Steam Input as the primary guest-input path (optional, SDK-gated)

Add SteamInputSource: initializes SteamAPI + Steam Input, loads a bundled action
manifest via SetInputActionManifestFilePath (no partner-backend config needed),
and reads the GameControls action set into CoopPadState -- falling back to XInput
per slot, and to pure XInput if Steam isn't available, so the host always runs.

Enabled automatically when the Steamworks SDK is vendored at
third_party/steamworks_sdk/ (auto-detected by CMake; gitignored and never
committed -- the build is XInput-only without it). Stages steam_api64.dll + the
manifest next to the host and builds coop_steam_input_probe (a console smoke test).

Verified: the probe initializes against the live Steam client and enumerates
controllers; the host degrades gracefully when launched standalone. All 5 tests
pass. Reading actual controller state needs a pad bound through Steam Input for
the running (donor) appid, which XInput otherwise covers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -10,3 +10,7 @@ CMakeUserPresets.json
 x64
 Debug
 Release
+
+# Steamworks SDK: vendored locally, not redistributable -- never commit it.
+/steamworks_sdk_*.zip
+/third_party/steamworks_sdk/

README.md

@@ -19,7 +19,7 @@ XInput game becomes Remote-Play-Together-able.
 
 | Concern | Mechanism | Component | Status |
 | --- | --- | --- | --- |
-| Receive guest input | XInput (RPT delivers guest pads to the focused window) | `coop_host.exe` | done |
+| Receive guest input | Steam Input (primary, when built with the Steamworks SDK) with XInput fallback; RPT delivers guest pads to the focused window | `coop_host.exe` | done |
 | Forward input to game | DLL injection + XInput hook (SafetyHook) — game sees *only* our pad | `coop_hook.dll` | done |
 | Keep game running unfocused | Hook spoofs focus so the game polls while the host holds OS focus | `coop_hook.dll` | done |
 | Mirror video | Windows Graphics Capture of the game window, letterboxed into the host window | `coop_host.exe` | done |
@@ -155,10 +155,21 @@ Done:
   all subsystems hooked, and the IPC channels (status, audio ring, video share,
   log) all flow across the x64↔x86 boundary.
 
-Future work, roughly in priority order:
+- **Steam Input. ✅** When the host is built with the Steamworks SDK
+  (auto-detected under `third_party/steamworks_sdk/`), guest input comes through
+  the Steam Input API (action-based) as the primary path, falling back to XInput
+  per slot — and to pure XInput if Steam isn't available, so the host always runs.
+  It initializes SteamAPI + Steam Input and loads a bundled action manifest
+  (`steam_input_actions.vdf`) via `SetInputActionManifestFilePath`, so it needs no
+  partner-backend config. Verified to initialize against the live Steam client and
+  enumerate controllers (`coop_steam_input_probe`). Reading actual button/stick
+  state still requires a controller bound through Steam Input for the running
+  appid (the donor appid under RPT) — without that binding the XInput path carries
+  the guest input, as before.
 
-- **Steam Input:** consume guest input through the Steam Input API directly
-  rather than XInput.
+All planned phases are now implemented. Possible later work: per-stream audio
+mixing for multi-stream games, per-stream format detection for the audio hook, and
+hooking D3D9 / pure-D3D12 present paths for the video hook.
 
 ## Building
 
@@ -180,8 +191,14 @@ configured into `build/x86/`) that produces `coop_hook_x86.dll` and
 it with `-DCOOP_BUILD_X86_HELPER=OFF` if you don't need 32-bit support.
 
 Third-party dependencies (Dear ImGui, SafetyHook) are git submodules under
-`third_party/`; the Steamworks SDK is vendored manually there when wired. No
-vcpkg / package manager is used.
+`third_party/`. No vcpkg / package manager is used.
+
+**Steam Input is optional.** It's enabled automatically when the Steamworks SDK is
+vendored at `third_party/steamworks_sdk/` (extract the `steamworks_sdk_*.zip`
+there). The SDK isn't redistributable, so it's gitignored and never committed; if
+it's absent the host builds XInput-only (no other features depend on it). When
+present, the build links `steam_api64.lib`, stages `steam_api64.dll` and the
+action manifest next to the host, and also builds `coop_steam_input_probe`.
 
