diff --git a/Package.swift b/Package.swift
index 29968d06..25151e5d 100644
--- a/Package.swift
+++ b/Package.swift
@@ -111,6 +111,16 @@ let package = Package(
             swiftSettings: swiftSettings
         ),
 
+        // Test Target for swift-openapi-generator
+        .testTarget(
+            name: "OpenAPIGeneratorTests",
+            dependencies: [
+                "swift-openapi-generator", .product(name: "ArgumentParser", package: "swift-argument-parser"),
+            ],
+            resources: [.copy("Resources")],
+            swiftSettings: swiftSettings
+        ),
+
         // Generator CLI
         .executableTarget(
             name: "swift-openapi-generator",
diff --git a/Sources/_OpenAPIGeneratorCore/Diagnostics.swift b/Sources/_OpenAPIGeneratorCore/Diagnostics.swift
index 1108d298..f85c991d 100644
--- a/Sources/_OpenAPIGeneratorCore/Diagnostics.swift
+++ b/Sources/_OpenAPIGeneratorCore/Diagnostics.swift
@@ -165,7 +165,33 @@ public protocol DiagnosticCollector {
 
     /// Submits a diagnostic to the collector.
     /// - Parameter diagnostic: The diagnostic to submit.
-    func emit(_ diagnostic: Diagnostic)
+    /// - Throws: An error if the implementing type determines that one should be thrown.
+    func emit(_ diagnostic: Diagnostic) throws
+}
+
+/// A type that conforms to the `DiagnosticCollector` protocol.
+///
+/// It receives diagnostics and forwards them to an upstream `DiagnosticCollector`.
+///
+/// If a diagnostic with a severity of `.error` is emitted, this collector will throw the diagnostic as an error.
+public struct ErrorThrowingDiagnosticCollector: DiagnosticCollector {
+    let upstream: any DiagnosticCollector
+
+    /// Initializes a new `ErrorThrowingDiagnosticCollector` with an upstream `DiagnosticCollector`.
+    ///
+    /// The upstream collector is where this collector will forward all received diagnostics.
+    ///
+    /// - Parameter upstream: The `DiagnosticCollector` to which this collector will forward diagnostics.
+    public init(upstream: any DiagnosticCollector) { self.upstream = upstream }
+
+    /// Emits a diagnostic to the collector.
+    ///
+    /// - Parameter diagnostic: The diagnostic to be submitted.
+    /// - Throws: The diagnostic itself if its severity is `.error`.
+    public func emit(_ diagnostic: Diagnostic) throws {
+        try upstream.emit(diagnostic)
+        if diagnostic.severity == .error { throw diagnostic }
+    }
 }
 
 extension DiagnosticCollector {
@@ -180,8 +206,9 @@ extension DiagnosticCollector {
     ///   feature was detected.
     ///   - context: A set of key-value pairs that help the user understand
     ///   where the warning occurred.
-    func emitUnsupported(_ feature: String, foundIn: String, context: [String: String] = [:]) {
-        emit(Diagnostic.unsupported(feature, foundIn: foundIn, context: context))
+    /// - Throws: This method will throw the diagnostic if the severity of the diagnostic is `.error`.
+    func emitUnsupported(_ feature: String, foundIn: String, context: [String: String] = [:]) throws {
+        try emit(Diagnostic.unsupported(feature, foundIn: foundIn, context: context))
     }
 
     /// Emits a diagnostic for an unsupported schema found in the specified
@@ -193,9 +220,10 @@ extension DiagnosticCollector {
     ///   schema was detected.
     ///   - context: A set of key-value pairs that help the user understand
     ///   where the warning occurred.
-    func emitUnsupportedSchema(reason: String, schema: JSONSchema, foundIn: String, context: [String: String] = [:]) {
-        emit(Diagnostic.unsupportedSchema(reason: reason, schema: schema, foundIn: foundIn, context: context))
-    }
+    /// - Throws: This method will throw the diagnostic if the severity of the diagnostic is `.error`.
+    func emitUnsupportedSchema(reason: String, schema: JSONSchema, foundIn: String, context: [String: String] = [:])
+        throws
+    { try emit(Diagnostic.unsupportedSchema(reason: reason, schema: schema, foundIn: foundIn, context: context)) }
 
