diff --git a/docs/runbooks/adapter-1c-mcp.md b/docs/runbooks/adapter-1c-mcp.md
index 86b6454..f07eaef 100644
--- a/docs/runbooks/adapter-1c-mcp.md
+++ b/docs/runbooks/adapter-1c-mcp.md
@@ -780,6 +780,9 @@ concrete read-only `metadata.write.plan`. The same strict mode also requires
 the write-preflight smoke to discover an extension form by public
 `extension/ref/form/member` names, compose an allowed plan, and prove that the
 repository and support gates use the same resolved `extension:<GUID>` layer.
+It then submits the same public selector with a deliberately different GUID
+and requires a read-only `blocked / extension_selector_conflict` result, so a
+legacy permissive repository profile cannot authorize a mismatched layer.
 Run that check directly with
 `scripts/smoke_1c_write_preflight.py --require-name-first-extension-form`.
 
diff --git a/plugins/1c/connector/adapter_1c_server.py b/plugins/1c/connector/adapter_1c_server.py
index 4978fef..408e0fc 100644
--- a/plugins/1c/connector/adapter_1c_server.py
+++ b/plugins/1c/connector/adapter_1c_server.py
@@ -47447,6 +47447,18 @@ def repository_apply_gate(payload: dict[str, Any], method: str, mode: str) -> di
             "schema": "onec_repository_write_gate.v1", "method": method,
             "base_id": payload.get("base_id"), **context_error,
         }
+    layer_block = development_layer_write_block(payload)
+    if layer_block:
+        return {
+            "schema": "onec_repository_write_gate.v1",
+            "method": method,
+            "status": "blocked",
+            "base_id": payload.get("base_id"),
+            **layer_block,
+            "diagnostics": {
+                "message": "Saved-state apply is blocked because the public extension selector did not resolve to one matching development layer.",
+            },
+        }
     gate = repository_control.write_gate(payload)
     support = live_sql_support_gate(payload)
     if gate.get("allowed") is True and support.get("allowed") is True:
@@ -47491,9 +47503,12 @@ def metadata_write_preflight(payload: dict[str, Any]) -> dict[str, Any]:
         }
         repository_gate = repository_control.write_gate(gate_payload)
         support_gate = live_sql_support_gate(gate_payload)
+        layer_block = development_layer_write_block(payload)
         schedule_status = str(schedule_plan.get("status") or "blocked")
         status = "ready" if schedule_status in {"planned", "unchanged"} else schedule_status
-        if repository_gate.get("allowed") is not True:
+        if layer_block:
+            status = "blocked"
+        elif repository_gate.get("allowed") is not True:
             status = str(repository_gate.get("status") or "blocked")
         elif support_gate.get("allowed") is not True:
             status = str(support_gate.get("status") or "blocked")
@@ -47526,6 +47541,11 @@ def metadata_write_preflight(payload: dict[str, Any]) -> dict[str, Any]:
             },
             "repository": repository_gate,
             "support": support_gate,
+            **(
+                {"development_layer_resolution": payload["development_layer_resolution"]}
+                if isinstance(payload.get("development_layer_resolution"), dict)
+                else {}
+            ),
             "guards": {
                 "required": ["allow_saved_state_write", "expected_sha1", "backup", "readback_verification"],
                 "requires_saved_state_prepare": schedule_status == "needs_prepare",
@@ -47649,7 +47669,10 @@ def metadata_write_preflight(payload: dict[str, Any]) -> dict[str, Any]:
         gate_payload["target"] = {**caller_target, **plan_target}
     repository_gate = repository_control.write_gate(gate_payload)
     support_gate = live_sql_support_gate(gate_payload)
-    if repository_gate.get("allowed") is not True:
+    layer_block = development_layer_write_block(payload)
+    if layer_block:
+        status = "blocked"
+    elif repository_gate.get("allowed") is not True:
         status = str(repository_gate.get("status") or "blocked")
     elif support_gate.get("allowed") is not True:
         status = str(support_gate.get("status") or "blocked")
@@ -47682,6 +47705,11 @@ def metadata_write_preflight(payload: dict[str, Any]) -> dict[str, Any]:
         "saved_state": saved_state,
         "repository": repository_gate,
         "support": support_gate,
+        **(
+            {"development_layer_resolution": payload["development_layer_resolution"]}
+            if isinstance(payload.get("development_layer_resolution"), dict)
+            else {}
+        ),
         **({"owner_resolution": payload["owner_resolution"]} if isinstance(payload.get("owner_resolution"), dict) else {}),
         "guards": guards,
         "plan": {
@@ -54672,6 +54700,20 @@ def repository_resolve_form_guid_sql(*, method: str, base_id: str, form_guid: st
 _FORM_MODULE_FILE_RE = re.compile(r"^(?:(?P<extension>[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})__)?(?P<form>[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})\.0$", re.IGNORECASE)
 
 
+def development_layer_write_block(payload: dict[str, Any]) -> dict[str, Any] | None:
+    resolution = payload.get("development_layer_resolution") if isinstance(payload.get("development_layer_resolution"), dict) else {}
+    status = str(resolution.get("status") or "").strip().casefold()
+    if not status or status == "resolved":
+        return None
+    diagnostics = resolution.get("diagnostics") if isinstance(resolution.get("diagnostics"), dict) else {}
+    error = str(diagnostics.get("error") or "extension_layer_not_resolved")
+    return {
+        "error": error,
+        "layer_id": resolution.get("layer_id") or "extension:unresolved",
+        "development_layer_resolution": resolution,
+    }
+
+
 def resolve_write_gate_context(payload: dict[str, Any]) -> dict[str, Any]:
     """Derive the exact development layer and form ownership before gates.
