apps/client/src/components/ui/agent-avatar-stack.test.tsx

@@ -2,7 +2,8 @@ import { describe, it, expect, vi } from "vitest";
 import { render, screen, fireEvent } from "@testing-library/react";
 import { MantineProvider } from "@mantine/core";
 import { Provider, createStore } from "jotai";
-import { AgentAvatarStack, agentGlyphBackground } from "./agent-avatar-stack";
+import { AgentAvatarStack } from "./agent-avatar-stack";
+import { avatarStyle } from "@/lib/avatar-palette";
 import {
   activeAiChatIdAtom,
   aiChatWindowOpenAtom,
@@ -13,14 +14,16 @@ import {
 
 type Props = React.ComponentProps<typeof AgentAvatarStack>;
 
-// The DOM normalizes an inline `background: hsl(...)` to `rgb(...)`. Push the
+// The DOM normalizes an inline hex `background-color` to `rgb(...)`. Push the
 // expected color through the same CSSOM path so the comparison stays exact and
 // non-vacuous (an empty string — i.e. no inline background, as in the pre-fix
-// Avatar approach — can never match a real color).
+// Avatar approach — can never match a real color). NOTE: jsdom's CSSOM does not
+// round-trip a `linear-gradient` in the `background` shorthand, which is why the
+// glyph carries an explicit solid `background-color` we assert on here.
 function normalizeColor(value: string): string {
   const probe = document.createElement("div");
-  probe.style.background = value;
-  return probe.style.background;
+  probe.style.backgroundColor = value;
+  return probe.style.backgroundColor;
 }
 
 function renderStack(props: Props) {
@@ -36,26 +39,6 @@ function renderStack(props: Props) {
   return { store, ...utils };
 }
 
-describe("agentGlyphBackground", () => {
-  it("is deterministic for a given agent name", () => {
-    expect(agentGlyphBackground("Researcher")).toBe(
-      agentGlyphBackground("Researcher"),
-    );
-  });
-
-  it("gives categorically different colors to different agents", () => {
-    // The two agents that looked identically violet in the report must differ.
-    expect(agentGlyphBackground("Структурный редактор")).not.toBe(
-      agentGlyphBackground("Фактчекер"),
-    );
-    expect(agentGlyphBackground("Researcher")).not.toBe(
-      agentGlyphBackground("Нарратор"),
-    );
-    // Every color is a dark hsl circle drawn from the palette.
-    expect(agentGlyphBackground("Нарратор")).toMatch(/^hsl\(\d+, \d+%, \d+%\)$/);
-  });
-});
-
 describe("AgentAvatarStack", () => {
   it("internal chat WITH role: emoji glyph + human launcher badge in front", () => {
     const { container } = renderStack({
@@ -73,8 +56,8 @@ describe("AgentAvatarStack", () => {
     expect(screen.getByText("Alice")).toBeDefined();
   });
 
-  it("emoji glyph applies its per-agent color as an inline DOM background", () => {
-    // Pins the actual fix: the hashed color must reach the DOM as an inline
+  it("emoji glyph applies its per-agent gradient as an inline DOM background", () => {
+    // Pins the actual fix: the hashed gradient must reach the DOM as an inline
     // `background` on the glyph Box. The pre-fix `Avatar variant="filled"` set no
     // inline background (Mantine's --avatar-bg overrode it), so this fails there.
     const agent = { name: "Researcher", emoji: "🔬", avatarUrl: null };
@@ -88,20 +71,19 @@ describe("AgentAvatarStack", () => {
       '[data-testid="agent-glyph"]',
     );
     expect(glyph).not.toBeNull();
-    // Non-vacuous: compare against the function output (normalized the same way),
-    // not a frozen literal. Empty against the pre-fix Avatar (no inline bg).
-    expect(glyph!.style.background).not.toBe("");
-    expect(glyph!.style.background).toBe(
-      normalizeColor(agentGlyphBackground(agent.name)),
-    );
+    const expected = normalizeColor(avatarStyle(agent.name).bg);
+    // Non-vacuous: the pre-fix Avatar set no inline background at all.
+    expect(expected).not.toBe("");
+    expect(glyph!.style.backgroundColor).toBe(expected);
+    // (The gradient overlay is a browser-only enhancement — jsdom's CSSOM does
+    // not round-trip linear-gradient — so its stops/angle are covered by the
+    // avatarStyle unit tests above, not asserted on the DOM here.)
   });
 
