diff --git a/editor/CMakeLists.txt b/editor/CMakeLists.txt
index e1132e8..0c34f5a 100644
--- a/editor/CMakeLists.txt
+++ b/editor/CMakeLists.txt
@@ -3928,4 +3928,13 @@ target_link_libraries(step560_test PRIVATE
     tree_sitter_javascript tree_sitter_typescript
     tree_sitter_java tree_sitter_rust tree_sitter_go)
 
+add_executable(step561_test tests/step561_test.cpp)
+target_include_directories(step561_test PRIVATE src)
+target_link_libraries(step561_test PRIVATE
+    nlohmann_json::nlohmann_json
+    unofficial::tree-sitter::tree-sitter
+    tree_sitter_python tree_sitter_cpp tree_sitter_elisp
+    tree_sitter_javascript tree_sitter_typescript
+    tree_sitter_java tree_sitter_rust tree_sitter_go)
+
 # Step 12: Dear ImGui shell scaffolding created (main.cpp exists but not built due to dependencies)
diff --git a/editor/src/TraceTimelineModel.h b/editor/src/TraceTimelineModel.h
new file mode 100644
index 0000000..d74ac58
--- /dev/null
+++ b/editor/src/TraceTimelineModel.h
@@ -0,0 +1,58 @@
+#pragma once
+// Step 561: Trace Timeline Model
+
+#include <algorithm>
+#include <string>
+#include <vector>
+
+struct TraceEvent {
+    long long ts = 0;
+    std::string type;   // pause/step/continue/exception/resume/stop
+    std::string sessionId;
+    std::string detail;
+};
+
+class TraceTimelineModel {
+public:
+    bool append(const TraceEvent& event) {
+        if (!isValid(event)) return false;
+        events_.push_back(event);
+        std::sort(events_.begin(), events_.end(),
+                  [](const TraceEvent& a, const TraceEvent& b) {
+                      if (a.ts != b.ts) return a.ts < b.ts;
+                      return a.type < b.type;
+                  });
+        return true;
+    }
+
+    std::vector<TraceEvent> all() const { return events_; }
+
+    std::vector<TraceEvent> byType(const std::string& type) const {
+        std::vector<TraceEvent> out;
+        for (const auto& e : events_) if (e.type == type) out.push_back(e);
+        return out;
+    }
+
+    std::vector<TraceEvent> bySession(const std::string& sessionId) const {
+        std::vector<TraceEvent> out;
+        for (const auto& e : events_) if (e.sessionId == sessionId) out.push_back(e);
+        return out;
+    }
+
+    std::vector<TraceEvent> window(long long startTs, long long endTs) const {
+        std::vector<TraceEvent> out;
+        for (const auto& e : events_) {
+            if (e.ts >= startTs && e.ts <= endTs) out.push_back(e);
+        }
+        return out;
+    }
+
+private:
+    std::vector<TraceEvent> events_;
+
+    static bool isValid(const TraceEvent& event) {
+        if (event.ts < 0) return false;
+        if (event.type.empty() || event.sessionId.empty()) return false;
+        return true;
+    }
+};
diff --git a/editor/tests/step561_test.cpp b/editor/tests/step561_test.cpp
new file mode 100644
index 0000000..5276c0e
--- /dev/null
+++ b/editor/tests/step561_test.cpp
@@ -0,0 +1,144 @@
+// Step 561: Trace Timeline Model (12 tests)
+
+#include "TraceTimelineModel.h"
+
+#include <iostream>
+
+static int passed = 0, failed = 0;
+#define TEST(name) { std::cout << "  " << #name << "... "; }
+#define PASS() { std::cout << "PASS\n"; ++passed; }
+#define FAIL(msg) { std::cout << "FAIL: " << msg << "\n"; ++failed; }
+#define CHECK(cond, msg) if (!(cond)) { FAIL(msg); return; } else {}
+
+void test_append_valid_event() {
+    TEST(append_valid_event);
+    TraceTimelineModel m;
+    CHECK(m.append({10, "pause", "s1", "hit bp"}), "append should succeed");
+    CHECK(m.all().size() == 1, "one event expected");
+    PASS();
+}
+
+void test_reject_event_with_negative_timestamp() {
+    TEST(reject_event_with_negative_timestamp);
+    TraceTimelineModel m;
+    CHECK(!m.append({-1, "pause", "s1", "bad"}), "negative ts should fail");
+    PASS();
+}
+
+void test_reject_event_with_empty_type() {
+    TEST(reject_event_with_empty_type);
+    TraceTimelineModel m;
+    CHECK(!m.append({1, "", "s1", "bad"}), "empty type should fail");
+    PASS();
+}
+
+void test_reject_event_with_empty_session() {
+    TEST(reject_event_with_empty_session);
+    TraceTimelineModel m;
+    CHECK(!m.append({1, "pause", "", "bad"}), "empty session should fail");
+    PASS();
+}
+
+void test_events_are_sorted_by_timestamp() {
+    TEST(events_are_sorted_by_timestamp);
+    TraceTimelineModel m;
+    CHECK(m.append({30, "step", "s1", "c"}), "append 30");
+    CHECK(m.append({10, "pause", "s1", "a"}), "append 10");
+    CHECK(m.append({20, "continue", "s1", "b"}), "append 20");
+    auto all = m.all();
+    CHECK(all[0].ts == 10 && all[1].ts == 20 && all[2].ts == 30, "timestamp ordering mismatch");
+    PASS();
+}
+
+void test_query_by_type_filters_events() {
+    TEST(query_by_type_filters_events);
+    TraceTimelineModel m;
+    m.append({10, "pause", "s1", "a"});
