diff --git a/core/block.ts b/core/block.ts
index de8f16680..14b34972e 100644
--- a/core/block.ts
+++ b/core/block.ts
@@ -1066,11 +1066,12 @@ export class Block implements IASTNodeLocation, IDeletable {
    * @returns List of variable ids.
    */
   getVars(): string[] {
-    const vars = [];
+    const vars: string[] = [];
     for (let i = 0, input; input = this.inputList[i]; i++) {
       for (let j = 0, field; field = input.fieldRow[j]; j++) {
         if (field.referencesVariables()) {
-          vars.push(field.getValue());
+          // NOTE: This only applies to `FieldVariable`, a `Field<string>`
+          vars.push(field.getValue() as string);
         }
       }
     }
diff --git a/core/field.ts b/core/field.ts
index cf9148e1c..0bcf99119 100644
--- a/core/field.ts
+++ b/core/field.ts
@@ -45,12 +45,30 @@ import * as WidgetDiv from './widgetdiv.js';
 import type {WorkspaceSvg} from './workspace_svg.js';
 import * as Xml from './xml.js';
 
-export type FieldValidator<T = any> = (value?: T) => T|null|undefined;
+/**
+ * A function that is called to validate changes to the field's value before
+ * they are set.
+ *
+ * **NOTE:** Validation returns one option between `T`, `null`, and `undefined`.
+ *
+ * @see {@link https://developers.google.com/blockly/guides/create-custom-blocks/fields/validators#return_values}
+ * @param newValue The value to be validated.
+ * @returns One of three instructions for setting the new value: `T`, `null`,
+ * or `undefined`.
+ *
+ * - `T` to set this function's returned value instead of `newValue`.
+ *
+ * - `null` to invoke `doValueInvalid_` and not set a value.
+ *
+ * - `undefined` to set `newValue` as is.
+ */
+export type FieldValidator<T = any> = (newValue: T) => T|null|undefined;
 
 /**
  * Abstract class for an editable field.
  *
  * @alias Blockly.Field
+ * @typeParam T - The value stored on the field.
  */
 export abstract class Field<T = any> implements IASTNodeLocationSvg,
                                                 IASTNodeLocationWithBlock,
@@ -76,6 +94,9 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
    * instead.
    */
   static readonly SKIP_SETUP = new Sentinel();
+  static isSentinel<T>(value: T|Sentinel): value is Sentinel {
+    return value === Field.SKIP_SETUP;
+  }
 
   /**
    * Name of field.  Unique within each block.
@@ -207,9 +228,7 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
     /** The size of the area rendered by the field. */
     this.size_ = new Size(0, 0);
 
-    if (value === Field.SKIP_SETUP) {
-      return;
-    }
+    if (Field.isSentinel(value)) return;
     if (opt_config) {
       this.configure_(opt_config);
     }
@@ -372,7 +391,8 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
    * @internal
    */
   fromXml(fieldElement: Element) {
-    this.setValue(fieldElement.textContent);
+    // Any because gremlins live here. No touchie!
+    this.setValue(fieldElement.textContent as any);
   }
 
   /**
@@ -384,7 +404,8 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
    * @internal
    */
   toXml(fieldElement: Element): Element {
-    fieldElement.textContent = this.getValue();
+    // Any because gremlins live here. No touchie!
+    fieldElement.textContent = this.getValue() as any;
     return fieldElement;
   }
 
@@ -619,7 +640,7 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
    *
    * @returns Validation function, or null.
    */
-  getValidator(): Function|null {
+  getValidator(): FieldValidator<T>|null {
     return this.validator_;
   }
 
@@ -965,41 +986,37 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
       return;
     }
 
-    let validatedValue = this.doClassValidation_(newValue);
-    // Class validators might accidentally forget to return, we'll ignore that.
-    newValue = this.processValidation_(newValue, validatedValue);
-    if (newValue instanceof Error) {
+    const classValidation = this.doClassValidation_(newValue);
+    const classValue = this.processValidation_(newValue, classValidation);
+    if (classValue instanceof Error) {
       doLogging && console.log('invalid class validation, return');
       return;
     }
 