-  it("agents with distinct hashed colors reach the DOM as distinct backgrounds", () => {
+  it("agents with distinct styles reach the DOM as distinct backgrounds", () => {
     // "Researcher" and "Нарратор" hash to different palette entries, so their
     // applied DOM backgrounds must differ — pins "distinct colors reach the DOM".
-    expect(agentGlyphBackground("Researcher")).not.toBe(
-      agentGlyphBackground("Нарратор"),
-    );
+    expect(avatarStyle("Researcher").bg).not.toBe(avatarStyle("Нарратор").bg);
 
     const a = renderStack({
       agent: { name: "Researcher", emoji: "🔬", avatarUrl: null },
@@ -120,14 +102,9 @@ describe("AgentAvatarStack", () => {
     const glyphB = b.container.querySelector<HTMLElement>(
       '[data-testid="agent-glyph"]',
     );
-    expect(glyphA!.style.background).toBe(
-      normalizeColor(agentGlyphBackground("Researcher")),
-    );
-    expect(glyphB!.style.background).toBe(
-      normalizeColor(agentGlyphBackground("Нарратор")),
-    );
-    // Different colors reach the DOM (the normalized rgb values also differ).
-    expect(glyphA!.style.background).not.toBe(glyphB!.style.background);
+    expect(glyphA!.style.backgroundColor).not.toBe("");
+    // Different base colors reach the DOM (the serialized rgb values differ).
+    expect(glyphA!.style.backgroundColor).not.toBe(glyphB!.style.backgroundColor);
   });
 
   it("showName=false: renders only the avatars, no inline name label", () => {

apps/client/src/components/ui/agent-avatar-stack.tsx

@@ -4,6 +4,7 @@ import { useCallback } from "react";
 import { useTranslation } from "react-i18next";
 import { useSetAtom } from "jotai";
 import { CustomAvatar } from "@/components/ui/custom-avatar.tsx";
+import { avatarStyle, avatarBackgroundCss } from "@/lib/avatar-palette";
 import {
   activeAiChatIdAtom,
   aiChatWindowOpenAtom,
@@ -29,54 +30,11 @@ const LAUNCHER_SIZE = 22;
 // sits as a small badge over that corner (above the glyph) and stays fully visible.
 const LAUNCHER_OVERHANG = 8;
 
-// Small deterministic string hash (same algorithm as custom-avatar's initials
-// hash) used to pick a stable per-agent glyph color.
-function hashName(input: string): number {
-  let hash = 0;
-  for (let i = 0; i < input.length; i += 1) {
-    hash = (hash << 5) - hash + input.charCodeAt(i);
-    hash |= 0;
-  }
-  return Math.abs(hash);
-}
-
-// A palette of categorically-DISTINCT dark circle colors for emoji/sparkles agent
-// glyphs. Every entry is intentionally dark (low lightness) so a bright emoji or
-// the white sparkles icon stays readable on top; the hues are spread across the
-// wheel (red → orange → amber → green → teal → cyan → blue → indigo → violet →
-// magenta + a neutral slate) so two different agents read as DIFFERENT colors,
-// not merely different shades of the same violet.
-const GLYPH_COLORS = [
-  "hsl(355, 60%, 34%)", // red
-  "hsl(18, 62%, 32%)", // vermilion
-  "hsl(32, 60%, 30%)", // orange
-  "hsl(45, 55%, 28%)", // amber
-  "hsl(75, 45%, 26%)", // olive-green
-  "hsl(140, 48%, 26%)", // green
-  "hsl(165, 52%, 26%)", // teal
-  "hsl(188, 58%, 28%)", // cyan
-  "hsl(205, 58%, 32%)", // sky blue
-  "hsl(225, 52%, 36%)", // blue
-  "hsl(250, 48%, 38%)", // indigo
-  "hsl(280, 46%, 36%)", // violet
-  "hsl(312, 48%, 34%)", // magenta
-  "hsl(210, 12%, 36%)", // slate / neutral
-];
-
-/**
- * Deterministic dark circle color for an emoji/sparkles agent glyph, picked from
- * GLYPH_COLORS by a hash of the agent name so distinct agents get categorically
- * distinct colors while every color stays dark enough to keep the glyph readable.
- */
-export function agentGlyphBackground(name: string): string {
-  return GLYPH_COLORS[hashName(name) % GLYPH_COLORS.length];
-}
-
 /**
  * The front avatar. Image-source priority (#300):
  *   1. agent.avatarUrl -> a real avatar image (external MCP agent account).
- *   2. agent.emoji     -> the role emoji on a per-agent dark circle.
- *   3. otherwise       -> the IconSparkles glyph on a per-agent dark circle (fallback).
+ *   2. agent.emoji     -> the role emoji on a per-agent gradient circle.
+ *   3. otherwise       -> the IconSparkles glyph on a per-agent gradient circle.
  */
 function AgentGlyph({ agent }: { agent: AgentInfo }) {
   if (agent.avatarUrl) {
@@ -89,10 +47,13 @@ function AgentGlyph({ agent }: { agent: AgentInfo }) {
     );
   }
 
