diff --git a/FuzzTesting/.gitignore b/FuzzTesting/.gitignore
new file mode 100644
index 00000000..bb39a0ba
--- /dev/null
+++ b/FuzzTesting/.gitignore
@@ -0,0 +1,2 @@
+.build/
+Package.resolved
diff --git a/FuzzTesting/FuzzCorpus/FuzzAudioTranscriptionStreamResultDecoder/seed-delta.json b/FuzzTesting/FuzzCorpus/FuzzAudioTranscriptionStreamResultDecoder/seed-delta.json
new file mode 100644
index 00000000..1937c5e0
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzAudioTranscriptionStreamResultDecoder/seed-delta.json
@@ -0,0 +1 @@
+{"type":"transcript.text.delta","delta":"Hello"}
diff --git a/FuzzTesting/FuzzCorpus/FuzzAudioTranscriptionStreamResultDecoder/seed-done-with-logprobs.json b/FuzzTesting/FuzzCorpus/FuzzAudioTranscriptionStreamResultDecoder/seed-done-with-logprobs.json
new file mode 100644
index 00000000..93956436
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzAudioTranscriptionStreamResultDecoder/seed-done-with-logprobs.json
@@ -0,0 +1 @@
+{"type":"transcript.text.done","text":"Hello world","logprobs":[{"token":"Hello","bytes":[72,101,108,108,111],"logprob":-0.5},{"token":" world","bytes":[32,119,111,114,108,100],"logprob":-0.7}]}
diff --git a/FuzzTesting/FuzzCorpus/FuzzChatResultDecoder/seed-basic.json b/FuzzTesting/FuzzCorpus/FuzzChatResultDecoder/seed-basic.json
new file mode 100644
index 00000000..94d305e1
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzChatResultDecoder/seed-basic.json
@@ -0,0 +1 @@
+{"id":"chatcmpl-123","object":"chat.completion","created":1677652288,"model":"gpt-4","choices":[{"index":0,"message":{"role":"assistant","content":"Hello, world!","annotations":[],"tool_calls":[]},"finish_reason":"stop"}],"usage":{"prompt_tokens":9,"completion_tokens":12,"total_tokens":21},"system_fingerprint":"fp_fc9f1d7035"}
diff --git a/FuzzTesting/FuzzCorpus/FuzzChatResultDecoder/seed-with-tool-calls.json b/FuzzTesting/FuzzCorpus/FuzzChatResultDecoder/seed-with-tool-calls.json
new file mode 100644
index 00000000..e4fd1b18
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzChatResultDecoder/seed-with-tool-calls.json
@@ -0,0 +1 @@
+{"id":"chatcmpl-xyz","object":"chat.completion","created":1677652288,"model":"gpt-4o","choices":[{"index":0,"message":{"role":"assistant","content":null,"refusal":null,"tool_calls":[{"id":"call_1","type":"function","function":{"name":"get_weather","arguments":"{\"city\":\"Kyiv\"}"}}]},"finish_reason":"tool_calls"}],"usage":{"prompt_tokens":80,"completion_tokens":24,"total_tokens":104}}
diff --git a/FuzzTesting/FuzzCorpus/FuzzChatStreamResultDecoder/seed-content-delta.json b/FuzzTesting/FuzzCorpus/FuzzChatStreamResultDecoder/seed-content-delta.json
new file mode 100644
index 00000000..c2450989
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzChatStreamResultDecoder/seed-content-delta.json
@@ -0,0 +1 @@
+{"id":"chatcmpl-stream-1","object":"chat.completion.chunk","created":1738577084,"model":"gpt-4o","choices":[{"index":0,"delta":{"role":"assistant","content":"Hi"},"logprobs":null,"finish_reason":null}]}
diff --git a/FuzzTesting/FuzzCorpus/FuzzChatStreamResultDecoder/seed-finish-stop.json b/FuzzTesting/FuzzCorpus/FuzzChatStreamResultDecoder/seed-finish-stop.json
new file mode 100644
index 00000000..6d534791
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzChatStreamResultDecoder/seed-finish-stop.json
@@ -0,0 +1 @@
