diff --git a/.chronus/changes/http-specs-union-extends-2026-09-14.md b/.chronus/changes/http-specs-union-extends-2026-09-14.md
new file mode 100644
index 00000000000..cd27db3e8fc
--- /dev/null
+++ b/.chronus/changes/http-specs-union-extends-2026-09-14.md
@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/http-specs"
+---
+
+Add a round-trip scenario for unions constrained to common base types.
diff --git a/.chronus/changes/spector-scenario-tspconfig-2026-09-14.md b/.chronus/changes/spector-scenario-tspconfig-2026-09-14.md
new file mode 100644
index 00000000000..11521f4492e
--- /dev/null
+++ b/.chronus/changes/spector-scenario-tspconfig-2026-09-14.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/spector"
+---
+
+Honor project `tspconfig.yaml` settings when validating scenarios and mock APIs.
diff --git a/packages/http-specs/spec-summary.md b/packages/http-specs/spec-summary.md
index 9c7f6485dc6..e5d42c12dba 100644
--- a/packages/http-specs/spec-summary.md
+++ b/packages/http-specs/spec-summary.md
@@ -9422,6 +9422,85 @@ Expected request to send body:
 }
 ```
 
+### Type_Union_Extends_Explicit_roundTrip
+
+- Endpoint: `put /type/union/extends/explicit`
+
+Send and receive a union whose variants explicitly extend its base type.
+
+Expected request and response body:
+
+```json
+[
+  {
+    "name": "mittens",
+    "toy": "ball"
+  },
+  {
+    "name": "rex",
+    "food": "bones"
+  }
+]
+```
+
+### Type_Union_Extends_Multiple_roundTrip
+
+- Endpoint: `put /type/union/extends/multiple`
+
+Send and receive the same variants through unions constrained to different base types.
+
+Expected request and response body:
+
+```json
+{
+  "byName": [
+    {
+      "name": "mittens",
+      "food": "fish",
+      "toy": "ball"
+    },
+    {
+      "name": "rex",
+      "food": "bones",
+      "bark": true
+    }
+  ],
+  "byFood": [
+    {
+      "name": "mittens",
+      "food": "fish",
+      "toy": "ball"
+    },
+    {
+      "name": "rex",
+      "food": "bones",
+      "bark": true
+    }
+  ]
+}
+```
+
+### Type_Union_Extends_Structural_roundTrip
+
+- Endpoint: `put /type/union/extends/structural`
+
+Send and receive a union whose variants structurally satisfy its base type.
+
+Expected request and response body:
+
+```json
+[
+  {
+    "name": "mittens",
+    "toy": "ball"
+  },
+  {
+    "name": "rex",
+    "food": "bones"
+  }
+]
+```
+
 ### Type_Union_FloatsOnly_get
 
 - Endpoint: `get /type/union/floats-only`
diff --git a/packages/http-specs/specs/type/union/extends/main.tsp b/packages/http-specs/specs/type/union/extends/main.tsp
new file mode 100644
index 00000000000..f3b705ca8c6
--- /dev/null
+++ b/packages/http-specs/specs/type/union/extends/main.tsp
@@ -0,0 +1,170 @@
+import "@typespec/http";
+import "@typespec/spector";
+
+using Http;
+using Spector;
+
+/**
+ * Describe unions constrained to a common base type.
+ */
+@scenarioService("/type/union/extends")
+namespace Type.Union.Extends;
+
+model StructuralPetBase {
+  name: string;
+}
+
+model StructuralCat {
+  name: string;
+  toy: string;
+}
+
+model StructuralDog {
+  name: string;
+  food: string;
+}
+
+union StructuralPet extends StructuralPetBase {
+  cat: StructuralCat,
+  dog: StructuralDog,
+}
+
+model ExplicitPetBase {
+  name: string;
+}
+
+model ExplicitCat extends ExplicitPetBase {
+  toy: string;
+}
+
+model ExplicitDog extends ExplicitPetBase {
+  food: string;
+}
+
+union ExplicitPet extends ExplicitPetBase {
+  cat: ExplicitCat,
+  dog: ExplicitDog,
+}
+
+model MultipleNameBase {
+  name: string;
+}
+
+model MultipleFoodBase {
+  food: string;
+}
+
+model MultipleCat {
+  name: string;
+  food: string;
+  toy: string;
+}
+
+model MultipleDog {
+  name: string;
+  food: string;
+  bark: boolean;
+}
+
+union MultiplePetWithName extends MultipleNameBase {
+  cat: MultipleCat,
+  dog: MultipleDog,
+}
+
+union MultiplePetWithFood extends MultipleFoodBase {
+  cat: MultipleCat,
+  dog: MultipleDog,
+}
+
+model MultipleCases {
+  byName: MultiplePetWithName[];
+  byFood: MultiplePetWithFood[];
+}
+
+@route("/structural")
+interface Structural {
+  @scenario
+  @scenarioDoc("""
+    Send and receive a union whose variants structurally satisfy its base type.
+    
+    Expected request and response body:
+    ```json
+    [
+      {
+        "name": "mittens",
+        "toy": "ball"
+      },
+      {
+        "name": "rex",
+        "food": "bones"
+      }
+    ]
+    ```
+    """)
+  @put
+  roundTrip(@body input: StructuralPet[]): StructuralPet[];
+}
+
+@route("/explicit")
+interface Explicit {
+  @scenario
+  @scenarioDoc("""
+    Send and receive a union whose variants explicitly extend its base type.
+    
+    Expected request and response body:
+    ```json
+    [
+      {
+        "name": "mittens",
+        "toy": "ball"
+      },
+      {
+        "name": "rex",
+        "food": "bones"
+      }
+    ]
+    ```
+    """)
+  @put
+  roundTrip(@body input: ExplicitPet[]): ExplicitPet[];
+}
+
+@route("/multiple")
+interface Multiple {
+  @scenario
+  @scenarioDoc("""
+    Send and receive the same variants through unions constrained to different base types.
+    
+    Expected request and response body:
+    ```json
+    {
+      "byName": [
+        {
+          "name": "mittens",
+          "food": "fish",
+          "toy": "ball"
+        },
+        {
+          "name": "rex",
+          "food": "bones",
+          "bark": true
+        }
+      ],
+      "byFood": [
+        {
+          "name": "mittens",
+          "food": "fish",
+          "toy": "ball"
+        },
+        {
+          "name": "rex",
+          "food": "bones",
+          "bark": true
+        }
+      ]
+    }
+    ```
+    """)
+  @put
+  roundTrip(@body input: MultipleCases): MultipleCases;
+}
diff --git a/packages/http-specs/specs/type/union/extends/mockapi.ts b/packages/http-specs/specs/type/union/extends/mockapi.ts
new file mode 100644
index 00000000000..ae35a6aaf9e
--- /dev/null
+++ b/packages/http-specs/specs/type/union/extends/mockapi.ts
@@ -0,0 +1,81 @@
+import type { ScenarioMockApi } from "@typespec/spec-api";
+import { json, passOnSuccess } from "@typespec/spec-api";
+
+export const Scenarios: Record<string, ScenarioMockApi> = {};
+
+const pets = [
+  {
+    name: "mittens",
+    toy: "ball",
+  },
+  {
+    name: "rex",
+    food: "bones",
+  },
+];
+
+const multiple = {
+  byName: [
+    {
+      name: "mittens",
+      food: "fish",
+      toy: "ball",
+    },
+    {
+      name: "rex",
+      food: "bones",
+      bark: true,