-    const localValidator = this.getValidator();
-    if (localValidator) {
-      validatedValue = localValidator.call(this, newValue);
-      // Local validators might accidentally forget to return, we'll ignore
-      // that.
-      newValue = this.processValidation_(newValue, validatedValue);
-      if (newValue instanceof Error) {
-        doLogging && console.log('invalid local validation, return');
-        return;
-      }
+    const localValidation = this.getValidator()?.call(this, classValue);
+    const localValue = this.processValidation_(classValue, localValidation);
+    if (localValue instanceof Error) {
+      doLogging && console.log('invalid local validation, return');
+      return;
     }
+
     const source = this.sourceBlock_;
     if (source && source.disposed) {
       doLogging && console.log('source disposed, return');
       return;
     }
+
     const oldValue = this.getValue();
-    if (oldValue === newValue) {
+    if (oldValue === localValue) {
       doLogging && console.log('same, doValueUpdate_, return');
-      this.doValueUpdate_(newValue);
+      this.doValueUpdate_(localValue);
       return;
     }
 
-    this.doValueUpdate_(newValue);
+    this.doValueUpdate_(localValue);
     if (source && eventUtils.isEnabled()) {
       eventUtils.fire(new (eventUtils.get(eventUtils.BLOCK_CHANGE))(
-          source, 'field', this.name || null, oldValue, newValue));
+          source, 'field', this.name || null, oldValue, localValue));
     }
     if (this.isDirty_) {
       this.forceRerender();
@@ -1015,8 +1032,7 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
    * @returns New value, or an Error object.
    */
   private processValidation_(
-      newValue: AnyDuringMigration,
-      validatedValue: AnyDuringMigration): AnyDuringMigration {
+      newValue: AnyDuringMigration, validatedValue: T|null|undefined): T|Error {
     if (validatedValue === null) {
       this.doValueInvalid_(newValue);
       if (this.isDirty_) {
@@ -1024,10 +1040,7 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
       }
       return Error();
     }
-    if (validatedValue !== undefined) {
-      newValue = validatedValue;
-    }
-    return newValue;
+    return validatedValue === undefined ? newValue as T : validatedValue;
   }
 
   /**
@@ -1035,23 +1048,39 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
    *
    * @returns Current value.
    */
-  getValue(): AnyDuringMigration {
+  getValue(): T|null {
     return this.value_;
   }
 
   /**
-   * Used to validate a value. Returns input by default. Can be overridden by
-   * subclasses, see FieldDropdown.
+   * Validate the changes to a field's value before they are set. See
+   * **FieldDropdown** for an example of subclass implementation.
    *
-   * @param opt_newValue The value to be validated.
-   * @returns The validated value, same as input by default.
+   * **NOTE:** Validation returns one option between `T`, `null`, and
+   * `undefined`. **Field**'s implementation will never return `undefined`, but
+   * it is valid for a subclass to return `undefined` if the new value is
+   * compatible with `T`.
+   *
+   * @see {@link https://developers.google.com/blockly/guides/create-custom-blocks/fields/validators#return_values}
+   * @param newValue - The value to be validated.
+   * @returns One of three instructions for setting the new value: `T`, `null`,
+   * or `undefined`.
+   *
+   * - `T` to set this function's returned value instead of `newValue`.
+   *
+   * - `null` to invoke `doValueInvalid_` and not set a value.
+   *
+   * - `undefined` to set `newValue` as is.
    */
-  protected doClassValidation_(opt_newValue?: AnyDuringMigration):
-      AnyDuringMigration {
-    if (opt_newValue === null || opt_newValue === undefined) {
+  protected doClassValidation_(newValue: T): T|null|undefined;
+  protected doClassValidation_(newValue?: AnyDuringMigration): T|null;
+  protected doClassValidation_(newValue?: T|AnyDuringMigration): T|null
+      |undefined {
+    if (newValue === null || newValue === undefined) {
       return null;
     }
-    return opt_newValue;
+
+    return newValue as T;
   }
 