+{"id":"chatcmpl-stream-3","object":"chat.completion.chunk","created":1738577084,"model":"gpt-4o","choices":[{"index":0,"delta":{},"logprobs":null,"finish_reason":"stop"}]}
diff --git a/FuzzTesting/FuzzCorpus/FuzzChatStreamResultDecoder/seed-tool-call-delta.json b/FuzzTesting/FuzzCorpus/FuzzChatStreamResultDecoder/seed-tool-call-delta.json
new file mode 100644
index 00000000..9273d201
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzChatStreamResultDecoder/seed-tool-call-delta.json
@@ -0,0 +1 @@
+{"id":"chatcmpl-stream-2","object":"chat.completion.chunk","created":1738577084,"model":"gpt-4o","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"call_1","type":"function","function":{"name":"get_weather","arguments":"{\""}}]},"finish_reason":null}]}
diff --git a/FuzzTesting/FuzzCorpus/FuzzResponseObjectDecoder/seed-completed.json b/FuzzTesting/FuzzCorpus/FuzzResponseObjectDecoder/seed-completed.json
new file mode 100644
index 00000000..f1dc4f25
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzResponseObjectDecoder/seed-completed.json
@@ -0,0 +1 @@
+{"id":"resp_abc","object":"response","created_at":1700000000,"status":"completed","model":"gpt-4o","output":[{"id":"msg_1","type":"message","role":"assistant","status":"completed","content":[{"type":"output_text","text":"Hello","annotations":[]}]}],"parallel_tool_calls":true,"tool_choice":"auto","tools":[],"usage":{"input_tokens":10,"output_tokens":5,"total_tokens":15}}
diff --git a/FuzzTesting/FuzzCorpus/FuzzResponseStreamEventDecoder/seed-function-call-args-done.json b/FuzzTesting/FuzzCorpus/FuzzResponseStreamEventDecoder/seed-function-call-args-done.json
new file mode 100644
index 00000000..0dcaff83
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzResponseStreamEventDecoder/seed-function-call-args-done.json
@@ -0,0 +1 @@
+{"type":"response.function_call_arguments.done","item_id":"item-abc","name":"get_weather","output_index":1,"arguments":"{ \"arg\": 123 }","sequence_number":1}
diff --git a/FuzzTesting/FuzzCorpus/FuzzResponseStreamEventDecoder/seed-mcp-call-arguments-done.json b/FuzzTesting/FuzzCorpus/FuzzResponseStreamEventDecoder/seed-mcp-call-arguments-done.json
new file mode 100644
index 00000000..338e5d4f
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzResponseStreamEventDecoder/seed-mcp-call-arguments-done.json
@@ -0,0 +1 @@
+{"type":"response.mcp_call_arguments.done","item_id":"mcp_1","output_index":0,"arguments":"{}","sequence_number":3}
diff --git a/FuzzTesting/FuzzCorpus/FuzzResponseStreamEventDecoder/seed-output-text-delta.json b/FuzzTesting/FuzzCorpus/FuzzResponseStreamEventDecoder/seed-output-text-delta.json
new file mode 100644
index 00000000..9439a0ac
--- /dev/null
+++ b/FuzzTesting/FuzzCorpus/FuzzResponseStreamEventDecoder/seed-output-text-delta.json
@@ -0,0 +1 @@
+{"type":"response.output_text.delta","item_id":"msg_1","output_index":0,"content_index":0,"delta":"Hi","sequence_number":1}
diff --git a/FuzzTesting/Package.swift b/FuzzTesting/Package.swift
new file mode 100644
index 00000000..c0b5f86e
--- /dev/null
+++ b/FuzzTesting/Package.swift
@@ -0,0 +1,47 @@
+// swift-tools-version: 5.10
+//
+// FuzzTesting/Package.swift
+//
+// A separate SwiftPM package for libFuzzer harnesses. Lives under
+// `FuzzTesting/` so that the main `OpenAI` package manifest stays
+// unchanged for library consumers.
+//
+// Pattern adapted from grpc-swift's FuzzTesting setup. See
+// `FuzzTesting/README.md` for how to build and run.
