diff --git a/src/auto-reply/envelope.test.ts b/src/auto-reply/envelope.test.ts
index ecb35f0dd9..179bd69abb 100644
--- a/src/auto-reply/envelope.test.ts
+++ b/src/auto-reply/envelope.test.ts
@@ -23,7 +23,7 @@ describe("formatAgentEnvelope", () => {
 
     process.env.TZ = originalTz;
 
-    expect(body).toBe("[WebChat user1 mac-mini 10.0.0.5 2025-01-02T03:04Z] hello");
+    expect(body).toBe("[WebChat user1 mac-mini 10.0.0.5 Thu 2025-01-02T03:04Z] hello");
   });
 
   it("formats timestamps in local timezone by default", () => {
@@ -39,7 +39,7 @@ describe("formatAgentEnvelope", () => {
 
     process.env.TZ = originalTz;
 
-    expect(body).toMatch(/\[WebChat 2025-01-01 19:04 [^\]]+\] hello/);
+    expect(body).toMatch(/\[WebChat Wed 2025-01-01 19:04 [^\]]+\] hello/);
   });
 
   it("formats timestamps in UTC when configured", () => {
@@ -56,7 +56,7 @@ describe("formatAgentEnvelope", () => {
 
     process.env.TZ = originalTz;
 
-    expect(body).toBe("[WebChat 2025-01-02T03:04Z] hello");
+    expect(body).toBe("[WebChat Thu 2025-01-02T03:04Z] hello");
   });
 
   it("formats timestamps in user timezone when configured", () => {
@@ -68,7 +68,7 @@ describe("formatAgentEnvelope", () => {
       body: "hello",
     });
 
-    expect(body).toMatch(/\[WebChat 2025-01-02 04:04 [^\]]+\] hello/);
+    expect(body).toMatch(/\[WebChat Thu 2025-01-02 04:04 [^\]]+\] hello/);
   });
 
   it("omits timestamps when configured", () => {
diff --git a/src/auto-reply/envelope.ts b/src/auto-reply/envelope.ts
index af10b15ef1..6e01048139 100644
--- a/src/auto-reply/envelope.ts
+++ b/src/auto-reply/envelope.ts
@@ -107,13 +107,35 @@ function formatTimestamp(
     return undefined;
   }
   const zone = resolveEnvelopeTimezone(resolved);
-  if (zone.mode === "utc") {
-    return formatUtcTimestamp(date);
+  // Include a weekday prefix so models do not need to derive DOW from the date
+  // (small models are notoriously unreliable at that).
+  const weekday = (() => {
+    try {
+      if (zone.mode === "utc") {
+        return new Intl.DateTimeFormat("en-US", { timeZone: "UTC", weekday: "short" }).format(date);
+      }
+      if (zone.mode === "local") {
+        return new Intl.DateTimeFormat("en-US", { weekday: "short" }).format(date);
+      }
+      return new Intl.DateTimeFormat("en-US", { timeZone: zone.timeZone, weekday: "short" }).format(
+        date,
+      );
+    } catch {
+      return undefined;
+    }
+  })();
+
+  const formatted =
+    zone.mode === "utc"
+      ? formatUtcTimestamp(date)
+      : zone.mode === "local"
+        ? formatZonedTimestamp(date)
+        : formatZonedTimestamp(date, { timeZone: zone.timeZone });
+
+  if (!formatted) {
+    return undefined;
   }
-  if (zone.mode === "local") {
-    return formatZonedTimestamp(date);
-  }
-  return formatZonedTimestamp(date, { timeZone: zone.timeZone });
+  return weekday ? `${weekday} ${formatted}` : formatted;
 }
 
 export function formatAgentEnvelope(params: AgentEnvelopeParams): string {
diff --git a/test/helpers/envelope-timestamp.ts b/test/helpers/envelope-timestamp.ts
index aa63d612d9..22aa3580d1 100644
--- a/test/helpers/envelope-timestamp.ts
+++ b/test/helpers/envelope-timestamp.ts
@@ -7,13 +7,30 @@ type EnvelopeTimestampZone = string;
 
 export function formatEnvelopeTimestamp(date: Date, zone: EnvelopeTimestampZone = "utc"): string {
   const normalized = zone.trim().toLowerCase();
+  const weekday = (() => {
+    try {
+      if (normalized === "utc" || normalized === "gmt") {
+        return new Intl.DateTimeFormat("en-US", { timeZone: "UTC", weekday: "short" }).format(date);
+      }
+      if (normalized === "local" || normalized === "host") {
+        return new Intl.DateTimeFormat("en-US", { weekday: "short" }).format(date);
+      }
+      return new Intl.DateTimeFormat("en-US", { timeZone: zone, weekday: "short" }).format(date);
+    } catch {
+      return undefined;
+    }
+  })();
+
   if (normalized === "utc" || normalized === "gmt") {
-    return formatUtcTimestamp(date);
+    const ts = formatUtcTimestamp(date);
+    return weekday ? `${weekday} ${ts}` : ts;
   }
   if (normalized === "local" || normalized === "host") {
-    return formatZonedTimestamp(date) ?? formatUtcTimestamp(date);
+    const ts = formatZonedTimestamp(date) ?? formatUtcTimestamp(date);
+    return weekday ? `${weekday} ${ts}` : ts;
   }
-  return formatZonedTimestamp(date, { timeZone: zone }) ?? formatUtcTimestamp(date);
+  const ts = formatZonedTimestamp(date, { timeZone: zone }) ?? formatUtcTimestamp(date);
+  return weekday ? `${weekday} ${ts}` : ts;
 }
 
 export function formatLocalEnvelopeTimestamp(date: Date): string {