Debug_Mcp

IAR-style non-halting live-attach debug tool for Cortex-M via pyOCD

Debug_Mcp is a read-only MCP (Model Context Protocol) server that connects to a running Cortex-M target without resetting or halting it (connect_mode=attach). It provides DWARF-based C variable resolution, symbol lookup, memory read, register snapshot, and more - all through a JSON-lines stdio interface.

Features

• Non-halting live attach - connects to a running Cortex-M via pyOCD without reset or halt (connect_mode=attach, auto_unlock=false, jlink.power=false)

• C variable expression resolver - resolves symbols through DWARF debug info from the matching AXF/ELF/OUT file

• Read-only safety - no write, flash, erase, breakpoint, reset, or run-control operations are exposed

• Snapshot mode - halts the target briefly, reads multiple variables coherently, then resumes

• Register read - captures r0-r12, sp, lr, pc, xpsr (halts briefly if running)

• Memory read - chunked reads up to a 64 KB safety cap

• Auto-discovery of AXF/ELF/OUT files under a project root directory

• Symtab fallback - when DWARF is not available, falls back to the ELF symbol table

• Bitfield support - decodes DWARF4 LSB-based and DWARF3 MSB-based bitfields

• Enum mapping - resolves enum names from debug info

• JSON-lines stdio - stdin/stdout MCP protocol

Related MCP server: jlink-mcp

Operations (MCP tools)

C Expression Support

The DWARF resolver supports:

• symbol

• symbol.member

• symbol.arr[5]

• symbol.member[2].subfield

• Pointer dereference: *ptr or ptr->field

Function calls, address-of, and casts are rejected.

Requirements

• Python 3.10+

• pyocd >= 0.45.0

• pyelftools >= 0.33

• cmsis-pack-manager >= 0.6.0

• pylink-square >= 1.7.0

• A supported debug probe (J-Link, ST-Link, CMSIS-DAP, etc.)

• A Cortex-M target built with debug symbols (AXF/ELF/OUT, e.g. Keil MDK output)

Quick Start

1. Install dependencies

pip install -r requirements.txt

2. List probes

python -m pyocd list

Take note of the probe serial number, e.g. J-Link [000123456789].

3. Connect as an MCP server

The server speaks JSON-lines over stdin/stdout. Register it in your MCP client (Claude Desktop, Claude Code, etc.):

{
  "command": "python",
  "args": ["live_attach.py"]
}

Example JSON-lines transcript:

{"id": 1, "operation": "attach", "arguments": {"probe_serial": "000123456789", "protocol": "swd", "clock_hz": 2000000}}
{"id": 2, "operation": "read_variable", "arguments": {"axf_file": "C:/firmware/project.axf", "expression": "g_bms.pack[2].soc"}}
{"id": 3, "operation": "read_registers"}
{"id": 4, "operation": "shutdown"}

4. Using the pre-built executable

A pre-built Windows executable (DebugMcp.exe) is included. Run it directly; it behaves exactly like live_attach.py:

DebugMcp.exe

Safety

• connect_mode=attach - the target is never reset or halted on connect

• No write tools - there are no variable-write, memory-write, flash, erase, breakpoint, reset, or run-control operations

• Read failure is not zero - errors are returned faithfully; callers must not treat a failed read as a zero value

• Probe state preserved - the probe is left in its original connection state when the session ends

Notes

• This project does not use Keil UVSC64.dll / UVSOCK; it talks to the probe directly via pyOCD, so it does not require Keil MDK.

• It works with any toolchain that produces DWARF debug info: Keil MDK (.axf), GCC (.elf), IAR (.out).

• snapshot and read_registers halt the target briefly; everything else is non-halting.

License

MIT - see LICENSE.