debugium-dap-debugger

name	Debugium DAP Debugger
description	Drive live debug sessions for Python, Node.js, TypeScript, C, C++, Rust, Java, Scala, and WebAssembly using the Debugium MCP tools. Use when asked to debug code, set breakpoints, inspect variables, step through execution, trace bugs, or find why something crashes.

Debugium Debugger Skill

Debugium is a DAP (Debug Adapter Protocol) client with an MCP interface and a real-time web UI. You can control any active debug session — set breakpoints, step through code, inspect live values, record findings, and annotate the source editor.

---

Setup & Connection

1. Install

cargo install --path crates/debugium-server

2. Register MCP server

Add to .mcp.json (project root) or ~/.claude.json:

{
  "mcpServers": {
    "debugium": { "command": "debugium", "args": ["mcp"] }
  }
}

3. Install language prerequisites

Language	Install
Python	pip install debugpy
Node.js / TypeScript	js-debug (build from microsoft/vscode-js-debug) + npm i -g tsx for TS
C / C++ / Rust	lldb-dap (ships with LLVM: brew install llvm)
Java	microsoft/java-debug adapter JAR
Scala	Running Metals language server with DAP

4. Launch a debug session

Preferred: use a dap.json config from examples/. All paths must be absolute.

# Python — multiple breakpoints with -b
debugium launch /abs/path/script.py --config examples/python.dap.json \
  -b /abs/path/script.py:42 -b /abs/path/script.py:67

# Comma-separated lines in one file
debugium launch /abs/path/script.py --adapter python \
  --breakpoint /abs/path/script.py:10,15,20

# Node.js
debugium launch /abs/path/app.js --config examples/node.dap.json \
  -b /abs/path/app.js:15 -b /abs/path/app.js:30

# TypeScript
debugium launch /abs/path/app.ts --config examples/typescript.dap.json \
  -b /abs/path/app.ts:10

# C / C++ (compile with debug symbols first: cc -g -O0)
debugium launch /tmp/a.out --config examples/c-cpp.dap.json \
  -b /abs/path/main.c:20

# Rust (cargo build first)
debugium launch ./target/debug/myapp --config examples/c-cpp.dap.json \
  -b /abs/path/src/main.rs:10

# Remote attach (debugpy already listening on 127.0.0.1:5678)
python3 -m debugpy --listen 127.0.0.1:5678 --wait-for-client app.py &
debugium launch app.py --config examples/remote-python.dap.json \
  -b /abs/path/app.py:42

Shorthand (Python only): debugium launch script.py --adapter python -b ...

Auto-discovery: place dap.json in project root, then just debugium launch program -b ...

5. Verify connection

Call get_sessions — if empty, the server isn't running. Launch a session first.

---

Web UI

The debugger launches a web UI automatically (unless --no-open-browser). Features:

• Layout presets: Slim (source only), Std (console collapsed), Full (everything open)
• Light/Dark mode: toggle with Ctrl/Cmd+D or the header button
• Panels: Source, Console, Variables, Stack, Breakpoints, Watch, Findings, Timeline
• Icon toolbar: Continue, Pause, Step In/Over/Out, Stop, Restart (F-key shortcuts)
• AI activity: LLM tool calls are shown in real-time via the console

---

Debugging Workflow

Standard loop

1. launch_session        – start a debug session (or attach_session for remote targets)
   attach_session        – attach to a running process (debugpy, JDWP, Node inspector)
2. get_debug_context     – orient: paused_at + locals + call_stack + source_window + breakpoints in ONE call
3. evaluate / get_variables  – inspect specific values
4. step_over / step_in   – advance (blocking: waits for pause, safe to chain)
5. get_debug_context     – re-orient after stepping
6. annotate / add_finding – record conclusions in the UI
7. Repeat 3–6 as needed
8. stop_session          – clean up when done

Key insight: get_debug_context replaces the old 7-call chain of get_threads → get_stack_trace → get_scopes → get_variables. Use it first.

---

Tool Reference

Session

launch_session

Launch a new debug session autonomously — no human intervention needed. Spawns the adapter, sets breakpoints, waits until paused.

{ "program": "/abs/path/script.py", "adapter": "python", "breakpoints": ["/abs/path/script.py:42"] }

Returns { "session_id": "...", "status": "paused" | "running" }. Use the returned session_id for all subsequent tool calls.

attach_session

Attach to a running debug target (JVM via JDWP, Python via debugpy, Node via inspector, or a remote DAP server). Spawns or connects a debug adapter, sets breakpoints, waits until paused.

