Gdbstub

Let your AI agent debug a kernel, and tell you why it triple-faulted.

gdbstub-mcp is an MCP server that gives AI agents (Claude Code, or any MCP client) a debugger for bare-metal code running in QEMU. It speaks the GDB Remote Serial Protocol directly to QEMU's built-in gdbstub, so you don't need a gdb binary. That matters most on Windows and macOS, where a cross-architecture gdb is a pain to get.

It's the companion to qemu-mcp. qemu-mcp lets an agent boot a kernel and see the crash. gdbstub-mcp lets it find out why.

What it looks like

An agent debugging a hobby kernel. This is real output, captured from the test kernel in tests/fixtures/kernel:

> debug_break kernel.c:10
#1 sw breakpoint at 0x100029 <add+0x9> at kernel.c:10

> debug_continue
Stopped: SIGTRAP (breakpoint/step) - breakpoint #1 (kernel.c:10)
pc = 0x100029 <add+0x9> at kernel.c:10
next: mov eax, dword ptr [ebp + 8]

> debug_backtrace
#0  0x100029 <add+0x9> at kernel.c:10
#1  0x100071 <compute+0x31> at kernel.c:16
#2  0x1000b2 <kmain+0x12> at kernel.c:28
#3  0x100016 <_start+0xa> at boot.s:21

> debug_registers eip,eflags,cr0
     eip = 0x00100029  <add+0x9> at kernel.c:10
  eflags = 0x00000093  [SF AF CF]
     cr0 = 0x00000011  [ET PE]

And when the machine silently reboots, debug_explain_fault reads QEMU's interrupt log and explains the crash:

> debug_explain_fault
A TRIPLE FAULT occurred: the CPU reset itself.

[0] #UD invalid opcode at triple_fault+0xa (kernel.c:23)
[1] #GP general protection fault at triple_fault+0xa (kernel.c:23)
     selector 0x32: IDT entry 6 = #UD
     -> the IDT entry for vector 0x6 is missing or invalid - the CPU couldn't
        even start your handler. ...
[2] #DF double fault at triple_fault+0xa (kernel.c:23)

Root cause: the chain started with #UD at triple_fault+0xa.
It escalated to a triple fault because the handler for #UD (IDT entry 0x6)
could not be invoked. Fix the IDT entry so the original exception reaches
your handler, then fix the #UD itself.

A real QEMU log of a crash is thousands of lines of timer interrupts and register dumps. This tool picks out the exception chain, decodes the error codes, and points at the cause.

Why another debugger MCP?

Existing GDB MCP servers wrap a gdb binary and target userland programs or core dumps. The embedded ones target microcontrollers through probe-rs or OpenOCD. None of them is built for the osdev loop: a kernel you wrote yourself, running in QEMU, crashing before it can print anything.

• No gdb required. It's pure Python (pyelftools + capstone). Run pip install gdbstub-mcp and you're set.
• Any architecture QEMU supports. The register layout comes from the stub's own target.xml. Control registers (CR0/CR2/CR3/CR4, EFER) are there on x86, and flag registers are decoded.
• Symbols and source lines. Break on kmain, kmain+0x10 or kernel.c:42, and every stop shows function+offset at file:line. Prefixed ABIs work too: kmain resolves to mort_kmain when that name is unique.
• Backtraces that work on real kernels. It walks the frame-pointer chain, and handles x86 prologues and epilogues correctly. For -O2 kernels without frame pointers it can scan the stack for return addresses instead.
• Triple-fault diagnosis. debug_explain_fault, described above. I haven't found another tool that does this.

Install

pip install gdbstub-mcp

Claude Code:

claude mcp add gdbstub -- gdbstub-mcp

Other MCP clients use the same idea: run gdbstub-mcp as a stdio server. Python 3.10+.

Usage

Start QEMU with its gdbstub on, with the CPU halted:

qemu-system-i386 -kernel kernel.elf -s -S -d int,cpu_reset -D int.log -no-reboot

• -s opens the stub on tcp::1234.
• -S halts the CPU before the first instruction.
• -d int,cpu_reset -D int.log -no-reboot is only needed for debug_explain_fault.

With qemu-mcp, pass the same flags through qemu_boot's extra_args: extra_args="-s -S -d int,cpu_reset -D int.log -no-reboot".

Then ask your agent something like "connect to the kernel with kernel.elf, break in kmain, and step until the page fault".

Addresses can be written as 0x1234, symbol, symbol+0x10, file.c:42 or $esp+8.

Tools

Limits (honest ones)

• Machine-level, not source-level. There's no next/finish, and no local variables or C expressions yet (see BACKLOG.md). You get symbols, lines, registers, memory and disassembly, which covers most kernel debugging.
• Stack scanning is a heuristic. It reports any stack word that points just after a call inside a known function, so stale frames can appear. Build with -fno-omit-frame-pointer for exact backtraces.
• debug_explain_fault understands x86 logs only.
• Tested against QEMU's gdbstub. Other RSP stubs (OpenOCD, Bochs, real-hardware probes) speak the same protocol and will likely work, but they're untested.

Tests

pip install ".[test]" anyio "ruff==0.16.0" "mypy==2.3.0"
ruff check src tests && mypy --strict src
pytest tests

Unit tests use a fake gdbstub on a real TCP socket, plus a real QEMU log of a triple fault, so they need no QEMU. tests/test_qemu_integration.py boots the fixture kernel in real QEMU (CI installs qemu-system-x86). It checks breakpoints, watchpoints, line numbers, a 4-deep backtrace and an end-to-end triple fault, and skips itself if QEMU isn't installed. To rebuild the fixture kernel, run python tests/fixtures/kernel/build.py, which needs pip install ziglang.

License

MIT