-  // Emoji/sparkles glyph on a per-agent dark circle (color hashed from the agent
-  // name). Rendered as a plain Box, NOT a Mantine `Avatar variant="filled"`, so
-  // the background is guaranteed instead of being overridden by Mantine's
-  // `--avatar-bg` (which was falling back to the theme's violet for every agent).
+  // Emoji/sparkles glyph on a per-agent gradient circle (color, gradient partner
+  // and split angle all hashed from the agent name via avatarStyle — see
+  // @/lib/avatar-palette). Rendered as a plain Box, NOT a Mantine
+  // `Avatar variant="filled"` — Mantine's `--avatar-bg` overrode the background
+  // (every agent fell back to the theme's violet). The foreground (the sparkles
+  // icon) uses the ring's WCAG-checked readable text color.
+  const style = avatarStyle(agent.name);
   return (
     <Box
       data-testid="agent-glyph"
@@ -100,8 +61,14 @@ function AgentGlyph({ agent }: { agent: AgentInfo }) {
         width: GLYPH_SIZE,
         height: GLYPH_SIZE,
         borderRadius: "50%",
-        background: agentGlyphBackground(agent.name),
-        color: "var(--mantine-color-white)",
+        // Solid base color is the fallback (and the testable value); the gradient
+        // paints over it in browsers that support it.
+        backgroundColor: style.bg,
+        backgroundImage: avatarBackgroundCss(style),
+        color:
+          style.text === "white"
+            ? "var(--mantine-color-white)"
+            : "var(--mantine-color-black)",
         display: "flex",
         alignItems: "center",
         justifyContent: "center",