{ "port": 5005, "adapter": "java", "host": "127.0.0.1", "breakpoints": ["/abs/path/App.java:42"] }

Parameters:

• port (required): Remote debug port (e.g. 5005 for JDWP, 5678 for debugpy, 9229 for Node inspector)
• adapter: "java", "python", "node", "lldb" — auto-detected from program extension if omitted
• host: Remote host (default: "127.0.0.1")
• program: Path to source file (for breakpoints and source context)
• breakpoints: Array of "file:line" strings
• attach_args: Custom args merged into the DAP attach request (overrides defaults)
• session_id: Optional custom session ID

Returns { "session_id": "...", "status": "paused" | "running" }.

stop_session

Stop and clean up a debug session. Sends disconnect, kills adapter, removes from registry.

{ "session_id": "session-123" }

get_sessions / list_sessions

List active sessions. Empty = server not started.

{}

---

Source

get_source

Read a source file with optional line zoom.

{ "path": "/abs/path/to/file.py", "around_line": 42, "context_lines": 10 }

---

Breakpoints

set_breakpoints

Set breakpoints in a file (replaces all existing in that file).

{ "file": "/abs/path/to/script.py", "lines": [42, 67, 103] }

set_breakpoint

Set or update a single breakpoint with an optional condition.

{ "file": "/abs/path/to/script.py", "line": 42, "condition": "x > 10" }

list_breakpoints / clear_breakpoints

{}

set_function_breakpoints

Break on a function name.

{ "names": ["my_function", "ClassName.method"] }

set_exception_breakpoints

{ "filter": "raised" }   // or "uncaught"

---

Execution Control

continue_execution

Resume until the next breakpoint. Returns console_line_count — pass it to wait_for_output as from_line to avoid matching stale output.

{ "thread_id": 1 }

Returns: { "status": "running", "console_line_count": 42, "hint": "..." }

step_over / step_in / step_out

Blocking — waits for the adapter to pause before returning. Safe to chain back-to-back without sleeps. Returns confirmation + "Call get_debug_context for location."

{ "thread_id": 1 }

pause / restart / terminate / disconnect

{ "thread_id": 1 }

---

Inspection (use when paused)

get_debug_context ★ START HERE

Single call returning: paused_at, locals, call_stack, source_window (±5 lines), breakpoints, frame_id, thread_id. Replaces the old get_threads→stack→scopes→vars chain.

{}

get_threads

{}

get_stack_trace

{ "thread_id": 1, "depth": 20 }

Returns frames with id (frame_id), name, line, source.

get_scopes

{ "frame_id": 2 }

Returns scopes (Locals, Globals) each with variablesReference.

get_variables

{ "variables_reference": 6 }

Nested objects have their own variablesReference — call recursively to drill in.

evaluate

Evaluate any expression in the current frame.

{ "expression": "len(my_list)", "frame_id": 2, "context": "repl" }

set_variable

Mutate a variable in the current scope.

{ "variables_reference": 6, "name": "counter", "value": "0" }

get_exception_info

Details about the current exception (when stopped on exception).

{}

get_capabilities

What the adapter supports.

{}

---

Console Output

get_console_output

Last N lines of stdout/stderr.

{ "lines": 50 }

wait_for_output

Poll until stdout matches a regex (or timeout). Use from_line from continue_execution to only match new output — not output from earlier in the session.

{ "pattern": "Error.*line", "from_line": 42, "timeout_secs": 10 }

---

History & Timeline

get_timeline

Every stop in this session: file, line, changed variables, stack summary.

{ "limit": 50 }

get_variable_history

How one variable's value changed across all stops. Answers "when did X go wrong?"

{ "name": "counter" }

---

Annotations & Findings (record conclusions in the UI)

annotate

Pin a colored marker on a source line, visible to the human in the editor.

{ "file": "/abs/path/to/file.py", "line": 42, "message": "off-by-one here", "color": "red" }

get_annotations

Read back all annotations you've already placed — do this at session start to avoid re-investigating known lines.

{}

add_finding

Record a structured conclusion in the Findings panel.

{ "message": "counter overflows at iteration 256", "level": "error" }

Levels: info, warning, error.

get_findings

Read back all findings — do this at session start to avoid restating known conclusions.

{}

---

Watch Expressions

add_watch / remove_watch

Expressions evaluated automatically at every stop.

{ "expression": "len(queue)" }

get_watches

Current expressions + their last evaluated values.

{}

---

Compound / LLM Tools

step_until

Step until an expression becomes true (up to max_steps).

{ "condition": "i == 10", "max_steps": 50 }

run_until_exception

Continue until any exception is raised.