   /**
@@ -1060,7 +1089,7 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
    *
    * @param newValue The value to be saved.
    */
-  protected doValueUpdate_(newValue: AnyDuringMigration) {
+  protected doValueUpdate_(newValue: T) {
     this.value_ = newValue;
     this.isDirty_ = true;
   }
@@ -1086,7 +1115,7 @@ export abstract class Field<T = any> implements IASTNodeLocationSvg,
     }
     const gesture = (this.sourceBlock_.workspace as WorkspaceSvg).getGesture(e);
     if (gesture) {
-      gesture.setStartField(this as Field);
+      gesture.setStartField(this);
     }
   }
 
diff --git a/core/field_angle.ts b/core/field_angle.ts
index 448147c65..70cce0f4e 100644
--- a/core/field_angle.ts
+++ b/core/field_angle.ts
@@ -137,9 +137,7 @@ export class FieldAngle extends FieldInput<number> {
       opt_config?: FieldAngleConfig) {
     super(Field.SKIP_SETUP);
 
-    if (opt_value === Field.SKIP_SETUP) {
-      return;
-    }
+    if (Field.isSentinel(opt_value)) return;
     if (opt_config) {
       this.configure_(opt_config);
     }
@@ -404,25 +402,24 @@ export class FieldAngle extends FieldInput<number> {
    *
    * @param e Keyboard event.
    */
-  protected override onHtmlInputKeyDown_(e: Event) {
+  protected override onHtmlInputKeyDown_(e: KeyboardEvent) {
     super.onHtmlInputKeyDown_(e);
     const block = this.getSourceBlock();
     if (!block) {
       throw new UnattachedFieldError();
     }
 
-    const keyboardEvent = e as KeyboardEvent;
     let multiplier;
-    if (keyboardEvent.keyCode === KeyCodes.LEFT) {
+    if (e.keyCode === KeyCodes.LEFT) {
       // decrement (increment in RTL)
       multiplier = block.RTL ? 1 : -1;
-    } else if (keyboardEvent.keyCode === KeyCodes.RIGHT) {
+    } else if (e.keyCode === KeyCodes.RIGHT) {
       // increment (decrement in RTL)
       multiplier = block.RTL ? -1 : 1;
-    } else if (keyboardEvent.keyCode === KeyCodes.DOWN) {
+    } else if (e.keyCode === KeyCodes.DOWN) {
       // decrement
       multiplier = -1;
-    } else if (keyboardEvent.keyCode === KeyCodes.UP) {
+    } else if (e.keyCode === KeyCodes.UP) {
       // increment
       multiplier = 1;
     }
diff --git a/core/field_checkbox.ts b/core/field_checkbox.ts
index 1007e1884..3b8c08a92 100644
--- a/core/field_checkbox.ts
+++ b/core/field_checkbox.ts
@@ -20,14 +20,16 @@ import {Field, FieldConfig, FieldValidator} from './field.js';
 import * as fieldRegistry from './field_registry.js';
 import type {Sentinel} from './utils/sentinel.js';
 
-export type FieldCheckboxValidator = FieldValidator<boolean>;
+type BoolString = 'TRUE'|'FALSE';
+type CheckboxBool = BoolString|boolean;
+export type FieldCheckboxValidator = FieldValidator<CheckboxBool>;
 
 /**
  * Class for a checkbox field.
  *
  * @alias Blockly.FieldCheckbox
  */
-export class FieldCheckbox extends Field<boolean> {
+export class FieldCheckbox extends Field<CheckboxBool> {
   /** Default character for the checkmark. */
   static readonly CHECK_CHAR = '✓';
   private checkChar_: string;
@@ -42,7 +44,12 @@ export class FieldCheckbox extends Field<boolean> {
    * Mouse cursor style when over the hotspot that initiates editability.
    */
   override CURSOR = 'default';
-  override value_: AnyDuringMigration;
+
+  /**
+   * NOTE: The default value is set in `Field`, so maintain that value instead
+   * of overwriting it here or in the constructor.
+   */
+  override value_: boolean|null = this.value_;
 