 
@@ -54705,16 +54747,47 @@ def resolve_write_gate_context(payload: dict[str, Any]) -> dict[str, Any]:
     else:
         requested_extension = str(requested_extension_value or "").strip()
     base_id = str(payload.get("base_id") or "").strip()
-    resolved_extension_guid = explicit_extension_guid
+    resolved_extension_guid = ""
     extension_error: dict[str, Any] | None = None
-    if not resolved_extension_guid and requested_extension and base_id:
-        resolved_extension_guid, extension_error = extension_filter_to_guid(
+    selector_conflict: dict[str, Any] | None = None
+    if explicit_extension_guid and not is_guid_text(explicit_extension_guid):
+        selector_conflict = {
+            "status": "invalid_argument",
+            "error": "invalid_extension_guid",
+            "extension_guid": explicit_extension_guid,
+        }
+    resolved_name_guid: str | None = None
+    if requested_extension and base_id and selector_conflict is None:
+        resolved_name_guid, extension_error = extension_filter_to_guid(
             base_id,
             requested_extension,
             method="write.gate_context",
         )
+        if extension_error:
+            selector_conflict = extension_error
+        elif explicit_extension_guid and resolved_name_guid != explicit_extension_guid:
+            selector_conflict = {
+                "status": "conflict",
+                "error": "extension_selector_conflict",
+                "base_id": base_id,
+                "query": {
+                    "extension": requested_extension,
+                    "extension_guid": explicit_extension_guid,
+                },
+                "resolved_extension_guid": resolved_name_guid,
+                "diagnostics": {
+                    "message": "The public extension name resolves to a different GUID than the supplied write context.",
+                },
+            }
+    elif requested_extension and not base_id and selector_conflict is None:
+        selector_conflict = {
+            "status": "not_resolved",
+            "error": "extension_resolution_requires_base_id",
+            "base_id": None,
+        }
+    if selector_conflict is None:
+        resolved_extension_guid = str(resolved_name_guid or explicit_extension_guid or "").lower()
     if resolved_extension_guid:
-        resolved_extension_guid = resolved_extension_guid.lower()
         result["extension_guid"] = resolved_extension_guid
         target["extension_guid"] = resolved_extension_guid
         target["origin"] = {
@@ -54736,18 +54809,24 @@ def resolve_write_gate_context(payload: dict[str, Any]) -> dict[str, Any]:
             },
             "layer_id": f"extension:{resolved_extension_guid}",
         }
-    elif requested_extension:
-        # Never let an unresolved public extension selector inherit the base
-        # repository/support policy.  The synthetic layer cannot match a
-        # configured GUID layer and therefore keeps every write gate closed.
+    elif requested_extension or selector_conflict:
+        # An explicit preflight/apply block consumes this resolution.  The
+        # synthetic layer also prevents accidental inheritance of base policy.
