Ltspice

I wanted to learn about MCP, slash commands and claude.md files. Are DSLs a thing of the past? I created the following with Claude:

An MCP (Model Context Protocol) server that gives an LLM direct, tool-based access to LTspice circuit simulations — read schematics, set component values, run simulations, parse results, and generate plots, all from a Claude conversation.

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What it does

1. Read & modify schematics — parse .asc files, inspect components and parameters, patch values
2. Run simulations — invoke LTspice headlessly via Wine (-b batch mode), get back log and output files
3. Analyze results — parse .raw / .op.raw binary output: waveforms, node voltages, device currents
4. Generate plots — auto-generated matplotlib scripts saved next to the schematic as a reproducible record

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MCP Tools

Tool	Description
list_schematics	Discover .asc files and associated outputs in a directory
read_schematic	Parse components, params, includes, directives from .asc
read_parameters	Read .param values from a .inc or .asc file
write_parameters	Write/overwrite .param values in a .inc file
run_simulation	Run LTspice via Wine (-b batch), return log + output file list
read_waveforms	Parse transient/AC .raw output — returns waveforms
read_op_point	Parse DC operating point .op.raw — returns node voltages/currents
modify_component	Patch SYMATTR Value of a named component directly in .asc
write_schematic	Generate a new .asc schematic from a component/netlist description

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/analyze-circuit slash command

The flagship workflow. Run it from Claude Code on any .asc schematic:

/analyze-circuit examples/rc_filter/mystery_circuit.asc
/analyze-circuit /absolute/path/to/my_filter.asc
/analyze-circuit          ← lists available schematics and prompts

It will:

1. Run the simulation via run_simulation
2. Read the DC operating point — transistor bias, operating region
3. Discover all waveforms (names, ranges, simulation type)
4. Detect high-frequency signals and switch to full-resolution rendering automatically
5. Write and execute a <stem>_analysis.py plot script next to the schematic
6. Display the plot inline and provide a written circuit explanation

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Example Analysis Output: AM Modulator (mystery_circuit.asc)

A collector-modulated AM transmitter built around a single 2N3904 NPN BJT.

; Sources
V1  30 V DC                        ; supply
V2  SINE(0 30m 600k)               ; RF carrier — 30 mV, 600 kHz
V3  SINE(0 3.5 1k)                 ; audio modulating signal — 3.5 V, 1 kHz

; Bias network
R1  56k    ; base voltage divider (top)
R2  15k    ; base voltage divider (bottom)
R3  10k    ; collector load
R4  4.7k   ; emitter resistor — V3 (audio) connects at its bottom terminal

; Signal coupling
C1  100n   ; AC-couples carrier (V2) into base
C2  100n   ; emitter bypass (shorts R4 at RF frequencies)
C3  470p   ; output coupling cap — collector → V(n002) load output

Q1  2N3904 ; common-emitter RF amplifier
R5  1k     ; output load

How it works: V3 (audio, 3.5 V) sits in series with R4 between the emitter and ground. As the audio signal swings, it shifts the effective emitter voltage at 1 kHz, varying Vbe and hence gm. Since the RF carrier is simultaneously being amplified at the base, its gain varies at the audio rate — producing AM modulation. C3 AC-couples the collector output to the load.

DC bias (Q1 in active region):

Parameter	Value
Vcc	30.0 V
Vc (collector)	18.0 V
Vbe	0.66 V
Vce	12.4 V
Ic	1.20 mA
β	334

/analyze-circuit output:

The top panel uses LTspice-style min/max-per-pixel rendering to produce the AM "football" shape at full 2 ms resolution — the green filled band is the 600 kHz carrier, its amplitude envelope tracing the 1 kHz audio signal (red dashed). The bottom panel zooms to 10–30 µs to show individual carrier cycles.

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Setup

# Install in editable mode
pip install -e .

# LTspice runs via Wine — set path if different from default:
export LTSPICE_EXE="/home/user/.wine/drive_c/Program Files/LTC/LTspiceXVII/XVIIx64.exe"

MCP config (~/.config/claude/claude_desktop_config.json)

{
  "mcpServers": {
    "ltspice": {
      "command": "ltspice-mcp"
    }
  }
}

Or without installing the package:

{
  "mcpServers": {
    "ltspice": {
      "command": "python",
      "args": ["-m", "ltspice_mcp.server"],
      "cwd": "/path/to/ltspice-mcp/src"
    }
  }
}

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Workflow example: RC filter optimization

list_schematics(directory="examples/rc_filter/")
read_schematic(path="rc_filter.asc")           # inspect components
read_parameters(path="params.inc")             # current R=1k, C=100n → fc≈1.6kHz
write_parameters(path="params.inc", parameters={"R": 2200, "C": 47e-9})
run_simulation(schematic_path="rc_filter.asc")
read_waveforms(raw_path="rc_filter.raw", signals=["V(out)"])
# adjust values, repeat

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Project structure

src/ltspice_mcp/
  server.py           — MCP server, all tool definitions
  raw_parser.py       — .raw file parser (binary UTF-16-LE and ASCII)
  runner.py           — Wine subprocess runner (two-step: netlist → sim)
  schematic_writer.py — .asc generator (auto-layout + wire routing)
examples/
  rc_filter/
    rc_filter.asc                   — RC low-pass filter demo
    mystery_circuit.asc             — AM modulator (this example)
    mystery_circuit_analysis.py     — auto-generated plot script
    mystery_circuit_analysis.png    — analysis output
.claude/commands/
  analyze-circuit.md  — /analyze-circuit slash command definition

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Requirements

• Python 3.10+
• Wine with LTspice XVII installed
• matplotlib, numpy (for generated plot scripts)
• MCP-compatible client (Claude Desktop or Claude Code)