feat(gateway): inject timestamps into agent handler messages

Messages arriving through the gateway agent method (TUI, web, spawned
subagents, sessions_send, heartbeats) now get a timestamp prefix
automatically. This gives all agent contexts date/time awareness
without modifying the system prompt (which is cached for stability).

Channel messages (Discord, Telegram, etc.) already have timestamps
via envelope formatting in a separate code path and never reach
the agent handler, so there is no double-stamping risk.

Cron jobs also inject their own 'Current time:' prefix and are
detected and skipped.

Extracted as a pure function (injectTimestamp) with 12 unit tests
covering: timezone handling, 12/24h format, midnight boundaries,
envelope detection, cron detection, and empty messages.

Integration test verifies the agent handler wires it in correctly.

Closes #3658
Refs: #1897, #1928, #2108

src/gateway/server-methods/agent-timestamp.test.ts

@@ -0,0 +1,135 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { injectTimestamp, timestampOptsFromConfig } from "./agent-timestamp.js";
+
+describe("injectTimestamp", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    // Wednesday, January 28, 2026 at 8:30 PM EST
+    vi.setSystemTime(new Date("2026-01-29T01:30:00.000Z"));
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("prepends a formatted timestamp to a plain message", () => {
+    const result = injectTimestamp("Is it the weekend?", {
+      timezone: "America/New_York",
+      timeFormat: "12",
+    });
+
+    expect(result).toMatch(/^\[.+\] Is it the weekend\?$/);
+    expect(result).toContain("Wednesday");
+    expect(result).toContain("January 28");
+    expect(result).toContain("2026");
+    expect(result).toContain("8:30 PM");
+  });
+
+  it("formats in 24-hour time when configured", () => {
+    const result = injectTimestamp("hello", {
+      timezone: "America/New_York",
+      timeFormat: "24",
+    });
+
+    expect(result).toContain("20:30");
+    expect(result).not.toContain("PM");
+  });
+
+  it("uses the configured timezone", () => {
+    const result = injectTimestamp("hello", {
+      timezone: "America/Chicago",
+      timeFormat: "12",
+    });
+
+    // 8:30 PM EST = 7:30 PM CST
+    expect(result).toContain("7:30 PM");
+  });
+
+  it("defaults to UTC when no timezone specified", () => {
+    const result = injectTimestamp("hello", {});
+
+    // 2026-01-29T01:30:00Z
+    expect(result).toContain("January 29"); // UTC date, not EST
+    expect(result).toContain("1:30 AM");
+  });
+
+  it("returns empty/whitespace messages unchanged", () => {
+    expect(injectTimestamp("", { timezone: "UTC" })).toBe("");
+    expect(injectTimestamp("   ", { timezone: "UTC" })).toBe("   ");
+  });
+
+  it("does NOT double-stamp messages with channel envelope timestamps", () => {
+    const enveloped = "[Discord user1 2026-01-28 20:30 EST] hello there";
+    const result = injectTimestamp(enveloped, { timezone: "America/New_York" });
+
+    expect(result).toBe(enveloped);
+  });
+
+  it("does NOT double-stamp messages with cron-injected timestamps", () => {
+    const cronMessage =
+      "[cron:abc123 my-job] do the thing\nCurrent time: Wednesday, January 28th, 2026 — 8:30 PM (America/New_York)";
+    const result = injectTimestamp(cronMessage, { timezone: "America/New_York" });
+
+    expect(result).toBe(cronMessage);
+  });
+
+  it("handles midnight correctly", () => {
+    vi.setSystemTime(new Date("2026-02-01T05:00:00.000Z")); // midnight EST
+
+    const result = injectTimestamp("hello", {
+      timezone: "America/New_York",
+      timeFormat: "12",
+    });
+
+    expect(result).toContain("February 1");
+    expect(result).toContain("12:00 AM");
+  });
+
+  it("handles date boundaries (just before midnight)", () => {
+    vi.setSystemTime(new Date("2026-02-01T04:59:00.000Z")); // 11:59 PM Jan 31 EST
+
+    const result = injectTimestamp("hello", {
+      timezone: "America/New_York",
+      timeFormat: "12",
+    });
+
+    expect(result).toContain("January 31");
+    expect(result).toContain("11:59 PM");
+  });
+
+  it("accepts a custom now date", () => {
+    const customDate = new Date("2025-07-04T16:00:00.000Z"); // July 4, noon EST
+
+    const result = injectTimestamp("fireworks?", {
+      timezone: "America/New_York",
+      timeFormat: "12",
+      now: customDate,
+    });
+
+    expect(result).toContain("July 4");
+    expect(result).toContain("2025");
+  });
+});
+
+describe("timestampOptsFromConfig", () => {
+  it("extracts timezone and timeFormat from config", () => {
+    const opts = timestampOptsFromConfig({
+      agents: {
+        defaults: {
+          userTimezone: "America/Chicago",
+          timeFormat: "24",
+        },
+      },
+    } as any);
+
+    expect(opts.timezone).toBe("America/Chicago");
+    expect(opts.timeFormat).toBe("24");
+  });
+
+  it("falls back gracefully with empty config", () => {
+    const opts = timestampOptsFromConfig({} as any);
+
+    expect(opts.timezone).toBeDefined(); // resolveUserTimezone provides a default
+    expect(opts.timeFormat).toBeUndefined();
+  });
+});