 ### clangd / IDE setup
 

host/CMakeLists.txt

@@ -37,3 +37,50 @@ target_link_libraries(coop_host PRIVATE
 	ole32)
 
 set_target_properties(coop_host PROPERTIES OUTPUT_NAME "coop_host")
+
+# Optional Steam Input: enabled automatically when the Steamworks SDK is vendored
+# under third_party/steamworks_sdk (it isn't redistributable, so it's gitignored
+# and never committed). Without it, the host builds XInput-only.
+set(COOP_STEAM_SDK_DIR "${CMAKE_SOURCE_DIR}/third_party/steamworks_sdk/sdk")
+if(EXISTS "${COOP_STEAM_SDK_DIR}/public/steam/steam_api.h")
+	message(STATUS "Steamworks SDK found -> Steam Input enabled")
+	target_sources(coop_host PRIVATE src/input/steam_input_source.cpp)
+	target_compile_definitions(coop_host PRIVATE COOP_WITH_STEAM)
+	target_include_directories(coop_host SYSTEM PRIVATE "${COOP_STEAM_SDK_DIR}/public")
+	# The SDK headers trip /W4 (strncpy, non-UTF-8 bytes); silence for this TU only.
+	set_source_files_properties(src/input/steam_input_source.cpp PROPERTIES
+		COMPILE_OPTIONS "/wd4996;/wd4828")
+	target_link_libraries(coop_host PRIVATE
+		"${COOP_STEAM_SDK_DIR}/redistributable_bin/win64/steam_api64.lib")
+	# Stage steam_api64.dll + the action manifest next to coop_host.exe.
+	add_custom_command(TARGET coop_host POST_BUILD
+		COMMAND ${CMAKE_COMMAND} -E copy_if_different
+			"${COOP_STEAM_SDK_DIR}/redistributable_bin/win64/steam_api64.dll"
+			"$<TARGET_FILE_DIR:coop_host>/steam_api64.dll"
+		COMMAND ${CMAKE_COMMAND} -E copy_if_different
+			"${CMAKE_CURRENT_SOURCE_DIR}/assets/steam_input_actions.vdf"
+			"$<TARGET_FILE_DIR:coop_host>/steam_input_actions.vdf"
+		VERBATIM)
+
+	# Console smoke test for Steam Input (reuses the shipping source). Not a ctest
+	# (needs Steam running + an appid); run manually from bin/<config>/.
+	add_executable(coop_steam_input_probe
+		${CMAKE_SOURCE_DIR}/tools/steam_input_probe/main.cpp
+		src/input/steam_input_source.cpp
+		src/input/xinput_source.cpp)
+	target_include_directories(coop_steam_input_probe PRIVATE src)
+	target_include_directories(coop_steam_input_probe SYSTEM PRIVATE "${COOP_STEAM_SDK_DIR}/public")
+	target_compile_definitions(coop_steam_input_probe PRIVATE COOP_WITH_STEAM)
+	set_source_files_properties(src/input/steam_input_source.cpp PROPERTIES COMPILE_OPTIONS "/wd4996;/wd4828")
+	target_link_libraries(coop_steam_input_probe PRIVATE
+		coop_common
+		xinput
+		"${COOP_STEAM_SDK_DIR}/redistributable_bin/win64/steam_api64.lib")
+	add_custom_command(TARGET coop_steam_input_probe POST_BUILD
+		COMMAND ${CMAKE_COMMAND} -E copy_if_different
+			"${COOP_STEAM_SDK_DIR}/redistributable_bin/win64/steam_api64.dll"
+			"$<TARGET_FILE_DIR:coop_steam_input_probe>/steam_api64.dll"
+		VERBATIM)
+else()
+	message(STATUS "Steamworks SDK not found -> Steam Input disabled (XInput only)")
+endif()

