fix(cron): anchor every jobs to lastRunAtMs to prevent drift

Fixes #1972 - nextRunAtMs was recalculating from current time on
updates/restarts, causing interval jobs to drift forward.

For 'every' schedule jobs, now uses lastRunAtMs as anchor when available,
falling back to anchorMs then nowMs. Cron expressions unchanged.

🤖 Generated with Clawdbot

Co-Authored-By: Clawd <noreply@anthropic.com>

src/cron/service/jobs.test.ts

@@ -0,0 +1,73 @@
+import { describe, expect, it } from "vitest";
+
+import type { CronJob } from "../types.js";
+import { computeJobNextRunAtMs } from "./jobs.js";
+
+function makeEveryJob(overrides: Partial<CronJob> = {}): CronJob {
+  return {
+    id: "test-job",
+    enabled: true,
+    createdAtMs: 1000,
+    updatedAtMs: 1000,
+    schedule: { kind: "every", everyMs: 3600_000 }, // 1 hour
+    sessionTarget: "main",
+    wakeMode: "next-heartbeat",
+    payload: { kind: "systemEvent", text: "test" },
+    state: {},
+    ...overrides,
+  };
+}
+
+describe("computeJobNextRunAtMs", () => {
+  it("anchors to lastRunAtMs for every jobs that have run", () => {
+    const job = makeEveryJob({
+      state: { lastRunAtMs: 1000 },
+    });
+    const nowMs = 5000;
+    const next = computeJobNextRunAtMs(job, nowMs);
+    // Should be lastRunAtMs + everyMs, NOT nowMs + everyMs
+    expect(next).toBe(1000 + 3600_000);
+  });
+
+  it("falls back to anchorMs when no lastRunAtMs", () => {
+    const job = makeEveryJob({
+      schedule: { kind: "every", everyMs: 3600_000, anchorMs: 500 },
+      state: {}, // No lastRunAtMs
+    });
+    const next = computeJobNextRunAtMs(job, 1000);
+    expect(next).toBe(500 + 3600_000);
+  });
+
+  it("falls back to nowMs when neither lastRunAtMs nor anchorMs exist", () => {
+    const job = makeEveryJob({
+      schedule: { kind: "every", everyMs: 3600_000 }, // No anchorMs
+      state: {}, // No lastRunAtMs
+    });
+    const next = computeJobNextRunAtMs(job, 1000);
+    expect(next).toBe(1000 + 3600_000);
+  });
+
+  it("does not affect cron expression schedules", () => {
+    const job: CronJob = {
+      id: "test-job",
+      enabled: true,
+      createdAtMs: 1000,
+      updatedAtMs: 1000,
+      schedule: { kind: "cron", expr: "0 9 * * *", tz: "UTC" }, // Daily at 9am UTC
+      sessionTarget: "main",
+      wakeMode: "next-heartbeat",
+      payload: { kind: "systemEvent", text: "test" },
+      state: { lastRunAtMs: 500 }, // Should be ignored for cron expr
+    };
+    const nowMs = Date.parse("2025-12-13T00:00:00.000Z");
+    const next = computeJobNextRunAtMs(job, nowMs);
+    // Should be next 9am UTC, not anchored to lastRunAtMs
+    expect(next).toBe(Date.parse("2025-12-13T09:00:00.000Z"));
+  });
+
+  it("returns undefined for disabled jobs", () => {
+    const job = makeEveryJob({ enabled: false });
+    const next = computeJobNextRunAtMs(job, 1000);
+    expect(next).toBeUndefined();
+  });
+});

src/cron/service/jobs.ts

@@ -40,6 +40,12 @@ export function computeJobNextRunAtMs(job: CronJob, nowMs: number): number | und
     if (job.state.lastStatus === "ok" && job.state.lastRunAtMs) return undefined;
     return job.schedule.atMs;
   }
+  // For "every" jobs, anchor to lastRunAtMs to prevent drift on updates/restarts
+  if (job.schedule.kind === "every") {
+    const anchor = job.state.lastRunAtMs ?? job.schedule.anchorMs ?? nowMs;
+    return computeNextRunAtMs({ ...job.schedule, anchorMs: anchor }, nowMs);
+  }
+  // "cron" expressions are clock-based
   return computeNextRunAtMs(job.schedule, nowMs);
 }