     /// Emits a diagnostic for an unsupported feature found in the specified
     /// type name.
@@ -206,8 +234,9 @@ extension DiagnosticCollector {
     ///   - foundIn: The type name related to where the issue was detected.
     ///   - context: A set of key-value pairs that help the user understand
     ///   where the warning occurred.
-    func emitUnsupported(_ feature: String, foundIn: TypeName, context: [String: String] = [:]) {
-        emit(Diagnostic.unsupported(feature, foundIn: foundIn.description, context: context))
+    /// - Throws: This method will throw the diagnostic if the severity of the diagnostic is `.error`.
+    func emitUnsupported(_ feature: String, foundIn: TypeName, context: [String: String] = [:]) throws {
+        try emit(Diagnostic.unsupported(feature, foundIn: foundIn.description, context: context))
     }
 
     /// Emits a diagnostic for an unsupported feature found in the specified
@@ -222,9 +251,12 @@ extension DiagnosticCollector {
     ///   feature was detected.
     ///   - context: A set of key-value pairs that help the user understand
     ///   where the warning occurred.
-    func emitUnsupportedIfNotNil(_ test: Any?, _ feature: String, foundIn: String, context: [String: String] = [:]) {
+    /// - Throws: This method will throw the diagnostic if the severity of the diagnostic is `.error`.
+    func emitUnsupportedIfNotNil(_ test: Any?, _ feature: String, foundIn: String, context: [String: String] = [:])
+        throws
+    {
         if test == nil { return }
-        emitUnsupported(feature, foundIn: foundIn, context: context)
+        try emitUnsupported(feature, foundIn: foundIn, context: context)
     }
 
     /// Emits a diagnostic for an unsupported feature found in the specified
@@ -239,14 +271,15 @@ extension DiagnosticCollector {
     ///   feature was detected.
     ///   - context: A set of key-value pairs that help the user understand
     ///   where the warning occurred.
+    /// - Throws: This method will throw the diagnostic if the severity of the diagnostic is `.error`.
     func emitUnsupportedIfNotEmpty<C: Collection>(
         _ test: C?,
         _ feature: String,
         foundIn: String,
         context: [String: String] = [:]
-    ) {
+    ) throws {
         guard let test = test, !test.isEmpty else { return }
-        emitUnsupported(feature, foundIn: foundIn, context: context)
+        try emitUnsupported(feature, foundIn: foundIn, context: context)
     }
 
     /// Emits a diagnostic for an unsupported feature found in the specified
@@ -261,9 +294,11 @@ extension DiagnosticCollector {
     ///   feature was detected.
     ///   - context: A set of key-value pairs that help the user understand
     ///   where the warning occurred.
-    func emitUnsupportedIfTrue(_ test: Bool, _ feature: String, foundIn: String, context: [String: String] = [:]) {
+    /// - Throws: This method will throw the diagnostic if the severity of the diagnostic is `.error`.
+    func emitUnsupportedIfTrue(_ test: Bool, _ feature: String, foundIn: String, context: [String: String] = [:]) throws
+    {
         if !test { return }
-        emitUnsupported(feature, foundIn: foundIn, context: context)
+        try emitUnsupported(feature, foundIn: foundIn, context: context)
     }
 }
 