host/assets/steam_input_actions.vdf

@@ -0,0 +1,84 @@
+// Steam Input In-Game Actions (IGA) manifest for CoopAllTheThings.
+//
+// SteamInputSource points Steam Input at this file via
+// ISteamInput::SetInputActionManifestFilePath, so the host doesn't depend on a
+// partner-backend-registered config. It declares one action set, "GameControls",
+// with the standard gamepad buttons + two sticks + two triggers. The action names
+// here must match host/src/input/steam_input_source.cpp.
+"In Game Actions"
+{
+	"actions"
+	{
+		"GameControls"
+		{
+			"title"		"#Set_GameControls"
+			"legacy_set"	"1"
+
+			"StickPadGyro"
+			{
+				"LeftStick"
+				{
+					"title"		"#Action_LeftStick"
+					"input_mode"	"joystick_move"
+				}
+				"RightStick"
+				{
+					"title"		"#Action_RightStick"
+					"input_mode"	"joystick_move"
+				}
+			}
+
+			"AnalogTrigger"
+			{
+				"LeftTrigger"	"#Action_LeftTrigger"
+				"RightTrigger"	"#Action_RightTrigger"
+			}
+
+			"Button"
+			{
+				"A"		"#Action_A"
+				"B"		"#Action_B"
+				"X"		"#Action_X"
+				"Y"		"#Action_Y"
+				"LB"		"#Action_LB"
+				"RB"		"#Action_RB"
+				"Back"		"#Action_Back"
+				"Start"		"#Action_Start"
+				"LStickClick"	"#Action_LStickClick"
+				"RStickClick"	"#Action_RStickClick"
+				"DPadUp"	"#Action_DPadUp"
+				"DPadDown"	"#Action_DPadDown"
+				"DPadLeft"	"#Action_DPadLeft"
+				"DPadRight"	"#Action_DPadRight"
+				"Guide"		"#Action_Guide"
+			}
+		}
+	}
+
+	"localization"
+	{
+		"english"
+		{
+			"Set_GameControls"	"Game Controls"
+			"Action_LeftStick"	"Left Stick"
+			"Action_RightStick"	"Right Stick"
+			"Action_LeftTrigger"	"Left Trigger"
+			"Action_RightTrigger"	"Right Trigger"
+			"Action_A"		"A"
+			"Action_B"		"B"
+			"Action_X"		"X"
+			"Action_Y"		"Y"
+			"Action_LB"		"Left Bumper"
+			"Action_RB"		"Right Bumper"
+			"Action_Back"		"Back"
+			"Action_Start"		"Start"
+			"Action_LStickClick"	"Left Stick Click"
+			"Action_RStickClick"	"Right Stick Click"
+			"Action_DPadUp"		"D-Pad Up"
+			"Action_DPadDown"	"D-Pad Down"
+			"Action_DPadLeft"	"D-Pad Left"
+			"Action_DPadRight"	"D-Pad Right"
+			"Action_Guide"		"Guide"
+		}
+	}
+}

