diff --git a/codegraph/analysis/__init__.py b/codegraph/analysis/__init__.py
index 85c3117..debaef0 100644
--- a/codegraph/analysis/__init__.py
+++ b/codegraph/analysis/__init__.py
@@ -1,6 +1,7 @@
 """Whole-project analyses operating on the codegraph store."""
 from codegraph.analysis.blast_radius import BlastRadiusResult, blast_radius
 from codegraph.analysis.cycles import Cycle, CycleReport, find_cycles
+from codegraph.analysis.dataflow import DataFlow, FlowHop, match_route, trace
 from codegraph.analysis.dead_code import DeadNode, find_dead_code
 from codegraph.analysis.hotspots import Hotspot, find_hotspots
 from codegraph.analysis.metrics import GraphMetrics, compute_metrics
@@ -11,7 +12,9 @@
     "BlastRadiusResult",
     "Cycle",
     "CycleReport",
+    "DataFlow",
     "DeadNode",
+    "FlowHop",
     "GraphMetrics",
     "Hotspot",
     "UntestedNode",
@@ -22,4 +25,6 @@
     "find_dead_code",
     "find_hotspots",
     "find_untested",
+    "match_route",
+    "trace",
 ]
diff --git a/codegraph/analysis/dataflow.py b/codegraph/analysis/dataflow.py
new file mode 100644
index 0000000..d017f4d
--- /dev/null
+++ b/codegraph/analysis/dataflow.py
@@ -0,0 +1,126 @@
+"""End-to-end data-flow tracing across the structural / behavioural / dataflow
+graph layers.
+
+This module exposes two complementary functions:
+
+* :func:`match_route` — given a frontend ``FETCH_CALL`` URL + method, find the
+  qualname of the backend handler whose ``ROUTE`` edge matches. Implemented by
+  the DF3 stitcher (URL pattern matching with parameter normalisation).
+
+* :func:`trace` — given an entry symbol (function qualname or ``url:METHOD path``
+  shape), walk the call graph + cross-layer edges to produce an ordered
+  :class:`DataFlow`. Implemented by DF4.
+
+The dataclasses are stable contract — agents fill in the function bodies but
+never modify the shapes here without coordination.
+"""
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from typing import Any
+
+import networkx as nx
+
+
+@dataclass
+class FlowHop:
+    """One step in a traced data-flow.
+
+    ``layer`` distinguishes ``frontend`` / ``backend`` / ``db`` so consumers
+    (CLI, MCP, dashboard) can render lanes. ``confidence`` is the per-hop match
+    quality — 1.0 for direct call-graph edges, lower for fuzzy URL matches.
+    """
+
+    layer: str  # "frontend" | "backend" | "db"
+    qualname: str
+    file: str = ""
+    line: int = 0
+    method: str | None = None  # HTTP verb, when applicable
+    path: str | None = None  # URL path, when applicable
+    args: list[str] = field(default_factory=list)
+    kwargs: dict[str, str] = field(default_factory=dict)
+    role: str | None = None  # HANDLER / SERVICE / COMPONENT / REPO if known
+    confidence: float = 1.0
+
+    def to_dict(self) -> dict[str, Any]:
+        out: dict[str, Any] = {
+            "layer": self.layer,
+            "qualname": self.qualname,
+            "file": self.file,
+            "line": self.line,
+            "args": list(self.args),
+            "kwargs": dict(self.kwargs),
+            "confidence": self.confidence,
+        }
+        if self.method is not None:
+            out["method"] = self.method
+        if self.path is not None:
+            out["path"] = self.path
+        if self.role is not None:
+            out["role"] = self.role
+        return out
+
+
+@dataclass
+class DataFlow:
+    """Ordered sequence of :class:`FlowHop` objects describing one trace.
+
+    ``confidence`` is the minimum across hops — the chain is only as strong as
+    its weakest match.
+    """
+
+    entry: str
+    hops: list[FlowHop] = field(default_factory=list)
+    confidence: float = 1.0
+
+    def to_dict(self) -> dict[str, Any]:
+        return {
+            "entry": self.entry,
+            "hops": [h.to_dict() for h in self.hops],
+            "confidence": self.confidence,
+        }
+
+
+def match_route(
+    graph: nx.MultiDiGraph,
+    fetch_url: str,
+    fetch_method: str = "GET",
+) -> tuple[str, float] | None:
+    """Return ``(handler_qualname, confidence)`` for the route matching this
+    frontend fetch, or ``None`` if no route matches.
+
+    Implemented by DF3 (agent A1). Stub returns ``None`` so DF4 can build
+    against the contract before A1 lands.
+
+    Confidence rubric (DF3 will encode):
+      * 1.0 — exact path + method match
+      * 0.9 — path with placeholders matches (``/users/{id}`` vs ``/users/42``)
+      * 0.7 — method matches, path matches with body-key heuristic
+      * 0.5 — only path prefix matches (last-resort)
+    """
+    return None
+
+
+def trace(
+    graph: nx.MultiDiGraph,
+    entry: str,
+    *,
+    max_depth: int = 6,
+) -> DataFlow | None:
+    """Trace a data-flow starting from ``entry``.
+
+    ``entry`` may be:
+
+      * a fully-qualified symbol name — walk forwards over CALLS edges
+      * ``"url:METHOD /path"`` — start from a frontend ``FETCH_CALL`` and stitch
+        through the matching ROUTE handler
+
+    Returns ``None`` when ``entry`` cannot be located in the graph.
+
+    Implemented by DF4 (agent A2). Stub returns ``None`` so the CLI / MCP layer
+    can register the public surface.
+    """
+    return None
+
+
+__all__ = ["DataFlow", "FlowHop", "match_route", "trace"]