+    m.append({20, "step", "s1", "b"});
+    m.append({30, "pause", "s1", "c"});
+    auto pauses = m.byType("pause");
+    CHECK(pauses.size() == 2, "two pause events expected");
+    PASS();
+}
+
+void test_query_by_session_filters_events() {
+    TEST(query_by_session_filters_events);
+    TraceTimelineModel m;
+    m.append({10, "pause", "s1", "a"});
+    m.append({20, "step", "s2", "b"});
+    m.append({30, "pause", "s1", "c"});
+    auto s1 = m.bySession("s1");
+    CHECK(s1.size() == 2, "two s1 events expected");
+    PASS();
+}
+
+void test_window_query_inclusive_bounds() {
+    TEST(window_query_inclusive_bounds);
+    TraceTimelineModel m;
+    m.append({10, "pause", "s1", "a"});
+    m.append({20, "step", "s1", "b"});
+    m.append({30, "pause", "s1", "c"});
+    auto w = m.window(10, 20);
+    CHECK(w.size() == 2, "window should include bounds");
+    PASS();
+}
+
+void test_window_query_empty_when_no_matches() {
+    TEST(window_query_empty_when_no_matches);
+    TraceTimelineModel m;
+    m.append({10, "pause", "s1", "a"});
+    auto w = m.window(100, 200);
+    CHECK(w.empty(), "no matches expected");
+    PASS();
+}
+
+void test_same_timestamp_orders_by_type_name() {
+    TEST(same_timestamp_orders_by_type_name);
+    TraceTimelineModel m;
+    m.append({10, "step", "s1", "b"});
+    m.append({10, "pause", "s1", "a"});
+    auto all = m.all();
+    CHECK(all[0].type == "pause" && all[1].type == "step", "type tiebreak ordering mismatch");
+    PASS();
+}
+
+void test_detail_payload_preserved() {
+    TEST(detail_payload_preserved);
+    TraceTimelineModel m;
+    m.append({10, "exception", "s1", "RuntimeError: boom"});
+    auto all = m.all();
+    CHECK(all[0].detail == "RuntimeError: boom", "detail payload mismatch");
+    PASS();
+}
+
+void test_mixed_event_types_supported() {
+    TEST(mixed_event_types_supported);
+    TraceTimelineModel m;
+    m.append({10, "pause", "s1", "a"});
+    m.append({15, "step", "s1", "b"});
+    m.append({20, "exception", "s1", "c"});
+    m.append({25, "resume", "s1", "d"});
+    m.append({30, "stop", "s1", "e"});
+    CHECK(m.all().size() == 5, "five mixed events expected");
+    PASS();
+}
+
+int main() {
+    std::cout << "Step 561: Trace Timeline Model\n";
+
+    test_append_valid_event();                 // 1
+    test_reject_event_with_negative_timestamp();// 2
+    test_reject_event_with_empty_type();       // 3
+    test_reject_event_with_empty_session();    // 4
+    test_events_are_sorted_by_timestamp();     // 5
+    test_query_by_type_filters_events();       // 6
+    test_query_by_session_filters_events();    // 7
+    test_window_query_inclusive_bounds();      // 8
+    test_window_query_empty_when_no_matches(); // 9
+    test_same_timestamp_orders_by_type_name(); // 10
+    test_detail_payload_preserved();           // 11
+    test_mixed_event_types_supported();        // 12
+
+    std::cout << "\nResults: " << passed << "/" << (passed + failed) << " passed\n";
+    return failed == 0 ? 0 : 1;
+}
diff --git a/progress.md b/progress.md
index 2465dc4..e7a6514 100644
--- a/progress.md
+++ b/progress.md
@@ -10010,3 +10010,37 @@ locals display fields and watch lifecycle management.
 - `editor/src/LocalsWatchesPanelModel.h` within header-size limit (`90` <= `600`)
 - `editor/tests/step560_test.cpp` within test-file size guidance (`138` lines)
 - Header-only architecture and naming conventions remain aligned with `ARCHITECTURE.md`
+
+### Step 561: Trace Timeline Model
+**Status:** PASS (12/12 tests)
+
+Implements a runtime trace timeline data model for ordered pause/step/exception
+state transitions with session/type/window query support.
+
+**Files added:**
+- `editor/src/TraceTimelineModel.h` - trace timeline module:
+  - event append with payload validation
+  - deterministic timeline ordering
+  - query helpers by event type, session, and time window
+  - support for mixed runtime event categories
+- `editor/tests/step561_test.cpp` - 12 tests covering:
+  - event validation behavior
+  - ordering behavior
+  - type/session/window query behavior
+  - timestamp tie-break behavior
+  - detail payload preservation
+  - mixed-event support behavior
+
+**Files modified:**
+- `editor/CMakeLists.txt` - `step561_test` target
+
+**Verification run:**
+- `cmake -S editor -B editor/build-native` - PASS
+- `cmake --build editor/build-native --target step561_test step560_test` - PASS
+- `./editor/build-native/step561_test` - PASS (12/12)
+- `./editor/build-native/step560_test` - PASS (12/12) regression coverage
+
+**Architecture gate check:**
+- `editor/src/TraceTimelineModel.h` within header-size limit (`58` <= `600`)
+- `editor/tests/step561_test.cpp` within test-file size guidance (`144` lines)
+- Header-only architecture and naming conventions remain aligned with `ARCHITECTURE.md`