+        result.pop("extension_guid", None)
+        target.pop("extension_guid", None)
         result["layer_id"] = "extension:unresolved"
         target["layer_id"] = "extension:unresolved"
+        unresolved_status = "conflict" if str((selector_conflict or {}).get("error") or "") == "extension_selector_conflict" else "not_resolved"
         result["development_layer_resolution"] = {
-            "status": "not_resolved",
+            "status": unresolved_status,
             "source": "extension_name",
-            "extension": {"name": requested_extension},
+            "extension": {
+                **({"name": requested_extension} if requested_extension else {}),
+                **({"supplied_guid": explicit_extension_guid} if explicit_extension_guid else {}),
+            },
             "layer_id": "extension:unresolved",
-            "diagnostics": extension_error
+            "diagnostics": selector_conflict
+            or extension_error
             or {
                 "status": "not_resolved",
                 "error": "extension_resolution_requires_base_id",
@@ -54756,6 +54835,8 @@ def resolve_write_gate_context(payload: dict[str, Any]) -> dict[str, Any]:
         }
     if target:
         result["target"] = target
+    if development_layer_write_block(result):
+        return result
     existing = payload.get("owner_resolution") if isinstance(payload.get("owner_resolution"), dict) else {}
     if existing.get("status") == "resolved" and str(existing.get("repository_object") or "").strip():
         return result
diff --git a/scripts/check_1c_verify_reports.py b/scripts/check_1c_verify_reports.py
index 6ccc517..59f3f76 100644
--- a/scripts/check_1c_verify_reports.py
+++ b/scripts/check_1c_verify_reports.py
@@ -401,7 +401,7 @@ def validate_write_preflight(
         failures.append({"code": "write_preflight_not_ok", "label": label, "path": str(path), "status": report.get("status")})
     if report.get("failures"):
         failures.append({"code": "write_preflight_failures_present", "label": label, "path": str(path), "failures": report.get("failures")})
-    for check in ("method_exposed", "effective_path_preflight", "concrete_saved_state_preflight", "name_first_extension_form_preflight"):
+    for check in ("method_exposed", "effective_path_preflight", "concrete_saved_state_preflight", "name_first_extension_form_preflight", "conflicting_extension_selector_preflight"):
         if check not in checks:
             failures.append({"code": "write_preflight_check_missing", "label": label, "check": check, "path": str(path)})
     expect_check(checks, failures, label, path, "method_exposed", {"status": "ok"}, failure_code="write_preflight_check_field_unexpected")
@@ -470,6 +470,31 @@ def validate_write_preflight(
                 "repository_layer_id": repository_layer or None,
                 "support_layer_id": support_layer or None,
             })
+    conflicting_extension = checks.get("conflicting_extension_selector_preflight") if isinstance(checks.get("conflicting_extension_selector_preflight"), dict) else {}
+    if conflicting_extension.get("status") == "skipped_no_public_extension_form_target":
+        if require_name_first_form:
+            failures.append({
+                "code": "write_preflight_extension_conflict_check_required",
+                "label": label,
+                "path": str(path),
+            })
+    else:
+        expect_check(
+            checks,
+            failures,
+            label,
+            path,
+            "conflicting_extension_selector_preflight",
+            {
+                "schema": "onec_metadata_write_preflight.v1",
+                "status": "blocked",
+                "allowed": False,
+                "resolution_status": "conflict",
+                "error": "extension_selector_conflict",
+                "layer_id": "extension:unresolved",
+            },
+            failure_code="write_preflight_check_field_unexpected",
+        )
     if require_mcp_initialize:
         if "mcp.initialize" not in checks:
             failures.append({"code": "write_preflight_check_missing", "label": label, "check": "mcp.initialize", "path": str(path)})
@@ -1633,6 +1658,14 @@ def write_self_test_reports(report_dir: Path, *, base_id: str, composed: bool, s
                 "name_first": True,
                 "extension": "test2",
             },
+            "conflicting_extension_selector_preflight": {
+                "schema": "onec_metadata_write_preflight.v1",
+                "status": "blocked",
+                "allowed": False,
+                "resolution_status": "conflict",
+                "error": "extension_selector_conflict",