src/gateway/server-methods/agent-timestamp.ts

@@ -0,0 +1,69 @@
+import {
+  formatUserTime,
+  resolveUserTimeFormat,
+  resolveUserTimezone,
+} from "../../agents/date-time.js";
+import type { MoltbotConfig } from "../../config/types.js";
+
+/**
+ * Envelope pattern used by channel plugins (Discord, Telegram, etc.):
+ *   [Channel sender 2026-01-28 20:31 EST] message text
+ *
+ * Messages arriving through channels already have timestamps.
+ * We skip injection for those to avoid double-stamping.
+ */
+const ENVELOPE_PATTERN = /^\[[\w]+ .+ \d{4}-\d{2}-\d{2}/;
+
+/**
+ * Cron jobs inject "Current time: ..." into their messages.
+ * Skip injection for those too.
+ */
+const CRON_TIME_PATTERN = /Current time: /;
+
+export interface TimestampInjectionOptions {
+  timezone?: string;
+  timeFormat?: "12" | "24";
+  now?: Date;
+}
+
+/**
+ * Injects a timestamp prefix into a message if one isn't already present.
+ *
+ * Used by the gateway agent handler to give all agent contexts (TUI, web,
+ * spawned subagents, sessions_send, heartbeats) date/time awareness without
+ * modifying the system prompt (which is cached for stability).
+ *
+ * Channel messages (Discord, Telegram, etc.) already have timestamps via
+ * envelope formatting and take a separate code path — they never reach
+ * the agent handler, so there's no double-stamping risk.
+ *
+ * @see https://github.com/moltbot/moltbot/issues/3658
+ */
+export function injectTimestamp(message: string, opts?: TimestampInjectionOptions): string {
+  if (!message.trim()) return message;
+
+  // Already has a channel envelope timestamp
+  if (ENVELOPE_PATTERN.test(message)) return message;
+
+  // Already has a cron-injected timestamp
+  if (CRON_TIME_PATTERN.test(message)) return message;
+
+  const now = opts?.now ?? new Date();
+  const timezone = opts?.timezone ?? "UTC";
+  const timeFormat = opts?.timeFormat ?? "12";
+
+  const formatted = formatUserTime(now, timezone, resolveUserTimeFormat(timeFormat));
+  if (!formatted) return message;
+
+  return `[${formatted}] ${message}`;
+}
+
+/**
+ * Build TimestampInjectionOptions from a MoltbotConfig.
+ */
+export function timestampOptsFromConfig(cfg: MoltbotConfig): TimestampInjectionOptions {
+  return {
+    timezone: resolveUserTimezone(cfg.agents?.defaults?.userTimezone),
+    timeFormat: cfg.agents?.defaults?.timeFormat as "12" | "24" | undefined,
+  };
+}

