diff --git a/routing/test/polyline.test.mjs b/routing/test/polyline.test.mjs
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+++ b/routing/test/polyline.test.mjs
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+#!/usr/bin/env node
+/*
+ * polyline.test.mjs — polyline6 codec vectors (P1–P8).
+ * -----------------------------------------------------
+ * RED first (plan 05-01 task 2): every case fails while
+ * routing/src/polyline.ts does not exist — the lazy-import pattern
+ * keeps each case reporting its own failure.
+ *
+ * Vectors are computed ground truth (the plan's <polyline_vectors> block,
+ * derived with the reference algorithm in the planning session), NOT
+ * guesses:
+ *   P1  zero vector          — decode("??") -> [{lat:0,lng:0}]; encode inverts
+ *   P2  KL point vector      — encode([{3.1578,101.7117}]) === "oqv_Egt~~`E"
+ *                              (note: the string contains a BACKTICK)
+ *   P3  delta accumulation   — the 3-point KL vector (deltas 100/100/100/200)
+ *   P4  negative zigzag      — the V4 vector (the ~ inversion branch)
+ *   P5  external OSRM vector — live demo-server geometry (KLCC -> KL Sentral,
+ *                              probed 2026-10-24; RESEARCH Finding 1): first
+ *                              point decodes to {3.157846, 101.711716} (OSRM
+ *                              snapped the origin onto the road graph) and
+ *                              encode(decode(s)) === s byte-identically
+ *   P6  empty input          — decode("") -> []; encode([]) === ""
+ *   P7  malformed refusal    — decode("A") / decode("g") THROW (truncated
+ *                              varint — never a partial/NaN point, T-05-02)
+ *   P8  round-trip law       — encode(decode(s)) === s for EVERY vector here
+ *
+ * Run:  npm run routing:test   (exit 0 = all green)
+ */
+import { strict as assert } from "node:assert";
+import { test } from "./harness.mjs";
+
+// Lazy codec load — missing module => per-case ERR_MODULE_NOT_FOUND (the RED state).
+let codecPromise = null;
+function loadCodec() {
+  if (!codecPromise) {
+    codecPromise = import("../src/polyline.ts");
+  }
+  return codecPromise;
+}
+
+/* ————— the pinned vectors ————— */
+
+const V_ZERO = "??";
+const V_KL_POINT = "oqv_Egt~~`E";
+const V_KL_THREE = "oqv_Egt~~`EgEgEgEoK";
+const V_NEGATIVE = "~tpzA_pihC_pihCnt}oC";
+const V_EXTERNAL =
+  "ktv_Egu~~`EsAlFaDnGeF`EmGvEmWbSySjOiTfUiOb`@{n@xZ}FhBsGx@cJ?";
+
+const KL_POINT = { lat: 3.1578, lng: 101.7117 };
+const KL_THREE_POINTS = [
+  { lat: 3.1578, lng: 101.7117 },
+  { lat: 3.1579, lng: 101.7118 },
+  { lat: 3.158, lng: 101.712 },
+];
+const NEGATIVE_POINTS = [
+  { lat: -1.5, lng: 2.25 },
+  { lat: 0.75, lng: -0.125 },
+];
+
+/* ————— P1–P8 ————— */
+
+test("P1 zero vector — decode('??') -> [{lat:0,lng:0}]; encode inverts exactly", async () => {
+  const { decodePolyline6, encodePolyline6 } = await loadCodec();
+  assert.deepEqual(decodePolyline6(V_ZERO), [{ lat: 0, lng: 0 }]);
+  assert.equal(encodePolyline6([{ lat: 0, lng: 0 }]), V_ZERO);
+});
+
+test("P2 KL point vector — encode([{3.1578,101.7117}]) === 'oqv_Egt~~`E' and decode inverts", async () => {
+  const { decodePolyline6, encodePolyline6 } = await loadCodec();
+  assert.equal(encodePolyline6([KL_POINT]), V_KL_POINT);
+  assert.deepEqual(decodePolyline6(V_KL_POINT), [KL_POINT]);
+});
+
+test("P3 delta accumulation — the 3-point KL vector encodes/decodes exactly", async () => {
+  const { decodePolyline6, encodePolyline6 } = await loadCodec();
+  assert.equal(encodePolyline6(KL_THREE_POINTS), V_KL_THREE);
+  assert.deepEqual(decodePolyline6(V_KL_THREE), KL_THREE_POINTS);
+});
+
+test("P4 negative zigzag — the V4 vector exercises the ~ inversion branch", async () => {
+  const { decodePolyline6, encodePolyline6 } = await loadCodec();
+  assert.equal(encodePolyline6(NEGATIVE_POINTS), V_NEGATIVE);
+  assert.deepEqual(decodePolyline6(V_NEGATIVE), NEGATIVE_POINTS);
+});
+
+test("P5 external OSRM vector — live demo geometry decodes and re-encodes byte-identically", async () => {
+  const { decodePolyline6, encodePolyline6 } = await loadCodec();
+  const first = decodePolyline6(V_EXTERNAL)[0];
+  assert.deepEqual(
+    first,
+    { lat: 3.157846, lng: 101.711716 },
+    "OSRM snapped the origin onto the road graph — the small offset from 3.1578/101.7117 is expected",
+  );
+  assert.equal(
+    encodePolyline6(decodePolyline6(V_EXTERNAL)),
+    V_EXTERNAL,
+    "the external ground truth must round-trip byte-identically (RESEARCH Finding 1)",
+  );
+});
+
+test("P6 empty input — decode('') -> []; encode([]) === ''", async () => {
+  const { decodePolyline6, encodePolyline6 } = await loadCodec();
+  assert.deepEqual(decodePolyline6(""), []);
+  assert.equal(encodePolyline6([]), "");
+});
+
+test("P7 malformed refusal — truncated varints THROW, never return partial/NaN points (T-05-02)", async () => {
+  const { decodePolyline6 } = await loadCodec();
+  assert.throws(() => decodePolyline6("A"), "a lone 'A' is a truncated coordinate — must throw");
+  assert.throws(() => decodePolyline6("g"), "a dangling continuation chunk — must throw");
+  for (const bad of ["A", "g", "oqv_E"]) {
+    let points = null;
+    try {
+      points = decodePolyline6(bad);
+    } catch {
+      continue; // throwing is the contract
+    }
+    assert.fail(`decode(${JSON.stringify(bad)}) returned ${JSON.stringify(points)} instead of throwing`);
+  }
+});
+
+test("P8 round-trip law — encode(decode(s)) === s for EVERY vector string in this suite", async () => {
+  const { decodePolyline6, encodePolyline6 } = await loadCodec();
+  const all = [V_ZERO, V_KL_POINT, V_KL_THREE, V_NEGATIVE, V_EXTERNAL];
+  for (const s of all) {
+    assert.equal(
+      encodePolyline6(decodePolyline6(s)),
+      s,
+      `round-trip law violated for ${s.slice(0, 12)}… — the corpus gate (plan 05-02) leans on this`,
+    );
+  }
+});