   /**
    * @param opt_value The initial value of the field. Should either be 'TRUE',
@@ -59,8 +66,7 @@ export class FieldCheckbox extends Field<boolean> {
    * for a list of properties this parameter supports.
    */
   constructor(
-      opt_value?: string|boolean|Sentinel,
-      opt_validator?: FieldCheckboxValidator,
+      opt_value?: CheckboxBool|Sentinel, opt_validator?: FieldCheckboxValidator,
       opt_config?: FieldCheckboxConfig) {
     super(Field.SKIP_SETUP);
 
@@ -70,9 +76,7 @@ export class FieldCheckbox extends Field<boolean> {
      */
     this.checkChar_ = FieldCheckbox.CHECK_CHAR;
 
-    if (opt_value === Field.SKIP_SETUP) {
-      return;
-    }
+    if (Field.isSentinel(opt_value)) return;
     if (opt_config) {
       this.configure_(opt_config);
     }
@@ -153,7 +157,7 @@ export class FieldCheckbox extends Field<boolean> {
    * @returns A valid value ('TRUE' or 'FALSE), or null if invalid.
    */
   protected override doClassValidation_(opt_newValue?: AnyDuringMigration):
-      string|null {
+      BoolString|null {
     if (opt_newValue === true || opt_newValue === 'TRUE') {
       return 'TRUE';
     }
@@ -169,7 +173,7 @@ export class FieldCheckbox extends Field<boolean> {
    * @param newValue The value to be saved. The default validator guarantees
    *     that this is a either 'TRUE' or 'FALSE'.
    */
-  protected override doValueUpdate_(newValue: AnyDuringMigration) {
+  protected override doValueUpdate_(newValue: BoolString) {
     this.value_ = this.convertValueToBool_(newValue);
     // Update visual.
     if (this.textElement_) {
@@ -182,7 +186,7 @@ export class FieldCheckbox extends Field<boolean> {
    *
    * @returns The value of this field.
    */
-  override getValue(): string {
+  override getValue(): BoolString {
     return this.value_ ? 'TRUE' : 'FALSE';
   }
 
@@ -191,8 +195,8 @@ export class FieldCheckbox extends Field<boolean> {
    *
    * @returns The boolean value of this field.
    */
-  getValueBoolean(): boolean {
-    return this.value_ as boolean;
+  getValueBoolean(): boolean|null {
+    return this.value_;
   }
 
   /**
@@ -213,12 +217,9 @@ export class FieldCheckbox extends Field<boolean> {
    * @param value The value to convert.
    * @returns The converted value.
    */
-  private convertValueToBool_(value: AnyDuringMigration): boolean {
-    if (typeof value === 'string') {
-      return value === 'TRUE';
-    } else {
-      return !!value;
-    }
+  private convertValueToBool_(value: CheckboxBool|null): boolean {
+    if (typeof value === 'string') return value === 'TRUE';
+    return !!value;
   }
 
   /**
diff --git a/core/field_colour.ts b/core/field_colour.ts
index 729ebe54f..db56acf53 100644
--- a/core/field_colour.ts
+++ b/core/field_colour.ts
@@ -80,7 +80,7 @@ export class FieldColour extends Field<string> {
   static COLUMNS = 7;
 
   /** The field's colour picker element. */
-  private picker_: Element|null = null;
+  private picker_: HTMLElement|null = null;
 
   /** Index of the currently highlighted element. */
   private highlightedIndex_: number|null = null;
@@ -134,9 +134,6 @@ export class FieldColour extends Field<string> {
    * setting. By default use the global constants for columns.
    */
   private columns_ = 0;
-  override size_: AnyDuringMigration;
-  override clickTarget_: AnyDuringMigration;
-  override value_: AnyDuringMigration;
 
