diff --git a/src/app/(dashboard)/dashboard/orchestration/model/compareRuns.ts b/src/app/(dashboard)/dashboard/orchestration/model/compareRuns.ts
new file mode 100644
index 0000000000..2cfb3c6072
--- /dev/null
+++ b/src/app/(dashboard)/dashboard/orchestration/model/compareRuns.ts
@@ -0,0 +1,149 @@
+/**
+ * Pure model to compare two Orchestration Canvas history runs side by side (History tab,
+ * PR-A / Task A1). No React, no side effects.
+ *
+ * `normalizeRunSide` turns a `HistoryItem` plus its raw detail payload (the JSON body of
+ * `GET /api/a2a/tasks/[id]` for `source === "a2a"`, or the in-memory `CloudAgentTask` for
+ * `source === "cloud-agent"`) into a `RunSide` with a normalized `events` timeline and
+ * `memoryHits` list. `detail` is untrusted: for A2A it round-trips through
+ * `reconstituteHistoricalTask` (`src/app/api/a2a/tasks/[id]/route.ts`) plus client-supplied
+ * `metadata` with no Zod validation, so every field is read defensively — a malformed shape
+ * drops the offending item (or yields `[]`) instead of throwing. See Fase 2 lesson in
+ * `historyModel.ts` / global-constraints: `memoryHits: "boom"` used to crash the drawer.
+ *
+ * `buildComparison` derives signed deltas (`right - left`) for `durationMs`/`cost` only when
+ * both sides have a finite value, and always reports `eventCount`'s delta and whether the two
+ * sides share the same (source, identity) pair.
+ */
+import type { HistoryItem } from "./historyModel";
+
+export interface RunEvent {
+  label: string;
+  timestamp: string | null;
+}
+
+export interface RunMemoryHit {
+  id: string;
+  key: string;
+  type: string;
+  snippet: string;
+}
+
+export interface RunSide {
+  item: HistoryItem;
+  events: RunEvent[];
+  memoryHits: RunMemoryHit[];
+}
+
+export interface RunDeltas {
+  durationMs: number | null;
+  cost: number | null;
+  eventCount: number;
+  sameIdentity: boolean;
+}
+
+export interface RunComparison {
+  left: RunSide;
+  right: RunSide;
+  deltas: RunDeltas;
+}
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}
+
+function isNonEmptyString(value: unknown): value is string {
+  return typeof value === "string" && value.length > 0;
+}
+
+/** `{ timestamp: string; state: string; message?: string }` — reconstituteHistoricalTask's shape. */
+function a2aEventsFrom(detail: Record<string, unknown>): RunEvent[] {
+  const events = detail.events;
+  if (!Array.isArray(events)) return [];
+  const out: RunEvent[] = [];
+  for (const raw of events) {
+    if (!isRecord(raw)) continue;
+    const { state, message, timestamp } = raw;
+    if (!isNonEmptyString(state)) continue;
+    if (message !== undefined && typeof message !== "string") continue;
+    if (timestamp !== undefined && timestamp !== null && typeof timestamp !== "string") continue;
+    out.push({
+      label: typeof message === "string" ? message : state,
+      timestamp: typeof timestamp === "string" ? timestamp : null,
+    });
+  }
+  return out;
+}
+
+/** `CloudAgentActivity[]` (`src/lib/cloudAgent/types.ts`) — each item has `type` and `content`. */
+function cloudAgentEventsFrom(detail: Record<string, unknown>): RunEvent[] {
+  const activities = detail.activities;
+  if (!Array.isArray(activities)) return [];
+  const out: RunEvent[] = [];
+  for (const raw of activities) {
+    if (!isRecord(raw)) continue;
+    const { content, timestamp } = raw;
+    if (!isNonEmptyString(content)) continue;
+    if (timestamp !== undefined && timestamp !== null && typeof timestamp !== "string") continue;
+    out.push({
+      label: content,
+      timestamp: typeof timestamp === "string" ? timestamp : null,
+    });
+  }
+  return out;
+}
+
+function memoryHitsFrom(detail: Record<string, unknown>): RunMemoryHit[] {
+  const metadata = detail.metadata;
+  if (!isRecord(metadata)) return [];
+  const hits = metadata.memoryHits;
+  if (!Array.isArray(hits)) return [];
+  const out: RunMemoryHit[] = [];
+  for (const raw of hits) {
+    if (!isRecord(raw)) continue;
+    const { id, key, type, snippet } = raw;
+    if (
+      typeof id === "string" &&
+      typeof key === "string" &&
+      typeof type === "string" &&
+      typeof snippet === "string"
+    ) {
+      out.push({ id, key, type, snippet });
+    }
+  }
+  return out;
+}
+
+/**
+ * Builds one side of a comparison. NEVER throws — `detail` is untrusted JSON (persisted rows
+ * for A2A history, client-supplied `metadata` with no Zod) so any shape mismatch just drops
+ * the offending item; a missing/non-object `detail` yields empty `events`/`memoryHits`.
