diff --git a/agent_assembly/core/assembly.py b/agent_assembly/core/assembly.py
index 82f6ba65..ae3637b2 100644
--- a/agent_assembly/core/assembly.py
+++ b/agent_assembly/core/assembly.py
@@ -28,6 +28,7 @@
     resolve_gateway_url,
 )
 from agent_assembly.core.runtime_interceptor import (
+    ENFORCE_MODE,
     _local_posture_is_enforce,
     _native_core_available,
     build_governance_interceptor,
@@ -215,6 +216,14 @@ def init_assembly(
     a ``deny`` blocks the tool before it runs. The SDK never calls a core HTTP
     endpoint directly for registration or policy checks.
 
+    :param mode: Interception layer to activate (see :data:`RuntimeMode`).
+        ``"sdk-only"`` is the in-process-only layer that starts no network
+        sidecar, so it is also the one mode where a gateway that cannot be
+        registered with only **warns** instead of failing init (AAASM-6155) —
+        unless ``enforcement_mode="enforce"`` is passed explicitly, which asks
+        for the fail-closed posture by name and aborts in every mode. Governed
+        tool calls are unaffected either way: the interceptor still denies under
+        enforce when no authoritative decision is available.
     :param control_plane_url: Optional URL of the control-plane HTTP API. When
         supplied, the SDK issues its remaining HTTP routes (topology edges,
         secret dispatch) against it instead of ``gateway_url``. When omitted it
@@ -308,6 +317,7 @@ def init_assembly(
                 runtime_client=runtime_client,
                 agent_id=resolved_agent_id,
                 enforcement_mode=enforcement_mode,
+                mode=mode,
                 gateway_endpoint=resolve_gateway_grpc_endpoint(gateway_url, allow_insecure=allow_insecure),
                 native_available=native_available,
                 team_id=team_id,
@@ -456,11 +466,48 @@ def _warn_audit_not_recorded(disposition: AuditSinkDisposition) -> None:
     )
 
 
+def _register_failure_is_fatal(*, mode: RuntimeMode, enforcement_mode: EnforcementMode | None) -> bool:
+    """Whether a failed ``register`` must abort init rather than warn (AAASM-6155).
+
+    Two rules, in order:
+
+    * An **explicit** ``enforce`` always aborts, in every ``mode``. The caller
+      asked for the fail-closed posture by name, so a gateway it cannot register
+      with is a misconfiguration and init must not come up.
+    * Otherwise ``mode="sdk-only"`` warns and continues. That mode is documented
+      as the in-process-only layer that starts no network sidecar and is "the best
+      choice for deterministic, offline examples and tests"; the quick-start
+      states the offline path *warns* that the agent is unregistered. Requiring a
+      reachable gateway there contradicts the one mode whose purpose is to run
+      without one.
+
+    Every other ``mode`` keeps the :func:`_local_posture_is_enforce` posture, so an
+    unset ``enforcement_mode`` of ``None`` still aborts (AAASM-4130): ``None``
+    registers under the gateway's server-side default of live ``enforce``, and
+    ``auto`` / ``proxy`` / ``ebpf`` all do bring up an interception layer that the
+    gateway is expected to back.
+
+    Relaxing init here does not make an ``sdk-only`` session run ungoverned. The
+    registration warning is unconditional (see :func:`_warn_agent_unregistered`),
+    ``AssemblyContext.registered`` reports ``False``, and the interceptor the
+    adapters are handed keeps its own fail-closed posture under enforce — an
+    unreachable runtime or an unauthoritative ``query_policy`` still **denies** the
+    tool call (AAASM-3106, AAASM-4760). What changes is only whether ``init_assembly``
+    raises instead of warning.
+    """
+    if enforcement_mode == ENFORCE_MODE:
+        return True
+    if mode == "sdk-only":
+        return False
+    return _local_posture_is_enforce(enforcement_mode)
+
+
 def _register_agent_with_gateway(
     *,
     runtime_client: Any | None,
     agent_id: str,
     enforcement_mode: EnforcementMode | None,
+    mode: RuntimeMode,
     gateway_endpoint: str,
     native_available: bool,
     team_id: str | None = None,
@@ -488,8 +535,9 @@ def _register_agent_with_gateway(
     ``register`` raises, the failure is no longer silent: a loud
     :func:`_warn_agent_unregistered` fires and ``False`` is returned (AAASM-4547).
     Init still proceeds so the proxy / eBPF layers stay authoritative — except
-    under an enforce posture (the ``None`` default or explicit ``enforce``), where a
-    ``register`` failure propagates so a misconfigured gateway fails init closed.
+    where :func:`_register_failure_is_fatal` says the failure must abort, which is
+    an explicit ``enforce`` in any mode, or the ``None`` default in any mode other
+    than ``sdk-only`` (AAASM-6155).
     """
     if runtime_client is None:
         if native_available:
@@ -512,7 +560,7 @@ def _register_agent_with_gateway(
             parent_agent_id=parent_agent_id,
         )
     except Exception as error:
-        if _local_posture_is_enforce(enforcement_mode):
+        if _register_failure_is_fatal(mode=mode, enforcement_mode=enforcement_mode):
             raise
         _warn_agent_unregistered(f"registration failed: {error}")
         return False
diff --git a/docs/concepts/index.md b/docs/concepts/index.md
index 71e8435c..e3d3d023 100644
--- a/docs/concepts/index.md
+++ b/docs/concepts/index.md
@@ -130,6 +130,14 @@ Two independent knobs control governance. It's worth keeping them straight:
 | `proxy` | Routes outbound traffic through the `aasm` sidecar proxy — network-egress policy without modifying the agent's source. |
 | `ebpf` | Kernel-level interception via eBPF. **Linux only** — raises `ConfigurationError` elsewhere. |
 