host/src/input/steam_input_source.cpp

@@ -0,0 +1,205 @@
+#include "input/steam_input_source.hpp"
+
+#include <cstdio>
+
+#include <windows.h>
+#include <xinput.h> // XINPUT_GAMEPAD_* button bits
+
+#include <steam/steam_api.h>
+
+namespace coop
+{
+
+namespace
+{
+
+// Digital actions in the manifest, paired with the XInput button bit they map to.
+// Names must match host/assets/steam_input_actions.vdf.
+struct ButtonAction
+{
+	const char* action;
+	std::uint16_t xinput_bit;
+};
+const ButtonAction kButtons[kSteamButtonActions] = {
+    {"A", XINPUT_GAMEPAD_A},
+    {"B", XINPUT_GAMEPAD_B},
+    {"X", XINPUT_GAMEPAD_X},
+    {"Y", XINPUT_GAMEPAD_Y},
+    {"LB", XINPUT_GAMEPAD_LEFT_SHOULDER},
+    {"RB", XINPUT_GAMEPAD_RIGHT_SHOULDER},
+    {"Back", XINPUT_GAMEPAD_BACK},
+    {"Start", XINPUT_GAMEPAD_START},
+    {"LStickClick", XINPUT_GAMEPAD_LEFT_THUMB},
+    {"RStickClick", XINPUT_GAMEPAD_RIGHT_THUMB},
+    {"DPadUp", XINPUT_GAMEPAD_DPAD_UP},
+    {"DPadDown", XINPUT_GAMEPAD_DPAD_DOWN},
+    {"DPadLeft", XINPUT_GAMEPAD_DPAD_LEFT},
+    {"DPadRight", XINPUT_GAMEPAD_DPAD_RIGHT},
+    {"Guide", 0x0400}, // unofficial XINPUT_GAMEPAD_GUIDE
+};
+
+std::int16_t to_axis(float v)
+{
+	if (v > 1.0f)
+	{
+		v = 1.0f;
+	}
+	if (v < -1.0f)
+	{
+		v = -1.0f;
+	}
+	return static_cast<std::int16_t>(v * 32767.0f);
+}
+
+std::uint8_t to_trigger(float v)
+{
+	if (v > 1.0f)
+	{
+		v = 1.0f;
+	}
+	if (v < 0.0f)
+	{
+		v = 0.0f;
+	}
+	return static_cast<std::uint8_t>(v * 255.0f);
+}
+
+} // namespace
+
+SteamInputSource::~SteamInputSource()
+{
+	shutdown();
+}
+
+bool SteamInputSource::init(const std::string& manifest_absolute_path)
+{
+	// Running standalone (not launched by Steam) without a steam_appid.txt makes
+	// SteamAPI_Init fail; that's fine -- we degrade to XInput.
+	if (!SteamAPI_Init())
+	{
+		std::printf("SteamInput: SteamAPI_Init failed (not under Steam?); using XInput.\n");
+		return false;
+	}
+	if (SteamInput() == nullptr)
+	{
+		std::printf("SteamInput: ISteamInput unavailable; using XInput.\n");
+		SteamAPI_Shutdown();
+		return false;
+	}
+	// Point Steam Input at our bundled action manifest so we don't depend on a
+	// partner-backend-registered config. Must be called before Init().
+	if (!manifest_absolute_path.empty())
+	{
+		SteamInput()->SetInputActionManifestFilePath(manifest_absolute_path.c_str());
+	}
+	if (!SteamInput()->Init(/*bExplicitlyCallRunFrame=*/false))
+	{
+		std::printf("SteamInput: ISteamInput::Init failed; using XInput.\n");
+		SteamAPI_Shutdown();
+		return false;
+	}
+
+	steam_ready_ = true;
+	name_ = "Steam Input (XInput fallback)";
+	resolve_handles();
+	std::printf("SteamInput: initialized (manifest=%s).\n", manifest_absolute_path.c_str());
+	return true;
+}
+
+void SteamInputSource::shutdown()
+{
+	if (steam_ready_)
+	{
+		SteamInput()->Shutdown();
+		SteamAPI_Shutdown();
+		steam_ready_ = false;
+		name_ = "XInput";
+	}
+}
+
+void SteamInputSource::resolve_handles()
+{
+	action_set_ = SteamInput()->GetActionSetHandle("GameControls");
+	for (int i = 0; i < kSteamButtonActions; ++i)
+	{
+		button_handles_[i] = SteamInput()->GetDigitalActionHandle(kButtons[i].action);
+	}
+	left_stick_ = SteamInput()->GetAnalogActionHandle("LeftStick");
+	right_stick_ = SteamInput()->GetAnalogActionHandle("RightStick");
+	left_trigger_ = SteamInput()->GetAnalogActionHandle("LeftTrigger");
+	right_trigger_ = SteamInput()->GetAnalogActionHandle("RightTrigger");
+}
+
+bool SteamInputSource::read_steam_pad(std::uint64_t controller, PadInfo& out) const
+{
+	SteamInput()->ActivateActionSet(controller, action_set_);
+
+	CoopPadState st{};
+	st.connected = 1;
+	bool any_active = false;
+
+	for (int i = 0; i < kSteamButtonActions; ++i)
+	{
+		const InputDigitalActionData_t d = SteamInput()->GetDigitalActionData(controller, button_handles_[i]);
+		any_active = any_active || d.bActive;
+		if (d.bState)
+		{
+			st.buttons |= kButtons[i].xinput_bit;
+		}
+	}
+
+	const InputAnalogActionData_t ls = SteamInput()->GetAnalogActionData(controller, left_stick_);
+	const InputAnalogActionData_t rs = SteamInput()->GetAnalogActionData(controller, right_stick_);
+	const InputAnalogActionData_t lt = SteamInput()->GetAnalogActionData(controller, left_trigger_);
+	const InputAnalogActionData_t rt = SteamInput()->GetAnalogActionData(controller, right_trigger_);
+	any_active = any_active || ls.bActive || rs.bActive || lt.bActive || rt.bActive;
+
+	st.thumb_lx = to_axis(ls.x);
+	st.thumb_ly = to_axis(ls.y);
+	st.thumb_rx = to_axis(rs.x);
+	st.thumb_ry = to_axis(rs.y);
+	st.left_trigger = to_trigger(lt.x);
+	st.right_trigger = to_trigger(rt.x);
+
+	// No action is bound/active (e.g. the controller isn't using our manifest) ->
+	// let the XInput fallback handle this slot instead of reporting an empty pad.
+	if (!any_active)
+	{
+		return false;
+	}
+
+	static std::uint32_t s_packet = 0;
+	st.packet = ++s_packet;
+	out.connected = true;
+	out.state = st;
+	out.source = "Steam Input";
+	return true;
+}
+
+void SteamInputSource::poll()
+{
+	// XInput first so every slot has a fallback value; Steam overrides where active.
+	xinput_.poll();
+	pads_ = xinput_.pads();
+
+	if (!steam_ready_)
+	{
+		steam_count_ = 0;
+		return;
+	}
+
+	SteamAPI_RunCallbacks();
+	InputHandle_t handles[STEAM_INPUT_MAX_COUNT] = {};
+	steam_count_ = SteamInput()->GetConnectedControllers(handles);
+
+	for (int i = 0; i < steam_count_ && i < static_cast<int>(kMaxPads); ++i)
+	{
+		PadInfo steam_pad;
+		if (read_steam_pad(handles[i], steam_pad))
+		{
+			pads_[i] = steam_pad; // Steam controller active on this slot -> use it
+		}
+	}
+}
+
+} // namespace coop