   /**
    * @param opt_value The initial value of the field. Should be in '#rrggbb'
@@ -157,9 +154,7 @@ export class FieldColour extends Field<string> {
       opt_config?: FieldColourConfig) {
     super(Field.SKIP_SETUP);
 
-    if (opt_value === Field.SKIP_SETUP) {
-      return;
-    }
+    if (Field.isSentinel(opt_value)) return;
     if (opt_config) {
       this.configure_(opt_config);
     }
@@ -230,15 +225,14 @@ export class FieldColour extends Field<string> {
    * @param newValue The value to be saved. The default validator guarantees
    *     that this is a colour in '#rrggbb' format.
    */
-  protected override doValueUpdate_(newValue: AnyDuringMigration) {
+  protected override doValueUpdate_(newValue: string) {
     this.value_ = newValue;
     if (this.borderRect_) {
-      this.borderRect_.style.fill = newValue as string;
+      this.borderRect_.style.fill = newValue;
     } else if (
         this.sourceBlock_ && this.sourceBlock_.rendered &&
         this.sourceBlock_ instanceof BlockSvg) {
-      this.sourceBlock_.pathObject.svgPath.setAttribute(
-          'fill', newValue as string);
+      this.sourceBlock_.pathObject.svgPath.setAttribute('fill', newValue);
       this.sourceBlock_.pathObject.svgPath.setAttribute('stroke', '#fff');
     }
   }
@@ -289,16 +283,12 @@ export class FieldColour extends Field<string> {
   /** Create and show the colour field's editor. */
   protected override showEditor_() {
     this.dropdownCreate_();
-    // AnyDuringMigration because:  Argument of type 'Element | null' is not
-    // assignable to parameter of type 'Node'.
-    dropDownDiv.getContentDiv().appendChild(this.picker_ as AnyDuringMigration);
+    dropDownDiv.getContentDiv().appendChild(this.picker_!);
 
     dropDownDiv.showPositionedByField(this, this.dropdownDispose_.bind(this));
 
     // Focus so we can start receiving keyboard events.
-    // AnyDuringMigration because:  Property 'focus' does not exist on type
-    // 'Element'.
-    (this.picker_ as AnyDuringMigration)!.focus({preventScroll: true});
+    this.picker_!.focus({preventScroll: true});
   }
 
   /**
@@ -519,9 +509,7 @@ export class FieldColour extends Field<string> {
       cell.setAttribute('data-colour', colours[i]);
       cell.title = titles[i] || colours[i];
       cell.id = idGenerator.getNextUniqueId();
-      // AnyDuringMigration because:  Argument of type 'number' is not
-      // assignable to parameter of type 'string'.
-      cell.setAttribute('data-index', i as AnyDuringMigration);
+      cell.setAttribute('data-index', String(i));
       aria.setRole(cell, aria.Role.GRIDCELL);
       aria.setState(cell, aria.State.LABEL, colours[i]);
       aria.setState(cell, aria.State.SELECTED, colours[i] === selectedColour);
@@ -584,7 +572,7 @@ export class FieldColour extends Field<string> {
   static fromJson(options: FieldColourFromJsonConfig): FieldColour {
     // `this` might be a subclass of FieldColour if that class doesn't override
     // the static fromJson method.
-    return new this(options['colour'], undefined, options);
+    return new this(options.colour, undefined, options);
   }
 }
 
diff --git a/core/field_dropdown.ts b/core/field_dropdown.ts
index eb14d766f..ced3a4bfe 100644
--- a/core/field_dropdown.ts
+++ b/core/field_dropdown.ts
@@ -92,7 +92,7 @@ export class FieldDropdown extends Field<string> {
    */
   override suffixField: string|null = null;
   // TODO(b/109816955): remove '!', see go/strict-prop-init-fix.
-  private selectedOption_!: Array<string|ImageProperties>;
+  private selectedOption_!: MenuOption;
   override clickTarget_: SVGElement|null = null;
 