+
+import PackageDescription
+
+let package = Package(
+    name: "FuzzTesting",
+    platforms: [.macOS(.v10_15)],
+    dependencies: [
+        .package(name: "OpenAI", path: ".."),
+    ],
+    targets: [
+        .executableTarget(
+            name: "FuzzResponseStreamEventDecoder",
+            dependencies: [.product(name: "OpenAI", package: "OpenAI")],
+            path: "Sources/FuzzResponseStreamEventDecoder"
+        ),
+        .executableTarget(
+            name: "FuzzChatResultDecoder",
+            dependencies: [.product(name: "OpenAI", package: "OpenAI")],
+            path: "Sources/FuzzChatResultDecoder"
+        ),
+        .executableTarget(
+            name: "FuzzChatStreamResultDecoder",
+            dependencies: [.product(name: "OpenAI", package: "OpenAI")],
+            path: "Sources/FuzzChatStreamResultDecoder"
+        ),
+        .executableTarget(
+            name: "FuzzResponseObjectDecoder",
+            dependencies: [.product(name: "OpenAI", package: "OpenAI")],
+            path: "Sources/FuzzResponseObjectDecoder"
+        ),
+        .executableTarget(
+            name: "FuzzAudioTranscriptionStreamResultDecoder",
+            dependencies: [.product(name: "OpenAI", package: "OpenAI")],
+            path: "Sources/FuzzAudioTranscriptionStreamResultDecoder"
+        ),
+    ]
+)
diff --git a/FuzzTesting/README.md b/FuzzTesting/README.md
new file mode 100644
index 00000000..e08a74a6
--- /dev/null
+++ b/FuzzTesting/README.md
@@ -0,0 +1,84 @@
+# FuzzTesting
+
+libFuzzer harnesses for the `OpenAI` Swift package. Lives in its own
+SwiftPM package so the main library's `Package.swift` stays clean for
+library consumers.
+
+This is the in-repo half of the work tracked in [#241 — Integrate
+OSS-Fuzz](https://github.com/MacPaw/OpenAI/issues/241). The upstream
+`google/oss-fuzz` project submission is a separate piece.
+
+## Targets
+
+| Target                                          | What it fuzzes                                                  |
+| ----------------------------------------------- | --------------------------------------------------------------- |
+| `FuzzResponseStreamEventDecoder`                | `JSONDecoder().decode(ResponseStreamEvent.self, …)`             |
+| `FuzzChatResultDecoder`                         | `JSONDecoder().decode(ChatResult.self, …)`                      |
+| `FuzzChatStreamResultDecoder`                   | `JSONDecoder().decode(ChatStreamResult.self, …)`                |
+| `FuzzResponseObjectDecoder`                     | `JSONDecoder().decode(ResponseObject.self, …)`                  |
+| `FuzzAudioTranscriptionStreamResultDecoder`     | `JSONDecoder().decode(AudioTranscriptionStreamResult.self, …)`  |
+
+All five targets fuzz Codable decoders that consume bytes off the wire:
+
+- `ResponseStreamEvent.init(from:)` runs several fallible decode passes
+  before falling back to a generated raw event with ~50 oneOf cases.
+- `ChatResult` / `ChatStreamResult` cover the non-streaming and
+  streaming `/v1/chat/completions` response shapes, including tool
+  calls, annotations, refusals, and usage breakdowns.
+- `ResponseObject` covers the non-streaming `/v1/responses` envelope
+  with reasoning, tool outputs, and rich output items.
+- `AudioTranscriptionStreamResult` has a hand-written `init(from:)`
+  that branches on `type` and optionally reads `logprobs`.
+
+## Replay mode (no libFuzzer toolchain required)
+
+```sh
+cd FuzzTesting
+swift build
+swift run FuzzResponseStreamEventDecoder FuzzCorpus/FuzzResponseStreamEventDecoder/seed-function-call-args-done.json
+```
+
+Replay mode runs the harness against a single input file and exits. Use
+it to reproduce libFuzzer crash artifacts locally, or to smoke-test the
+harness against seed inputs.
+
+## Fuzzing mode (libFuzzer)
+
+Requires a Swift toolchain built with libFuzzer support
+([reference](https://github.com/apple/swift/blob/main/docs/libFuzzerIntegration.md)).
