",
+        medium: "medium",
+        high: "high",
+      }
+    `);
+
+    expect(isUnionEnum(Priority)).toBe(true);
+  });
+
+  it("returns false for union with non-string variant types", async () => {
+    const { Mixed } = await runner.compile(t.code`
+      model Foo { x: string; }
+      union ${t.union("Mixed")} {
+        Foo,
+        "bar",
+      }
+    `);
+
+    expect(isUnionEnum(Mixed)).toBe(false);
+  });
+});
+
+describe("union-as-enum rendering", () => {
+  it("renders union with unnamed string literals", async () => {
+    const { Priority } = await runner.compile(t.code`
+      union ${t.union("Priority")} {
+        "low",
+        "medium",
+        "high",
+      }
+    `);
+
+    expect(
+      <Wrapper>
+        <UnionEnumDecl union={Priority} />
+      </Wrapper>,
+    ).toRenderTo(`
+      public enum Priority
+      {
+          [JsonStringEnumMemberName("low")]
+          Low,
+          [JsonStringEnumMemberName("medium")]
+          Medium,
+          [JsonStringEnumMemberName("high")]
+          High
+      }
+    `);
+  });
+
+  it("renders union with unnamed string literals and null (null is skipped)", async () => {
+    const { ReasoningEffort } = await runner.compile(t.code`
+      union ${t.union("ReasoningEffort")} {
+        "none",
+        "minimal",
+        "low",
+        "medium",
+        "high",
+        null,
+      }
+    `);
+
+    expect(
+      <Wrapper>
+        <UnionEnumDecl union={ReasoningEffort} />
+      </Wrapper>,
+    ).toRenderTo(`
+      public enum ReasoningEffort
+      {
+          [JsonStringEnumMemberName("none")]
+          None,
+          [JsonStringEnumMemberName("minimal")]
+          Minimal,
+          [JsonStringEnumMemberName("low")]
+          Low,
+          [JsonStringEnumMemberName("medium")]
+          Medium,
+          [JsonStringEnumMemberName("high")]
+          High
+      }
+    `);
+  });
+
+  it("renders union with named variants and null (null is skipped)", async () => {
+    const { ReasoningEffort } = await runner.compile(t.code`
+      union ${t.union("ReasoningEffort")} {
+        none: "none",
+        medium: "medium",
+        high: "high",
+        null,
+      }
+    `);
+
+    expect(
+      <Wrapper>
+        <UnionEnumDecl union={ReasoningEffort} />
+      </Wrapper>,
+    ).toRenderTo(`
+      public enum ReasoningEffort
+      {
+          [JsonStringEnumMemberName("none")]
+          None,
+          [JsonStringEnumMemberName("medium")]
+          Medium,
+          [JsonStringEnumMemberName("high")]
+          High
+      }
+    `);
+  });
+
+  it("renders extensible union with string base, named variants, and null", async () => {
+    const { ReasoningEffort } = await runner.compile(t.code`
+      union ${t.union("ReasoningEffort")} {
+        string,
+        none: "none",
+        medium: "medium",
+        high: "high",
+        null,
+      }
+    `);
+
+    expect(
+      <Wrapper>
+        <UnionEnumDecl union={ReasoningEffort} />
+      </Wrapper>,
+    ).toRenderTo(`
+      public enum ReasoningEffort
+      {
+          [JsonStringEnumMemberName("none")]
+          None,
+          [JsonStringEnumMemberName("medium")]
+          Medium,
+          [JsonStringEnumMemberName("high")]
+          High
+      }
+    `);
+  });
+
+  it("renders union with inline anonymous union of string literals and null", async () => {
+    const { ReasoningEffort } = await runner.compile(t.code`
+      union ${t.union("ReasoningEffort")} {
+        "none" | "minimal" | "low" | "medium" | "high",
+        null,
+      }
+    `);
+
+    expect(
+      <Wrapper>
+        <UnionEnumDecl union={ReasoningEffort} />
+      </Wrapper>,
+    ).toRenderTo(`
+      public enum ReasoningEffort
+      {
+          [JsonStringEnumMemberName("none")]
+          None,
+          [JsonStringEnumMemberName("minimal")]
+          Minimal,
+          [JsonStringEnumMemberName("low")]
+          Low,
+          [JsonStringEnumMemberName("medium")]
+          Medium,
+          [JsonStringEnumMemberName("high")]
+          High
+      }
+    `);
+  });
+});
diff --git a/packages/http-server-csharp/src/components/enums/enums.tsx b/packages/http-server-csharp/src/components/enums/enums.tsx
index 9f3be70673b..8fdcf3d9fa7 100644
--- a/packages/http-server-csharp/src/components/enums/enums.tsx
+++ b/packages/http-server-csharp/src/components/enums/enums.tsx
@@ -133,35 +133,62 @@ export function Enums(props: EnumsProps): Children {
   );
 }
 