diff --git a/Sources/_OpenAPIGeneratorCore/DiagnosticsCollectorProvider.swift b/Sources/_OpenAPIGeneratorCore/DiagnosticsCollectorProvider.swift
new file mode 100644
index 00000000..965eebb0
--- /dev/null
+++ b/Sources/_OpenAPIGeneratorCore/DiagnosticsCollectorProvider.swift
@@ -0,0 +1,36 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+/// Prepares a diagnostics collector.
+/// - Parameter outputPath: A file path where to persist the YAML file. If `nil`, diagnostics will be printed to stderr.
+/// - Returns: A tuple containing:
+///   - An instance of `DiagnosticCollector` conforming to `Sendable`.
+///   - A closure to finalize the diagnostics collection
+public func preparedDiagnosticsCollector(outputPath: URL?) -> (any DiagnosticCollector & Sendable, () throws -> Void) {
+    let innerDiagnostics: any DiagnosticCollector & Sendable
+    let finalizeDiagnostics: () throws -> Void
+
+    if let outputPath {
+        let _diagnostics = _YamlFileDiagnosticsCollector(url: outputPath)
+        finalizeDiagnostics = _diagnostics.finalize
+        innerDiagnostics = _diagnostics
+    } else {
+        innerDiagnostics = StdErrPrintingDiagnosticCollector()
+        finalizeDiagnostics = {}
+    }
+    let diagnostics = ErrorThrowingDiagnosticCollector(upstream: innerDiagnostics)
+    return (diagnostics, finalizeDiagnostics)
+}
diff --git a/Sources/_OpenAPIGeneratorCore/GeneratorPipeline.swift b/Sources/_OpenAPIGeneratorCore/GeneratorPipeline.swift
index e38df8d2..8f4a8cf1 100644
--- a/Sources/_OpenAPIGeneratorCore/GeneratorPipeline.swift
+++ b/Sources/_OpenAPIGeneratorCore/GeneratorPipeline.swift
@@ -110,7 +110,7 @@ func makeGeneratorPipeline(
     }
     let validateDoc = { (doc: OpenAPI.Document) -> OpenAPI.Document in
         let validationDiagnostics = try validator(doc, config)
-        for diagnostic in validationDiagnostics { diagnostics.emit(diagnostic) }
+        for diagnostic in validationDiagnostics { try diagnostics.emit(diagnostic) }
         return doc
     }
     return .init(
diff --git a/Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateObjectStruct.swift b/Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateObjectStruct.swift
index a55517b4..4b48e2e1 100644
--- a/Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateObjectStruct.swift
+++ b/Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateObjectStruct.swift
@@ -32,7 +32,6 @@ extension TypesFileTranslator {
         objectContext: JSONSchema.ObjectContext,
         isDeprecated: Bool
     ) throws -> Declaration {
-
         let documentedProperties: [PropertyBlueprint] = try objectContext.properties
             .filter { key, value in
 
@@ -42,7 +41,7 @@ extension TypesFileTranslator {
                 // have a proper definition in the `properties` map are skipped, as they
                 // often imply a typo or a mistake in the document. So emit a diagnostic as well.
                 guard !value.inferred else {
-                    diagnostics.emit(
+                    try diagnostics.emit(
                         .warning(
                             message:
                                 "A property name only appears in the required list, but not in the properties map - this is likely a typo; skipping this property.",
@@ -63,7 +62,7 @@ extension TypesFileTranslator {
                 // allowed in object properties, explicitly filter these out
                 // here.
                 if value.isString && value.formatString == "binary" {
-                    diagnostics.emitUnsupportedSchema(
+                    try diagnostics.emitUnsupportedSchema(
                         reason: "Binary properties in object schemas.",
                         schema: value,
                         foundIn: foundIn
diff --git a/Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateSchema.swift b/Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateSchema.swift
index f7668e4f..e92b5605 100644
--- a/Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateSchema.swift
+++ b/Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateSchema.swift
@@ -76,7 +76,7 @@ extension TypesFileTranslator {
 
         // Attach any warnings from the parsed schema as a diagnostic.
         for warning in schema.warnings {
-            diagnostics.emit(
+            try diagnostics.emit(
                 .warning(
                     message: "Schema warning: \(warning.description)",
                     context: [
diff --git a/Sources/_OpenAPIGeneratorCore/Translator/Content/ContentInspector.swift b/Sources/_OpenAPIGeneratorCore/Translator/Content/ContentInspector.swift
index b8df47b5..f78f5e9f 100644
--- a/Sources/_OpenAPIGeneratorCore/Translator/Content/ContentInspector.swift
+++ b/Sources/_OpenAPIGeneratorCore/Translator/Content/ContentInspector.swift
@@ -123,7 +123,7 @@ extension FileTranslator {
         -> SchemaContent?
     {
         guard !map.isEmpty else { return nil }
-        if map.count > 1 { diagnostics.emitUnsupported("Multiple content types", foundIn: foundIn) }
+        if map.count > 1 { try diagnostics.emitUnsupported("Multiple content types", foundIn: foundIn) }
         let mapWithContentTypes = try map.map { key, content in try (type: key.asGeneratorContentType, value: content) }
 
