Kconfig

kconfig-mcp

"Why can't I enable CONFIG_X?", answered with file:line and a fix that's been checked.

kconfig-mcp is an MCP server that lets an AI agent reason about Kconfig, the configuration system used by the Linux kernel, Zephyr, U-Boot, Buildroot and others.

Everyone who has configured a kernel has hit this: the option you want isn't in menuconfig, or it's there but greyed out, and the reason is five depends on hops away in a different directory. kconfig-mcp walks that chain for you. Then it searches for the smallest set of changes that would unlock the option, and applies them to the real tree to verify they work before it shows them to you.

What it looks like

This is real output on the current Linux tree (18,913 symbols) for x86:

> kconfig_why_not X86_X2APIC
X86_X2APIC = n (default), needs y - bool at arch/x86/Kconfig:463
  its prompt (arch/x86/Kconfig:463) is visible only if
  X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST), which is n:
    needs any one of (2 options, all currently too low):
      IRQ_REMAP = n (default), needs y - bool at drivers/iommu/Kconfig:201
        its prompt is visible only if X86_64 && X86_IO_APIC && PCI_MSI && ACPI && IOMMU_SUPPORT:
          PCI_MSI = n (default), needs y - bool at drivers/pci/Kconfig:44
            its prompt is visible only if PCI, which is n:
              PCI = n (default) - set PCI=y directly.
      HYPERVISOR_GUEST = n (default), needs y - bool at arch/x86/Kconfig:787
        its prompt is visible - set HYPERVISOR_GUEST=y directly.

Verified fix (2 change(s)): HYPERVISOR_GUEST=y, X86_X2APIC=y
  HYPERVISOR_GUEST: arch/x86/Kconfig:787
  X86_X2APIC: arch/x86/Kconfig:463
Unverified alternatives: PCI=y, PCI_MSI=y, IRQ_REMAP=y, X86_X2APIC=y

> kconfig_set HYPERVISOR_GUEST y
CONFIG_HYPERVISOR_GUEST: 'n' -> 'y'.
1 other symbol(s) changed as a consequence: X86_X2APIC n->y

The same question on a 32-bit build (ARCH=i386) ends somewhere no symbol change can reach, and the tool says so instead of inventing a fix:

64BIT = n (default), needs y - bool at arch/x86/Kconfig:3
  requires "i386" = "x86", which compares two constants and is always false here.
  One side was probably expanded from an environment variable (like $(ARCH)) when
  the tree was loaded: no symbol change can fix this - reload with a different env.
No verified fix: every route ends in something no assignment can change.

Why it's built this way

• Fixes are verified, not guessed. Kconfig is full of edge cases: tristate m needs MODULES, bools promote m to y, choices deselect their other members, select overrides depends on, and default y turns things on silently. So every candidate plan is applied to the loaded tree and checked. Only plans that really reach the goal are shown as fixes, together with their side effects (what else select/imply/defaults switch on or off).
• It parses current Linux. It's built on Kconfiglib, which hasn't had a release since 2020 and can't parse Linux's newer modules, transitional and depends on X if Y syntax. kconfig-mcp rewrites those lines as files are read, keeping every line number exact. depends on X if Y is lowered with a line-by-line port of expr_trans_compare from scripts/kconfig/expr.c, so it behaves exactly as kconfig does, including how kconfig treats !S when S is m.
• Environment problems are named. Linux sources arch/$(SRCARCH)/Kconfig, and an unset variable silently becomes "". kconfig-mcp tracks undefined variables and names them in the error (undefined variable(s): SRCARCH (the arch/ directory to use, ...)) instead of leaving you with arch//Kconfig not found.

Install

pip install kconfig-mcp
claude mcp add kconfig -- kconfig-mcp

Python 3.10+. You don't need a kernel build environment. A source tree with its Kconfig files is enough, even a sparse checkout:

git clone --depth 1 --filter=blob:none --sparse https://github.com/torvalds/linux.git
cd linux && git sparse-checkout set --no-cone '/Kconfig' '**/Kconfig*' '/arch/*/configs/*'

Loading trees

shell controls the tree's $(shell,...) calls, which is how Linux probes the compiler:

• "run" (default) runs them with a POSIX sh, as make menuconfig would. Compiler-dependent symbols then reflect your real toolchain.
• "dry" runs nothing. Linux's probe idioms get fixed answers: feature probes read as absent, and the toolchain reports as GCC 13.2. $(error-if) checks become warnings. Results then carry a note that compiler-dependent symbols such as CC_HAS_* read as absent.

Tools

Symbol names work with or without CONFIG_. A misspelt name gets "Did you mean: ...".

Limitations

• transitional symbols are always n. Real kconfig reads them from an old .config to migrate a value during a rename. Kconfiglib has no equivalent.
• The fix search is bounded. Selects and defaults are only explored when a symbol can't be reached through its own prompt, and the search has an expansion budget. If no verified fix is found, the result says whether the budget ran out or every route is genuinely blocked.
• why_not covers bool and tristate symbols. For int/hex/string symbols, use kconfig_symbol and kconfig_set.
• Dry mode answers compiler probes generically (see above), so trust CC_HAS_*-style symbols only with shell="run" on a real toolchain.
• It depends on Kconfiglib 14.1.0, pinned because compat.py hooks two of its internal methods. CI parses the latest Linux tree every week, so the next new keyword shows up as a failing build rather than a silent wrong answer.

Tests

pip install ".[test]" "ruff==0.16.0" "mypy==2.3.0"
ruff check src tests && mypy --strict src
pytest tests                                   # unit + stdio tests, fixture trees
LINUX_SRC=/path/to/linux pytest tests/test_linux_integration.py

The fixture tree in tests/fixtures/tree exercises every construct the explainer handles: depends on, select, imply, choice, visible if, menuconfig/if blocks, defaults, tristate and modules, and an arch directory chosen by $(SRCARCH). The Linux tests assert real facts, such as x2APIC's dependencies, the i386 dead end, and the x86_64 defconfig already enabling it. CI runs them against a sparse checkout of the current tree.

License

MIT. Kconfiglib is ISC-licensed.