+The two knobs are independent with one exception: what a **failed agent registration** does to
+`init_assembly()`. Under `sdk-only` — the mode that starts no sidecar and is documented as
+needing no gateway — an unreachable gateway **warns** and init continues. Every other mode, and
+any mode where you pass `enforcement_mode="enforce"` explicitly, treats it as a misconfiguration
+and fails init closed (AAASM-6155). Only whether `init_assembly()` raises differs: in both cases
+`ctx.registered` reports `False`, the warning is unconditional, and a governed tool call with no
+authoritative decision behind it is still **denied** under enforce.
+
 ### Enforcement modes
 
 `enforcement_mode` is the governance posture sent to the gateway at registration. Leaving it
diff --git a/test/unit/core/test_init_registration.py b/test/unit/core/test_init_registration.py
index 32fcdc9a..270fef8c 100644
--- a/test/unit/core/test_init_registration.py
+++ b/test/unit/core/test_init_registration.py
@@ -686,6 +686,138 @@ def test_native_present_but_no_runtime_client_warns_and_marks_unregistered(
         context.shutdown()
 
 
+def test_sdk_only_default_posture_warns_on_register_failure(
+    monkeypatch: pytest.MonkeyPatch,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """AAASM-6155 regression: ``mode="sdk-only"`` with no ``enforcement_mode`` must
+    warn on a register failure, not abort init.
+
+    ``sdk-only`` is documented as the in-process-only layer that starts no network
+    sidecar and needs no gateway, so requiring a reachable one there contradicted
+    the mode's purpose. The AAASM-4130 fail-closed posture for an unset
+    ``enforcement_mode`` still applies to every other mode — see the ``auto`` /
+    ``proxy`` / ``ebpf`` cases below, which are what keeps this from being a blanket
+    relaxation.
+    """
+    runtime_client = FakeRuntimeClient(decision="allow")
+    runtime_client.register_should_raise = RuntimeError("gateway gRPC endpoint is unreachable for registration")
+    install_fake_core(monkeypatch, runtime_client)
+    _no_network(monkeypatch)
+    monkeypatch.setattr(core_assembly, "_register_adapters", lambda **_kwargs: ([], AUDIT_SINK_ABSENT))
+
+    context = init_assembly(gateway_url=_GW_URL, api_key=_API_KEY, agent_id="offline-demo", mode="sdk-only")
+    try:
+        # Init came up, and it is honest about what it could not do.
+        assert context.registered is False
+        err = capsys.readouterr().err
+        assert "NOT registered" in err
+        assert "registration failed" in err
+    finally:
+        context.shutdown()
+
+
+@pytest.mark.parametrize("mode", ["auto", "proxy", "ebpf"])
+def test_non_sdk_only_default_posture_still_propagates_register_failure(
+    monkeypatch: pytest.MonkeyPatch,
+    mode: str,
+) -> None:
+    """The AAASM-6155 relaxation is scoped to ``sdk-only`` and nothing else.
+
+    Every mode that does bring up an interception layer keeps the AAASM-4130
+    posture: an unset ``enforcement_mode`` registers under the gateway's
+    server-side default of live ``enforce``, so a gateway it cannot register with
+    fails init closed. Without these cases the ``sdk-only`` test above would also
+    pass if the guard had been removed outright.
+    """
+    runtime_client = FakeRuntimeClient(decision="allow")
+    runtime_client.register_should_raise = RuntimeError("gateway gRPC endpoint is unreachable for registration")
+    install_fake_core(monkeypatch, runtime_client)
+    # Patched so the assertion is about registration, not about which network
+    # layer this host happens to support (``ebpf`` is Linux-only).
+    _no_network(monkeypatch)