         let chosenContent: (type: ContentType, schema: SchemaContent, content: OpenAPI.Content)?
@@ -137,7 +137,7 @@ extension FileTranslator {
                 contentValue
             )
         } else {
-            diagnostics.emitUnsupported("Unsupported content", foundIn: foundIn)
+            try diagnostics.emitUnsupported("Unsupported content", foundIn: foundIn)
             chosenContent = nil
         }
         if let chosenContent {
@@ -145,7 +145,7 @@ extension FileTranslator {
             if contentType.lowercasedType == "multipart"
                 || contentType.lowercasedTypeAndSubtype.contains("application/x-www-form-urlencoded")
             {
-                diagnostics.emitUnsupportedIfNotNil(
+                try diagnostics.emitUnsupportedIfNotNil(
                     chosenContent.content.encoding,
                     "Custom encoding for multipart/formEncoded content",
                     foundIn: "\(foundIn), content \(contentType.originallyCasedTypeAndSubtype)"
@@ -181,7 +181,7 @@ extension FileTranslator {
     ) throws -> SchemaContent? {
         let contentType = try contentKey.asGeneratorContentType
         if contentType.lowercasedTypeAndSubtype.contains("application/x-www-form-urlencoded") {
-            diagnostics.emitUnsupportedIfNotNil(
+            try diagnostics.emitUnsupportedIfNotNil(
                 contentValue.encoding,
                 "Custom encoding for formEncoded content",
                 foundIn: "\(foundIn), content \(contentType.originallyCasedTypeAndSubtype)"
@@ -191,7 +191,7 @@ extension FileTranslator {
         if contentType.isUrlEncodedForm { return .init(contentType: contentType, schema: contentValue.schema) }
         if contentType.isMultipart {
             guard isRequired else {
-                diagnostics.emit(
+                try diagnostics.emit(
                     .warning(
                         message:
                             "Multipart request bodies must always be required, but found an optional one - skipping. Mark as `required: true` to get this body generated.",
@@ -205,7 +205,7 @@ extension FileTranslator {
         if !excludeBinary, contentType.isBinary {
             return .init(contentType: contentType, schema: .b(.string(contentEncoding: .binary)))
         }
-        diagnostics.emitUnsupported("Unsupported content", foundIn: foundIn)
+        try diagnostics.emitUnsupported("Unsupported content", foundIn: foundIn)
         return nil
     }
 }
diff --git a/Sources/_OpenAPIGeneratorCore/Translator/Multipart/MultipartContentInspector.swift b/Sources/_OpenAPIGeneratorCore/Translator/Multipart/MultipartContentInspector.swift
index b9edc3b8..1c27dbd9 100644
--- a/Sources/_OpenAPIGeneratorCore/Translator/Multipart/MultipartContentInspector.swift
+++ b/Sources/_OpenAPIGeneratorCore/Translator/Multipart/MultipartContentInspector.swift
@@ -312,7 +312,7 @@ extension FileTranslator {
         }
         let contentType = finalContentTypeSource.contentType
         if finalContentTypeSource.contentType.isMultipart {
-            diagnostics.emitUnsupported("Multipart part cannot nest another multipart content.", foundIn: foundIn)
+            try diagnostics.emitUnsupported("Multipart part cannot nest another multipart content.", foundIn: foundIn)
             return nil
         }
         let info = MultipartPartInfo(repetition: repetitionKind, contentTypeSource: finalContentTypeSource)
diff --git a/Sources/_OpenAPIGeneratorCore/Translator/Parameters/TypedParameter.swift b/Sources/_OpenAPIGeneratorCore/Translator/Parameters/TypedParameter.swift
index d84f9d3d..66a0ce26 100644
--- a/Sources/_OpenAPIGeneratorCore/Translator/Parameters/TypedParameter.swift
+++ b/Sources/_OpenAPIGeneratorCore/Translator/Parameters/TypedParameter.swift
@@ -140,7 +140,7 @@ extension FileTranslator {
             switch location {
             case .query:
                 guard case .form = style else {
-                    diagnostics.emitUnsupported(
+                    try diagnostics.emitUnsupported(
                         "Query params of style \(style.rawValue), explode: \(explode)",
                         foundIn: foundIn
                     )
@@ -148,14 +148,14 @@ extension FileTranslator {
                 }
             case .header, .path:
                 guard case .simple = style else {
-                    diagnostics.emitUnsupported(
+                    try diagnostics.emitUnsupported(
                         "\(location.rawValue) params of style \(style.rawValue), explode: \(explode)",
                         foundIn: foundIn
                     )
                     return nil
                 }
             case .cookie:
-                diagnostics.emitUnsupported("Cookie params", foundIn: foundIn)
+                try diagnostics.emitUnsupported("Cookie params", foundIn: foundIn)
                 return nil
             }
 