apps/client/src/lib/avatar-palette.test.ts

@@ -0,0 +1,107 @@
+import { describe, it, expect } from "vitest";
+import {
+  PALETTE,
+  avatarStyle,
+  avatarBackgroundCss,
+  normalizeName,
+  minPairwiseDistance,
+  relativeLuminance,
+  contrastRatio,
+  oklchToSrgb,
+  isInGamut,
+} from "./avatar-palette";
+
+/** Parse "#rrggbb" into sRGB components on the 0..1 scale relativeLuminance expects. */
+function hexToRgb01(hex: string): [number, number, number] {
+  return [
+    parseInt(hex.slice(1, 3), 16) / 255,
+    parseInt(hex.slice(3, 5), 16) / 255,
+    parseInt(hex.slice(5, 7), 16) / 255,
+  ];
+}
+
+describe("avatar-palette validation", () => {
+  it("palette colors stay distinguishable", () => {
+    // 0.06 in OKLab is ~4-5 JNDs — safely distinct at avatar size. If a future
+    // RINGS tweak drops this, "almost identical" colors would reappear.
+    expect(minPairwiseDistance().distance).toBeGreaterThanOrEqual(0.06);
+    expect(PALETTE.length).toBe(20);
+  });
+
+  it("every palette entry is WCAG-readable and in sRGB gamut", () => {
+    // white text = luminance 1, black text = luminance 0 (per buildPalette).
+    const textLum = { white: 1, black: 0 } as const;
+    for (const entry of PALETTE) {
+      expect(entry.hex).toMatch(/^#[0-9a-f]{6}$/);
+
+      // (a) The chosen text color really clears the code's 3:1 threshold on the
+      // actual background hex — recomputed independently from the hex, not from
+      // the build-time luminance. A slot that picked the wrong text (or a color
+      // too dim for either text) would fail here.
+      const hexLum = relativeLuminance(hexToRgb01(entry.hex));
+      const chosen = contrastRatio(textLum[entry.text], hexLum);
+      expect(chosen).toBeGreaterThanOrEqual(3);
+      // buildPalette prefers white and only falls back to black when white
+      // fails 3:1. Mirror that decision: black is used *only* when white would
+      // not clear the threshold — so a mis-assigned "black" on a dark color
+      // (where white was fine) fails here.
+      if (entry.text === "black") {
+        expect(contrastRatio(textLum.white, hexLum)).toBeLessThan(3);
+      }
+
+      // (b) The entry's OKLCH is inside the sRGB gamut after chroma clamping;
+      // an out-of-gamut slot (e.g. un-clamped chroma) would produce components
+      // outside [0,1] and fail here.
+      expect(isInGamut(oklchToSrgb(entry.L, entry.C, entry.h))).toBe(true);
+    }
+  });
+});
+
+describe("avatarStyle", () => {
+  it("name-to-avatar mapping is frozen (golden values)", () => {
+    // Golden slice: if this breaks, all existing avatars change — make sure
+    // that is intentional (a config change in avatar-palette.ts).
+    const s = avatarStyle("Backend Developer");
+    expect([s.bg, s.bg2, s.angleDeg]).toEqual(["#a55795", "#90355e", 150]);
+    expect(s.text).toBe("white");
+  });
+
+  it("is deterministic and normalizes the name", () => {
+    expect(avatarStyle("Researcher")).toEqual(avatarStyle("Researcher"));
+    // Casing, surrounding and repeated whitespace must not change the avatar.
+    expect(avatarStyle("  RESEARCHER ")).toEqual(avatarStyle("researcher"));
+    expect(avatarStyle("Backend   Developer")).toEqual(
+      avatarStyle("backend developer"),
+    );
+    expect(normalizeName("  PM ")).toBe("pm");
+  });
+
+  it("returns a valid base color, angle and matching text", () => {
+    const s = avatarStyle("Нарратор");
+    const idx = PALETTE.findIndex((e) => e.hex === s.bg);
+    expect(idx).toBe(s.paletteIndex);
+    expect(idx).toBeGreaterThanOrEqual(0); // bg is a palette entry
+    // Text color comes from the chosen palette entry.
+    expect(s.text).toBe(PALETTE[idx].text);
+    // Split angle is one of the SPLIT_ANGLE_STEPS (24) directions → multiples of 15.
+    expect(s.angleDeg % 15).toBe(0);
+    expect(s.angleDeg).toBeGreaterThanOrEqual(0);
+    expect(s.angleDeg).toBeLessThan(360);
+  });
+
+  it("distinguishes the agents that used to collide as violet", () => {
+    // "Структурный редактор" and "Фактчекер" looked identically violet before.
+    expect(avatarStyle("Структурный редактор")).not.toEqual(
+      avatarStyle("Фактчекер"),
+    );
+  });
+});
+
+describe("avatarBackgroundCss", () => {
+  it("renders a two-stop gradient with a soft boundary", () => {
+    const s = avatarStyle("Backend Developer");
+    expect(avatarBackgroundCss(s)).toBe(
+      "linear-gradient(150deg, #a55795 42%, #90355e 58%)",
+    );
+  });
+});

