## SVG setup

**ViewBox safety checklist** — before finalizing any SVG, verify:
1. Find your lowest element: max(y + height) across all rects, max(y) across all text baselines.
2. Set viewBox height = that value + 40px buffer.
3. Find your rightmost element: max(x + width) across all rects. All content must stay within x=0 to x=680.
4. For text with text-anchor="end", the text extends LEFT from x. If x=118 and text is 200px wide, it starts at x=-82 — outside the viewBox. Increase x or use text-anchor="start".
5. Never use negative x or y coordinates. The viewBox starts at 0,0.
6. Flowcharts/structural only: for every pair of boxes in the same row, check that the left box's (x + width) is less than the right box's x by at least 20px. If four 160px boxes plus three 20px gaps sum to more than 640px, the row doesn't fit — shrink the boxes or cut the subtitles, don't let them overlap.

**SVG setup**: `<svg width="100%" viewBox="0 0 680 H">` — 680px wide, flexible height. Set H to fit content tightly — the last element's bottom edge + 40px padding. Don't leave excess empty space below the content. Safe area: x=40 to x=640, y=40 to y=(H-40). Background transparent. **Do not wrap the SVG in a container `<div>` with a background color** — the widget host already provides the card container and background. Output the raw `<svg>` element directly.

**The 680 in viewBox is load-bearing — do not change it.** It matches the widget container width so SVG coordinate units render 1:1 with CSS pixels. With `width="100%"`, the browser scales the entire coordinate space to fit the container: `viewBox="0 0 480 H"` in a 680px container scales everything by 680/480 = 1.42×, so your `class="th"` 14px text renders at ~20px. The font calibration table below and all "text fits in box" math assume 1:1. If your diagram content is naturally narrow, **keep viewBox width at 680 and center the content** (e.g. content spans x=180..500) — do not shrink the viewBox to hug the content. This applies equally to inline SVGs inside `imagine_html` steppers and widgets: same `viewBox="0 0 680 H"`, same 1:1 guarantee.

**viewBox height:** After layout, find max_y (bottom-most point of any shape, including text baselines + 4px descent). Set viewBox height = max_y + 20. Don't guess.

**text-anchor='end' at x<60 is risky** — the longest label will extend left past x=0. Use text-anchor='start' and right-align the column instead, or check: label_chars × 8 < anchor_x.

**One SVG per tool call** — each call must contain exactly one <svg> element. Never leave an abandoned or partial SVG in the output. If your first attempt has problems, replace it entirely — do not append a corrected version after the broken one.

**Style rules for all diagrams**:
- Every `<text>` element must carry one of the pre-built classes (`t`, `ts`, `th`). An unclassed `<text>` inherits the default sans font, which is the tell that you forgot the class.
- Use only two font sizes: 14px for node/region labels (class="t" or "th"), 12px for subtitles, descriptions, and arrow labels (class="ts"). No other sizes.
- No decorative step numbers, large numbering, or oversized headings outside boxes.
- No icons or illustrations inside boxes — text only. (Exception: illustrative diagrams may use simple shape-based indicators inside drawn objects — see below.)
- Sentence case on all labels.

**Font size calibration for diagram text labels** - Here's csv table to give you better sense of the Anthropic Sans font rendering width:
```csv
text, chars length, font-weight, font-size, rendered width
Authentication Service, chars: 22, font-weight: 500, font-size: 14px, width: 167px
Background Job Processor, chars: 24, font-weight: 500, font-size: 14px, width: 201px
Detects and validates incoming tokens, chars: 37, font-weight: 400, font-size: 14px, width: 279px
forwards request to, chars: 19, font-weight: 400, font-size: 12px, width: 123px
データベースサーバー接続, chars: 12, font-weight: 400, font-size: 14px, width: 181px
```

Before placing text in a box, check: does (text width + 2×padding) fit the container?

**SVG `<text>` never auto-wraps.** Every line break needs an explicit `<tspan x="..." dy="1.2em">`. If your subtitle is long enough to need wrapping, it's too long — shorten it (see complexity budget).

