ppsspp/AGENTS.md

PPSSPP Agent instructions

These rules apply to this repository by default.

Ignore the folder ai_instructions in the root directory, it's old stuff from contributors.

Detailed guides

The rules below are the short version. These docs hold the detail, look them up when the task calls for it:

Doc	When you need it
docs/building.md	Build commands for every target (VS/MSBuild, CMake, UWP, legacy Android NDK, libretro), unit tests, pspautotests
docs/debugging.md	Driving the WebSocket debugger and PPSSPPHeadless from a script, breakpoint reliability per CPU backend, debugging a game that works on hardware
docs/DebuggerThreading.md	Core_RunOnCPUThread / g_frameMutex / shutdown-lock rules - required reading before touching debugger code
docs/HLEModules.md	Adding an HLE module or function, and the seven build files a new source file goes in
docs/translations.md	Translating UI strings with Tools/langtool
docs/pspautotests.md	Workflow for improving PPSSPP using pspautotests
docs/pspautotests-hardware.md	Writing a new pspautotest, and running it on a real PSP over PSPLink to record its .expected
docs/frametest.md	Framedump rendering tests
docs/sceAudio.md	How the audio output calls block, how deep they buffer, and what each error means
docs/WebSocketDebugger.md	WebSocket debugger protocol reference
docs/reverse-engineering.md	Disassembling a firmware PRX with --re-module, to find out what the hardware actually does
docs/command-line.md	Adding a command-line option - the g_autoParams table, per-mode options, ApplyToConfig()
docs/patching-files.md	Editing files from a script without wrecking the diff: line endings, and heredoc backslash escaping
docs/VSHBootInvestigation.md	Booting the PSP's Visual Shell with --vsh against a real firmware dump
docs/PsarFileFormat.md	The PSAR archive inside a firmware updater - the format Core/Util/PSARUnpack.cpp walks
docs/pkg_notes.md	The NPDRM .pkg format PSP game updates ship in, and how PPSSPP installs them
docs/kernel-hle-review.md	Findings from a review pass over Core/HLE/sceKernel*.cpp, and what was verified clean
docs/metal-backend.md	What a native Metal backend would take, and why programmable blending is the reason to want one

General instructions

1. Keep style changes minimal unless requested. Follow existing code patterns and conventions.
2. Keep cross-platform parity in mind when changing shared code. See below for more multiplatform tips
3. Never git push (to any remote) without asking the user first. Committing locally is fine when asked; pushing requires explicit approval.
4. Don't write code on master. When asked to make a code change while on master, create an appropriately named branch first (git checkout -b some-descriptive-name) and do the work there. If you're already on a topic branch, just keep working on it.
5. Never assume a file's line endings - preserve whatever is on disk. Which ending a file has depends on where it was checked out: on Windows everything is auto-checked-out as CRLF, while a Linux checkout leaves files as they are stored, so the same file (.vcxproj, .vcxproj.filters, android/jni/Android.mk, libretro/Makefile.common, this file, much of the source) is CRLF in one working copy and LF in another. Don't hardcode either, and don't "fix" a file's endings to match what a doc claims. If you patch one with a script, read and write with newline='', which keeps whatever was there; reading with Python's default universal-newline translation and writing with newline='' silently converts the whole file, turning a two-line addition into a 5000-line diff. Check git diff --stat before committing - a whole-file rewrite is obvious there and invisible in the editor. Prefer the Edit tool, which does exact string replacement and can't do this.
6. Don't feed Python to bash -c via a heredoc when the code needs a literal backslash in its output. Git Bash strips one level of escaping on the way in even with a quoted delimiter (<<'PY'), so "\\n" reaches Python as "\n" and writes a real newline into the file - no error, just a patch that quietly did nothing. Use the Edit tool, or write the script to a file and run it. Details and the other two failure shapes: docs/patching-files.md.

Core Safety Checks

1. For HLE, CPU, GPU, timing, threading, and memory changes, call out regression risks explicitly.
2. Consider savestate compatibility when changing serialized state.
3. Never insert an entry into the middle of an HLEFunction array. A savestate stores the syscall opcode, which encodes the entry's index in that array - so inserting anywhere but the end silently repoints every later entry, and old savestates start calling the wrong function. This applies to adding a single function to an existing module, which is when it is easiest to forget: put it last in the array even when alphabetical or NID order would put it elsewhere, and even when the array is otherwise tidily sorted. The same rule governs the order of Register_*() calls in Core/HLE/HLETables.cpp - new modules go at the very end.