diff --git a/Sources/_OpenAPIGeneratorCore/Translator/Parameters/translateParameter.swift b/Sources/_OpenAPIGeneratorCore/Translator/Parameters/translateParameter.swift
index c422bec0..0176a8cd 100644
--- a/Sources/_OpenAPIGeneratorCore/Translator/Parameters/translateParameter.swift
+++ b/Sources/_OpenAPIGeneratorCore/Translator/Parameters/translateParameter.swift
@@ -114,7 +114,7 @@ extension ClientFileTranslator {
             containerExpr = .identifierPattern(requestVariableName)
             supportsStyleAndExplode = true
         default:
-            diagnostics.emitUnsupported(
+            try diagnostics.emitUnsupported(
                 "Parameter of type \(parameter.location.rawValue)",
                 foundIn: parameter.description
             )
@@ -198,7 +198,7 @@ extension ServerFileTranslator {
                     ])
             )
         default:
-            diagnostics.emitUnsupported(
+            try diagnostics.emitUnsupported(
                 "Parameter of type \(parameter.location)",
                 foundIn: "\(typedParameter.description)"
             )
diff --git a/Sources/_OpenAPIGeneratorCore/Translator/TypeAssignment/isSchemaSupported.swift b/Sources/_OpenAPIGeneratorCore/Translator/TypeAssignment/isSchemaSupported.swift
index e4735b0b..49fb4466 100644
--- a/Sources/_OpenAPIGeneratorCore/Translator/TypeAssignment/isSchemaSupported.swift
+++ b/Sources/_OpenAPIGeneratorCore/Translator/TypeAssignment/isSchemaSupported.swift
@@ -64,7 +64,7 @@ extension FileTranslator {
         switch try isSchemaSupported(schema, referenceStack: &referenceStack) {
         case .supported: return true
         case .unsupported(reason: let reason, schema: let schema):
-            diagnostics.emitUnsupportedSchema(reason: reason.description, schema: schema, foundIn: foundIn)
+            try diagnostics.emitUnsupportedSchema(reason: reason.description, schema: schema, foundIn: foundIn)
             return false
         }
     }
@@ -82,7 +82,7 @@ extension FileTranslator {
         switch try isSchemaSupported(schema, referenceStack: &referenceStack) {
         case .supported: return true
         case .unsupported(reason: let reason, schema: let schema):
-            diagnostics.emitUnsupportedSchema(reason: reason.description, schema: schema, foundIn: foundIn)
+            try diagnostics.emitUnsupportedSchema(reason: reason.description, schema: schema, foundIn: foundIn)
             return false
         }
     }
@@ -100,7 +100,7 @@ extension FileTranslator {
         switch try isObjectOrRefToObjectSchemaAndSupported(schema, referenceStack: &referenceStack) {
         case .supported: return true
         case .unsupported(reason: let reason, schema: let schema):
-            diagnostics.emitUnsupportedSchema(reason: reason.description, schema: schema, foundIn: foundIn)
+            try diagnostics.emitUnsupportedSchema(reason: reason.description, schema: schema, foundIn: foundIn)
             return false
         }
     }
diff --git a/Sources/_OpenAPIGeneratorCore/Translator/TypesTranslator/translateBoxedTypes.swift b/Sources/_OpenAPIGeneratorCore/Translator/TypesTranslator/translateBoxedTypes.swift
index 02921018..ce2da20d 100644
--- a/Sources/_OpenAPIGeneratorCore/Translator/TypesTranslator/translateBoxedTypes.swift
+++ b/Sources/_OpenAPIGeneratorCore/Translator/TypesTranslator/translateBoxedTypes.swift
@@ -33,7 +33,7 @@ extension TypesFileTranslator {
         var decls = decls
         for (index, decl) in decls.enumerated() {
             guard let name = decl.name, boxedNames.contains(name) else { continue }
-            diagnostics.emit(
+            try diagnostics.emit(
                 .note(
                     message: "Detected a recursive type; it will be boxed to break the reference cycle.",
                     context: ["name": name]
diff --git a/Sources/swift-openapi-generator/YamlFileDiagnosticsCollector.swift b/Sources/_OpenAPIGeneratorCore/YamlFileDiagnosticsCollector.swift
similarity index 98%
rename from Sources/swift-openapi-generator/YamlFileDiagnosticsCollector.swift
rename to Sources/_OpenAPIGeneratorCore/YamlFileDiagnosticsCollector.swift
index 5035d7cb..17e65726 100644
--- a/Sources/swift-openapi-generator/YamlFileDiagnosticsCollector.swift
+++ b/Sources/_OpenAPIGeneratorCore/YamlFileDiagnosticsCollector.swift
@@ -13,7 +13,6 @@
 //===----------------------------------------------------------------------===//
 import Foundation
 import Yams
-import _OpenAPIGeneratorCore
 
