Don't clear gist description on update when omitted

update_gist always sent Description as a pointer to the OptionalParam
zero value (""), so omitting description would overwrite an existing gist
description with an empty string. Only set UpdateGistRequest.Description
when the caller actually provided the argument; an explicit empty string
still clears it. Adds a test asserting the description key is absent from
the PATCH body when omitted and present when set.

This addresses a pre-existing behavior surfaced while migrating to the
v89 gist request types.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: tommaso-moro

pkg/github/gists.go

@@ -331,9 +331,17 @@ func UpdateGist(t translations.TranslationHelperFunc) inventory.ServerTool {
 				Content:  github.Ptr(content),
 			}
 
+			// Only set Description when the caller actually provided it, so
+			// omitting it preserves the gist's existing description. Passing an
+			// explicit empty string still clears it.
+			var descriptionPtr *string
+			if _, ok := args["description"]; ok {
+				descriptionPtr = github.Ptr(description)
+			}
+
 			gist := github.UpdateGistRequest{
 				Files:       files,
-				Description: github.Ptr(description),
+				Description: descriptionPtr,
 			}
 
 			client, err := deps.GetClient(ctx)

pkg/github/gists_test.go

@@ -581,3 +581,66 @@ func Test_UpdateGist(t *testing.T) {
 		})
 	}
 }
+
+func Test_UpdateGist_DescriptionOnlySentWhenProvided(t *testing.T) {
+	serverTool := UpdateGist(translations.NullTranslationHelper)
+
+	updatedGist := &github.Gist{
+		ID:      github.Ptr("existing-gist-id"),
+		HTMLURL: github.Ptr("https://gist.github.com/user/existing-gist-id"),
+	}
+
+	cases := []struct {
+		name            string
+		args            map[string]any
+		wantDescPresent bool
+		wantDescValue   string
+	}{
+		{
+			name: "omitted description is not sent, preserving the existing one",
+			args: map[string]any{
+				"gist_id":  "existing-gist-id",
+				"filename": "updated.go",
+				"content":  "package main",
+			},
+			wantDescPresent: false,
+		},
+		{
+			name: "explicit empty description is sent to clear it",
+			args: map[string]any{
+				"gist_id":     "existing-gist-id",
+				"filename":    "updated.go",
+				"content":     "package main",
+				"description": "",
+			},
+			wantDescPresent: true,
+			wantDescValue:   "",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			var gotBody map[string]any
+			client := mustNewGHClient(t, MockHTTPClientWithHandlers(map[string]http.HandlerFunc{
+				PatchGistsByGistID: func(w http.ResponseWriter, r *http.Request) {
+					require.NoError(t, json.NewDecoder(r.Body).Decode(&gotBody))
+					w.WriteHeader(http.StatusOK)
+					_, _ = w.Write(MustMarshal(updatedGist))
+				},
+			}))
+			deps := BaseDeps{Client: client}
+			handler := serverTool.Handler(deps)
+			request := createMCPRequest(tc.args)
+
+			result, err := handler(ContextWithDeps(context.Background(), deps), &request)
+			require.NoError(t, err)
+			require.False(t, result.IsError)
+
+			desc, present := gotBody["description"]
+			assert.Equal(t, tc.wantDescPresent, present, "description key presence in PATCH body")
+			if tc.wantDescPresent {
+				assert.Equal(t, tc.wantDescValue, desc)
+			}
+		})
+	}
+}