Add LTspice simulation engine via mcltspice

Wire up LTspice as a second simulation engine using mcltspice (Wine-based
LTspice runner). The backend architecture already had the ABC + factory
pattern; this connects the ltspice branch.

- Extract shared raw_to_waveform() into raw_convert.py with Protocol typing
  so both ngspice and mcltspice RawFile objects work without coupling
- Add LtspiceEngine with deferred mcltspice import for graceful degradation
- Register "ltspice" in get_engine() with availability check
  (mcltspice + Wine + LTspice.exe must all be present)
- Add startup validation log for LTspice availability
- Add mcltspice as optional dependency: pip install spicebook[ltspice]
- Integration tests auto-skip when LTspice is unavailable (Docker/CI)

backend/Dockerfile

@@ -1,6 +1,9 @@
 FROM ghcr.io/astral-sh/uv:python3.12-bookworm-slim AS base
 
 # System dependencies -- ngspice is required for simulation
+# NOTE: LTspice requires Wine + a Windows LTspice install, which is only
+# available on the dev host (not in this container). The backend gracefully
+# degrades -- LTspice engine returns UnsupportedEngineError in Docker.
 RUN apt-get update && \
     apt-get install -y --no-install-recommends ngspice && \
     rm -rf /var/lib/apt/lists/*

backend/pyproject.toml

@@ -17,6 +17,7 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
+ltspice = ["mcltspice>=2026.02.14"]
 dev = [
     "pytest>=8.0",
     "pytest-asyncio>=0.23",

backend/src/spicebook/engine/__init__.py

@@ -1,15 +1,45 @@
 """SPICE simulation engine registry."""
 
+import logging
+
 from spicebook.engine.base import SpiceEngine
 from spicebook.engine.ngspice import NgspiceEngine
 
+logger = logging.getLogger(__name__)
+
 
 class UnsupportedEngineError(ValueError):
     """Raised when an unknown engine name is requested."""
 
 
+def _ltspice_available() -> bool:
+    """Check whether mcltspice + Wine + LTspice.exe are all present."""
+    try:
+        from mcltspice.config import validate_installation
+
+        ok, msg = validate_installation()
+        if not ok:
+            logger.debug("LTspice not available: %s", msg)
+        return ok
+    except ImportError:
+        return False
+    except Exception as exc:
+        logger.warning("LTspice availability check failed: %s", exc)
+        return False
+
+
 def get_engine(engine_name: str) -> SpiceEngine:
     """Resolve a simulation engine by name."""
     if engine_name == "ngspice":
         return NgspiceEngine()
+
+    if engine_name == "ltspice":
+        if not _ltspice_available():
+            raise UnsupportedEngineError(
+                "LTspice engine is not available (requires mcltspice + Wine + LTspice.exe)"
+            )
+        from spicebook.engine.ltspice import LtspiceEngine
+
+        return LtspiceEngine()
+
     raise UnsupportedEngineError(f"Unsupported engine: '{engine_name}'")