   /**
@@ -398,8 +398,7 @@ export class FieldDropdown extends Field<string> {
    * @param opt_newValue The input value.
    * @returns A valid language-neutral option, or null if invalid.
    */
-  protected override doClassValidation_(opt_newValue?: MenuOption[1]): string
-      |null {
+  protected override doClassValidation_(opt_newValue?: string): string|null {
     const options = this.getOptions(true);
     const isValueValid = options.some((option) => option[1] === opt_newValue);
 
@@ -421,7 +420,7 @@ export class FieldDropdown extends Field<string> {
    * @param newValue The value to be saved. The default validator guarantees
    *     that this is one of the valid dropdown options.
    */
-  protected override doValueUpdate_(newValue: MenuOption[1]) {
+  protected override doValueUpdate_(newValue: string) {
     super.doValueUpdate_(newValue);
     const options = this.getOptions(true);
     for (let i = 0, option; option = options[i]; i++) {
@@ -465,7 +464,7 @@ export class FieldDropdown extends Field<string> {
     // Show correct element.
     const option = this.selectedOption_ && this.selectedOption_[0];
     if (option && typeof option === 'object') {
-      this.renderSelectedImage_((option));
+      this.renderSelectedImage_(option);
     } else {
       this.renderSelectedText_();
     }
diff --git a/core/field_image.ts b/core/field_image.ts
index 728be7e94..ab0a6bb37 100644
--- a/core/field_image.ts
+++ b/core/field_image.ts
@@ -60,7 +60,6 @@ export class FieldImage extends Field<string> {
 
   /** Alt text of this image. */
   private altText_ = '';
-  override value_: AnyDuringMigration;
 
   /**
    * @param src The URL of the image.
@@ -179,7 +178,7 @@ export class FieldImage extends Field<string> {
    * @param newValue The value to be saved. The default validator guarantees
    *     that this is a string.
    */
-  protected override doValueUpdate_(newValue: AnyDuringMigration) {
+  protected override doValueUpdate_(newValue: string) {
     this.value_ = newValue;
     if (this.imageElement_) {
       this.imageElement_.setAttributeNS(
diff --git a/core/field_input.ts b/core/field_input.ts
index b7e36d05b..88946b47a 100644
--- a/core/field_input.ts
+++ b/core/field_input.ts
@@ -32,14 +32,16 @@ import * as WidgetDiv from './widgetdiv.js';
 import type {WorkspaceSvg} from './workspace_svg.js';
 
 export type InputTypes = string|number;
-export type FieldInputValidator<T extends InputTypes> = FieldValidator<T>;
+export type FieldInputValidator<T extends InputTypes> =
+    FieldValidator<string|T>;
 
 /**
- * Class for an editable text field.
+ * Abstract class for an editable input field.
  *
  * @alias Blockly.FieldInput
+ * @typeParam T - The value stored on the field.
  */
-export abstract class FieldInput<T extends InputTypes> extends Field<T> {
+export abstract class FieldInput<T extends InputTypes> extends Field<string|T> {
   /**
    * Pixel size of input border radius.
    * Should match blocklyText's border-radius in CSS.
@@ -83,9 +85,6 @@ export abstract class FieldInput<T extends InputTypes> extends Field<T> {
 
   /** Mouse cursor style when over the hotspot that initiates the editor. */
   override CURSOR = 'text';
-  override clickTarget_: AnyDuringMigration;
-  override value_: AnyDuringMigration;
-  override isDirty_: AnyDuringMigration;
 
   /**
    * @param opt_value The initial value of the field. Should cast to a string.
@@ -106,9 +105,7 @@ export abstract class FieldInput<T extends InputTypes> extends Field<T> {
       opt_config?: FieldInputConfig) {
     super(Field.SKIP_SETUP);
 
-    if (opt_value === Field.SKIP_SETUP) {
-      return;
-    }
+    if (Field.isSentinel(opt_value)) return;
     if (opt_config) {
       this.configure_(opt_config);
     }
@@ -161,20 +158,6 @@ export abstract class FieldInput<T extends InputTypes> extends Field<T> {
     this.createTextElement_();
   }
 