+On Linux, this typically means a Swift toolchain installed with the
+fuzzer runtime; on macOS, support is more limited.
+
+```sh
+cd FuzzTesting
+swift build -c debug \
+  -Xswiftc -sanitize=fuzzer,address \
+  -Xswiftc -parse-as-library \
+  -Xswiftc -DFUZZING_ENABLED
+./.build/debug/FuzzResponseStreamEventDecoder \
+  FuzzCorpus/FuzzResponseStreamEventDecoder \
+  -max_total_time=60
+```
+
+`-DFUZZING_ENABLED` removes each harness's `@main` replay entry point
+so libFuzzer's own `main` (provided by `-sanitize=fuzzer`) drives
+`LLVMFuzzerTestOneInput`.
+
+## Adding a corpus input
+
+Drop any byte sequence into `FuzzCorpus/<TargetName>/`. Real-world
+response chunks captured during integration testing make the best seeds.
+The fuzzer will mutate from these.
+
+## Adding a new harness
+
+1. Add an `.executableTarget` to `Package.swift`.
+2. Create `Sources/<TargetName>/<TargetName>.swift` with the same shape
+   as the existing harnesses:
+   - One `@_cdecl("LLVMFuzzerTestOneInput")` function with the body
+     under test.
+   - A `#if !FUZZING_ENABLED` replay `@main` for local use.
+3. Add a `FuzzCorpus/<TargetName>/` directory with at least one seed.
+
+Pick targets that consume untrusted bytes: parsers, decoders, anything
+on the network-input path.
diff --git a/FuzzTesting/Sources/FuzzAudioTranscriptionStreamResultDecoder/FuzzAudioTranscriptionStreamResultDecoder.swift b/FuzzTesting/Sources/FuzzAudioTranscriptionStreamResultDecoder/FuzzAudioTranscriptionStreamResultDecoder.swift
new file mode 100644
index 00000000..1216c737
--- /dev/null
+++ b/FuzzTesting/Sources/FuzzAudioTranscriptionStreamResultDecoder/FuzzAudioTranscriptionStreamResultDecoder.swift
@@ -0,0 +1,43 @@
+//
+//  FuzzAudioTranscriptionStreamResultDecoder.swift
+//  OpenAI / FuzzTesting
+//
+//  libFuzzer harness for `AudioTranscriptionStreamResult` JSON decoding.
+//  The streamed audio transcription event has a hand-written
+//  `init(from:)` (see Public/Models/AudioTranscriptionStreamResult.swift)
+//  that branches on `type` and optionally reads `logprobs`, so it has a
+//  decent surface for malformed inputs.
+//
+//  See FuzzTesting/README.md.
+
+import Foundation
+import OpenAI
+
+private let decoder = JSONDecoder()
+
+@inline(__always)
+private func decodeOnce(_ data: Data) {
+    _ = try? decoder.decode(AudioTranscriptionStreamResult.self, from: data)
+}
+
+@_cdecl("LLVMFuzzerTestOneInput")
+public func LLVMFuzzerTestOneInput(_ start: UnsafePointer<UInt8>, _ count: Int) -> Int32 {
+    decodeOnce(Data(bytes: start, count: count))
+    return 0
+}
+
+#if !FUZZING_ENABLED
+@main
+enum Replay {
+    static func main() throws {
+        let args = CommandLine.arguments
+        guard args.count == 2 else {
+            FileHandle.standardError.write(Data("usage: \(args.first ?? "FuzzAudioTranscriptionStreamResultDecoder") <input-file>\n".utf8))
+            exit(2)
+        }
+        let data = try Data(contentsOf: URL(fileURLWithPath: args[1]))
+        decodeOnce(data)
+        print("decoded \(data.count) bytes without crashing")
+    }
+}
+#endif
diff --git a/FuzzTesting/Sources/FuzzChatResultDecoder/FuzzChatResultDecoder.swift b/FuzzTesting/Sources/FuzzChatResultDecoder/FuzzChatResultDecoder.swift
new file mode 100644
index 00000000..125cd34d
--- /dev/null
+++ b/FuzzTesting/Sources/FuzzChatResultDecoder/FuzzChatResultDecoder.swift
@@ -0,0 +1,43 @@
+//
+//  FuzzChatResultDecoder.swift
+//  OpenAI / FuzzTesting
+//
+//  libFuzzer harness for `ChatResult` JSON decoding. `ChatResult` is the
+//  non-streaming response body for `/v1/chat/completions` — bytes come
+//  straight off the wire and pass through a Codable initializer that
+//  recursively decodes message content, tool calls, annotations, and
+//  usage details, any of which can have unexpected shapes.