backend/src/spicebook/engine/ltspice.py

@@ -0,0 +1,98 @@
+"""LTspice simulation engine -- delegates to mcltspice via Wine."""
+
+import logging
+import time
+from pathlib import Path
+
+from spicebook.engine.base import SpiceEngine
+from spicebook.engine.raw_convert import raw_to_waveform
+from spicebook.models.simulation import SimulationResponse
+
+logger = logging.getLogger(__name__)
+
+# Wine overhead means LTspice needs more headroom than native ngspice
+LTSPICE_TIMEOUT_SECONDS = 120
+
+
+class LtspiceEngine(SpiceEngine):
+    """Run simulations via LTspice through Wine using mcltspice."""
+
+    async def run(self, netlist: str, work_dir: Path) -> SimulationResponse:
+        t0 = time.monotonic()
+
+        # Deferred import so the module loads even without mcltspice installed
+        try:
+            from mcltspice.runner import run_netlist
+        except ImportError as exc:
+            logger.error("mcltspice.runner import failed: %s", exc)
+            return SimulationResponse(
+                success=False,
+                error=f"mcltspice runner unavailable: {exc}",
+                elapsed_seconds=time.monotonic() - t0,
+            )
+
+        # Ensure netlist ends with .end
+        netlist_stripped = netlist.rstrip()
+        if not netlist_stripped.lower().endswith(".end"):
+            netlist_stripped += "\n.end\n"
+        else:
+            netlist_stripped += "\n"
+
+        cir_path = work_dir / "input.cir"
+        try:
+            cir_path.write_text(netlist_stripped)
+        except OSError as exc:
+            return SimulationResponse(
+                success=False,
+                error=f"Failed to write netlist to disk: {exc}",
+                elapsed_seconds=time.monotonic() - t0,
+            )
+
+        try:
+            result = await run_netlist(
+                netlist_path=cir_path,
+                timeout=LTSPICE_TIMEOUT_SECONDS,
+            )
+        except Exception:
+            logger.exception("LTspice execution failed unexpectedly")
+            return SimulationResponse(
+                success=False,
+                error="LTspice execution failed (see server logs)",
+                elapsed_seconds=time.monotonic() - t0,
+            )
+
+        log_text = result.stdout + result.stderr
+
+        if not result.success:
+            return SimulationResponse(
+                success=False,
+                log=log_text,
+                error=result.error or "LTspice simulation failed",
+                elapsed_seconds=result.elapsed_seconds,
+            )
+
+        if result.raw_data is None:
+            return SimulationResponse(
+                success=False,
+                log=log_text,
+                error=result.error or "No waveform data produced",
+                elapsed_seconds=result.elapsed_seconds,
+            )
+
+        try:
+            waveform = raw_to_waveform(result.raw_data)
+        except Exception:
+            logger.exception("Failed to convert LTspice raw data to waveform")
+            return SimulationResponse(
+                success=False,
+                log=log_text,
+                error="Failed to parse LTspice output (see server logs)",
+                elapsed_seconds=result.elapsed_seconds,
+            )
+
+        return SimulationResponse(
+            success=True,
+            waveform=waveform,
+            log=log_text,
+            elapsed_seconds=result.elapsed_seconds,
+        )

backend/src/spicebook/engine/ngspice.py

@@ -11,11 +11,8 @@ import numpy as np
 
 from spicebook.config import settings
 from spicebook.engine.base import SpiceEngine
-from spicebook.models.simulation import (
-    SimulationResponse,
-    WaveformData,
-    WaveformVariable,
-)
+from spicebook.engine.raw_convert import raw_to_waveform
+from spicebook.models.simulation import SimulationResponse
 
 logger = logging.getLogger(__name__)
 
@@ -300,7 +297,7 @@ class NgspiceEngine(SpiceEngine):
                 elapsed_seconds=time.monotonic() - t0,
             )
 
-        waveform = _raw_to_waveform(raw)
+        waveform = raw_to_waveform(raw)
 