-  /**
-   * Ensure that the input value casts to a valid string.
-   *
-   * @param opt_newValue The input value.
-   * @returns A valid string, or null if invalid.
-   */
-  protected override doClassValidation_(opt_newValue?: AnyDuringMigration):
-      AnyDuringMigration {
-    if (opt_newValue === null || opt_newValue === undefined) {
-      return null;
-    }
-    return String(opt_newValue);
-  }
-
   /**
    * Called by setValue if the text input is not valid. If the field is
    * currently being edited it reverts value of the field to the previous
@@ -207,7 +190,7 @@ export abstract class FieldInput<T extends InputTypes> extends Field<T> {
    * @param newValue The value to be saved. The default validator guarantees
    *     that this is a string.
    */
-  protected override doValueUpdate_(newValue: AnyDuringMigration) {
+  protected override doValueUpdate_(newValue: string|T) {
     this.isDirty_ = true;
     this.isTextValid_ = true;
     this.value_ = newValue;
@@ -380,7 +363,7 @@ export abstract class FieldInput<T extends InputTypes> extends Field<T> {
     div!.appendChild(htmlInput);
 
     htmlInput.value = htmlInput.defaultValue = this.getEditorText_(this.value_);
-    htmlInput.setAttribute('data-untyped-default-value', this.value_);
+    htmlInput.setAttribute('data-untyped-default-value', String(this.value_));
 
     this.resizeEditor_();
 
@@ -457,29 +440,19 @@ export abstract class FieldInput<T extends InputTypes> extends Field<T> {
    *
    * @param e Keyboard event.
    */
-  protected onHtmlInputKeyDown_(e: Event) {
-    // AnyDuringMigration because:  Property 'keyCode' does not exist on type
-    // 'Event'.
-    if ((e as AnyDuringMigration).keyCode === KeyCodes.ENTER) {
+  protected onHtmlInputKeyDown_(e: KeyboardEvent) {
+    if (e.keyCode === KeyCodes.ENTER) {
       WidgetDiv.hide();
       dropDownDiv.hideWithoutAnimation();
-      // AnyDuringMigration because:  Property 'keyCode' does not exist on type
-      // 'Event'.
-    } else if ((e as AnyDuringMigration).keyCode === KeyCodes.ESC) {
+    } else if (e.keyCode === KeyCodes.ESC) {
       this.setValue(
           this.htmlInput_!.getAttribute('data-untyped-default-value'));
       WidgetDiv.hide();
       dropDownDiv.hideWithoutAnimation();
-      // AnyDuringMigration because:  Property 'keyCode' does not exist on type
-      // 'Event'.
-    } else if ((e as AnyDuringMigration).keyCode === KeyCodes.TAB) {
+    } else if (e.keyCode === KeyCodes.TAB) {
       WidgetDiv.hide();
       dropDownDiv.hideWithoutAnimation();
-      // AnyDuringMigration because:  Property 'shiftKey' does not exist on type
-      // 'Event'. AnyDuringMigration because:  Argument of type 'this' is not
-      // assignable to parameter of type 'Field'.
-      (this.sourceBlock_ as BlockSvg)
-          .tab(this as AnyDuringMigration, !(e as AnyDuringMigration).shiftKey);
+      (this.sourceBlock_ as BlockSvg).tab(this, !e.shiftKey);
       e.preventDefault();
     }
   }
diff --git a/core/field_label.ts b/core/field_label.ts
index 2a4730ff2..22a55a3b7 100644
--- a/core/field_label.ts
+++ b/core/field_label.ts
@@ -51,9 +51,7 @@ export class FieldLabel extends Field<string> {
       opt_config?: FieldLabelConfig) {
     super(Field.SKIP_SETUP);
 