+                "layer_id": "extension:unresolved",
+            },
         },
         "failures": [],
     }
diff --git a/scripts/smoke_1c_write_preflight.py b/scripts/smoke_1c_write_preflight.py
index bbc1f98..9656aa0 100644
--- a/scripts/smoke_1c_write_preflight.py
+++ b/scripts/smoke_1c_write_preflight.py
@@ -207,6 +207,14 @@ def first_public_extension_form_candidate(result: dict[str, Any]) -> dict[str, A
     return candidate
 
 
+def different_extension_guid(layer_id: str) -> str | None:
+    if not EXTENSION_GUID_LAYER_RE.fullmatch(str(layer_id or "")):
+        return None
+    guid = str(layer_id).split(":", 1)[1].lower()
+    replacement = "0" if guid[-1] != "0" else "1"
+    return f"{guid[:-1]}{replacement}"
+
+
 def run_smoke(
     endpoint_url: str,
     base_id: str,
@@ -312,6 +320,9 @@ def run_smoke(
             "search_status": forms.get("status"),
             "forms": int((forms.get("counts") or {}).get("forms") or 0),
         }
+        checks["conflicting_extension_selector_preflight"] = {
+            "status": "skipped_no_public_extension_form_target",
+        }
         if require_name_first_extension_form:
             failures.append("name-first extension form preflight target is required but was not discovered")
     else:
@@ -355,6 +366,49 @@ def run_smoke(
         require(name_first, "extension form preflight input must remain name-first", failures)
         require(bool(EXTENSION_GUID_LAYER_RE.fullmatch(repository_layer)), "repository gate must use a resolved extension GUID layer", failures)
         require(repository_layer == support_layer, "repository and support gates must use the same extension layer", failures)
+        conflicting_guid = different_extension_guid(repository_layer)
+        if not conflicting_guid:
+            checks["conflicting_extension_selector_preflight"] = {
+                "status": "skipped_no_resolved_extension_layer",
+            }
+        else:
+            conflicting = rpc_call(
+                endpoint_url,
+                "metadata.write.preflight",
+                {
+                    "base_id": base_id,
+                    "extension_guid": conflicting_guid,
+                    "target": form_candidate["target"],
+                    "edits": [form_candidate["edit"]],
+                    "resolve_origin": False,
+                    "timeout_seconds": int(timeout),
+                },
+                timeout,
+                transport=transport,
+                session_id=session_id,
+            )
+            layer_resolution = (
+                conflicting.get("development_layer_resolution")
+                if isinstance(conflicting.get("development_layer_resolution"), dict)
+                else {}
+            )
+            layer_diagnostics = (
+                layer_resolution.get("diagnostics")
+                if isinstance(layer_resolution.get("diagnostics"), dict)
+                else {}
+            )
+            checks["conflicting_extension_selector_preflight"] = {
+                "schema": conflicting.get("schema"),
+                "status": conflicting.get("status"),
+                "allowed": conflicting.get("allowed"),
+                "resolution_status": layer_resolution.get("status"),
+                "error": layer_diagnostics.get("error"),
+                "layer_id": layer_resolution.get("layer_id"),
+            }
+            require(conflicting.get("schema") == "onec_metadata_write_preflight.v1", "conflicting extension selector preflight must return expected schema", failures)
+            require(conflicting.get("status") == "blocked" and conflicting.get("allowed") is False, "conflicting extension name/GUID must block preflight", failures)
+            require(layer_resolution.get("status") == "conflict", "conflicting extension selector must report conflict resolution status", failures)
+            require(layer_diagnostics.get("error") == "extension_selector_conflict", "conflicting extension selector must report extension_selector_conflict", failures)
 
     return {
         "schema": "onec_write_preflight_smoke.v1",
diff --git a/scripts/verify_1c_adapter_deployment.ps1 b/scripts/verify_1c_adapter_deployment.ps1
index 471b7a0..137c9d3 100644
--- a/scripts/verify_1c_adapter_deployment.ps1
+++ b/scripts/verify_1c_adapter_deployment.ps1
@@ -206,7 +206,7 @@ function Assert-WritePreflightReport {
     if ($report.failures -and $report.failures.Count -gt 0) {
         throw "$Label report contains failures: $Path"
     }
-    foreach ($check in @("method_exposed", "effective_path_preflight", "concrete_saved_state_preflight", "name_first_extension_form_preflight")) {
