diff --git a/src/ClaudeDo.Worker/Report/WeekReportPromptBuilder.cs b/src/ClaudeDo.Worker/Report/WeekReportPromptBuilder.cs
new file mode 100644
index 0000000..0bf4414
--- /dev/null
+++ b/src/ClaudeDo.Worker/Report/WeekReportPromptBuilder.cs
@@ -0,0 +1,71 @@
+using System.Globalization;
+using System.Text;
+
+namespace ClaudeDo.Worker.Report;
+
+public static class WeekReportPromptBuilder
+{
+    private const string Instructions = """
+        You are generating a concise weekly standup report for a software developer.
+        Summarize what they accomplished between {0} and {1}.
+
+        Rules:
+        - Write the ENTIRE report in German.
+        - Group by day. One "## {{Wochentag}}, {{dd.MM.yyyy}}" section per day that has
+          activity (German weekday names). Omit days with no activity entirely.
+        - Within each day: 3-5 first-person, past-tense bullets ("- Habe X umgesetzt",
+          "- Y behoben"). Merge related small work into one bullet.
+        - Drop trivia: typo fixes, pure exploration, false starts, tooling/log noise.
+        - Blend the developer's own notes and the derived activity into ONE deduplicated
+          bullet list per day. The developer's notes are authoritative - never omit or
+          contradict their substance.
+        - Name the project/repo when it adds clarity.
+        - Output ONLY the dated sections. No preamble, no intro, no closing remarks.
+        """;
+
+    public static string Build(
+        DateOnly start, DateOnly end,
+        IReadOnlyList<RepoActivity> activity,
+        IReadOnlyDictionary<DateOnly, List<string>> notesByDay)
+    {
+        var sb = new StringBuilder();
+        sb.AppendLine(string.Format(CultureInfo.InvariantCulture, Instructions,
+            start.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture),
+            end.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture)));
+        sb.AppendLine();
+
+        var days = new SortedDictionary<DateOnly, List<(string Repo, DayActivity Day)>>();
+        foreach (var repo in activity)
+            foreach (var day in repo.Days)
+            {
+                if (!days.TryGetValue(day.Date, out var list))
+                    days[day.Date] = list = new();
+                list.Add((repo.RepoPath, day));
+            }
+        foreach (var d in notesByDay.Keys)
+            if (!days.ContainsKey(d)) days[d] = new();
+
+        sb.AppendLine("== Activity (from session history) ==");
+        foreach (var (date, repos) in days)
+        {
+            sb.AppendLine($"### {date:yyyy-MM-dd}");
+            foreach (var (repoPath, day) in repos)
+            {
+                sb.AppendLine($"Repo: {repoPath}");
+                foreach (var p in day.Prompts) sb.AppendLine($"  Prompt: {p}");
+                foreach (var s in day.Summaries) sb.AppendLine($"  Summary: {s}");
+            }
+            sb.AppendLine();
+        }
+
+        sb.AppendLine("== Developer notes ==");
+        foreach (var (date, _) in days)
+        {
+            if (!notesByDay.TryGetValue(date, out var notes) || notes.Count == 0) continue;
+            sb.AppendLine($"### {date:yyyy-MM-dd}");
+            foreach (var n in notes) sb.AppendLine($"  - {n}");
+        }
+
+        return sb.ToString();
+    }
+}
diff --git a/tests/ClaudeDo.Worker.Tests/Report/WeekReportPromptBuilderTests.cs b/tests/ClaudeDo.Worker.Tests/Report/WeekReportPromptBuilderTests.cs
new file mode 100644
index 0000000..8589d3a
--- /dev/null
+++ b/tests/ClaudeDo.Worker.Tests/Report/WeekReportPromptBuilderTests.cs
@@ -0,0 +1,37 @@
+using ClaudeDo.Worker.Report;
+
+namespace ClaudeDo.Worker.Tests.Report;
+
+public class WeekReportPromptBuilderTests
+{
+    [Fact]
+    public void Build_IsDayMajor_AndIncludesNotesAndInstructions()
+    {
+        var repoA = new RepoActivity { RepoPath = @"C:\Dev\App" };
+        var d1 = new DayActivity { Date = new DateOnly(2026, 6, 1) };
+        d1.Prompts.Add("Add login");
+        d1.Summaries.Add("Implemented login");
+        repoA.Days.Add(d1);
+        var d2 = new DayActivity { Date = new DateOnly(2026, 6, 2) };
+        d2.Prompts.Add("Fix bug");
+        repoA.Days.Add(d2);
+
+        var notes = new Dictionary<DateOnly, List<string>>
+        {
+            [new DateOnly(2026, 6, 1)] = new() { "Standup um 9" },
+        };
+
+        var prompt = WeekReportPromptBuilder.Build(
+            new DateOnly(2026, 5, 28), new DateOnly(2026, 6, 3),
+            new[] { repoA }, notes);
+
+        Assert.Contains("Write the ENTIRE report in German", prompt);
+        var idxJun1 = prompt.IndexOf("2026-06-01", StringComparison.Ordinal);
+        var idxJun2 = prompt.IndexOf("2026-06-02", StringComparison.Ordinal);
+        Assert.True(idxJun1 >= 0 && idxJun2 > idxJun1);
+        Assert.Contains("Add login", prompt);
+        Assert.Contains("Implemented login", prompt);
+        Assert.Contains("Standup um 9", prompt);
+        Assert.Contains(@"C:\Dev\App", prompt);
+    }
+}