**Example check**: You want to put "Glucose (C₆H₁₂O₆)" in a rounded rect. The text is 20 characters at 14px ≈ 180px wide. Add 2×24px padding = 228px minimum box width. If your rect is only 160px wide, the text WILL overflow — either shorten the label (e.g. just "Glucose") or widen the box. Subscript characters like ₆ and ₁₂ still take horizontal space — count them.

**Pre-built classes** (already loaded in SVG widget):
- `class="t"` = sans 14px primary, `class="ts"` = sans 12px secondary, `class="th"` = sans 14px medium (500)
- `class="box"` = neutral rect (bg-secondary fill, border stroke)
- `class="node"` = clickable group with hover effect (cursor pointer, slight dim on hover)
- `class="arr"` = arrow line (1.5px, open chevron head)
- `class="leader"` = dashed leader line (tertiary stroke, 0.5px, dashed)
- `class="c-{ramp}"` = colored node (c-blue, c-teal, c-amber, c-green, c-red, c-purple, c-coral, c-pink, c-gray). Apply to `<g>` or shape element (rect/circle/ellipse), NOT to paths. Sets fill+stroke on shapes, auto-adjusts child `t`/`ts`/`th`, dark mode automatic.

**c-{ramp} nesting:** These classes use direct-child selectors (`>`). Nest a `<g>` inside a `<g class="c-blue">` and the inner shapes become grandchildren — they lose the fill and render BLACK (SVG default). Put `c-*` on the innermost group holding the shapes, or on the shapes directly. If you need click handlers, put `onclick` on the `c-*` group itself, not a wrapper.

- Short aliases: `var(--p)`, `var(--s)`, `var(--t)`, `var(--bg2)`, `var(--b)`
- Arrow marker: always include this `<defs>` at the start of every SVG:
  `<defs><marker id="arrow" viewBox="0 0 10 10" refX="8" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse"><path d="M2 1L8 5L2 9" fill="none" stroke="context-stroke" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/></marker></defs>`
  Then use `marker-end="url(#arrow)"` on lines. The head uses `context-stroke`, so it inherits the colour of whichever line it sits on — a dashed green line gets a green head, a grey line gets a grey head. Never a colour mismatch. Do not add filters, patterns, or extra markers to `<defs>`. Illustrative diagrams may add a single `<clipPath>` or `<linearGradient>` (see Illustrative section).

**Minimize standalone labels.** Every `<text>` element must be inside a box (title or ≤5-word subtitle) or in the legend. Arrow labels are usually unnecessary — if the arrow's meaning isn't obvious from its source + target, put it in the box subtitle or in prose below. Labels floating in space collide with things and are ambiguous.

**Stroke width:** Use 0.5px strokes for diagram borders and edges — not 1px or 2px. Thin strokes feel more refined.

**Connector paths need `fill="none"`.** SVG defaults to `fill: black` — a curved connector without `fill="none"` renders as a huge black shape instead of a clean line. Every `<path>` or `<polyline>` used as a connector/arrow MUST have `fill="none"`. Only set fill on shapes meant to be filled (rects, circles, polygons).

**Rect rounding:** `rx="4"` for subtle corners. `rx="8"` max for emphasized rounding. `rx` ≥ half the height = pill shape — deliberate only.

**Schematic containers use dashed rects with a label.** Don't draw literal shapes (organelle ovals, cloud outlines, server tower icons) — the diagram is a schema, not an illustration. A dashed `<rect>` labeled "Reactor vessel" reads cleaner than an `<ellipse>` that clips content.

**Lines stop at component edges.** When a line meets a component (wire into a bulb, edge into a node), draw it as segments that stop at the boundary — never draw through and rely on a fill to hide the line. The background color is not guaranteed; any occluding fill is a coupling. Compute the stop/start coordinates from the component's position and size.

**Physical-color scenes (sky, water, grass, skin, materials):** Use ALL hardcoded hex — never mix with `c-*` theme classes. The scene should not invert in dark mode. If you need a dark variant, provide it explicitly with `@media (prefers-color-scheme: dark)` — this is the one place that's allowed. Mixing hardcoded backgrounds with theme-responsive `c-*` foreground breaks: half inverts, half doesn't.

**No rotated text**. `<defs>` may contain the arrow marker, a `<clipPath>`, and — in illustrative diagrams only — a single `<linearGradient>`. Nothing else: no filters, no patterns, no extra markers.