diff --git a/lsp_client/__init__.py b/lsp_client/__init__.py
index e05ba96..f69be8d 100644
--- a/lsp_client/__init__.py
+++ b/lsp_client/__init__.py
@@ -5,9 +5,14 @@
     CancelRequest,
     ClientCapabilities,
     ClientInfo,
+    CodeDescription,
     CompletionRequest,
     ContentChange,
     DefinitionRequest,
+    Diagnostic,
+    DiagnosticRelatedInformation,
+    DiagnosticSeverity,
+    DiagnosticTag,
     ErrorCodes,
     ExitNotification,
     GeneralClientCapabilities,
@@ -16,6 +21,7 @@
     InitializeRequest,
     InitializedNotification,
     LanguageKind,
+    Location,
     LSPErrorCodes,
     Message,
     NotificationMessage,
@@ -59,9 +65,14 @@
     "CancelRequest",
     "ClientCapabilities",
     "ClientInfo",
+    "CodeDescription",
     "CompletionRequest",
     "ContentChange",
     "DefinitionRequest",
+    "Diagnostic",
+    "DiagnosticRelatedInformation",
+    "DiagnosticSeverity",
+    "DiagnosticTag",
     "ErrorCodes",
     "ExitNotification",
     "GeneralClientCapabilities",
@@ -72,6 +83,7 @@
     "LSPClient",
     "LSPErrorCodes",
     "LanguageKind",
+    "Location",
     "Message",
     "NotificationMessage",
     "PositionEncodingKind",
diff --git a/lsp_client/protocol.py b/lsp_client/protocol.py
index 251196a..5d392cb 100644
--- a/lsp_client/protocol.py
+++ b/lsp_client/protocol.py
@@ -432,6 +432,85 @@ class Range(BaseModel):
     end: Position
 
 
+# Location
+# See https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#location  # noqa: E501
+
+
+class Location(BaseModel):
+    """A range inside a text document, identified by its URI."""
+
+    uri: str
+    range: Range
+
+
+# Diagnostic
+# See https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#diagnostic  # noqa: E501
+
+
+class DiagnosticSeverity(IntEnum):
+    """How severe a diagnostic is."""
+
+    Error = 1
+    Warning = 2
+    Information = 3
+    Hint = 4
+
+
+class DiagnosticTag(IntEnum):
+    """Additional metadata about a diagnostic. @since 3.15.0"""
+
+    #: Unused or unnecessary code — clients may render this faded out.
+    Unnecessary = 1
+    #: Deprecated or obsolete code — clients may render this struck through.
+    Deprecated = 2
+
+
+class CodeDescription(BaseModel):
+    """A structure describing a diagnostic's error code. @since 3.16.0"""
+
+    #: A URI to open with more information about the diagnostic error.
+    href: str
+
+
+class DiagnosticRelatedInformation(BaseModel):
+    """A related message and source location for a diagnostic.
+
+    Used e.g. when symbol names within a scope collide, to point at every
+    colliding definition.
+    """
+
+    location: Location
+    message: str
+
+
+class Diagnostic(BaseModel):
+    """A diagnostic, such as a compiler error or warning.
+
+    Diagnostic objects are only valid in the scope of a resource.
+    """
+
+    #: The range at which the message applies.
+    range: Range
+    #: The diagnostic's severity. If omitted the client interprets it.
+    severity: DiagnosticSeverity | None = None
+    #: The diagnostic's code, which may appear in the user interface.
+    code: int | str | None = None
+    #: An optional structure describing the error code. @since 3.16.0
+    codeDescription: CodeDescription | None = None
+    #: A human-readable description of the diagnostic's source, e.g.
+    #: ``"typescript"`` or ``"super lint"``.
+    source: str | None = None
+    #: The diagnostic's message.
+    message: str
+    #: Additional metadata about the diagnostic. @since 3.15.0
+    tags: list[DiagnosticTag] | None = None
+    #: An array of related diagnostic information.
