diff --git a/airbyte/mcp/_arg_resolvers.py b/airbyte/mcp/_arg_resolvers.py
index 52f600937..85a2a597c 100644
--- a/airbyte/mcp/_arg_resolvers.py
+++ b/airbyte/mcp/_arg_resolvers.py
@@ -113,9 +113,27 @@ def resolve_manifest_yaml(manifest_yaml: str | Path | None) -> str | Path | None
     if is_trusted_execution_enabled():
         return Path(manifest_yaml)
 
+    if _looks_like_file_path(manifest_yaml):
+        raise AirbyteLibInputError(
+            message="`manifest_yaml` looks like a file path, which this server cannot read.",
+            guidance=(
+                "This MCP server does not run on your machine and cannot read local files. "
+                "Pass the manifest content inline as the `manifest_yaml` value."
+            ),
+            context={"manifest_yaml": manifest_yaml},
+        )
+
     return manifest_yaml
 
 
+def _looks_like_file_path(value: str) -> bool:
+    """Return True for a single-line value shaped like a manifest file path."""
+    stripped = value.strip()
+    return stripped.lower().endswith((".yaml", ".yml", ".json")) or stripped.startswith(
+        ("/", "~", "./", "../")
+    )
+
+
 def resolve_connector_config(  # noqa: PLR0912
     config: dict | str | None = None,
     config_file: str | Path | None = None,
diff --git a/airbyte/mcp/cloud.py b/airbyte/mcp/cloud.py
index e9bfb4ffd..bc1fb73d3 100644
--- a/airbyte/mcp/cloud.py
+++ b/airbyte/mcp/cloud.py
@@ -2991,8 +2991,8 @@ def publish_custom_source_definition(
         Field(
             description=(
                 "The Low-code CDK manifest as inline YAML or a file path. File paths are "
-                "honored only when trusted execution is enabled; otherwise, the value is "
-                "treated as inline YAML. Required for YAML connectors."
+                "honored only when trusted execution is enabled; otherwise, pass the manifest "
+                "content inline (path-like values are rejected). Required for YAML connectors."
             ),
             default=None,
         ),
@@ -3234,7 +3234,8 @@ def update_custom_source_definition(
         Field(
             description=(
                 "New manifest as inline YAML or a file path. File paths are honored only when "
-                "trusted execution is enabled; otherwise, the value is treated as inline YAML. "
+                "trusted execution is enabled; otherwise, pass the manifest content inline "
+                "(path-like values are rejected). "
                 "Optional; omit to update only testing values."
             ),
             default=None,
diff --git a/tests/unit_tests/test_mcp_trusted_execution.py b/tests/unit_tests/test_mcp_trusted_execution.py
index 75ea06a5d..1ff1fc401 100644
--- a/tests/unit_tests/test_mcp_trusted_execution.py
+++ b/tests/unit_tests/test_mcp_trusted_execution.py
@@ -178,7 +178,7 @@ def test_resolve_manifest_yaml_rejects_path_when_untrusted(
 @pytest.mark.parametrize(
     "manifest_yaml",
     [
-        pytest.param("manifest.yaml", id="single-line-text"),
+        pytest.param("not-a-manifest", id="single-line-text"),
         pytest.param(
             '{"version": "0.1.0", "type": "DeclarativeSource"}', id="inline-json"
         ),
@@ -196,6 +196,27 @@ def test_resolve_manifest_yaml_preserves_inline_values_when_untrusted(
     assert resolved == manifest_yaml
 
 
+@pytest.mark.parametrize(
+    "manifest_yaml",
+    [
+        pytest.param("manifest.yaml", id="relative-file"),
+        pytest.param("/private/tmp/session/bill_manifest.yaml", id="absolute-yaml"),
+        pytest.param("~/connectors/manifest.YML", id="home-yml"),
+        pytest.param("./manifest", id="dot-relative"),
+    ],
+)
+def test_resolve_manifest_yaml_rejects_path_like_text_when_untrusted(
+    monkeypatch: MonkeyPatch,
+    manifest_yaml: str,
+) -> None:
+    """Untrusted execution rejects single-line path-like text and asks for inline content."""
+    _set_trusted(monkeypatch, enabled=False)
+    with pytest.raises(AirbyteLibInputError) as exc_info:
+        resolve_manifest_yaml(manifest_yaml)
+    assert exc_info.value.guidance is not None
+    assert "inline" in exc_info.value.guidance
+
+
 def test_resolve_manifest_yaml_preserves_existing_path_when_trusted(
     monkeypatch: MonkeyPatch,
     tmp_path: Path,
@@ -228,13 +249,13 @@ def test_resolve_manifest_yaml_does_not_read_file_when_untrusted(
     monkeypatch: MonkeyPatch,
     tmp_path: Path,
 ) -> None:
-    """A path string remains inline YAML when trusted execution is disabled."""
+    """A path string is rejected, not read, when trusted execution is disabled."""
     _set_trusted(monkeypatch, enabled=False)
     manifest_path = tmp_path / "manifest.yaml"
     manifest_path.write_text("version: 0.1.0\ntype: DeclarativeSource\n")
-    resolved = resolve_manifest_yaml(str(manifest_path))
-    assert isinstance(resolved, str)
-    assert resolved == str(manifest_path)
+    with pytest.raises(AirbyteLibInputError) as exc_info:
+        resolve_manifest_yaml(str(manifest_path))
+    assert "version: 0.1.0" not in str(exc_info.value)
 
 
 _UNTRUSTED_LOCAL_HELPERS: list[Callable[[], object]] = [