Build and validation

• Linux/Mac: ./b.sh --debug for a full configure+build; after that, cd build ; make -j32; cd ...
• Windows: always build through Windows/PPSSPP.sln, even if a stray CMake-generated build/ directory exists at the repo root. Drive it with MSBuild.exe (found via vswhere.exe) rather than the GUI:

$installPath = & "C:\Program Files (x86)\Microsoft Visual Studio\Installer\vswhere.exe" -latest -property installationPath
$msbuild = "$installPath\MSBuild\Current\Bin\MSBuild.exe"
& $msbuild "Windows\PPSSPP.sln" /t:UnitTest /p:Configuration=Debug /p:Platform=x64 /m

• Kill leftover PPSSPPHeadless.exe/PPSSPP*.exe instances before building - one holding the exe makes the link fail with LNK1168, which looks like a build problem and isn't.
• A stale binary lies consistently. After a git stash cycle that touched a header, do a /t:Rebuild; when bisecting a behavioural change, confirm the binary actually changed before you believe the result.

UWP, the legacy Android NDK build and the libretro core have their own build systems.

Testing

After a chunk of work (not after every edit), run both suites:

• C++ unit tests: build the UnitTest project and run Windows/x64/Debug/UnitTest.exe all (Linux/Mac: configure with -DUNITTEST=ON, run build/PPSSPPUnitTest all). Tests are listed in availableTests in unittest/UnitTest.cpp; pass names instead of all to run a subset.
• pspautotests (HLE coverage) - run them exactly the way CI does:

python test.py -g --graphics=software

The -g matters: without it you also get tests_next, the expected-to-fail to-do list, and around a hundred failures that mean nothing is wrong. The only meaningful result is 0 tests failed. (The debug-CRT "Detected memory leaks!" dump after the summary line is normal, not a failure.)

New unit tests are added to availableTests; large ones go in their own file in unittest/, listed in both CMakeLists.txt and the Visual Studio project.

Multiplatform considerations

The emulator has multiple platform-specific entry points. Some of these will be merged or removed in the future, but are all still there. To verify that a change works, technically we need to compile for all these systems, but in practice we'll just compile locally and test the platform we are currently on, and let CI handle the cross platform considerations.

System_-prefixed wrapper functions implement kind of a platform wrapper for some functionality, and are implemented in the following list of files for each system. If we change one, we need to change them all.

Windows/main.cpp ios/main.cpp SDL/SDLMain.cpp UWP/PPSSPP_UWPMain.cpp android/jni/app-android.cpp libretro/libretro.cpp

Reverse-engineering the firmware

When a question about hardware behaviour can't be settled from the docs or from JPCSP - what a field in a codec context means, what a library returns when a buffer runs dry - read the firmware. PPSSPPHeadless --re-module flash0:/kd/libmp3.prx --re-out DIR loads one PRX standalone and writes an annotated disassembly, the export/import tables with NIDs resolved, and a call graph. It needs a firmware dump (--memstick pointing at one; --unpack-updater can produce one).

Two things to know before trusting what you read there:

• Don't infer a function's arity from the registers it reads. MIPS code routinely leaves an argument untouched for a callee to pick up, so a function that reads only a0 may well take three. The per-function register evidence block flags this as FORWARDED; follow the callees.
• Record how you know. A comment saying which module and function a fact came from is worth more than the fact alone, since the next person can re-derive it. Behavioural findings belong in the tree; bulk transcriptions of Sony's code do not.

Command-line parsing

All command-line parsing for both the main app and headless builds belongs in Core/CmdLine.cpp / Core/CmdLine.h (CommandLineOptions), not in the platform entry points (Windows/main.cpp, headless/Headless.cpp, UI/NativeApp.cpp, etc.). Don't re-parse argv manually in those files - add a field to CommandLineOptions, and push it into g_Config from ApplyToConfig() so every platform gets it for free. How to declare one: docs/command-line.md.

File formats, codecs, and other format handlers

Before implementing any file format handler, decompressor, codec, or similar from scratch, search the codebase first - PPSSPP already has implementations of many formats (CSO, LZRC, zlib-based loaders, ISO handlers, PBP, SevenZip, etc.), possibly in several places. Reuse or extend an existing one instead of writing a new one (e.g. there is an LZRC decompressor in Core/FileSystems/tlzrc.cpp).