host/src/input/steam_input_source.hpp

@@ -0,0 +1,72 @@
+// Receives guest controllers through the Steam Input API (action-based) -- the
+// plan's "primary" input path -- and falls back to XInput per slot for any slot
+// Steam Input doesn't cover. If SteamAPI / Steam Input can't initialize (e.g. the
+// host isn't running under Steam, or no action manifest binds), this transparently
+// behaves as plain XInput, so it's always safe to use.
+//
+// Compiled only when the host is built with the Steamworks SDK (COOP_WITH_STEAM).
+#pragma once
+
+#include <array>
+#include <cstdint>
+#include <string>
+
+#include "input/input_source.hpp"
+#include "input/xinput_source.hpp"
+
+namespace coop
+{
+
+// Number of digital (button) actions in the bundled action manifest.
+inline constexpr int kSteamButtonActions = 15;
+
+class SteamInputSource final : public InputSource
+{
+public:
+	~SteamInputSource() override;
+
+	// Initializes SteamAPI + Steam Input and loads the action manifest at the given
+	// absolute path. Returns true if Steam Input is live; false means it will run
+	// as XInput-only (poll() still works either way).
+	bool init(const std::string& manifest_absolute_path);
+	void shutdown();
+
+	[[nodiscard]] const char* name() const override
+	{
+		return name_;
+	}
+	void poll() override;
+	[[nodiscard]] const std::array<PadInfo, kMaxPads>& pads() const override
+	{
+		return pads_;
+	}
+
+	[[nodiscard]] bool steam_active() const
+	{
+		return steam_ready_;
+	}
+	[[nodiscard]] int steam_controllers() const
+	{
+		return steam_count_;
+	}
+
+private:
+	void resolve_handles();
+	bool read_steam_pad(std::uint64_t controller, PadInfo& out) const;
+
+	XInputSource xinput_; // fallback for slots Steam Input doesn't fill
+	std::array<PadInfo, kMaxPads> pads_;
+
+	bool steam_ready_ = false;
+	int steam_count_ = 0;
+	const char* name_ = "XInput";
+
+	std::uint64_t action_set_ = 0;
+	std::uint64_t button_handles_[kSteamButtonActions] = {};
+	std::uint64_t left_stick_ = 0;
+	std::uint64_t right_stick_ = 0;
+	std::uint64_t left_trigger_ = 0;
+	std::uint64_t right_trigger_ = 0;
+};
+
+} // namespace coop

host/src/main.cpp

@@ -23,9 +23,31 @@
 #include "log_panel.hpp"
 #include "ui/app_chrome.hpp"
 