 struct _DiagnosticsYamlFileContent: Encodable {
     var uniqueMessages: [String]
diff --git a/Sources/swift-openapi-generator/GenerateOptions+runGenerator.swift b/Sources/swift-openapi-generator/GenerateOptions+runGenerator.swift
index 06b02d09..9f3fe83b 100644
--- a/Sources/swift-openapi-generator/GenerateOptions+runGenerator.swift
+++ b/Sources/swift-openapi-generator/GenerateOptions+runGenerator.swift
@@ -42,17 +42,7 @@ extension _GenerateOptions {
                 featureFlags: resolvedFeatureFlags
             )
         }
-        let diagnostics: any DiagnosticCollector & Sendable
-        let finalizeDiagnostics: () throws -> Void
-        if let diagnosticsOutputPath {
-            let _diagnostics = _YamlFileDiagnosticsCollector(url: diagnosticsOutputPath)
-            finalizeDiagnostics = _diagnostics.finalize
-            diagnostics = _diagnostics
-        } else {
-            diagnostics = StdErrPrintingDiagnosticCollector()
-            finalizeDiagnostics = {}
-        }
-
+        let (diagnostics, finalizeDiagnostics) = preparedDiagnosticsCollector(outputPath: diagnosticsOutputPath)
         let doc = self.docPath
         print(
             """
@@ -83,7 +73,7 @@ extension _GenerateOptions {
             try finalizeDiagnostics()
         } catch let error as Diagnostic {
             // Emit our nice Diagnostics message instead of relying on ArgumentParser output.
-            diagnostics.emit(error)
+            try diagnostics.emit(error)
             try finalizeDiagnostics()
             throw ExitCode.failure
         } catch {
diff --git a/Tests/OpenAPIGeneratorCoreTests/Test_DiagnosticsCollectorProvider.swift b/Tests/OpenAPIGeneratorCoreTests/Test_DiagnosticsCollectorProvider.swift
new file mode 100644
index 00000000..d66169cb
--- /dev/null
+++ b/Tests/OpenAPIGeneratorCoreTests/Test_DiagnosticsCollectorProvider.swift
@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+@testable import _OpenAPIGeneratorCore
+
+final class Test_DiagnosticsCollectorProvider: XCTestCase {
+
+    func testPreparedDiagnosticsCollectorWithOutputPath() throws {
+        let outputPath = URL(fileURLWithPath: "/path/to/diagnostics.yaml")
+        let (diagnostics, _) = preparedDiagnosticsCollector(outputPath: outputPath)
+        XCTAssertTrue(diagnostics is ErrorThrowingDiagnosticCollector)
+
+        if let errorThrowingCollector = diagnostics as? ErrorThrowingDiagnosticCollector {
+            XCTAssertTrue(errorThrowingCollector.upstream is _YamlFileDiagnosticsCollector)
+        } else {
+            XCTFail("Expected diagnostics to be `ErrorThrowingDiagnosticCollector`")
+        }
+    }
+
+    func testPreparedDiagnosticsCollectorWithoutOutputPath() throws {
+        let outputPath: URL? = nil
+        let (diagnostics, _) = preparedDiagnosticsCollector(outputPath: outputPath)
+        XCTAssertTrue(diagnostics is ErrorThrowingDiagnosticCollector)
+        if let errorThrowingCollector = diagnostics as? ErrorThrowingDiagnosticCollector {
+            XCTAssertTrue(errorThrowingCollector.upstream is StdErrPrintingDiagnosticCollector)
+        } else {
+            XCTFail("Expected diagnostics to be `ErrorThrowingDiagnosticCollector`")
+        }
+    }
+}
diff --git a/Tests/OpenAPIGeneratorReferenceTests/FileBasedReferenceTests.swift b/Tests/OpenAPIGeneratorReferenceTests/FileBasedReferenceTests.swift
index 1e9e6786..75b8be78 100644
--- a/Tests/OpenAPIGeneratorReferenceTests/FileBasedReferenceTests.swift
+++ b/Tests/OpenAPIGeneratorReferenceTests/FileBasedReferenceTests.swift
@@ -31,7 +31,7 @@ extension TestConfig {
 }
 