+    monkeypatch.setattr(core_assembly, "_register_adapters", lambda **_kwargs: ([], AUDIT_SINK_ABSENT))
+
+    with pytest.raises(ConfigurationError, match="Failed to initialize assembly runtime"):
+        init_assembly(gateway_url=_GW_URL, api_key=_API_KEY, agent_id=f"agent-{mode}", mode=mode)  # type: ignore[arg-type]
+
+
+def test_sdk_only_register_failure_still_denies_governed_tool_calls(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """The AAASM-6155 relaxation must not let an ``sdk-only`` session run ungoverned.
+
+    Only whether ``init_assembly`` *raises* changes. With registration failed and
+    the runtime unable to return an authoritative verdict, the interceptor the
+    adapters are handed keeps its enforce-posture fail-closed behaviour: the
+    governed tool call is denied (AAASM-3106). A relaxation that had also loosened
+    the interceptor would show ``allow`` here.
+    """
+    # ``query_failed`` is what an unreachable runtime yields — not an
+    # authoritative allow, so under enforce it must deny.
+    runtime_client = FakeRuntimeClient(decision="query_failed")
+    runtime_client.register_should_raise = RuntimeError("gateway gRPC endpoint is unreachable for registration")
+    install_fake_core(monkeypatch, runtime_client)
+    _no_network(monkeypatch)
+    adapter = _CapturingAdapter()
+    monkeypatch.setattr(core_assembly, "_register_adapters", _patched_register_adapters(adapter))
+
+    context = init_assembly(gateway_url=_GW_URL, api_key=_API_KEY, agent_id="offline-governed", mode="sdk-only")
+    try:
+        assert context.registered is False
+        assert adapter.interceptor is not None
+        interceptor: Any = adapter.interceptor
+        verdict = interceptor.check_tool_start(
+            serialized={"name": "web_search"},
+            input_str="q",
+            tool_name="web_search",
+            args={"q": "x"},
+        )
+        assert verdict["status"] == "deny"
+        assert verdict["status"] != "allow"
+    finally:
+        context.shutdown()
+
+
+@pytest.mark.parametrize(
+    ("mode", "enforcement_mode", "fatal"),
+    [
+        # sdk-only: the None default warns; an explicit enforce still aborts.
+        ("sdk-only", None, False),
+        ("sdk-only", "enforce", True),
+        ("sdk-only", "observe", False),
+        ("sdk-only", "disabled", False),
+        # Every other mode keeps the AAASM-4130 posture for the None default.
+        ("auto", None, True),
+        ("auto", "enforce", True),
+        ("auto", "observe", False),
+        ("auto", "disabled", False),
+        ("proxy", None, True),
+        ("proxy", "enforce", True),
+        ("proxy", "observe", False),
+        ("proxy", "disabled", False),
+        ("ebpf", None, True),
+        ("ebpf", "enforce", True),
+        ("ebpf", "observe", False),
+        ("ebpf", "disabled", False),
+    ],
+)
+def test_register_failure_fatality_matrix(mode: str, enforcement_mode: str | None, fatal: bool) -> None:
+    """The full ``mode`` × ``enforcement_mode`` contract for a failed register.
+
+    Spelled out exhaustively because the defect (AAASM-6155) was a single cell of
+    this matrix — ``sdk-only`` × ``None`` — flipping when the guard moved from an
+    equality test to :func:`_local_posture_is_enforce`.
+    """
+    assert (
+        core_assembly._register_failure_is_fatal(
+            mode=mode,  # type: ignore[arg-type]
+            enforcement_mode=enforcement_mode,  # type: ignore[arg-type]
+        )
+        is fatal
+    )
+
+
 def test_register_failure_under_observe_warns_and_marks_unregistered(
     monkeypatch: pytest.MonkeyPatch,
     capsys: pytest.CaptureFixture[str],