diff --git a/docs/handler-authoring.md b/docs/handler-authoring.md
index c77c9e4d..1c7ec8d7 100644
--- a/docs/handler-authoring.md
+++ b/docs/handler-authoring.md
@@ -929,8 +929,9 @@ mock-B).
 ## A2A transport
 
 `serve(MyAgent(), transport="a2a")` wires the same handler through the
-A2A protocol with auto-generated agent card (`/.well-known/agent.json`)
-derived from the `ADCPHandler` methods your class overrides.
+A2A protocol with auto-generated agent card (`/.well-known/agent-card.json`,
+with the 0.3 alias `/.well-known/agent.json` also served for backwards
+compatibility) derived from the `ADCPHandler` methods your class overrides.
 
 ### Durable task storage
 
diff --git a/src/adcp/server/a2a_server.py b/src/adcp/server/a2a_server.py
index e20dd0f0..4f3a4a53 100644
--- a/src/adcp/server/a2a_server.py
+++ b/src/adcp/server/a2a_server.py
@@ -917,8 +917,16 @@ def create_a2a_server(
     }
     if context_builder is not None:
         jsonrpc_kwargs["context_builder"] = context_builder
-    routes = list(create_agent_card_routes(agent_card=agent_card)) + list(
-        create_jsonrpc_routes(**jsonrpc_kwargs)
+    routes = (
+        list(create_agent_card_routes(agent_card=agent_card))
+        # 0.3 alias: A2A 0.3 buyer SDKs probe /.well-known/agent.json
+        # as a positive A2A signal. Same handler, no redirect round-trip.
+        + list(
+            create_agent_card_routes(
+                agent_card=agent_card, card_url="/.well-known/agent.json"
+            )
+        )
+        + list(create_jsonrpc_routes(**jsonrpc_kwargs))
     )
     app = Starlette(routes=routes)
 
diff --git a/src/adcp/server/auth.py b/src/adcp/server/auth.py
index 48e8ccb8..4b27c66c 100644
--- a/src/adcp/server/auth.py
+++ b/src/adcp/server/auth.py
@@ -750,7 +750,8 @@ def resolved_a2a_bearer_prefix_required(self) -> bool:
 _A2A_DISCOVERY_PATHS: frozenset[str] = frozenset(
     {
         _A2A_AGENT_CARD_PATH,  # 1.0 canonical: ``/.well-known/agent-card.json``.
-        "/.well-known/agent.json",  # Legacy 0.3 alias retained by enable_v0_3_compat=True.
+        # Legacy 0.3 alias — route registered explicitly in create_a2a_server.
+        "/.well-known/agent.json",
     }
 )
 
@@ -762,7 +763,8 @@ class A2ABearerAuthMiddleware:
     :func:`adcp.server.a2a_server.create_a2a_server` with this
     middleware to require a valid bearer header on every JSON-RPC
     request, while leaving the spec-mandated public discovery
-    surface (``/.well-known/agent-card.json``) accessible.
+    surface (``/.well-known/agent-card.json`` and the 0.3 alias
+    ``/.well-known/agent.json``) accessible.
 
     Designed to compose with a2a-sdk's
     :class:`DefaultServerCallContextBuilder`: on auth success the
diff --git a/tests/test_a2a_server.py b/tests/test_a2a_server.py
index 7d19bce8..bc3205a0 100644
--- a/tests/test_a2a_server.py
+++ b/tests/test_a2a_server.py
@@ -345,10 +345,13 @@ def test_create_a2a_server_creates_starlette_app():
     # Starlette app has .routes
     assert hasattr(app, "routes")
     route_paths = [r.path for r in app.routes]
-    # A2A well-known agent card endpoint
-    # 1.0 serves ``/.well-known/agent-card.json`` in addition to the
-    # legacy ``/.well-known/agent.json`` aliased path (compat shim).
-    assert any(p.startswith("/.well-known/agent-card") for p in route_paths)
+    # Both the 1.0 canonical path and the 0.3 alias must be registered.
+    assert any(p.startswith("/.well-known/agent-card") for p in route_paths), (
+        "canonical /.well-known/agent-card.json route missing"
+    )
+    assert "/.well-known/agent.json" in route_paths, (
+        "0.3 alias /.well-known/agent.json route missing from create_a2a_server"
+    )
 
 
 # ---------------------------------------------------------------------------
diff --git a/tests/test_serve_auth_both.py b/tests/test_serve_auth_both.py
index e99c0f0d..82415459 100644
--- a/tests/test_serve_auth_both.py
+++ b/tests/test_serve_auth_both.py
@@ -618,6 +618,29 @@ def test_discovery_paths_match_a2a_sdk_routes() -> None:
     )
 
 
+def test_all_discovery_paths_are_registered_routes() -> None:
+    """Inverse of test_discovery_paths_match_a2a_sdk_routes: every path in
+    _A2A_DISCOVERY_PATHS must actually be registered in the Starlette app
+    produced by create_a2a_server. A path in the frozenset but missing from
+    the routing table is auth-exempted but unserviceable — a 404 dressed up
+    as a bypass. This test would have caught the missing /.well-known/agent.json
+    route (issue #612)."""
+    from adcp.server.a2a_server import create_a2a_server
+    from adcp.server.auth import _A2A_DISCOVERY_PATHS
+
+    app = create_a2a_server(_OkHandler(), name="inverse-drift-guard", validation=None)
+    # Structural check only (r.path membership). Live dispatch is validated by
+    # test_a2a_agent_card_served_on_root_path in test_unified_mcp_a2a.py.
+    app_paths = {r.path for r in app.routes}
+
+    not_routed = [p for p in _A2A_DISCOVERY_PATHS if p not in app_paths]
+    assert not not_routed, (
+        f"_A2A_DISCOVERY_PATHS contains path(s) {not_routed!r} that are NOT "
+        f"registered in the Starlette app. Auth middleware exempts them but "
+        f"they 404 — add matching routes in create_a2a_server."
+    )
+
+
 def test_a2a_agent_card_constant_referenced_directly() -> None:
     """The 1.0 path uses ``a2a.utils.constants.AGENT_CARD_WELL_KNOWN_PATH``
     rather than a string literal. If a2a-sdk changes the constant,
diff --git a/tests/test_unified_mcp_a2a.py b/tests/test_unified_mcp_a2a.py
index 97683fbd..ed58a952 100644
--- a/tests/test_unified_mcp_a2a.py
+++ b/tests/test_unified_mcp_a2a.py
@@ -60,17 +60,14 @@ def unified_client():
 
 
 def test_a2a_agent_card_served_on_root_path(unified_client) -> None:
-    """``/.well-known/agent.json`` resolves through the dispatcher
-    to the A2A app. Even if a2a-sdk's wrapper returns 404 (variation
-    in card-path between versions), the request must NOT 405 / 502
-    / 500 — those would indicate the dispatcher routed wrong."""
+    """``/.well-known/agent.json`` (0.3 alias) resolves to a 200 agent-card
+    response. The route must be registered — a 404 means the alias was
+    stripped from create_a2a_server's route list."""
     resp = unified_client.get("/.well-known/agent.json")
-    assert resp.status_code in (200, 404)
-    assert resp.status_code not in (
-        405,
-        502,
-        500,
-    ), f"A2A agent-card path resolved to wrong inner app: status={resp.status_code}"
+    assert resp.status_code == 200, (
+        f"/.well-known/agent.json returned {resp.status_code}; "
+        "expected 200 — the 0.3 alias route is missing from create_a2a_server"
+    )
 
 
 def test_a2a_root_path_routed_to_a2a_app(unified_client) -> None: