diff --git a/researchers/web/trace.py b/researchers/web/trace.py
index 1c8e5fb..97c81ae 100644
--- a/researchers/web/trace.py
+++ b/researchers/web/trace.py
@@ -35,6 +35,22 @@ _INFO_ACTIONS = frozenset(
 
 _log = get_logger("marchwarden.researcher.trace")
 
+# Action pairings for duration tracking. When a starter action fires
+# we record a monotonic start time keyed by the starter name. When the
+# matching completer fires we compute the elapsed duration and attach
+# it as a field on the completer's entry, then clear the start.
+#
+# Synthesis has two possible completers (success or error), both
+# pointing back to synthesis_start.
+_DURATION_PAIRS: dict[str, str] = {
+    "web_search_complete": "web_search",
+    "fetch_url_complete": "fetch_url",
+    "synthesis_complete": "synthesis_start",
+    "synthesis_error": "synthesis_start",
+    "complete": "start",
+}
+_STARTER_ACTIONS = frozenset(_DURATION_PAIRS.values())
+
 
 class TraceLogger:
     """Logs research steps to a JSONL file.
@@ -65,6 +81,9 @@ class TraceLogger:
         self.file_path = self.trace_dir / f"{self.trace_id}.jsonl"
         self._step_counter = 0
         self._file = None
+        # action_name -> monotonic start time, populated by starter
+        # actions and consumed by their matching completer (Issue #35).
+        self._pending_starts: dict[str, float] = {}
 
     @property
     def _writer(self):
@@ -93,6 +112,24 @@ class TraceLogger:
         }
         entry.update(kwargs)
 
+        # Duration tracking (Issue #35). Record start times for starter
+        # actions; when the matching completer fires, attach elapsed time
+        # to both the trace entry and the operational log line.
+        now = time.monotonic()
+        if action in _STARTER_ACTIONS:
+            self._pending_starts[action] = now
+        duration_extras: dict[str, Any] = {}
+        if action in _DURATION_PAIRS:
+            starter = _DURATION_PAIRS[action]
+            start = self._pending_starts.pop(starter, None)
+            if start is not None:
+                elapsed = now - start
+                if action == "complete":
+                    duration_extras["total_duration_sec"] = round(elapsed, 3)
+                else:
+                    duration_extras["duration_ms"] = int(elapsed * 1000)
+        entry.update(duration_extras)
+
         self._writer.write(json.dumps(entry, default=str) + "\n")
         self._writer.flush()
 
@@ -101,7 +138,13 @@ class TraceLogger:
         # already bound in contextvars by WebResearcher.research, so
         # they automatically appear on every line.
         log_method = _log.info if action in _INFO_ACTIONS else _log.debug
-        log_method(action, step=self._step_counter, decision=decision, **kwargs)
+        log_method(
+            action,
+            step=self._step_counter,
+            decision=decision,
+            **kwargs,
+            **duration_extras,
+        )
 
         return entry
 
diff --git a/tests/test_trace.py b/tests/test_trace.py
index 8a42c39..3c77699 100644
--- a/tests/test_trace.py
+++ b/tests/test_trace.py
@@ -143,3 +143,74 @@ class TestTraceLogger:
             assert a.read_entries()[0]["query"] == "a"
             assert b.read_entries()[0]["query"] == "b"
             assert a.file_path != b.file_path
+
+
+# ---------------------------------------------------------------------------
+# Step duration tracking (Issue #35)
+# ---------------------------------------------------------------------------
+
+
+import time as _time
+
+
+class TestStepDurations:
+    def test_web_search_pair_records_duration_ms(self, tmp_path):
+        logger = TraceLogger(trace_dir=str(tmp_path))
+        logger.log_step("web_search", query="utah crops")
+        _time.sleep(0.02)
+        entry = logger.log_step("web_search_complete", result_count=5)
+        logger.close()
+        assert "duration_ms" in entry
+        assert entry["duration_ms"] >= 15
+
+    def test_fetch_url_pair_records_duration_ms(self, tmp_path):
+        logger = TraceLogger(trace_dir=str(tmp_path))
+        logger.log_step("fetch_url", url="https://example.com")
+        _time.sleep(0.01)
+        entry = logger.log_step("fetch_url_complete", success=True)
+        logger.close()
+        assert "duration_ms" in entry
+
+    def test_synthesis_complete_records_duration_ms(self, tmp_path):
+        logger = TraceLogger(trace_dir=str(tmp_path))
+        logger.log_step("synthesis_start")
+        _time.sleep(0.01)
+        entry = logger.log_step("synthesis_complete")
+        logger.close()
+        assert "duration_ms" in entry
+
+    def test_synthesis_error_also_records_duration_ms(self, tmp_path):
+        logger = TraceLogger(trace_dir=str(tmp_path))
+        logger.log_step("synthesis_start")
+        _time.sleep(0.01)
+        entry = logger.log_step("synthesis_error", parse_error="boom")
+        logger.close()
+        assert "duration_ms" in entry
+
+    def test_complete_records_total_duration_sec(self, tmp_path):
+        logger = TraceLogger(trace_dir=str(tmp_path))
+        logger.log_step("start", question="q")
+        _time.sleep(0.02)
+        entry = logger.log_step("complete", confidence=0.9)
+        logger.close()
+        assert "total_duration_sec" in entry
+        assert entry["total_duration_sec"] >= 0.015
+        # Sec precision, not ms
+        assert "duration_ms" not in entry
+
+    def test_unpaired_completer_does_not_crash(self, tmp_path):
+        logger = TraceLogger(trace_dir=str(tmp_path))
+        # No matching web_search starter
+        entry = logger.log_step("web_search_complete", result_count=0)
+        logger.close()
+        assert "duration_ms" not in entry
+
+    def test_existing_fields_preserved(self, tmp_path):
+        logger = TraceLogger(trace_dir=str(tmp_path))
+        logger.log_step("web_search", query="x")
+        entry = logger.log_step("web_search_complete", result_count=3, urls=["u1"])
+        logger.close()
+        assert entry["result_count"] == 3
+        assert entry["urls"] == ["u1"]
+        assert "step" in entry
+        assert "timestamp" in entry