         return SimulationResponse(
             success=True,
@@ -318,64 +315,3 @@ def _extract_error_lines(log: str) -> str:
         if "error" in lower or "fatal" in lower or "cannot" in lower:
             error_lines.append(line.strip())
     return "\n".join(error_lines) if error_lines else ""
-
-
-def _raw_to_waveform(raw: RawFile) -> WaveformData:
-    """Convert parsed RawFile into a WaveformData response model."""
-    is_complex = "complex" in raw.flags
-
-    # Determine x-axis variable (first variable is always the sweep)
-    x_var = raw.variables[0]
-    x_type = "frequency" if "frequency" in x_var.type.lower() else "time"
-
-    # Build variable list (skip x-axis variable)
-    variables = []
-    for v in raw.variables[1:]:
-        variables.append(WaveformVariable(name=v.name, type=v.type))
-
-    # Convert x data
-    x_array = raw.data[0]
-    if is_complex:
-        x_data = np.real(x_array).tolist()
-    else:
-        x_data = x_array.tolist()
-
-    # Convert y data
-    y_data: dict[str, list[float]] = {}
-    y_magnitude_db: dict[str, list[float]] | None = None
-    y_phase_deg: dict[str, list[float]] | None = None
-
-    if is_complex:
-        y_magnitude_db = {}
-        y_phase_deg = {}
-
-        for v in raw.variables[1:]:
-            arr = raw.data[v.index]
-            # Real part as the default y_data
-            y_data[v.name] = np.real(arr).tolist()
-
-            # Magnitude in dB
-            magnitude = np.abs(arr)
-            # Avoid log10(0) -- clamp to a very small value
-            magnitude = np.where(magnitude > 0, magnitude, 1e-30)
-            mag_db = 20.0 * np.log10(magnitude)
-            y_magnitude_db[v.name] = mag_db.tolist()
-
-            # Phase in degrees
-            phase = np.degrees(np.angle(arr))
-            y_phase_deg[v.name] = phase.tolist()
-    else:
-        for v in raw.variables[1:]:
-            arr = raw.data[v.index]
-            y_data[v.name] = arr.tolist()
-
-    return WaveformData(
-        variables=variables,
-        points=raw.points,
-        x_data=x_data,
-        y_data=y_data,
-        x_type=x_type,
-        is_complex=is_complex,
-        y_magnitude_db=y_magnitude_db,
-        y_phase_deg=y_phase_deg,
-    )

backend/src/spicebook/engine/raw_convert.py

@@ -0,0 +1,96 @@
+"""Shared raw-file-to-waveform conversion for all SPICE engines.
+
+Both ngspice and mcltspice produce RawFile objects with the same structural
+shape (data ndarray, variables list, flags, points). This module provides
+a single converter that works with either via Protocol typing.
+"""
+
+from collections.abc import Sequence
+from typing import Protocol
+
+import numpy as np
+
+from spicebook.models.simulation import WaveformData, WaveformVariable
+
+
+class RawVariable(Protocol):
+    @property
+    def index(self) -> int: ...
+    @property
+    def name(self) -> str: ...
+    @property
+    def type(self) -> str: ...
+
+
+class RawFilelike(Protocol):
+    @property
+    def flags(self) -> Sequence[str]: ...
+    @property
+    def variables(self) -> Sequence[RawVariable]: ...
+    @property
+    def points(self) -> int: ...
+    @property
+    def data(self) -> np.ndarray: ...
+
+
+def raw_to_waveform(raw: RawFilelike) -> WaveformData:
+    """Convert a parsed RawFile into a WaveformData response model.
+
+    Works with any raw-file object that satisfies the RawFilelike protocol
+    (ngspice RawFile, mcltspice RawFile, etc.).
+    """
+    if not raw.variables:
+        raise ValueError("Raw file contains no variables -- simulation produced no output")
+    if raw.data.shape[0] == 0 or (raw.data.ndim > 1 and raw.data.shape[1] == 0):
+        raise ValueError(f"Raw file data is empty (shape {raw.data.shape})")
+
+    n_rows = raw.data.shape[0]
+    for v in raw.variables:
+        if v.index >= n_rows:
+            raise ValueError(
+                f"Variable '{v.name}' index {v.index} exceeds data rows ({n_rows})"
+            )
+
+    is_complex = "complex" in raw.flags
+
+    # First variable is always the sweep axis
+    x_var = raw.variables[0]
+    x_type = "frequency" if "frequency" in x_var.type.lower() else "time"
+
+    variables = [
+        WaveformVariable(name=v.name, type=v.type) for v in raw.variables[1:]
+    ]
+
+    x_array = raw.data[0]
+    x_data = np.real(x_array).tolist() if is_complex else x_array.tolist()
+
+    y_data: dict[str, list[float]] = {}
+    y_magnitude_db: dict[str, list[float]] | None = None
+    y_phase_deg: dict[str, list[float]] | None = None
+
+    if is_complex:
+        y_magnitude_db = {}
+        y_phase_deg = {}
+
+        for v in raw.variables[1:]:
+            arr = raw.data[v.index]
+            y_data[v.name] = np.real(arr).tolist()
+
+            magnitude = np.abs(arr)
+            magnitude = np.where(magnitude > 0, magnitude, 1e-30)
+            y_magnitude_db[v.name] = (20.0 * np.log10(magnitude)).tolist()
+            y_phase_deg[v.name] = np.degrees(np.angle(arr)).tolist()
+    else:
+        for v in raw.variables[1:]:
+            y_data[v.name] = raw.data[v.index].tolist()
+
+    return WaveformData(
+        variables=variables,
+        points=raw.points,
+        x_data=x_data,
+        y_data=y_data,
+        x_type=x_type,
+        is_complex=is_complex,
+        y_magnitude_db=y_magnitude_db,
+        y_phase_deg=y_phase_deg,
+    )