apps/client/src/lib/avatar-palette.ts

@@ -0,0 +1,267 @@
+/**
+ * Deterministic avatar backgrounds for agent roles.
+ *
+ * The palette is generated from scratch at module load in OKLCH (a perceptually
+ * uniform color space), so every value below is tunable: change the ring
+ * configuration or the partner shifts and the whole palette regenerates.
+ *
+ * Pipeline: name -> normalize -> cyrb53 hash -> split into independent fields:
+ *   - base color index (one of the validated palette colors)
+ *   - partner hue shift: analogous 20..45deg (either side), complementary 180deg,
+ *     or triadic +/-120deg — classic color-wheel schemes; partner is also darker
+ *   - split angle (SPLIT_ANGLE_STEPS directions, soft boundary)
+ * The same name always yields the same avatar, on any platform, forever.
+ */
+
+// ------------------------- Tunable configuration -------------------------
+
+export interface RingConfig {
+  /** OKLCH lightness, 0..1 */
+  L: number;
+  /** OKLCH chroma target; clamped down per-hue to fit the sRGB gamut */
+  C: number;
+  /** Hue of the first color in the ring, degrees */
+  hueStart: number;
+  /** Number of evenly spaced hues in the ring */
+  count: number;
+}
+
+/**
+ * Two lightness rings. 12 light + 8 dark = 20 base colors with a validated
+ * min pairwise deltaE-OK of ~0.066 (clearly distinguishable at avatar size).
+ * Don't add more hues per ring without re-checking minPairwiseDistance():
+ * beyond ~20-24 colors humans stop telling them apart reliably.
+ */
+const RINGS: readonly RingConfig[] = [
+  { L: 0.70, C: 0.14, hueStart: 15, count: 12 }, // light ring
+  { L: 0.57, C: 0.13, hueStart: 20, count: 8 },  // darker ring
+];
+
+/** Partner color: lightness shifted by this much (negative = darker) */
+const PARTNER_L_SHIFT = -0.10;
+/** Analogous scheme: hue shift magnitude range, degrees (inclusive, 5-deg steps) */
+const ANALOG_MIN_SHIFT = 20;
+const ANALOG_SHIFT_STEP = 5;
+const ANALOG_SHIFT_STEPS = 6; // 20, 25, 30, 35, 40, 45
+/** Complementary scheme: fixed hue shift, degrees */
+const COMPLEMENTARY_SHIFT = 180;
+/** Triadic scheme: fixed hue shift magnitude, degrees (either side) */
+const TRIADIC_SHIFT = 120;
+/** Number of split directions (24 -> 15deg per step) */
+const SPLIT_ANGLE_STEPS = 24;
+/** Position of the color boundary, percent of the gradient axis */
+const SPLIT_PERCENT = 50;
+/** Width of the soft transition zone around the boundary, percent (0 = hard edge) */
+const SPLIT_SOFTNESS = 16;
+
+// ------------------------- OKLCH -> sRGB math -------------------------
+// Matrices from Bjorn Ottosson's OKLab reference implementation.
+
+function oklabToLinearSrgb(L: number, a: number, b: number): [number, number, number] {
+  const l_ = L + 0.3963377774 * a + 0.2158037573 * b;
+  const m_ = L - 0.1055613458 * a - 0.0638541728 * b;
+  const s_ = L - 0.0894841775 * a - 1.2914855480 * b;
+  const l = l_ ** 3, m = m_ ** 3, s = s_ ** 3;
+  return [
+    +4.0767416621 * l - 3.3077115913 * m + 0.2309699292 * s,
+    -1.2684380046 * l + 2.6097574011 * m - 0.3413193965 * s,
+    -0.0041960863 * l - 0.7034186147 * m + 1.7076147010 * s,
+  ];
+}
+
+function gammaEncode(c: number): number {
+  return c <= 0.0031308 ? 12.92 * c : 1.055 * c ** (1 / 2.4) - 0.055;
+}
+
+export function oklchToSrgb(L: number, C: number, hDeg: number): [number, number, number] {
+  const h = (hDeg * Math.PI) / 180;