+    foreach ($check in @("method_exposed", "effective_path_preflight", "concrete_saved_state_preflight", "name_first_extension_form_preflight", "conflicting_extension_selector_preflight")) {
         if ($report.checks.PSObject.Properties.Name -notcontains $check) {
             throw "$Label report is missing check '$check': $Path"
         }
diff --git a/tests/1c/test_payload_codec.py b/tests/1c/test_payload_codec.py
index f3f5b28..56611ba 100644
--- a/tests/1c/test_payload_codec.py
+++ b/tests/1c/test_payload_codec.py
@@ -20992,6 +20992,14 @@ def test_metadata_write_active_form_target_auto_prepares_when_apply_allowed(monk
 def test_metadata_write_missing_form_target_auto_prepares_and_retries(monkeypatch: pytest.MonkeyPatch) -> None:
     calls: dict[str, int] = {"apply": 0}
     seen: dict[str, Any] = {}
+    monkeypatch.setattr(
+        adapter_server,
+        "extension_filter_to_guid",
+        lambda base_id, extension, method: (
+            "fb26cf42-7609-11f1-828f-005056b0d483",
+            None,
+        ),
+    )
 
     def fake_apply(payload: dict[str, Any]) -> dict[str, Any]:
         calls["apply"] += 1
@@ -21856,6 +21864,104 @@ def test_write_gate_context_does_not_fall_back_to_base_for_unknown_extension(
     assert adapter_server.repository_control.development_layer_id(result) == "extension:unresolved"
 
 
+def test_write_gate_context_rejects_conflicting_extension_name_and_guid(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    name_guid = "fb26cf42-7609-11f1-828f-005056b0d483"
+    supplied_guid = "11111111-1111-1111-1111-111111111111"
+    monkeypatch.setattr(
+        adapter_server,
+        "extension_filter_to_guid",
+        lambda base_id, extension, method: (name_guid, None),
+    )
+
+    result = adapter_server.resolve_write_gate_context(
+        {
+            "base_id": "upo_test",
+            "extension_guid": supplied_guid,
+            "target": {
+                "kind": "form",
+                "extension": "test2",
+                "ref": "Catalog.test2",
+                "form": "t_Форма",
+            },
+        }
+    )
+
+    resolution = result["development_layer_resolution"]
+    assert "extension_guid" not in result
+    assert "extension_guid" not in result["target"]
+    assert resolution["status"] == "conflict"
+    assert resolution["layer_id"] == "extension:unresolved"
+    assert resolution["extension"] == {
+        "name": "test2",
+        "supplied_guid": supplied_guid,
+    }
+    assert resolution["diagnostics"]["error"] == "extension_selector_conflict"
+    assert resolution["diagnostics"]["resolved_extension_guid"] == name_guid
+
+
+def test_repository_apply_gate_blocks_unresolved_extension_before_policy_gates(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    def must_not_run(_payload: dict[str, Any]) -> dict[str, Any]:
+        raise AssertionError("repository/support policy gates must not authorize an unresolved extension layer")
+
+    monkeypatch.setattr(adapter_server.repository_control, "write_gate", must_not_run)
+    monkeypatch.setattr(adapter_server, "live_sql_support_gate", must_not_run)
+
+    result = adapter_server.repository_apply_gate(
+        {
+            "base_id": "upo_test",
+            "development_layer_resolution": {
+                "status": "conflict",
+                "layer_id": "extension:unresolved",
+                "diagnostics": {"error": "extension_selector_conflict"},
+            },
+        },
+        "metadata.write",
+        "apply",
+    )
+
+    assert result["status"] == "blocked"
+    assert result["error"] == "extension_selector_conflict"
+    assert result["layer_id"] == "extension:unresolved"
+
+
+def test_metadata_write_apply_blocks_conflicting_extension_name_and_guid(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(
+        adapter_server,
+        "extension_filter_to_guid",
+        lambda base_id, extension, method: (
+            "fb26cf42-7609-11f1-828f-005056b0d483",
+            None,
+        ),
+    )
+
+    result = adapter_server.metadata_write(
+        {
+            "base_id": "upo_test",
+            "extension_guid": "11111111-1111-1111-1111-111111111111",
+            "target": {
+                "kind": "form",
+                "extension": "test2",
+                "ref": "Catalog.test2",
+                "form": "t_Форма",
+                "command": "ЗаменаДомена",
+            },
+            "edits": [{"property": "Заголовок", "value": "Новое имя"}],
+            "mode": "apply",
+        }
+    )
+
+    assert result["schema"] == "onec_repository_write_gate.v1"
+    assert result["status"] == "blocked"
+    assert result["error"] == "extension_selector_conflict"
+    assert result["development_layer_resolution"]["status"] == "conflict"
+
+
