diff --git a/craft_application/application.py b/craft_application/application.py
index c92b5d154..bc2ba528c 100644
--- a/craft_application/application.py
+++ b/craft_application/application.py
@@ -34,8 +34,10 @@
 import craft_cli
 import craft_parts
 import craft_providers
+import pydantic
 from craft_parts.plugins.plugins import PluginType
 from platformdirs import user_cache_path
+from pydantic.v1.utils import deep_update
 
 from craft_application import _config, commands, errors, grammar, models, secrets, util
 from craft_application.errors import PathInvalidError
@@ -379,7 +381,7 @@ def get_project(
         craft_cli.emit.debug(f"Loading project file '{project_path!s}'")
 
         with project_path.open() as file:
-            yaml_data = util.safe_yaml_load(file)
+            yaml_data = util.safe_yaml_load_with_lines(file)
 
         host_arch = util.get_host_architecture()
         build_planner = self.app.BuildPlannerClass.from_yaml_data(
@@ -404,13 +406,27 @@ def get_project(
                 build_for = self._build_plan[0].build_for
 
         # validate project grammar
-        GrammarAwareProject.validate_grammar(yaml_data)
+        try:
+            GrammarAwareProject.validate_grammar(yaml_data)
+        except pydantic.ValidationError as err:
+            raise errors.CraftValidationError.from_pydantic(
+                err,
+                file_name=project_path.name,
+                doc_slug="common/craft-parts/reference/part_properties",
+                logpath_report=False,
+                validated_object=yaml_data,
+            ) from None
 
         build_on = host_arch
 
         # Setup partitions, some projects require the yaml data, most will not
         self._partitions = self._setup_partitions(yaml_data)
-        yaml_data = self._transform_project_yaml(yaml_data, build_on, build_for)
+
+        # Apply transformations to base yaml, then update to preserve line numbers
+        yaml_base = util.remove_yaml_lines(yaml_data)
+        yaml_update = self._transform_project_yaml(yaml_base, build_on, build_for)
+        yaml_data = deep_update(yaml_data, yaml_update)
+
         self.__project = self.app.ProjectClass.from_yaml_data(yaml_data, project_path)
 
         # check if mandatory adoptable fields exist if adopt-info not used
diff --git a/craft_application/errors.py b/craft_application/errors.py
index 7a637a9e6..4b7e10eb2 100644
--- a/craft_application/errors.py
+++ b/craft_application/errors.py
@@ -22,7 +22,7 @@
 
 import os
 from collections.abc import Sequence
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any
 
 import yaml
 from craft_cli import CraftError
@@ -72,6 +72,7 @@ def from_pydantic(
         error: pydantic.ValidationError,
         *,
         file_name: str = "yaml file",
+        validated_object: dict[str, Any] | None = None,
         **kwargs: str | bool | int | None,
     ) -> Self:
         """Convert this error from a pydantic ValidationError.
@@ -81,7 +82,9 @@ def from_pydantic(
         :param doc_slug: The optional slug to this error's docs.
         :param kwargs: additional keyword arguments get passed to CraftError
         """
-        message = format_pydantic_errors(error.errors(), file_name=file_name)
+        message = format_pydantic_errors(
+            error.errors(), file_name=file_name, validated_object=validated_object
+        )
         return cls(message, **kwargs)  # type: ignore[arg-type]
 
 
diff --git a/craft_application/grammar.py b/craft_application/grammar.py
index 3af035b81..ffa86d29b 100644
--- a/craft_application/grammar.py
+++ b/craft_application/grammar.py
@@ -121,6 +121,9 @@ def self_check(value: Any) -> bool:  # noqa: ANN401
     processor = GrammarProcessor(arch=arch, target_arch=target_arch, checker=self_check)
 
     for part_name, part_data in parts_yaml_data.items():
+        # Ignore line numbers coming from yaml reader
+        if part_name.startswith("__line__"):
+            continue
         parts_yaml_data[part_name] = process_part(
             part_yaml_data=part_data, processor=processor
         )
diff --git a/craft_application/models/base.py b/craft_application/models/base.py
index e0eaff5d9..caa614481 100644
--- a/craft_application/models/base.py
+++ b/craft_application/models/base.py
@@ -21,6 +21,7 @@
 from typing import Any
 
 import pydantic
+from pydantic import model_validator
 from typing_extensions import Self
 
 from craft_application import errors, util
@@ -42,6 +43,11 @@ class CraftBaseModel(pydantic.BaseModel):
         coerce_numbers_to_str=True,
     )
 