+  const [r, g, b] = oklabToLinearSrgb(L, C * Math.cos(h), C * Math.sin(h));
+  return [gammaEncode(r), gammaEncode(g), gammaEncode(b)];
+}
+
+export function isInGamut(rgb: readonly number[]): boolean {
+  return rgb.every((c) => c >= -1e-6 && c <= 1 + 1e-6);
+}
+
+/** Binary-search the max chroma <= C that fits into the sRGB gamut. */
+function clampChroma(L: number, C: number, hDeg: number): number {
+  if (isInGamut(oklchToSrgb(L, C, hDeg))) return C;
+  let lo = 0, hi = C;
+  for (let i = 0; i < 40; i++) {
+    const mid = (lo + hi) / 2;
+    if (isInGamut(oklchToSrgb(L, mid, hDeg))) lo = mid;
+    else hi = mid;
+  }
+  return lo;
+}
+
+function toHex(rgb: readonly number[]): string {
+  return (
+    "#" +
+    rgb
+      .map((c) => Math.round(Math.min(1, Math.max(0, c)) * 255).toString(16).padStart(2, "0"))
+      .join("")
+  );
+}
+
+/** WCAG relative luminance of an sRGB color (components 0..1). */
+export function relativeLuminance(rgb: readonly number[]): number {
+  const lin = rgb.map((c) => (c <= 0.04045 ? c / 12.92 : ((c + 0.055) / 1.055) ** 2.4));
+  return 0.2126 * lin[0] + 0.7152 * lin[1] + 0.0722 * lin[2];
+}
+
+export function contrastRatio(l1: number, l2: number): number {
+  return (Math.max(l1, l2) + 0.05) / (Math.min(l1, l2) + 0.05);
+}
+
+// ------------------------- Palette generation -------------------------
+
+export interface PaletteEntry {
+  /** Base background color */
+  hex: string;
+  /** OKLCH coordinates of the base color (used to derive partner colors) */
+  L: number;
+  C: number;
+  h: number;
+  /** Text/icon color with the best WCAG contrast on the base color */
+  text: "white" | "black";
+  /** OKLab coordinates of the base color (kept for validation) */
+  lab: readonly [number, number, number];
+}
+
+function buildPalette(): PaletteEntry[] {
+  const entries: PaletteEntry[] = [];
+  for (const ring of RINGS) {
+    const step = 360 / ring.count;
+    for (let i = 0; i < ring.count; i++) {
+      const h = (ring.hueStart + i * step) % 360;
+      const C = clampChroma(ring.L, ring.C, h);
+      const rgb = oklchToSrgb(ring.L, C, h);
+      const lum = relativeLuminance(rgb);
+      entries.push({
+        hex: toHex(rgb),
+        L: ring.L,
+        C,
+        h,
+        // White text needs >= 3:1 contrast; otherwise fall back to black.
+        text: contrastRatio(lum, 1) >= 3 ? "white" : "black",
+        lab: [
+          ring.L,
+          C * Math.cos((h * Math.PI) / 180),
+          C * Math.sin((h * Math.PI) / 180),
+        ],
+      });
+    }
+  }
+  return entries;
+}
+
+/** Partner color for the split: base hue shifted by shiftDeg, darker by PARTNER_L_SHIFT. */
+function partnerHex(entry: PaletteEntry, shiftDeg: number): string {
+  const h2 = (entry.h + shiftDeg + 360) % 360;
+  const L2 = entry.L + PARTNER_L_SHIFT;
+  return toHex(oklchToSrgb(L2, clampChroma(L2, entry.C, h2), h2));
+}
+
+/** Generated once at module load; regenerates on every build from the config above. */
+export const PALETTE: readonly PaletteEntry[] = buildPalette();
+
+// ------------------------- Name -> avatar style -------------------------
+
+/** Normalize so that "PM ", "pm" and "Pm" map to the same avatar. */
+export function normalizeName(name: string): string {
+  return name.normalize("NFC").trim().toLowerCase().replace(/\s+/g, " ");
+}
+
+/**
