fix: change snap_to_grid default grid from 2.54mm to 1.27mm (50 mil)

Half of all valid KiCAD schematic pin positions are on the 50-mil
(1.27mm) grid but not the 100-mil (2.54mm) grid — e.g. 26.67mm = 21 ×
1.27mm. Snapping to 2.54mm displaced those coordinates by 1.27mm,
moving labels off their pins and increasing floating-label count.

KiCAD source confirms: DEFAULT_CONNECTION_GRID_MILS = 50 and the ERC
off-grid check uses exact integer modulo against this value, so any
displacement breaks connectivity unconditionally.

Also update the kicad-source absolute path in CLAUDE.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,60 @@
+# KiCAD MCP Server
+
+## Related Source
+
+- KiCad source code is located at `../kicad-source/` (absolute: `/home/eugene/Projects/kicad-source/`)
+
+## Testing
+
+### When to Write Tests
+
+Write tests for every non-trivial change to Python handler or business logic code:
+
+- **New MCP tools** — add tests for schema validation, handler dispatch, parameter validation, and the core logic path (happy path + key error cases).
+- **Changes to existing tools** — add tests covering the changed behaviour; update any tests that no longer reflect reality.
+- **Bug fixes** — add a regression test that would have caught the bug before adding the fix.
+- **Refactors that delete or rename public methods** — add a test asserting the old name no longer exists (see `TestConnectionManagerOrphanedMethodsRemoved` for the pattern).
+
+You do **not** need tests for TypeScript/TS-layer glue code that only forwards calls to Python (the TS test runner is not yet configured).
+
+### Test Levels
+
+| Level       | Use for                                                                                            | Marker                     |
+| ----------- | -------------------------------------------------------------------------------------------------- | -------------------------- |
+| Unit        | Schema shape, parameter validation, pure logic, mock-heavy handler dispatch                        | `@pytest.mark.unit`        |
+| Integration | Real file I/O against a `.kicad_sch` / `.kicad_pcb` copy; WireManager, JunctionManager round-trips | `@pytest.mark.integration` |
+
+Keep unit tests free of file I/O. Keep integration tests free of business-logic assertions that belong in unit tests.
+
+### Where to Put Tests
+
+```
+tests/
+  test_<feature_name>.py       # all Python tests go here
+```
+
+Group related test classes inside a single file (e.g. `TestSchemas`, `TestHandlerDispatch`, `TestHandleAddSchematicWireRouting` all in `test_wire_junction_changes.py`). Name classes `Test<Area>` and methods `test_<what_is_verified>`.
+
+Use `python/templates/empty.kicad_sch` as the base fixture for integration tests — copy it to a `tempfile` directory, run the handler, then parse the result with `sexpdata`.
+
+### Running Tests
+
+Always use the `.venv` virtualenv for Python commands:
+
+```bash
+npm run test:py                 # pytest tests/ -v
+.venv/bin/pytest tests/ -v      # all Python tests
+.venv/bin/pytest -m unit        # unit tests only
+.venv/bin/pytest -m integration # integration tests only
+.venv/bin/pytest --cov=python   # with coverage report
+.venv/bin/mypy python/          # type checking
+```
+
+## Git Workflow
+
+- **Never open a pull request automatically.** Commit and push when asked, but always wait for explicit instructions before running `gh pr create` or any equivalent command.
+
+## Python Code Style
+
+- **Never use `assert` in production code** — raise a specific exception (`ValueError`, `RuntimeError`, etc.) instead. `assert` is stripped in optimised builds and gives poor error messages.
+- **Do not introduce logic-breaking workarounds to satisfy the type checker** (e.g. `x or ""` when `""` is not a valid substitute for `None`). Fix the types or narrow with a proper guard (`if x is None: raise ...`).

python/commands/schematic_snap.py

@@ -4,9 +4,18 @@ Snap-to-grid tool for KiCAD schematics.
 Snaps wire endpoints, junction positions, net labels, and optionally component
 positions to the nearest grid point. Modifies the schematic file in place.
 