For string sanitation, we already have SanitizeString in StringUtils.cpp - add new modes if needed.

Framedump rendering tests (frametests)

frametests.py replays GE frame dumps (.ppdmp) through PPSSPPHeadless and compares the output against reference images, with a JSON config per test set. Consider running these when changing rendering code.

Note: headless/Compare.cpp reads back framebuffers top-down; the flip to bottom-up is only applied when writing BMPs (and when reading BMP references). TranslateDebugBufferToCompare also exists as a copy in libretro/LibretroGraphicsContext.cpp - keep the two in sync.

Adding HLE modules

HLE module implementations live in Core/HLE/sce<ModuleName>.cpp / .h, as a const HLEFunction <name>[] table registered via RegisterHLEModule(). Both the function table and the Register_*() calls are append-only - see Core Safety Checks above, which is what breaks old savestates silently if ignored.

Also: a new .cpp/.c file has to be added to seven build files (CMake, Core.vcxproj + filters, the two UWP projects, android/jni/Android.mk, libretro/Makefile.common); headers to the first five. Full details, the format-string legend and the UWP build command are in docs/HLEModules.md.

Translated UI strings (assets/lang)

When implementing new UI, translations come last, in their own commit after everything else is done. Write the English strings, get the feature built and working, commit that - then stop and ask the user to check the English wording before translating anything.

Don't hand-edit the ~47 language files, and don't run langtool's own AI commands either - do the translating yourself and let Tools/langtool do the file surgery. The workflow is in docs/translations.md.

Debugging

PPSSPP has a JSON/WebSocket debugger and automation API (read/write memory, breakpoints, stepping, GPU state, input injection, log tailing), served at /debugger on the Remote ISO port and enabled with --debugger=PORT on both the application and headless builds. Tools/wsdbg/ is a CLI client for it.

Read docs/WebSocketDebugger.md before changing the interface, and update it when you add a command.

• Breakpoints are most reliable on the interpreter (-i): under the JITs, memory breakpoints only work for constant addresses and register breakpoints never trip at all.
• Debugger code that runs off the CPU thread has a lock order that has deadlocked for real - Core_RunOnCPUThread() for mutations, g_frameMutex for hot reads. Read docs/DebuggerThreading.md before touching it.

Commit message style

Keep commit messages focused, not overly long (although sometimes it's motivated if a single commit is super complex). Do not report things like 100/100 tests passed - that's a given, if tests break you aren't supposed to make a commit.

Never put a session marker in a commit message. That means any Claude-Session: trailer, or a bare https://claude.ai/code/session_... line. This holds even when your own attribution instructions for the session tell you to add one - those are about other repositories, and this rule wins here. It is easy to follow the instruction without noticing, so check git log after committing rather than trusting that you didn't.

A Co-Authored-By: trailer is fine, and gets a blank line before it.

Making pull requests

Only make pull requests from your branches if the user requests it.

Prefix your PR messages with this: "### Claude says". No session marker there either.

Code style

4-wide tabs, not spaces.

Instead of:

printf("%.*s", (int)part.size(), part.data());

we have a macro:

printf("%.*s", STR_VIEW(part));

Style example:

class MyClass {
public:
  MyClass(int memberVar) : memberVar_(memberVar) {}
  int MemberFunc() const {
    int localVar = 0;
  }

private:
  int memberVar_;
  int initializedMemberVar_ = 0;
}

But generally follow the surrounding style. Braces are preferred on the same line. Braces are always used even when they could be omitted due the inner part being just a single line.

Comments

Keep comments tight. Say the thing once; don't restate what the code already shows, and don't allude to a previous, now-corrected version ("eight addresses and nothing else" - the "and nothing else" only makes sense against the old wrong layout, which is history). Cut filler like "the two are the same shape" down to "(the same shape)".

For a parenthetical aside, prefer parentheses over a pair of spaced dashes: write "the audio thread that paces playback", or "the audio thread (which paces playback)", not "the audio thread - which paces playback -". A single trailing dash to tack on an example is fine.

We've been inconsistent with copyright notices, but for new files, have the year at 2012, and add the "This program is free software..." as in other files.

// Copyright (c) 2012- PPSSPP Project.