+    },
+  ],
+  byFood: [
+    {
+      name: "mittens",
+      food: "fish",
+      toy: "ball",
+    },
+    {
+      name: "rex",
+      food: "bones",
+      bark: true,
+    },
+  ],
+};
+
+Scenarios.Type_Union_Extends_Structural_roundTrip = passOnSuccess({
+  uri: "/type/union/extends/structural",
+  method: "put",
+  request: {
+    body: json(pets),
+  },
+  response: {
+    status: 200,
+    body: json(pets),
+  },
+  kind: "MockApiDefinition",
+});
+
+Scenarios.Type_Union_Extends_Explicit_roundTrip = passOnSuccess({
+  uri: "/type/union/extends/explicit",
+  method: "put",
+  request: {
+    body: json(pets),
+  },
+  response: {
+    status: 200,
+    body: json(pets),
+  },
+  kind: "MockApiDefinition",
+});
+
+Scenarios.Type_Union_Extends_Multiple_roundTrip = passOnSuccess({
+  uri: "/type/union/extends/multiple",
+  method: "put",
+  request: {
+    body: json(multiple),
+  },
+  response: {
+    status: 200,
+    body: json(multiple),
+  },
+  kind: "MockApiDefinition",
+});
diff --git a/packages/http-specs/tspconfig.yaml b/packages/http-specs/tspconfig.yaml
index e69de29bb2d..c4a50e9d0f1 100644
--- a/packages/http-specs/tspconfig.yaml
+++ b/packages/http-specs/tspconfig.yaml
@@ -0,0 +1,3 @@
+kind: project
+features:
+  - union-extends
diff --git a/packages/spector/src/actions/validate-mock-apis.ts b/packages/spector/src/actions/validate-mock-apis.ts
index b0ded1d0952..c85259a33ae 100644
--- a/packages/spector/src/actions/validate-mock-apis.ts
+++ b/packages/spector/src/actions/validate-mock-apis.ts
@@ -35,13 +35,21 @@ export async function validateMockApis({
   const diagnostics = createDiagnosticReporter();
   for (const { name, specFilePath } of scenarioFiles) {
     logger.debug(`Found scenario "${specFilePath}"`);
+    const [compilerOptions, configDiagnostics] = await specCompiler.resolveCompilerOptions(
+      specCompiler.NodeHost,
+      {
+        cwd: process.cwd(),
+        entrypoint: specFilePath,
+      },
+    );
     const program = await specCompiler.compile(specCompiler.NodeHost, specFilePath, {
+      ...compilerOptions,
       noEmit: true,
       warningAsError: true,
     });
 
     // Workaround https://github.com/Azure/cadl-azure/issues/2458
-    const programDiagnostics = program.diagnostics.filter(
+    const programDiagnostics = [...configDiagnostics, ...program.diagnostics].filter(
       (d) =>
         !(
           d.code === "@azure-tools/typespec-azure-core/casing-style" &&
diff --git a/packages/spector/src/scenarios-resolver.ts b/packages/spector/src/scenarios-resolver.ts
index 8662808f84c..a84539b4ea0 100644
--- a/packages/spector/src/scenarios-resolver.ts
+++ b/packages/spector/src/scenarios-resolver.ts
@@ -69,14 +69,22 @@ export async function loadScenarios(
 
   for (const { name, specFilePath } of scenarioFiles) {
     logger.debug(`Found scenario "${specFilePath}"`);
+    const [compilerOptions, configDiagnostics] = await typespecCompiler.resolveCompilerOptions(
+      typespecCompiler.NodeHost,
+      {
+        cwd: process.cwd(),
+        entrypoint: specFilePath,
+      },
+    );
     const program = await typespecCompiler.compile(typespecCompiler.NodeHost, specFilePath, {
+      ...compilerOptions,
       additionalImports: ["@typespec/spector"],
       noEmit: true,
       warningAsError: true,
     });
 
     // Workaround https://github.com/Azure/cadl-azure/issues/2458
-    const programDiagnostics = program.diagnostics.filter(
+    const programDiagnostics = [...configDiagnostics, ...program.diagnostics].filter(
       (d) =>
         !(
           d.code === "@azure-tools/typespec-azure-core/casing-style" &&