diff --git a/src/rius/__init__.py b/src/rius/__init__.py
index ca7fa66..3b61923 100644
--- a/src/rius/__init__.py
+++ b/src/rius/__init__.py
@@ -9,7 +9,7 @@
 from .observe import observe
 from .semconv import SpanKind
 from .session import session
-from .spans import Observation, start_as_current_span, start_span
+from .spans import Observation, current_trace_id, start_as_current_span, start_span
 
 __all__ = [
     "Generation",
@@ -20,6 +20,7 @@
     "SpanKind",
     "__version__",
     "build_span_exporter",
+    "current_trace_id",
     "get_tracer",
     "init",
     "observe",
diff --git a/src/rius/spans.py b/src/rius/spans.py
index 1726f0a..976edb3 100644
--- a/src/rius/spans.py
+++ b/src/rius/spans.py
@@ -11,6 +11,9 @@
 
 ``start_generation`` / ``start_as_current_generation`` are the LLM-specialized
 equivalents.
+
+``current_trace_id`` is the read side of the same context: it reports the trace
+the caller is already running inside, without creating anything.
 """
 
 from __future__ import annotations
@@ -20,7 +23,7 @@
 from typing import Any
 
 from opentelemetry import trace
-from opentelemetry.trace import Span
+from opentelemetry.trace import Span, format_trace_id
 
 from . import __version__
 from ._serde import serialize
@@ -131,3 +134,21 @@ def start_as_current_span(
         observation = Observation(span)
         _configure(observation, kind, input)
         yield observation
+
+
+def current_trace_id() -> str | None:
+    """Return the trace id of the caller's active trace, or ``None`` if untraced.
+
+    Reads the ambient OpenTelemetry context, so it reports whatever trace the
+    caller is running inside — the enclosing ``start_as_current_span`` /
+    ``@observe`` scope, or a context propagated in from upstream — not a trace
+    this SDK created. The id is the standard 32-character lowercase hex form,
+    ready to hand to the platform API or embed in a link.
+
+    ``None`` means there is genuinely no active trace (no span in context, or a
+    sampled-out placeholder), rather than the all-zeros invalid id.
+    """
+    span_context = trace.get_current_span().get_span_context()
+    if not span_context.is_valid:
+        return None
+    return format_trace_id(span_context.trace_id)
diff --git a/tests/test_spans.py b/tests/test_spans.py
index 32cfe65..b6246f9 100644
--- a/tests/test_spans.py
+++ b/tests/test_spans.py
@@ -1,8 +1,10 @@
+import re
+
 import pytest
 from opentelemetry.sdk.trace.export.in_memory_span_exporter import InMemorySpanExporter
-from opentelemetry.trace import StatusCode
+from opentelemetry.trace import INVALID_SPAN, StatusCode, format_trace_id, use_span
 
-from rius import start_as_current_span, start_span
+from rius import current_trace_id, start_as_current_span, start_span
 from rius.semconv import SpanKind
 
 # --- context manager: start_as_current_span ---
@@ -74,3 +76,33 @@ def test_manual_span_is_not_current(exported_spans: InMemorySpanExporter) -> Non
     obs.end()
     spans = {s.name: s for s in exported_spans.get_finished_spans()}
     assert spans["inner"].parent is None  # manual span is not activated as current
+
+
+# --- ambient trace id: current_trace_id ---
+
+
+def test_current_trace_id_inside_span_matches_span_context(
+    exported_spans: InMemorySpanExporter,
+) -> None:
+    with start_as_current_span("op"):
+        trace_id = current_trace_id()
+    span = exported_spans.get_finished_spans()[0]
+    assert trace_id == format_trace_id(span.context.trace_id)
+    assert re.fullmatch(r"[0-9a-f]{32}", trace_id or "")
+
+
+def test_current_trace_id_nested_span_reports_the_enclosing_trace() -> None:
+    with start_as_current_span("outer"):
+        outer = current_trace_id()
+        with start_as_current_span("inner"):
+            inner = current_trace_id()
+    assert inner == outer
+
+
+def test_current_trace_id_outside_any_span_is_none() -> None:
+    assert current_trace_id() is None
+
+
+def test_current_trace_id_ignores_the_invalid_span() -> None:
+    with use_span(INVALID_SPAN):
+        assert current_trace_id() is None