backend/src/spicebook/main.py

@@ -23,6 +23,7 @@ logger = logging.getLogger("spicebook")
 async def lifespan(app: FastAPI):
     _configure_logging()
     _validate_ngspice()
+    _validate_ltspice()
     _ensure_directories()
     yield
     await close_client()
@@ -119,6 +120,20 @@ def _validate_ngspice() -> None:
         )
 
 
+def _validate_ltspice() -> None:
+    """Check whether LTspice is available (never blocks startup)."""
+    try:
+        from mcltspice.config import validate_installation
+
+        ok, msg = validate_installation()
+        if ok:
+            logger.info("LTspice engine available: %s", msg)
+        else:
+            logger.info("LTspice engine not available: %s", msg)
+    except ImportError:
+        logger.info("LTspice engine not available: mcltspice not installed")
+
+
 def _ensure_directories() -> None:
     """Create notebook directories if they don't exist."""
     settings.notebook_dir.mkdir(parents=True, exist_ok=True)

backend/tests/test_ltspice.py

@@ -0,0 +1,127 @@
+"""Integration tests for LtspiceEngine with RC circuits.
+
+Skipped automatically when mcltspice + Wine + LTspice aren't available
+(e.g., Docker production, CI).
+"""
+
+import tempfile
+from pathlib import Path
+
+import pytest
+
+# Determine availability at import time for skip markers
+_ltspice_available = False
+try:
+    from mcltspice.config import validate_installation
+
+    _ltspice_available, _ = validate_installation()
+except ImportError:
+    pass
+
+pytestmark = pytest.mark.skipif(
+    not _ltspice_available,
+    reason="LTspice not available (mcltspice + Wine + LTspice.exe required)",
+)
+
+from spicebook.engine.ltspice import LtspiceEngine  # noqa: E402
+
+RC_TRANSIENT_NETLIST = """\
+RC Low-Pass Filter - Transient
+V1 in 0 PULSE(0 5 0 1n 1n 5m 10m)
+R1 in out 1k
+C1 out 0 1u
+.tran 100u 20m
+.end
+"""
+
+RC_AC_NETLIST = """\
+RC Low-Pass Filter - AC
+V1 in 0 AC 1
+R1 in out 1k
+C1 out 0 1u
+.ac dec 50 1 1Meg
+.end
+"""
+
+
+@pytest.fixture
+def engine():
+    return LtspiceEngine()
+
+
+@pytest.fixture
+def work_dir():
+    d = tempfile.mkdtemp(prefix="spicebook-ltspice-test-")
+    yield Path(d)
+    import shutil
+
+    shutil.rmtree(d, ignore_errors=True)
+
+
+@pytest.mark.asyncio
+async def test_transient_simulation(engine, work_dir):
+    """Run a transient analysis via LTspice and verify waveform output."""
+    result = await engine.run(RC_TRANSIENT_NETLIST, work_dir)
+
+    assert result.success, f"Simulation failed: {result.error}\n{result.log}"
+    assert result.waveform is not None
+    assert result.waveform.x_type == "time"
+    assert result.waveform.points > 0
+    assert len(result.waveform.x_data) == result.waveform.points
+    assert result.elapsed_seconds > 0
+
+    # Should have at least one signal in y_data
+    assert len(result.waveform.y_data) > 0
+
+    # Check that V(out) or similar exists
+    signal_names = list(result.waveform.y_data.keys())
+    has_out = any("out" in name.lower() for name in signal_names)
+    assert has_out, f"Expected 'out' signal in {signal_names}"
+
+    # Verify data length matches
+    for name, values in result.waveform.y_data.items():
+        assert len(values) == result.waveform.points, f"Length mismatch for {name}"
+
+
+@pytest.mark.asyncio
+async def test_ac_simulation(engine, work_dir):
+    """Run an AC analysis via LTspice and verify complex waveform output."""
+    result = await engine.run(RC_AC_NETLIST, work_dir)
+
+    assert result.success, f"Simulation failed: {result.error}\n{result.log}"
+    assert result.waveform is not None
+    assert result.waveform.x_type == "frequency"
+    assert result.waveform.is_complex
+    assert result.waveform.points > 0
+
+    # AC analysis should produce magnitude and phase data
+    assert result.waveform.y_magnitude_db is not None
+    assert result.waveform.y_phase_deg is not None
+    assert len(result.waveform.y_magnitude_db) > 0
+    assert len(result.waveform.y_phase_deg) > 0
+
+
+@pytest.mark.asyncio
+async def test_netlist_without_end_directive(engine, work_dir):
+    """Engine should auto-append .end if missing."""
+    netlist_no_end = """\
+Simple Resistor Divider
+V1 in 0 DC 10
+R1 in out 1k
+R2 out 0 1k
+.tran 1u 1m
+"""
+    result = await engine.run(netlist_no_end, work_dir)
+    assert result.success, f"Simulation failed: {result.error}\n{result.log}"
+
+
+@pytest.mark.asyncio
+async def test_invalid_netlist(engine, work_dir):
+    """Malformed netlist should return success=False with error info."""
+    bad_netlist = """\
+This is not a valid SPICE netlist
+.tran 1m 10m
+.end
+"""
+    result = await engine.run(bad_netlist, work_dir)
+    assert isinstance(result.success, bool)