-The standard KiCAD eeschema grid is 2.54 mm (0.1 inch). Off-grid coordinates
-cause wires that appear visually connected to fail ERC connectivity checks
-because KiCAD uses exact integer (IU) matching internally.
+The standard KiCAD schematic grid is 50 mil (1.27 mm). Component pins are
+placed at multiples of 1.27 mm relative to the symbol origin, so absolute pin
+coordinates end up as odd multiples of 1.27 mm (e.g. 26.67 mm = 21 × 1.27 mm).
+These are valid on-grid positions that must not be moved.
+
+The coarser 2.54 mm (100-mil) grid is a common mistake: exactly half of all
+valid 1.27 mm positions are not multiples of 2.54 mm and would be displaced by
+1.27 mm — moving labels or wire endpoints off their pins and breaking
+connectivity.
+
+Off-grid coordinates cause wires that appear visually connected to fail ERC
+connectivity checks because KiCAD uses exact integer (IU) matching internally.
 """
 
 import logging
@@ -18,7 +27,7 @@ from sexpdata import Symbol
 
 logger = logging.getLogger("kicad_interface")
 
-_DEFAULT_GRID_MM: float = 2.54
+_DEFAULT_GRID_MM: float = 1.27
 
 # Element type names exposed in the public API
 _VALID_ELEMENTS = frozenset({"wires", "junctions", "labels", "components"})
@@ -91,7 +100,10 @@ def snap_to_grid(
 
     Args:
         schematic_path: Path to the ``.kicad_sch`` file.
-        grid_size:      Grid spacing in mm (default 2.54 mm = 0.1 inch).
+        grid_size:      Grid spacing in mm (default 1.27 mm = 50 mil).
+                        Do NOT use 2.54 mm — half of all valid KiCAD pin
+                        positions fall between 2.54 mm grid lines and would
+                        be displaced 1.27 mm, breaking connectivity.
         elements:       List of element types to snap.  Valid values:
                         ``"wires"``, ``"junctions"``, ``"labels"``,
                         ``"components"``.  Defaults to

python/kicad_interface.py

@@ -3121,7 +3121,7 @@ class KiCADInterface:
             if not schematic_path:
                 return {"success": False, "message": "schematicPath is required"}
 
-            grid_size = float(params.get("gridSize", 2.54))
+            grid_size = float(params.get("gridSize", 1.27))
             elements = params.get("elements")  # None → defaults inside snap_to_grid
 
             result = snap_to_grid(Path(schematic_path), grid_size=grid_size, elements=elements)

python/schemas/tool_schemas.py

@@ -1902,12 +1902,12 @@ SCHEMATIC_TOOLS = [
                 "gridSize": {
                     "type": "number",
                     "description": (
-                        "Grid spacing in mm. "
-                        "Standard KiCAD schematic grid is 2.54 mm (0.1 inch). "
-                        "Use 1.27 mm for high-density layouts. "
-                        "Defaults to 2.54."
+                        "Grid spacing in mm (default: 1.27 — standard KiCAD schematic grid). "
+                        "Do NOT use 2.54: half of all valid KiCAD pin positions are at odd "
+                        "multiples of 1.27 mm and would be displaced 1.27 mm, breaking "
+                        "connectivity."
                     ),
-                    "default": 2.54,
+                    "default": 1.27,
                 },
                 "elements": {
                     "type": "array",

tests/test_snap_to_grid.py

@@ -203,7 +203,7 @@ class TestSnapDefaults:
         path = _make_temp_schematic(extra)
         result = snap_to_grid(path)  # defaults: grid=2.54, elements=None
         assert result["snapped"] >= 3
-        assert result["grid_size"] == pytest.approx(2.54)
+        assert result["grid_size"] == pytest.approx(1.27)
 
     def test_idempotent(self):
         path = _make_temp_schematic(_wire_sexp(2.51, 5.03, 7.56, 5.03))
@@ -213,6 +213,23 @@ class TestSnapDefaults:
         content_after_second = path.read_text(encoding="utf-8")
         assert content_after_first == content_after_second
 
+    def test_default_grid_is_1_27mm(self):
+        # Regression: default was 2.54 mm, which displaces valid KiCAD pin
+        # coordinates that fall on the 50-mil (1.27 mm) grid but not on the
+        # 100-mil (2.54 mm) grid — e.g. 26.67 mm = 21 × 1.27 mm.
+        # With the correct 1.27 mm default those coordinates must be left
+        # untouched (snapped == 0, already_on_grid >= 1).
+        # 26.67 / 2.54 == 10.5 → would snap to 25.40 mm (off by 1.27 mm).
+        # 26.67 / 1.27 == 21.0 → already on grid, no move.
+        path = _make_temp_schematic(_wire_sexp(335.28, 26.67, 350.52, 26.67))
+        result = snap_to_grid(path)  # default grid
+        assert result["grid_size"] == pytest.approx(1.27)
+        assert result["snapped"] == 0, (
+            "Wire at valid 50-mil pin coordinates was displaced by default snap — "
+            "default grid must be 1.27 mm, not 2.54 mm"
+        )
+        assert result["already_on_grid"] >= 1
+
     def test_custom_grid(self):
         # 1.27 mm grid — wire at 1.25 should snap to 1.27
         path = _make_temp_schematic(_wire_sexp(1.25, 1.25, 2.51, 2.51))