 def test_metadata_write_preflight_routes_public_extension_form_gates_to_extension_layer(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
@@ -21928,6 +22034,84 @@ def test_metadata_write_preflight_routes_public_extension_form_gates_to_extensio
     assert captured_layers == [expected_layer, expected_layer]
 
 
+def test_metadata_write_preflight_blocks_conflicting_extension_name_and_guid(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    name_guid = "fb26cf42-7609-11f1-828f-005056b0d483"
+    supplied_guid = "11111111-1111-1111-1111-111111111111"
+    monkeypatch.setattr(
+        adapter_server,
+        "extension_filter_to_guid",
+        lambda base_id, extension, method: (name_guid, None),
+    )
+    monkeypatch.setattr(
+        adapter_server,
+        "metadata_form_write_target_resolve",
+        lambda payload: {
+            "status": "ok",
+            "source": {
+                "table": "ConfigCASSave",
+                "file_name": f"{name_guid}__form-guid.0",
+            },
+            "counts": {"matches": 1},
+        },
+    )
+    monkeypatch.setattr(
+        adapter_server,
+        "metadata_saved_state_diff",
+        lambda payload: {
+            "status": "changed",
+            "target": {"table": payload["table"], "file_name": payload["file_name"]},
+            "freshness": {
+                "source": "live_sql",
+                "status": "live_sql_verified",
+                "verified_against_sql": True,
+            },
+        },
+    )
+    monkeypatch.setattr(
+        adapter_server.repository_control,
+        "write_gate",
+        lambda payload: {
+            "status": "not_configured",
+            "allowed": True,
+            "layer_id": adapter_server.repository_control.development_layer_id(payload),
+        },
+    )
+    monkeypatch.setattr(
+        adapter_server,
+        "live_sql_support_gate",
+        lambda payload: {
+            "status": "support_not_configured_legacy",
+            "allowed": True,
+            "layer_id": adapter_server.repository_control.development_layer_id(payload),
+        },
+    )
+
+    result = adapter_server.metadata_write_preflight(
+        {
+            "base_id": "upo_test",
+            "extension_guid": supplied_guid,
+            "target": {
+                "kind": "form",
+                "extension": "test2",
+                "ref": "Catalog.test2",
+                "form": "t_Форма",
+                "command": "ЗаменаДомена",
+            },
+            "edits": [{"property": "Заголовок", "value": "Новое имя"}],
+            "resolve_origin": False,
+        }
+    )
+
+    assert result["status"] == "blocked"
+    assert result["allowed"] is False
+    assert result["repository"]["allowed"] is True
+    assert result["support"]["allowed"] is True
+    assert result["development_layer_resolution"]["status"] == "conflict"
+    assert result["development_layer_resolution"]["diagnostics"]["error"] == "extension_selector_conflict"
+
+
 def test_metadata_write_plan_help_describes_name_first_form_and_module_targets() -> None:
     methods = {
         row["name"]: row
@@ -26459,6 +26643,14 @@ def test_metadata_write_missing_module_target_exposes_prepare_plan(monkeypatch:
 def test_metadata_write_missing_module_target_auto_prepares_and_retries(monkeypatch: pytest.MonkeyPatch) -> None:
     calls: dict[str, int] = {"search": 0}
     seen: dict[str, Any] = {}
+    monkeypatch.setattr(
+        adapter_server,
+        "extension_filter_to_guid",
+        lambda base_id, extension, method: (
+            "fb26cf42-7609-11f1-828f-005056b0d483",
+            None,
+        ),
+    )
 
     def fake_search(payload: dict[str, Any]) -> dict[str, Any]:
         calls["search"] += 1
diff --git a/tests/1c/test_write_preflight_smoke.py b/tests/1c/test_write_preflight_smoke.py
index 1c8f27e..717fd6e 100644
--- a/tests/1c/test_write_preflight_smoke.py
+++ b/tests/1c/test_write_preflight_smoke.py
@@ -112,6 +112,22 @@ def test_public_extension_form_candidate_is_name_first_and_prefers_title() -> No
     }
 
 
+def test_different_extension_guid_changes_only_last_hex_digit() -> None:
+    assert (
+        smoke.different_extension_guid(
+            "extension:fb26cf42-7609-11f1-828f-005056b0d483"
+        )
+        == "fb26cf42-7609-11f1-828f-005056b0d480"
+    )
+    assert (
+        smoke.different_extension_guid(
+            "extension:fb26cf42-7609-11f1-828f-005056b0d480"
+        )
+        == "fb26cf42-7609-11f1-828f-005056b0d481"
+    )
+    assert smoke.different_extension_guid("base") is None
+
+
 def test_name_first_extension_form_preflight_rejects_base_gate_layer(monkeypatch) -> None:
     def fake_rpc_call(
         _endpoint_url: str,