 /// Tests that the generator produces Swift files that match a reference.
-class FileBasedReferenceTests: XCTestCase {
+final class FileBasedReferenceTests: XCTestCase {
 
     /// Setup method called before the invocation of each test method in the class.
     override func setUp() {
diff --git a/Tests/OpenAPIGeneratorTests/Resources/Docs/malformed-openapi.yaml b/Tests/OpenAPIGeneratorTests/Resources/Docs/malformed-openapi.yaml
new file mode 100644
index 00000000..a31882d7
--- /dev/null
+++ b/Tests/OpenAPIGeneratorTests/Resources/Docs/malformed-openapi.yaml
@@ -0,0 +1,4 @@
+# openapi: '3.1.0' -> Missing openapi version
+info:
+  title: MalformedDocument
+  version: 1.0.0
diff --git a/Tests/OpenAPIGeneratorTests/Resources/Docs/openapi-generator-config.yaml b/Tests/OpenAPIGeneratorTests/Resources/Docs/openapi-generator-config.yaml
new file mode 100644
index 00000000..f68d9c85
--- /dev/null
+++ b/Tests/OpenAPIGeneratorTests/Resources/Docs/openapi-generator-config.yaml
@@ -0,0 +1,4 @@
+generate:
+  - types
+  - server
+accessModifier: internal
diff --git a/Tests/OpenAPIGeneratorTests/Test_GenerateOptions.swift b/Tests/OpenAPIGeneratorTests/Test_GenerateOptions.swift
new file mode 100644
index 00000000..6d73da7e
--- /dev/null
+++ b/Tests/OpenAPIGeneratorTests/Test_GenerateOptions.swift
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+import XCTest
+import _OpenAPIGeneratorCore
+import OpenAPIKit
+import ArgumentParser
+@testable import swift_openapi_generator
+
+final class Test_GenerateOptions: XCTestCase {
+
+    var resourcesDirectory: URL! = nil
+
+    /// Setup method called before the invocation of each test method in the class.
+    override func setUpWithError() throws {
+        resourcesDirectory = try XCTUnwrap(
+            Bundle.module.url(forResource: "Resources", withExtension: nil),
+            "Could not find reference test resources directory."
+        )
+    }
+
+    func testRunGeneratorThrowsErrorDiagnostic() async throws {
+        let outputDirectory = URL(fileURLWithPath: "/invalid/path")
+        let docsDirectory = resourcesDirectory.appendingPathComponent("Docs")
+        let docPath = docsDirectory.appendingPathComponent("malformed-openapi.yaml")
+        let configPath = docsDirectory.appendingPathComponent("openapi-generator-config.yaml")
+
+        let arguments = [docPath.path, "--config", configPath.path]
+        let generator = try _GenerateOptions.parse(arguments)
+
+        do {
+            try await generator.runGenerator(outputDirectory: outputDirectory, pluginSource: .build, isDryRun: false)
+            XCTFail("Expected to throw an error, but it did not throw")
+        } catch let diagnostic as Diagnostic {
+            XCTAssertEqual(diagnostic.severity, .error, "Expected diagnostic severity to be `.error`")
+        } catch { XCTFail("Expected to throw a Diagnostic `.error`, but threw a different error: \(error)") }
+    }
+}