+//
+//  See FuzzTesting/README.md.
+
+import Foundation
+import OpenAI
+
+private let decoder = JSONDecoder()
+
+@inline(__always)
+private func decodeOnce(_ data: Data) {
+    _ = try? decoder.decode(ChatResult.self, from: data)
+}
+
+@_cdecl("LLVMFuzzerTestOneInput")
+public func LLVMFuzzerTestOneInput(_ start: UnsafePointer<UInt8>, _ count: Int) -> Int32 {
+    decodeOnce(Data(bytes: start, count: count))
+    return 0
+}
+
+#if !FUZZING_ENABLED
+@main
+enum Replay {
+    static func main() throws {
+        let args = CommandLine.arguments
+        guard args.count == 2 else {
+            FileHandle.standardError.write(Data("usage: \(args.first ?? "FuzzChatResultDecoder") <input-file>\n".utf8))
+            exit(2)
+        }
+        let data = try Data(contentsOf: URL(fileURLWithPath: args[1]))
+        decodeOnce(data)
+        print("decoded \(data.count) bytes without crashing")
+    }
+}
+#endif
diff --git a/FuzzTesting/Sources/FuzzChatStreamResultDecoder/FuzzChatStreamResultDecoder.swift b/FuzzTesting/Sources/FuzzChatStreamResultDecoder/FuzzChatStreamResultDecoder.swift
new file mode 100644
index 00000000..77788360
--- /dev/null
+++ b/FuzzTesting/Sources/FuzzChatStreamResultDecoder/FuzzChatStreamResultDecoder.swift
@@ -0,0 +1,42 @@
+//
+//  FuzzChatStreamResultDecoder.swift
+//  OpenAI / FuzzTesting
+//
+//  libFuzzer harness for `ChatStreamResult` JSON decoding — the
+//  per-chunk type emitted by `/v1/chat/completions` when streaming.
+//  Each SSE `data:` line is decoded into this type, so any malformed
+//  byte sequence the parser tolerates ends up here.
+//
+//  See FuzzTesting/README.md.
+
+import Foundation
+import OpenAI
+
+private let decoder = JSONDecoder()
+
+@inline(__always)
+private func decodeOnce(_ data: Data) {
+    _ = try? decoder.decode(ChatStreamResult.self, from: data)
+}
+
+@_cdecl("LLVMFuzzerTestOneInput")
+public func LLVMFuzzerTestOneInput(_ start: UnsafePointer<UInt8>, _ count: Int) -> Int32 {
+    decodeOnce(Data(bytes: start, count: count))
+    return 0
+}
+
+#if !FUZZING_ENABLED
+@main
+enum Replay {
+    static func main() throws {
+        let args = CommandLine.arguments
+        guard args.count == 2 else {
+            FileHandle.standardError.write(Data("usage: \(args.first ?? "FuzzChatStreamResultDecoder") <input-file>\n".utf8))
+            exit(2)
+        }
+        let data = try Data(contentsOf: URL(fileURLWithPath: args[1]))
+        decodeOnce(data)
+        print("decoded \(data.count) bytes without crashing")
+    }
+}
+#endif
diff --git a/FuzzTesting/Sources/FuzzResponseObjectDecoder/FuzzResponseObjectDecoder.swift b/FuzzTesting/Sources/FuzzResponseObjectDecoder/FuzzResponseObjectDecoder.swift
new file mode 100644
index 00000000..1a8e298f
--- /dev/null
+++ b/FuzzTesting/Sources/FuzzResponseObjectDecoder/FuzzResponseObjectDecoder.swift
@@ -0,0 +1,42 @@
+//
+//  FuzzResponseObjectDecoder.swift
+//  OpenAI / FuzzTesting
+//
+//  libFuzzer harness for `ResponseObject` JSON decoding — the
+//  non-streaming response body for `/v1/responses`. Has a richer
+//  envelope than `ChatResult`, with reasoning blocks, tool outputs,
+//  output items of multiple kinds, and prompt references.