+ */
+export function normalizeRunSide(item: HistoryItem, detail: unknown): RunSide {
+  if (!isRecord(detail)) {
+    return { item, events: [], memoryHits: [] };
+  }
+  const events = item.source === "a2a" ? a2aEventsFrom(detail) : cloudAgentEventsFrom(detail);
+  const memoryHits = item.source === "a2a" ? memoryHitsFrom(detail) : [];
+  return { item, events, memoryHits };
+}
+
+function signedDelta(left: number | null, right: number | null): number | null {
+  if (!Number.isFinite(left) || !Number.isFinite(right)) return null;
+  return (right as number) - (left as number);
+}
+
+/** Combines two normalized sides into a comparison with signed `right - left` deltas. */
+export function buildComparison(left: RunSide, right: RunSide): RunComparison {
+  return {
+    left,
+    right,
+    deltas: {
+      durationMs: signedDelta(left.item.durationMs, right.item.durationMs),
+      cost: signedDelta(left.item.cost, right.item.cost),
+      eventCount: right.events.length - left.events.length,
+      sameIdentity:
+        left.item.source === right.item.source && left.item.identity === right.item.identity,
+    },
+  };
+}
diff --git a/tests/unit/ui/orchestrationCompare.test.ts b/tests/unit/ui/orchestrationCompare.test.ts
new file mode 100644
index 0000000000..61c41b5edc
--- /dev/null
+++ b/tests/unit/ui/orchestrationCompare.test.ts
@@ -0,0 +1,98 @@
+/**
+ * tests/unit/ui/orchestrationCompare.test.ts
+ * Pure model tests for comparing two Orchestration Canvas history runs (Task A1, PR-A).
+ * Run: node --import tsx/esm --test tests/unit/ui/orchestrationCompare.test.ts
+ */
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import {
+  buildComparison,
+  normalizeRunSide,
+} from "../../../src/app/(dashboard)/dashboard/orchestration/model/compareRuns.ts";
+
+const a2aItem = {
+  id: "a2a:1",
+  source: "a2a" as const,
+  identity: "smart-routing",
+  state: "succeeded" as const,
+  label: "smart-routing",
+  createdAt: "2026-09-01T10:00:00.000Z",
+  completedAt: "2026-09-01T10:00:30.000Z",
+  durationMs: 30000,
+  cost: null,
+  raw: {},
+};
+const caItem = {
+  id: "cloud-agent:9",
+  source: "cloud-agent" as const,
+  identity: "jules",
+  state: "succeeded" as const,
+  label: "fix login",
+  createdAt: "2026-09-01T10:00:00.000Z",
+  completedAt: "2026-09-01T10:00:10.000Z",
+  durationMs: 10000,
+  cost: 0.5,
+  raw: {},
+};
+
+test("normalizeRunSide reads a2a events from the historical task shape", () => {
+  const side = normalizeRunSide(a2aItem, {
+    events: [
+      { timestamp: "2026-09-01T10:00:00.000Z", state: "submitted" },
+      { timestamp: "2026-09-01T10:00:05.000Z", state: "working", message: "started" },
+    ],
+  });
+  assert.deepEqual(side.events, [
+    { label: "submitted", timestamp: "2026-09-01T10:00:00.000Z" },
+    { label: "started", timestamp: "2026-09-01T10:00:05.000Z" },
+  ]);
+});
+
+test("normalizeRunSide reads cloud-agent activities", () => {
+  const side = normalizeRunSide(caItem, {
+    activities: [
+      { id: "a", type: "log", content: "cloning", timestamp: "2026-09-01T10:00:01.000Z" },
+    ],
+  });
+  assert.deepEqual(side.events, [{ label: "cloning", timestamp: "2026-09-01T10:00:01.000Z" }]);
+});
+
+test("normalizeRunSide never throws on malformed detail", () => {
+  for (const bad of [null, undefined, "boom", 42, { events: "nope" }, { events: [null, 7, {}] }]) {
+    const side = normalizeRunSide(a2aItem, bad);
+    assert.deepEqual(side.events, []);
+    assert.deepEqual(side.memoryHits, []);
+  }
+});
+
+test("normalizeRunSide keeps only well-formed memoryHits", () => {
+  const side = normalizeRunSide(a2aItem, {
+    events: [],
+    metadata: {
+      memoryHits: [
+        { id: "m1", key: "k", type: "factual", snippet: "s" },
+        { id: "m2", key: { a: 1 }, type: "factual", snippet: "s" },
+        "nope",
+      ],
+    },
+  });
+  assert.equal(side.memoryHits.length, 1);
+  assert.equal(side.memoryHits[0].id, "m1");
+});
+
+test("buildComparison computes signed deltas and sameIdentity", () => {
+  const left = normalizeRunSide(a2aItem, { events: [{ timestamp: null, state: "submitted" }] });
+  const right = normalizeRunSide({ ...a2aItem, id: "a2a:2", durationMs: 45000 }, { events: [] });
+  const cmp = buildComparison(left, right);
+  assert.equal(cmp.deltas.durationMs, 15000);
+  assert.equal(cmp.deltas.eventCount, -1);
+  assert.equal(cmp.deltas.sameIdentity, true);
+});
+
+test("buildComparison returns null delta when either side lacks the value", () => {
+  const left = normalizeRunSide(a2aItem, {}); // cost null
+  const right = normalizeRunSide(caItem, {}); // cost 0.5
+  const cmp = buildComparison(left, right);
+  assert.equal(cmp.deltas.cost, null);
+  assert.equal(cmp.deltas.sameIdentity, false);
+});