+    @model_validator(mode="before")
+    @classmethod
+    def _flatten(cls, values: dict[str, Any]) -> dict[str, Any]:
+        return util.remove_yaml_lines(values)
+
     def marshal(self) -> dict[str, str | list[str] | dict[str, Any]]:
         """Convert to a dictionary."""
         return self.model_dump(mode="json", by_alias=True, exclude_unset=True)
@@ -84,6 +90,7 @@ def from_yaml_data(cls, data: dict[str, Any], filepath: pathlib.Path) -> Self:
                 file_name=filepath.name,
                 doc_slug=cls.model_reference_slug(),
                 logpath_report=False,
+                validated_object=data,
             ) from None
 
     def to_yaml_file(self, path: pathlib.Path) -> None:
diff --git a/craft_application/models/grammar.py b/craft_application/models/grammar.py
index 9338e1538..3b3342ef0 100644
--- a/craft_application/models/grammar.py
+++ b/craft_application/models/grammar.py
@@ -22,6 +22,7 @@
 from craft_grammar.models import Grammar  # type: ignore[import-untyped]
 from pydantic import ConfigDict
 
+from craft_application import util
 from craft_application.models.base import alias_generator
 from craft_application.models.constraints import SingleEntryDict
 
@@ -92,6 +93,7 @@ def _ensure_parts(cls, data: dict[str, Any]) -> dict[str, Any]:
         item defined, set it to an empty dictionary. This is distinct from having
         `parts` be invalid, which is not coerced here.
         """
+        data = util.remove_yaml_lines(data)
         data.setdefault("parts", {})
         return data
 
diff --git a/craft_application/util/__init__.py b/craft_application/util/__init__.py
index c95cb71f0..87ab102be 100644
--- a/craft_application/util/__init__.py
+++ b/craft_application/util/__init__.py
@@ -35,7 +35,12 @@
 )
 from craft_application.util.string import humanize_list, strtobool
 from craft_application.util.system import get_parallel_build_count
-from craft_application.util.yaml import dump_yaml, safe_yaml_load
+from craft_application.util.yaml import (
+    dump_yaml,
+    safe_yaml_load,
+    safe_yaml_load_with_lines,
+    remove_yaml_lines,
+)
 from craft_application.util.cli import format_timestamp
 
 __all__ = [
@@ -55,6 +60,8 @@
     "get_host_base",
     "dump_yaml",
     "safe_yaml_load",
+    "safe_yaml_load_with_lines",
+    "remove_yaml_lines",
     "retry",
     "get_parallel_build_count",
     "get_hostname",
diff --git a/craft_application/util/error_formatting.py b/craft_application/util/error_formatting.py
index 1aa7acd36..61e3c854d 100644
--- a/craft_application/util/error_formatting.py
+++ b/craft_application/util/error_formatting.py
@@ -17,8 +17,8 @@
 
 from __future__ import annotations
 
-from collections.abc import Iterable
-from typing import NamedTuple
+from collections.abc import Iterable, Sequence
+from typing import Any, NamedTuple
 
 from pydantic import error_wrappers
 
@@ -45,7 +45,11 @@ def from_str(cls, loc_str: str) -> FieldLocationTuple:
         return cls(field, location)
 
 
-def format_pydantic_error(loc: Iterable[str | int], message: str) -> str:
+def format_pydantic_error(
+    loc: Sequence[str | int],
+    message: str,
+    validated_object: dict[str, Any] | None = None,
+) -> str:
     """Format a single pydantic ErrorDict as a string.
 