src/gateway/server-methods/agent.test.ts

@@ -8,6 +8,7 @@ const mocks = vi.hoisted(() => ({
   updateSessionStore: vi.fn(),
   agentCommand: vi.fn(),
   registerAgentRunContext: vi.fn(),
+  loadConfigReturn: {} as Record<string, unknown>,
 }));
 
 vi.mock("../session-utils.js", () => ({
@@ -32,7 +33,7 @@ vi.mock("../../commands/agent.js", () => ({
 }));
 
 vi.mock("../../config/config.js", () => ({
-  loadConfig: () => ({}),
+  loadConfig: () => mocks.loadConfigReturn,
 }));
 
 vi.mock("../../agents/agent-scope.js", () => ({
@@ -115,6 +116,62 @@ describe("gateway agent handler", () => {
     expect(capturedEntry?.claudeCliSessionId).toBe(existingClaudeCliSessionId);
   });
 
+  it("injects a timestamp into the message passed to agentCommand", async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-01-29T01:30:00.000Z")); // Wed Jan 28, 8:30 PM EST
+    mocks.agentCommand.mockReset();
+
+    mocks.loadConfigReturn = {
+      agents: {
+        defaults: {
+          userTimezone: "America/New_York",
+          timeFormat: "12",
+        },
+      },
+    };
+
+    mocks.loadSessionEntry.mockReturnValue({
+      cfg: mocks.loadConfigReturn,
+      storePath: "/tmp/sessions.json",
+      entry: {
+        sessionId: "existing-session-id",
+        updatedAt: Date.now(),
+      },
+      canonicalKey: "agent:main:main",
+    });
+    mocks.updateSessionStore.mockResolvedValue(undefined);
+    mocks.agentCommand.mockResolvedValue({
+      payloads: [{ text: "ok" }],
+      meta: { durationMs: 100 },
+    });
+
+    const respond = vi.fn();
+    await agentHandlers.agent({
+      params: {
+        message: "Is it the weekend?",
+        agentId: "main",
+        sessionKey: "agent:main:main",
+        idempotencyKey: "test-timestamp-inject",
+      },
+      respond,
+      context: makeContext(),
+      req: { type: "req", id: "ts-1", method: "agent" },
+      client: null,
+      isWebchatConnect: () => false,
+    });
+
+    // Wait for the async agentCommand call
+    await vi.waitFor(() => expect(mocks.agentCommand).toHaveBeenCalled());
+
+    const callArgs = mocks.agentCommand.mock.calls[0][0];
+    expect(callArgs.message).toMatch(
+      /^\[.*Wednesday.*January 28.*2026.*8:30 PM.*\] Is it the weekend\?$/,
+    );
+
+    mocks.loadConfigReturn = {};
+    vi.useRealTimers();
+  });
+
   it("handles missing cliSessionIds gracefully", async () => {
     mocks.loadSessionEntry.mockReturnValue({
       cfg: {},

src/gateway/server-methods/agent.ts

@@ -2,6 +2,7 @@ import { randomUUID } from "node:crypto";
 import { agentCommand } from "../../commands/agent.js";
 import { listAgentIds } from "../../agents/agent-scope.js";
 import { loadConfig } from "../../config/config.js";
+import { injectTimestamp, timestampOptsFromConfig } from "./agent-timestamp.js";
 import {
   resolveAgentIdFromSessionKey,
   resolveExplicitAgentSessionKey,
@@ -136,6 +137,13 @@ export const agentHandlers: GatewayRequestHandlers = {
         return;
       }
     }
+
+    // Inject timestamp into messages that don't already have one.
+    // Channel messages (Discord, Telegram, etc.) get timestamps via envelope
+    // formatting in a separate code path — they never reach this handler.
+    // See: https://github.com/moltbot/moltbot/issues/3658
+    message = injectTimestamp(message, timestampOptsFromConfig(cfg));
+
     const isKnownGatewayChannel = (value: string): boolean => isGatewayMessageChannel(value);
     const channelHints = [request.channel, request.replyChannel]
       .filter((value): value is string => typeof value === "string")