+/**
+ * Returns true if an anonymous inline union contains only string literal variants.
+ */
+function isInlineStringLiteralUnion(type: Type): boolean {
+  if (type.kind !== "Union" || type.name) return false;
+  for (const variant of type.variants.values()) {
+    if (variant.type.kind !== "String") return false;
+  }
+  return type.variants.size > 0;
+}
+
 /**
  * Returns true if a named union can be represented as a C# enum.
  * Requires: named union, every named variant has a string value,
- * and optionally one unnamed scalar `string` variant (open/extensible).
+ * and optionally one unnamed scalar `string` variant (open/extensible)
+ * and/or a `null` variant. Also supports inline anonymous unions of
+ * string literals (e.g., `"a" | "b" | "c"` as a single variant).
  */
 export function isUnionEnum(union: Union): boolean {
   if (!union.name) return false;
 
   const variants = Array.from(union.variants.values());
-  let hasNamedStringVariant = false;
+  let hasStringVariant = false;
 
   for (const variant of variants) {
     // Allow a single open string scalar variant (extensible union)
     if (variant.type.kind === "Scalar" && variant.type.name === "string") {
       continue;
     }
+    // Allow null variant (nullable union)
+    if (variant.type.kind === "Intrinsic" && variant.type.name === "null") {
+      continue;
+    }
     // Named variant with a string literal value
     if (variant.type.kind === "String" && variant.name && typeof variant.name === "string") {
-      hasNamedStringVariant = true;
+      hasStringVariant = true;
+      continue;
+    }
+    // Unnamed variant with a string literal value (e.g., union { "low", "medium", "high" })
+    if (variant.type.kind === "String" && typeof variant.name === "symbol") {
+      hasStringVariant = true;
+      continue;
+    }
+    // Inline anonymous union of string literals (e.g., "a" | "b" | "c" as a single variant)
+    if (isInlineStringLiteralUnion(variant.type)) {
+      hasStringVariant = true;
       continue;
     }
     // Any other variant type means it's not a simple enum
     return false;
   }
 
-  return hasNamedStringVariant;
+  return hasStringVariant;
 }
 
-/** Gets the named string variants of a union-as-enum (skipping the open `string` variant). */
+/** Gets the named string variants of a union-as-enum (skipping the open `string` and `null` variants). */
 export function getUnionEnumMembers(
   union: Union,
 ): { name: string; value: string; variant: import("@typespec/compiler").UnionVariant }[] {
@@ -172,7 +199,22 @@ export function getUnionEnumMembers(
   }[] = [];
   for (const variant of union.variants.values()) {
     if (variant.type.kind === "String" && variant.name && typeof variant.name === "string") {
+      // Named variant with explicit key (e.g., none: "none")
       members.push({ name: variant.name, value: variant.type.value, variant });
+    } else if (variant.type.kind === "String" && typeof variant.name === "symbol") {
+      // Unnamed string literal variant (e.g., "none") — derive name from the value
+      members.push({ name: variant.type.value, value: variant.type.value, variant });
+    } else if (isInlineStringLiteralUnion(variant.type)) {
+      // Inline anonymous union of string literals — flatten into individual members
+      for (const innerVariant of (variant.type as Union).variants.values()) {
+        if (innerVariant.type.kind === "String") {
+          members.push({
+            name: innerVariant.type.value,
+            value: innerVariant.type.value,
+            variant: innerVariant,
+          });
+        }
+      }
     }
   }
   return members;