     :param loc: An iterable of strings and integers determining the error location.
@@ -56,9 +60,12 @@ def format_pydantic_error(loc: Iterable[str | int], message: str) -> str:
     """
     field_path = _format_pydantic_error_location(loc)
     message = _format_pydantic_error_message(message)
+    line_num = _get_line_number(loc, validated_object)
     field_name, location = FieldLocationTuple.from_str(field_path)
     if location != "top-level":
         location = repr(location)
+    if line_num is not None:
+        location += f" - line {line_num}"
 
     if message == "field required":
         return f"- field {field_name!r} required in {location} configuration"
@@ -68,11 +75,16 @@ def format_pydantic_error(loc: Iterable[str | int], message: str) -> str:
         return f"- duplicate {field_name!r} entry not permitted in {location} configuration"
     if field_path in ("__root__", ""):
         return f"- {message}"
-    return f"- {message} (in field {field_path!r})"
+    return f"- {message} (in field {field_path!r}" + (
+        f" - line {line_num})" if line_num else ")"
+    )
 
 
 def format_pydantic_errors(
-    errors: Iterable[error_wrappers.ErrorDict], *, file_name: str = "yaml file"
+    errors: Iterable[error_wrappers.ErrorDict],
+    *,
+    file_name: str = "yaml file",
+    validated_object: dict[str, Any] | None = None,
 ) -> str:
     """Format errors.
 
@@ -87,7 +99,10 @@ def format_pydantic_errors(
     - field: <some field 2>
       reason: <some reason 2>.
     """
-    messages = (format_pydantic_error(error["loc"], error["msg"]) for error in errors)
+    messages = (
+        format_pydantic_error(error["loc"], error["msg"], validated_object)
+        for error in errors
+    )
     return "\n".join((f"Bad {file_name} content:", *messages))
 
 
@@ -115,3 +130,24 @@ def _format_pydantic_error_message(msg: str) -> str:
     if msg:
         msg = msg[0].lower() + msg[1:]
     return msg
+
+
+def _get_line_number(
+    loc: Sequence[str | int], validated_object: dict[str, Any] | None
+) -> int | None:
+    """Return the line number of a key based on its location."""
+    if validated_object is None:
+        return None
+
+    object_value: dict[str, Any] | Sequence[Any] = validated_object
+    line_number: int | None = None
+
+    for i, location in enumerate(loc):
+        if isinstance(location, int) and isinstance(object_value, Sequence):
+            object_value = object_value[location]  # type: ignore[arg-type]
+        elif isinstance(location, str) and isinstance(object_value, dict):
+            if i == len(loc) - 1 and f"__line__{location}" in object_value:
+                line_number = object_value[f"__line__{location}"]
+            elif location in object_value:
+                object_value = object_value[location]
+    return line_number
diff --git a/craft_application/util/yaml.py b/craft_application/util/yaml.py
index 2b4c17991..646f17505 100644
--- a/craft_application/util/yaml.py
+++ b/craft_application/util/yaml.py
@@ -22,6 +22,10 @@
 from typing import TYPE_CHECKING, Any, TextIO, cast, overload
 
 import yaml
+from yaml.composer import Composer
+from yaml.constructor import Constructor
+from yaml.nodes import MappingNode, Node, ScalarNode
+from yaml.resolver import BaseResolver
 
 from craft_application import errors
 
@@ -97,6 +101,42 @@ def __init__(self, stream: TextIO) -> None:
         )
 