backend/uv.lock

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@@ -579,6 +585,18 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/94/54/e7d793b573f298e1c9013b8c4dade17d481164aa517d1d7148619c2cedbf/markdown_it_py-4.0.0-py3-none-any.whl", hash = "sha256:87327c59b172c5011896038353a81343b6754500a08cd7a4973bb48c6d578147", size = 87321, upload-time = "2025-08-11T12:57:51.923Z" },
 ]
 
+[[package]]
+name = "mcltspice"
+version = "2026.3.5"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "fastmcp" },
+    { name = "numpy" },
+]
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/d6/ef/2cadafe520e8b87a74c976dbc45c9a90a04d8b5364ded50b274d5c783232/mcltspice-2026.3.5-py3-none-any.whl", hash = "sha256:35107b8f98455805dbb10aaf5ee30441d9700268c018f487f4da79c61c8c5217", size = 111326, upload-time = "2026-03-05T21:38:32.895Z" },
+]
+
 [[package]]
 name = "mcp"
 version = "1.26.0"
@@ -1223,6 +1241,9 @@ dev = [
     { name = "pytest-asyncio" },
     { name = "ruff" },
 ]
+ltspice = [
+    { name = "mcltspice" },
+]
 
 [package.metadata]
 requires-dist = [
@@ -1230,6 +1251,7 @@ requires-dist = [
     { name = "fastmcp", specifier = ">=3.0.0" },
     { name = "httpx", specifier = ">=0.28.0" },
     { name = "httpx", marker = "extra == 'dev'", specifier = ">=0.27" },
+    { name = "mcltspice", marker = "extra == 'ltspice'", specifier = ">=2026.2.14" },
     { name = "numpy", specifier = ">=1.24.0" },
     { name = "pydantic", specifier = ">=2.0" },
     { name = "pytest", marker = "extra == 'dev'", specifier = ">=8.0" },
@@ -1239,7 +1261,7 @@ requires-dist = [
     { name = "uvicorn", extras = ["standard"], specifier = ">=0.30.0" },
     { name = "websockets", specifier = ">=12.0" },
 ]
-provides-extras = ["dev"]
+provides-extras = ["ltspice", "dev"]
 
 [[package]]
 name = "sse-starlette"