{}

compare_snapshots

Diff variable snapshots between two timeline stops.

{ "stop_a": 3, "stop_b": 7 }

find_first_change

Find the first timeline stop where a variable changed (optionally from an expected value).

{ "variable_name": "counter", "expected_value": "0" }

get_call_tree

Stack + locals for each frame in one call.

{ "max_depth": 5 }

step_until_change

Step until a variable's value changes.

{ "variable_name": "status", "max_steps": 20 }

explain_exception

When stopped on an exception, gather all relevant context in one call.

{}

restart_frame

Re-run execution from a specific stack frame (requires supportsRestartFrame).

{ "frame_id": 2 }

---

Navigation & Source Discovery

goto_targets

Get valid jump targets for a given source location (requires supportsGotoTargetsRequest).

{ "file": "/abs/path/to/file.py", "line": 43 }

goto

Jump execution to a target without running intermediate code.

{ "thread_id": 1, "target_id": 0 }

breakpoint_locations

Get valid breakpoint positions in a line range (requires supportsBreakpointLocationsRequest).

{ "file": "/abs/path/to/file.js", "line": 30, "end_line": 40 }

step_in_targets

List possible step-in targets when a line has multiple calls (requires supportsStepInTargetsRequest).

{ "frame_id": 0 }

loaded_sources

List all source files currently loaded by the adapter (requires supportsLoadedSourcesRequest).

{}

source_by_reference

Fetch source code for generated/internal code by sourceReference ID.

{ "source_reference": 2017626721 }

---

Mutation

set_expression

Set the value of an expression (requires supportsSetExpression).

{ "expression": "obj.field", "value": "42", "frame_id": 0 }

---

Memory & Disassembly (native debugging)

read_memory

Read raw bytes from debuggee memory (requires supportsReadMemoryRequest).

{ "memory_reference": "0x7fff5000", "count": 128 }

write_memory

Write raw bytes to debuggee memory (requires supportsWriteMemoryRequest).

{ "memory_reference": "0x7fff5000", "data": "AQIDBA==" }

disassemble

Disassemble machine instructions (requires supportsDisassembleRequest).

{ "memory_reference": "0x100003f00", "instruction_count": 20 }

---

Control

cancel_request

Cancel an in-flight request (requires supportsCancelRequest).

{ "request_id": 42 }

---

Data Breakpoints (watchpoints)

set_data_breakpoint

Break when a variable is written/read.

{ "name": "counter", "access_type": "write" }

list_data_breakpoints / clear_data_breakpoints

{}

---

Session Persistence

export_session

Export breakpoints, annotations, findings, and watches as a JSON bundle.

{}

import_session

Restore exported state into the current session.

{ "data": { "..." } }

---

Supported Adapters

Language	--adapter flag	Prerequisite	Verified
Python	python / debugpy	pip install debugpy	✅
Node.js	node / js	js-debug (bundled)	✅
TypeScript	typescript / ts / tsx	js-debug + tsx or ts-node	✅
C / C++	lldb / codelldb	lldb-dap	✅
Rust	lldb / rust	lldb-dap + cargo build	✅
Java	java / jvm	microsoft/java-debug adapter JAR	✅
Scala	--config scala-jvm.dap.json	scalac + Scala library JAR	✅
WebAssembly	--config wasm.dap.json	wasmtime + lldb-dap (LLVM ≥16)	✅
Any adapter	--config dap.json	See dap.json.example	✅

Remote attach is supported via attach_session MCP tool or dap.json with host + port fields.

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CLI Breakpoint Syntax

# Repeated -b flags (short for --breakpoint)
debugium launch app.py --adapter python -b app.py:10 -b app.py:20 -b other.py:5

# Comma-separated lines in one file
debugium launch app.py --adapter python --breakpoint app.py:10,15,20

# Mix both
debugium launch app.py --adapter python -b app.py:10,20 -b other.py:5

---

Tips

1. Start with get_debug_context — not get_source. Live runtime values beat static reading.
2. Steps are blocking — chain step_over calls freely; each confirms the pause before returning.
3. Thread console_line_count — pass it from continue_execution into wait_for_output as from_line to avoid false positives on old output.
4. Read before re-investigating — call get_annotations and get_findings at the start of each session to see what you already know.
5. Use evaluate before stepping — cheaper than advancing line-by-line. Narrow the bug first.
6. Use step_until — instead of manually looping, let the tool advance until your condition fires.
7. Drill nested variables — any variablesReference > 0 has children; call get_variables recursively.
8. Set all breakpoints at once — one set_breakpoints call beats many individual ones.