+    relatedInformation: list[DiagnosticRelatedInformation] | None = None
+    #: A data entry preserved between a ``textDocument/publishDiagnostics``
+    #: notification and a ``textDocument/codeAction`` request. @since 3.16.0
+    data: Any | None = None
+
+
 class ContentChange(BaseModel):
     text: str
     range: Optional[Range] = None
diff --git a/tests/test_protocol.py b/tests/test_protocol.py
index f927047..03a0c0a 100644
--- a/tests/test_protocol.py
+++ b/tests/test_protocol.py
@@ -7,9 +7,14 @@
     CancelRequest,
     ClientCapabilities,
     ClientInfo,
+    CodeDescription,
     CompletionRequest,
     ContentChange,
     DefinitionRequest,
+    Diagnostic,
+    DiagnosticRelatedInformation,
+    DiagnosticSeverity,
+    DiagnosticTag,
     ErrorCodes,
     ExitNotification,
     GeneralClientCapabilities,
@@ -18,6 +23,7 @@
     InitializeRequest,
     InitializedNotification,
     LanguageKind,
+    Location,
     LSPErrorCodes,
     Message,
     NotificationMessage,
@@ -456,6 +462,79 @@ def test_server_capabilities_position_encoding_optional():
     assert ServerCapabilities().model_dump(exclude_none=True) == {}
 
 
+def _range() -> Range:
+    return Range(start=Position(line=1, character=0), end=Position(line=1, character=8))
+
+
+def test_diagnostic_severity_values():
+    assert DiagnosticSeverity.Error == 1
+    assert DiagnosticSeverity.Warning == 2
+    assert DiagnosticSeverity.Information == 3
+    assert DiagnosticSeverity.Hint == 4
+
+
+def test_diagnostic_tag_values():
+    assert DiagnosticTag.Unnecessary == 1
+    assert DiagnosticTag.Deprecated == 2
+
+
+def test_location_structure():
+    loc = Location(uri="file:///tmp/a.py", range=_range())
+    assert loc.model_dump() == {
+        "uri": "file:///tmp/a.py",
+        "range": {
+            "start": {"line": 1, "character": 0},
+            "end": {"line": 1, "character": 8},
+        },
+    }
+
+
+def test_diagnostic_minimal():
+    diag = Diagnostic(range=_range(), message="undefined name 'x'")
+    data = diag.model_dump(exclude_none=True)
+    # Only range and message are required; optional fields are omitted.
+    assert set(data) == {"range", "message"}
+    assert data["message"] == "undefined name 'x'"
+
+
+def test_diagnostic_full():
+    diag = Diagnostic(
+        range=_range(),
+        severity=DiagnosticSeverity.Warning,
+        code="F821",
+        codeDescription=CodeDescription(href="https://example.com/F821"),
+        source="flake8",
+        message="undefined name 'x'",
+        tags=[DiagnosticTag.Unnecessary],
+        relatedInformation=[
+            DiagnosticRelatedInformation(
+                location=Location(uri="file:///tmp/b.py", range=_range()),
+                message="first defined here",
+            )
+        ],
+        data={"fixable": True},
+    )
+    data = diag.model_dump(exclude_none=True)
+    assert data["severity"] == 2
+    assert data["code"] == "F821"
+    assert data["codeDescription"] == {"href": "https://example.com/F821"}
+    assert data["source"] == "flake8"
+    assert data["tags"] == [1]
+    assert data["relatedInformation"][0]["location"]["uri"] == "file:///tmp/b.py"
+    assert data["relatedInformation"][0]["message"] == "first defined here"
+    assert data["data"] == {"fixable": True}
+
+
+def test_diagnostic_integer_code():
+    diag = Diagnostic(range=_range(), message="boom", code=42)
+    assert diag.model_dump(exclude_none=True)["code"] == 42
+
+
+def test_diagnostic_rejects_invalid_severity():
+    with pytest.raises(ValidationError):
+        Diagnostic(range=_range(), message="boom", severity=5)
+
+
 def test_error_codes_values():
     # ErrorCodes carries only the JSON-RPC defined codes and reserved markers.
     assert ErrorCodes.ParseError == -32700