fix(issues): allow delete:false in issue_write issue_fields

The `delete` property of issue_write's issue_fields items was declared with
Enum: []any{true}, making true its only legal value. The property is optional,
but a client that fills every property of a schema -- common, since
OpenAI-style strict function calling requires every property to appear in
`required` -- had no way to express "not deleting this field": there is no
false in the enum and no null in the type. `value` offers no alternative
either, being typed ["string","number","boolean"] with no null.

The MCP Go SDK validates arguments against the resolved input schema before
the handler runs, so delete: false was rejected at schema validation and never
reached optionalIssueWriteFields. Such clients sent delete: true alongside a
value instead and hit the handler's mutual-exclusion check, so issue_write
could never set an issue field for them.

Remove the enum so false is a legal no-op, and document that omitting the
property or setting it to false leaves the field unchanged. No handler change
is needed: the code already branches on `if deleteField`, so false falls
through to the normal value path, and the mutual-exclusion check for
delete: true still applies. Add tests for optionalIssueWriteFields, which had
none.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/github/__toolsnaps__/issue_write.snap

@@ -37,10 +37,7 @@
           "additionalProperties": false,
           "properties": {
             "delete": {
-              "description": "Set to true to clear this field's current value on the issue. Cannot be combined with 'value' or 'field_option_name'.",
-              "enum": [
-                true
-              ],
+              "description": "Set to true to clear this field's current value on the issue. Cannot be combined with 'value' or 'field_option_name'. Omit this property, or set it to false, to leave the field's current value unchanged.",
               "type": "boolean"
             },
             "field_name": {

pkg/github/issues.go

@@ -2447,9 +2447,10 @@ Options are:
 								},
 								"delete": {
 									Type: "boolean",
-									Enum: []any{true},
 									Description: "Set to true to clear this field's current value on the " +
-										"issue. Cannot be combined with 'value' or 'field_option_name'.",
+										"issue. Cannot be combined with 'value' or 'field_option_name'. " +
+										"Omit this property, or set it to false, to leave the field's " +
+										"current value unchanged.",
 								},
 							},
 							Required: []string{"field_name"},

pkg/github/issues_test.go

@@ -2115,6 +2115,63 @@ func Test_issueWriteHasNonFormParams(t *testing.T) {
 	}
 }
 
+// Test_optionalIssueWriteFields covers parsing of issue_write's issue_fields
+// items. The delete:false cases matter because the schema deliberately does not
+// constrain 'delete' to a single value: clients that populate every property of
+// a schema need a way to say "not deleting", and false must be a no-op that
+// falls through to the normal value path.
+func Test_optionalIssueWriteFields(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		item    map[string]any
+		want    issueWriteFieldInput
+		wantErr string
+	}{
+		{
+			name: "delete false alongside a value sets the value",
+			item: map[string]any{"field_name": "Start date", "value": "2026-08-14", "delete": false, "field_option_name": ""},
+			want: issueWriteFieldInput{FieldName: "Start date", Value: "2026-08-14"},
+		},
+		{
+			name: "delete true alone clears the field",
+			item: map[string]any{"field_name": "Start date", "delete": true},
+			want: issueWriteFieldInput{FieldName: "Start date", Delete: true},
+		},
+		{
+			name: "delete omitted with field_option_name",
+			item: map[string]any{"field_name": "Priority", "field_option_name": "High"},
+			want: issueWriteFieldInput{FieldName: "Priority", FieldOptionName: "High"},
+		},
+		{
+			name:    "delete true with a value is rejected",
+			item:    map[string]any{"field_name": "Start date", "value": "2026-08-14", "delete": true},
+			wantErr: "cannot specify 'delete' together with 'value' or 'field_option_name'",
+		},
+		{
+			name:    "delete false with nothing to set is rejected",
+			item:    map[string]any{"field_name": "Start date", "delete": false},
+			wantErr: "must specify either value or field_option_name",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			got, err := optionalIssueWriteFields(map[string]any{"issue_fields": []any{tc.item}})
+			if tc.wantErr != "" {
+				require.Error(t, err)
+				assert.Contains(t, err.Error(), tc.wantErr)
+				return
+			}
+			require.NoError(t, err)
+			require.Len(t, got, 1)
+			assert.Equal(t, tc.want, got[0])
+		})
+	}
+}
+
 // Test_issueWriteSchemaClassification fails when a schema property is added
 // without classifying it as either form-resendable (issueWriteFormParams) or
 // known-non-form (knownNonForm below). Without this guard, an unclassified