+//
+//  See FuzzTesting/README.md.
+
+import Foundation
+import OpenAI
+
+private let decoder = JSONDecoder()
+
+@inline(__always)
+private func decodeOnce(_ data: Data) {
+    _ = try? decoder.decode(ResponseObject.self, from: data)
+}
+
+@_cdecl("LLVMFuzzerTestOneInput")
+public func LLVMFuzzerTestOneInput(_ start: UnsafePointer<UInt8>, _ count: Int) -> Int32 {
+    decodeOnce(Data(bytes: start, count: count))
+    return 0
+}
+
+#if !FUZZING_ENABLED
+@main
+enum Replay {
+    static func main() throws {
+        let args = CommandLine.arguments
+        guard args.count == 2 else {
+            FileHandle.standardError.write(Data("usage: \(args.first ?? "FuzzResponseObjectDecoder") <input-file>\n".utf8))
+            exit(2)
+        }
+        let data = try Data(contentsOf: URL(fileURLWithPath: args[1]))
+        decodeOnce(data)
+        print("decoded \(data.count) bytes without crashing")
+    }
+}
+#endif
diff --git a/FuzzTesting/Sources/FuzzResponseStreamEventDecoder/FuzzResponseStreamEventDecoder.swift b/FuzzTesting/Sources/FuzzResponseStreamEventDecoder/FuzzResponseStreamEventDecoder.swift
new file mode 100644
index 00000000..f99fa162
--- /dev/null
+++ b/FuzzTesting/Sources/FuzzResponseStreamEventDecoder/FuzzResponseStreamEventDecoder.swift
@@ -0,0 +1,51 @@
+//
+//  FuzzResponseStreamEventDecoder.swift
+//  OpenAI / FuzzTesting
+//
+//  libFuzzer harness for `ResponseStreamEvent` JSON decoding.
+//
+//  `ResponseStreamEvent.init(from:)` has a non-trivial decoding pipeline
+//  with multiple `try?`/fallback paths (see Public/Schemas/Facade/
+//  ResponseStreamEvent.swift). It consumes untrusted bytes off the wire,
+//  which makes it a high-value target for fuzz testing.
+//
+//  See FuzzTesting/README.md for how to build & run.
+
+import Foundation
+import OpenAI
+
+private let decoder = JSONDecoder()
+
+/// The body of one fuzzer iteration. Catching all errors — we are looking
+/// for crashes, hangs, and sanitizer findings, not decode failures.
+@inline(__always)
+private func decodeOnce(_ data: Data) {
+    _ = try? decoder.decode(ResponseStreamEvent.self, from: data)
+}
+
+@_cdecl("LLVMFuzzerTestOneInput")
+public func LLVMFuzzerTestOneInput(_ start: UnsafePointer<UInt8>, _ count: Int) -> Int32 {
+    decodeOnce(Data(bytes: start, count: count))
+    return 0
+}
+
+#if !FUZZING_ENABLED
+// Replay mode for local use without libFuzzer: takes a path to a single
+// input file (e.g. a libFuzzer crash artifact) and runs one decode pass.
+// libFuzzer's own driver is linked in when built with `-sanitize=fuzzer`,
+// in which case this `@main` entry is excluded via the `FUZZING_ENABLED`
+// flag (set in `build-fuzzer.sh`).
+@main
+enum Replay {
+    static func main() throws {
+        let args = CommandLine.arguments
+        guard args.count == 2 else {
+            FileHandle.standardError.write(Data("usage: \(args.first ?? "FuzzResponseStreamEventDecoder") <input-file>\n".utf8))
+            exit(2)
+        }
+        let data = try Data(contentsOf: URL(fileURLWithPath: args[1]))
+        decodeOnce(data)
+        print("decoded \(data.count) bytes without crashing")
+    }
+}
+#endif