 
+class _SafeLineNoLoader(_SafeYamlLoader):
+    def __init__(self, stream: TextIO) -> None:
+        super().__init__(stream)
+
+        self.add_constructor(
+            yaml.resolver.BaseResolver.DEFAULT_MAPPING_TAG, _dict_constructor
+        )
+
+    def compose_node(self, parent: Node | None, index: int) -> Node | None:
+        # the line number where the previous token has ended (plus empty lines)
+        line = self.line
+        node = Composer.compose_node(self, parent, index)
+        setattr(node, "__line__", line + 1)  # noqa: B010 - used internally, prevent mypy error
+        return node
+
+    def construct_mapping(
+        self,
+        node: MappingNode,
+        deep: bool = False,  # noqa: FBT001, FBT002 - used internally by yaml.SafeLoader
+    ) -> dict[Hashable, Any]:
+        node_pair_lst = node.value
+        node_pair_lst_for_appending = []
+
+        for key_node, _ in node_pair_lst:
+            shadow_key_node = ScalarNode(
+                tag=BaseResolver.DEFAULT_SCALAR_TAG, value="__line__" + key_node.value
+            )
+            shadow_value_node = ScalarNode(
+                tag=BaseResolver.DEFAULT_SCALAR_TAG, value=key_node.__line__
+            )
+            node_pair_lst_for_appending.append((shadow_key_node, shadow_value_node))
+
+        node.value = node_pair_lst + node_pair_lst_for_appending
+        return Constructor.construct_mapping(self, node, deep=deep)  # type: ignore[arg-type]
+
+
 def safe_yaml_load(stream: TextIO) -> Any:  # noqa: ANN401 - The YAML could be anything
     """Equivalent to pyyaml's safe_load function, but constraining duplicate keys.
 
@@ -112,6 +152,21 @@ def safe_yaml_load(stream: TextIO) -> Any:  # noqa: ANN401 - The YAML could be a
         raise errors.YamlError.from_yaml_error(filename, error) from error
 
 
+def safe_yaml_load_with_lines(stream: TextIO) -> Any:  # noqa: ANN401 - The YAML could be anything
+    """Equivalent to pyyaml's safe_load function, but constraining duplicate keys and including line numbers.
+
+    :param stream: Any text-like IO object.
+    :returns: A dict object mapping the yaml.
+    """
+    try:
+        # Silencing S506 ("probable use of unsafe loader") because we override it by
+        # using our own safe loader.
+        return yaml.load(stream, Loader=_SafeLineNoLoader)  # noqa: S506
+    except yaml.YAMLError as error:
+        filename = pathlib.Path(stream.name).name
+        raise errors.YamlError.from_yaml_error(filename, error) from error
+
+
 @overload
 def dump_yaml(
     data: Any,  # noqa: ANN401 # Any gets passed to pyyaml
@@ -146,3 +201,17 @@ def dump_yaml(data: Any, stream: TextIO | None = None, **kwargs: Any) -> str | N
     return cast(  # This cast is needed for pyright but not mypy
         str | None, yaml.dump(data, stream, Dumper=yaml.SafeDumper, **kwargs)
     )
+
+
+def remove_yaml_lines(data: dict[str, Any] | list[Any]) -> dict[str, Any]:
+    """Recursively flattens a nested dictionary by removing the '__line__' fields."""
+    if type(data) is list:
+        return [remove_yaml_lines(v) for v in data]  # type: ignore[return-value]
+    if type(data) is not dict:
+        return data  # type: ignore[return-value]
+    # k is only None in one test case
+    return {
+        k: remove_yaml_lines(v)
+        for k, v in data.items()
+        if k is None or "__line__" not in k  # type: ignore[reportUnnecessaryComparison]
+    }
diff --git a/tests/unit/test_application.py b/tests/unit/test_application.py
index 898f1f75f..f78291383 100644
--- a/tests/unit/test_application.py
+++ b/tests/unit/test_application.py
@@ -2141,8 +2141,8 @@ def test_build_planner_errors(tmp_path, monkeypatch, fake_services):
 
     expected = (
         "Bad testcraft.yaml content:\n"
-        "- bad value1: 10 (in field 'value1')\n"
-        "- bad value2: banana (in field 'value2')"
+        "- bad value1: 10 (in field 'value1' - line 3)\n"
+        "- bad value2: banana (in field 'value2' - line 4)"
     )
     assert str(err.value) == expected