+#ifdef COOP_WITH_STEAM
+#include <string>
+
+#include "input/steam_input_source.hpp"
+#endif
+
 namespace
 {
 
+#ifdef COOP_WITH_STEAM
+// Absolute path to the bundled Steam Input action manifest (next to the exe).
+std::string steam_manifest_path()
+{
+	char buffer[MAX_PATH] = {};
+	const DWORD len = GetModuleFileNameA(nullptr, buffer, MAX_PATH);
+	std::string path(buffer, len);
+	const std::size_t slash = path.find_last_of("\\/");
+	if (slash != std::string::npos)
+	{
+		path.resize(slash + 1);
+	}
+	return path + "steam_input_actions.vdf";
+}
+#endif
+
 // When the overlay is hidden, briefly show a fading hint so the operator can find
 // the way back. The window is borderless and non-interactive so it never steals a
 // click or a frame from the mirror underneath.
@@ -64,7 +86,18 @@ int run()
 		return 1;
 	}
 
-	auto input = std::make_unique<coop::XInputSource>();
+	// Steam Input is the plan's primary guest-input path (it falls back to XInput
+	// per slot, and to pure XInput if Steam isn't available). When the host is built
+	// without the Steamworks SDK, use XInput directly.
+#ifdef COOP_WITH_STEAM
+	std::unique_ptr<coop::InputSource> input = [] {
+		auto steam = std::make_unique<coop::SteamInputSource>();
+		steam->init(steam_manifest_path());
+		return steam;
+	}();
+#else
+	std::unique_ptr<coop::InputSource> input = std::make_unique<coop::XInputSource>();
+#endif
 	coop::ControllersPanel controllers;
 	coop::InjectionPanel injection;
 	coop::AudioPanel audio;

tools/steam_input_probe/main.cpp

@@ -0,0 +1,59 @@
+// coop_steam_input_probe -- console smoke test for SteamInputSource. Brings up
+// Steam Input via the shipping host code (no GUI), prints whether it initialized,
+// how many Steam controllers are connected, and a few polled snapshots, then
+// exits. Needs Steam running and an appid context: either launch it under Steam,
+// or drop a steam_appid.txt (e.g. "480") next to the exe. Built only when the
+// Steamworks SDK is vendored.
+
+#include <cstdio>
+
+#include <windows.h>
+
+#include "input/steam_input_source.hpp"
+
+namespace
+{
+std::string manifest_path()
+{
+	char buffer[MAX_PATH] = {};
+	const DWORD len = GetModuleFileNameA(nullptr, buffer, MAX_PATH);
+	std::string path(buffer, len);
+	const std::size_t slash = path.find_last_of("\\/");
+	if (slash != std::string::npos)
+	{
+		path.resize(slash + 1);
+	}
+	return path + "steam_input_actions.vdf";
+}
+} // namespace
+
+int main()
+{
+	coop::SteamInputSource src;
+	const bool ok = src.init(manifest_path());
+	std::printf("init=%d  steam_active=%d  backend=\"%s\"\n", ok ? 1 : 0, src.steam_active() ? 1 : 0,
+	            src.name());
+
+	for (int frame = 0; frame < 20; ++frame)
+	{
+		src.poll();
+		Sleep(50);
+	}
+
+	std::printf("steam_controllers=%d\n", src.steam_controllers());
+	const auto& pads = src.pads();
+	for (std::uint32_t i = 0; i < coop::kMaxPads; ++i)
+	{
+		const coop::PadInfo& p = pads[i];
+		if (p.connected)
+		{
+			std::printf("  slot %u: source=\"%s\" buttons=0x%04X LX=%d LY=%d LT=%u RT=%u\n", i,
+			            p.source.c_str(), p.state.buttons, p.state.thumb_lx, p.state.thumb_ly,
+			            p.state.left_trigger, p.state.right_trigger);
+		}
+	}
+
+	src.shutdown();
+	std::printf("done.\n");
+	return 0;
+}