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This release is a pre-release and may not be stable for production use.

llmdbg

llmdbg is an early-stage debugger for stepping through language-model layers and presenting human-readable interpretations of internal vectors. It speaks the Debug Adapter Protocol so editor integrations can use a standard debugger UI.

The current prototype includes real GPT-2 Small loading, layer-state capture, logit-lens interpretations, and a VS Code debug extension. A deterministic built-in trace remains available when no model is configured.

pip install --pre llmdbg
llmdbg --version

VS Code prototype

Build and install the extension:

cd vscode-extension
npm install
npm run package
code --install-extension llmdbg-0.0.1.vsix

Create a gpt2.llmdbg file in the workspace:

model = "openai-community/gpt2"
revision = "main"
device = "cuda"
dtype = "bfloat16"
prompt = "The capital of France is"
stop_on_entry = false

submodule = "token_embedding"

layer = 0
submodule = "transformer.h.0.attention"
submodule = "transformer.h.0.mlp"

layer = 1
submodule = "transformer.h.1.attention"
submodule = "transformer.h.1.mlp"

Open that file and press F5. If VS Code asks for a debugger, choose llmdbg. No launch.json is required. If a workspace launch configuration is preferred, the complete minimal version is:

{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "llmdbg",
      "request": "launch",
      "name": "llmdbg: Active file",
      "config": "${file}"
    }
  ]
}

On first use, the extension creates a private Python environment in VS Code's extension storage. It does not install anything into the workspace. The first GPT-2 run downloads and loads the model, then keeps its tensors resident in a persistent model-host process. Stopping and restarting the debugger reuses the resident model and runs only a new forward pass. Closing/reloading VS Code stops the host and releases the model. Each submodule statement is an authored model execution stop. Set breakpoints on these statements or use Step Over to advance through them. Runtime vectors appear in debugger scopes; the source file is never rewritten.

To stop at a specific prompt token and layer, add a conditional breakpoint to the desired submodule statement and use a zero-based condition such as token.position == 3. The debugger prefills tokens before that position into the KV cache, then captures the selected token with a one-token forward pass. Stepping beyond the last selected layer advances to the next token using the updated cache.

License

MIT

Metadata

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