diff --git a/src/lib/jobs/__tests__/reaction-line-units.test.ts b/src/lib/jobs/__tests__/reaction-line-units.test.ts
new file mode 100644
index 000000000..d738bbb76
--- /dev/null
+++ b/src/lib/jobs/__tests__/reaction-line-units.test.ts
@@ -0,0 +1,80 @@
+/**
+ * The datum a reaction line reacts to is handed to the model in the reader's
+ * units, so the line cannot quote kilograms to a reader on pounds.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const measurementFindMany = vi.fn();
+const workoutFindFirst = vi.fn();
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    measurement: {
+      findMany: (...a: unknown[]) => measurementFindMany(...a),
+    },
+    workout: { findFirst: (...a: unknown[]) => workoutFindFirst(...a) },
+  },
+}));
+
+import { loadArrivalEvidence } from "../reaction-line";
+
+const row = { occurredAt: new Date("2026-09-30T07:00:00Z"), refId: "w1" };
+const imperial = {
+  timezone: "UTC",
+  sourcePriorityJson: null,
+  unitPreference: "imperial",
+  glucoseUnit: null,
+};
+
+beforeEach(() => {
+  measurementFindMany.mockReset();
+  workoutFindFirst.mockReset();
+});
+
+describe("loadArrivalEvidence — the reader's units", () => {
+  it("states a new weight in pounds for an imperial reader", async () => {
+    measurementFindMany.mockResolvedValue([{ type: "WEIGHT", value: 80 }]);
+    const evidence = await loadArrivalEvidence(
+      { userId: "u1", kind: "weight", localDate: "2026-09-30", revision: "r" },
+      row,
+      imperial,
+    );
+    expect(evidence).toBe("- Newly arrived reading: WEIGHT 176.4 lb.");
+  });
+
+  it("states blood pressure in mmHg for every reader", async () => {
+    measurementFindMany.mockResolvedValue([
+      { type: "BLOOD_PRESSURE_DIA", value: 80 },
+      { type: "BLOOD_PRESSURE_SYS", value: 125 },
+    ]);
+    const evidence = await loadArrivalEvidence(
+      {
+        userId: "u1",
+        kind: "blood_pressure",
+        localDate: "2026-09-30",
+        revision: "r",
+      },
+      row,
+      imperial,
+    );
+    expect(evidence).toContain("BLOOD_PRESSURE_SYS 125 mmHg.");
+  });
+
+  it("states a workout distance in miles, not metres", async () => {
+    measurementFindMany.mockResolvedValue([]);
+    workoutFindFirst.mockResolvedValue({
+      sportType: "running",
+      durationSec: 1800,
+      totalDistanceM: 5000,
+      totalEnergyKcal: 300,
+      avgHeartRate: 150,
+    });
+    const evidence = await loadArrivalEvidence(
+      { userId: "u1", kind: "workout", localDate: "2026-09-30", revision: "r" },
+      row,
+      imperial,
+    );
+    expect(evidence).toContain("distance 3.11 mi;");
+    expect(evidence).not.toContain("metres");
+  });
+});
diff --git a/src/lib/jobs/reaction-line.ts b/src/lib/jobs/reaction-line.ts
index 5d3084acf..3c32cf7d6 100644
--- a/src/lib/jobs/reaction-line.ts
+++ b/src/lib/jobs/reaction-line.ts
@@ -25,6 +25,12 @@
  * The deterministic digest adds only the same computed context rendered next
  * to the reaction.
  */
+import {
+  applyDisplayTransform,
+  getQuantityTransform,
+  getReadingTransform,
+  resolveUnitPreferences,
+} from "@/lib/measurements/display-transform";
 import type { Job } from "pg-boss";
 
 import { prisma } from "@/lib/db";
@@ -140,11 +146,19 @@ export function sanitiseReactionLine(
  * intentionally a narrow, labels-and-numbers-only projection: no notes or
  * other free text can enter the provider prompt through this path.
  */
-async function loadArrivalEvidence(
+export async function loadArrivalEvidence(
   job: ReactionLineJob,
   row: { occurredAt: Date; refId: string | null },
-  user: { timezone: string; sourcePriorityJson: unknown },
+  user: {
+    timezone: string;
+    sourcePriorityJson: unknown;
+    unitPreference: string | null;
+    glucoseUnit: string | null;
+  },
 ): Promise<string> {
+  // The line is written in the reader's units, so the new datum it reacts to
+  // is handed over in them.
+  const units = resolveUnitPreferences(user);
   if (job.kind === "weight" || job.kind === "blood_pressure") {
     const types =
       job.kind === "weight"
@@ -157,15 +171,15 @@ async function loadArrivalEvidence(
         measuredAt: row.occurredAt,
         deletedAt: null,
       },
-      select: { type: true, value: true, unit: true },
+      select: { type: true, value: true },
       orderBy: { type: "asc" },
     });
     if (readings.length > 0) {
       return readings
-        .map(
-          (reading) =>
-            `- Newly arrived reading: ${reading.type} ${reading.value} ${reading.unit}.`,
-        )
+        .map((reading) => {
+          const transform = getReadingTransform(reading.type, units);
+          return `- Newly arrived reading: ${reading.type} ${applyDisplayTransform(reading.value, transform)} ${transform.displayUnit}.`;
+        })
         .join("\n");
     }
   }
@@ -241,7 +255,12 @@ async function loadArrivalEvidence(
       },
     });
     if (workout) {
-      return `- Newly arrived workout: ${workout.sportType}; duration ${workout.durationSec} seconds; distance ${workout.totalDistanceM ?? "not reported"} metres; energy ${workout.totalEnergyKcal ?? "not reported"} kcal; average heart rate ${workout.avgHeartRate ?? "not reported"} bpm.`;
+      const distance = getQuantityTransform("distance", units.system);
+      const distanceText =
+        workout.totalDistanceM == null
+          ? "not reported"
+          : `${applyDisplayTransform(workout.totalDistanceM, distance)} ${distance.displayUnit}`;
+      return `- Newly arrived workout: ${workout.sportType}; duration ${workout.durationSec} seconds; distance ${distanceText}; energy ${workout.totalEnergyKcal ?? "not reported"} kcal; average heart rate ${workout.avgHeartRate ?? "not reported"} bpm.`;
     }
   }