+ * cyrb53: deterministic 53-bit string hash with good avalanche.
+ * Pure JS, cross-platform — never use language built-in hashing here.
+ */
+function cyrb53(str: string, seed = 0): number {
+  let h1 = 0xdeadbeef ^ seed;
+  let h2 = 0x41c6ce57 ^ seed;
+  for (let i = 0; i < str.length; i++) {
+    const ch = str.charCodeAt(i);
+    h1 = Math.imul(h1 ^ ch, 2654435761);
+    h2 = Math.imul(h2 ^ ch, 1597334677);
+  }
+  h1 = Math.imul(h1 ^ (h1 >>> 16), 2246822507) ^ Math.imul(h2 ^ (h2 >>> 13), 3266489909);
+  h2 = Math.imul(h2 ^ (h2 >>> 16), 2246822507) ^ Math.imul(h1 ^ (h1 >>> 13), 3266489909);
+  return 4294967296 * (2097151 & h2) + (h1 >>> 0);
+}
+
+export interface AvatarStyle {
+  /** Index of the base color in PALETTE */
+  paletteIndex: number;
+  /** Base color hex */
+  bg: string;
+  /** Second color hex (split partner) */
+  bg2: string;
+  /** Signed hue shift of the partner, degrees (e.g. -35, +45, 180, -120) */
+  hueShift: number;
+  /** Direction of the split, degrees */
+  angleDeg: number;
+  /** Text/icon color for the base color */
+  text: "white" | "black";
+}
+
+/** Pure function: the same (normalized) name always returns the same style. */
+export function avatarStyle(agentName: string): AvatarStyle {
+  const h = cyrb53(normalizeName(agentName));
+  // Slice the hash into independent fields, like digits of a number:
+  const paletteIndex = h % PALETTE.length;
+  let rest = Math.floor(h / PALETTE.length);
+  const angleDeg = (rest % SPLIT_ANGLE_STEPS) * (360 / SPLIT_ANGLE_STEPS);
+  rest = Math.floor(rest / SPLIT_ANGLE_STEPS);
+  // Scheme: 0,1 -> analogous (minus/plus); 2 -> complementary; 3 -> triadic
+  const scheme = rest % 4;
+  rest = Math.floor(rest / 4);
+  let hueShift: number;
+  if (scheme === 2) {
+    hueShift = COMPLEMENTARY_SHIFT;
+  } else if (scheme === 3) {
+    hueShift = rest % 2 ? TRIADIC_SHIFT : -TRIADIC_SHIFT;
+  } else {
+    const magnitude = ANALOG_MIN_SHIFT + (rest % ANALOG_SHIFT_STEPS) * ANALOG_SHIFT_STEP;
+    hueShift = scheme === 0 ? -magnitude : magnitude;
+  }
+  const entry = PALETTE[paletteIndex];
+  return {
+    paletteIndex,
+    bg: entry.hex,
+    bg2: partnerHex(entry, hueShift),
+    hueShift,
+    angleDeg,
+    text: entry.text,
+  };
+}
+
+/** CSS background value: two colors with a slightly blurred boundary. */
+export function avatarBackgroundCss(style: AvatarStyle): string {
+  const from = SPLIT_PERCENT - SPLIT_SOFTNESS / 2;
+  const to = SPLIT_PERCENT + SPLIT_SOFTNESS / 2;
+  return `linear-gradient(${style.angleDeg}deg, ${style.bg} ${from}%, ${style.bg2} ${to}%)`;
+}
+
+// ------------------------- Validation -------------------------
+
+/**
+ * Min pairwise deltaE-OK (euclidean distance in OKLab) between base colors.
+ * Re-check after tweaking RINGS: keep it >= ~0.06 so no two palette colors
+ * look alike. Intended for a unit test or a dev-time assertion.
+ */
+export function minPairwiseDistance(): { distance: number; pair: [string, string] } {
+  let min = Infinity;
+  let pair: [string, string] = ["", ""];
+  for (let i = 0; i < PALETTE.length; i++) {
+    for (let j = i + 1; j < PALETTE.length; j++) {
+      const a = PALETTE[i].lab, b = PALETTE[j].lab;
+      const d = Math.hypot(a[0] - b[0], a[1] - b[1], a[2] - b[2]);
+      if (d < min) {
+        min = d;
+        pair = [PALETTE[i].hex, PALETTE[j].hex];
+      }
+    }
+  }
+  return { distance: min, pair };
+}