-    if (opt_value === Field.SKIP_SETUP) {
-      return;
-    }
+    if (Field.isSentinel(opt_value)) return;
     if (opt_config) {
       this.configure_(opt_config);
     } else {
diff --git a/core/field_multilineinput.ts b/core/field_multilineinput.ts
index abd53193a..e6473811b 100644
--- a/core/field_multilineinput.ts
+++ b/core/field_multilineinput.ts
@@ -70,9 +70,7 @@ export class FieldMultilineInput extends FieldTextInput {
       opt_config?: FieldMultilineInputConfig) {
     super(Field.SKIP_SETUP);
 
-    if (opt_value === Field.SKIP_SETUP) {
-      return;
-    }
+    if (Field.isSentinel(opt_value)) return;
     if (opt_config) {
       this.configure_(opt_config);
     }
@@ -210,9 +208,11 @@ export class FieldMultilineInput extends FieldTextInput {
    * @param newValue The value to be saved. The default validator guarantees
    *     that this is a string.
    */
-  protected override doValueUpdate_(newValue: AnyDuringMigration) {
+  protected override doValueUpdate_(newValue: string) {
     super.doValueUpdate_(newValue);
-    this.isOverflowedY_ = this.value_.split('\n').length > this.maxLines_;
+    if (this.value_ !== null) {
+      this.isOverflowedY_ = this.value_.split('\n').length > this.maxLines_;
+    }
   }
 
   /** Updates the text of the textElement. */
@@ -300,7 +300,7 @@ export class FieldMultilineInput extends FieldTextInput {
       // absolute longest line, even if it would be truncated after editing.
       // Otherwise we would get wrong editor width when there are more
       // lines than this.maxLines_.
-      const actualEditorLines = this.value_.split('\n');
+      const actualEditorLines = String(this.value_).split('\n');
       const dummyTextElement = dom.createSvgElement(
           Svg.TEXT, {'class': 'blocklyText blocklyMultilineText'});
 
@@ -385,7 +385,7 @@ export class FieldMultilineInput extends FieldTextInput {
     div!.appendChild(htmlInput);
 
     htmlInput.value = htmlInput.defaultValue = this.getEditorText_(this.value_);
-    htmlInput.setAttribute('data-untyped-default-value', this.value_);
+    htmlInput.setAttribute('data-untyped-default-value', String(this.value_));
     htmlInput.setAttribute('data-old-value', '');
     if (userAgent.GECKO) {
       // In FF, ensure the browser reflows before resizing to avoid issue #2777.
@@ -428,10 +428,8 @@ export class FieldMultilineInput extends FieldTextInput {
    *
    * @param e Keyboard event.
    */
-  protected override onHtmlInputKeyDown_(e: Event) {
-    // AnyDuringMigration because:  Property 'keyCode' does not exist on type
-    // 'Event'.
-    if ((e as AnyDuringMigration).keyCode !== KeyCodes.ENTER) {
+  protected override onHtmlInputKeyDown_(e: KeyboardEvent) {
+    if (e.keyCode !== KeyCodes.ENTER) {
       super.onHtmlInputKeyDown_(e);
     }
   }
diff --git a/core/field_number.ts b/core/field_number.ts
index a891f1c58..20b799555 100644
--- a/core/field_number.ts
+++ b/core/field_number.ts
@@ -77,9 +77,7 @@ export class FieldNumber extends FieldInput<number> {
     // Pass SENTINEL so that we can define properties before value validation.
     super(Field.SKIP_SETUP);
 
-    if (opt_value === Field.SKIP_SETUP) {
-      return;
-    }
+    if (Field.isSentinel(opt_value)) return;
     if (opt_config) {
       this.configure_(opt_config);
     } else {
@@ -260,6 +258,7 @@ export class FieldNumber extends FieldInput<number> {
     if (opt_newValue === null) {
       return null;
     }
+
     // Clean up text.
     let newValue = String(opt_newValue);
     // TODO: Handle cases like 'ten', '1.203,14', etc.
diff --git a/core/field_textinput.ts b/core/field_textinput.ts
index 2a2f1c6db..5ef633dc4 100644
--- a/core/field_textinput.ts
+++ b/core/field_textinput.ts
@@ -22,6 +22,11 @@ import type {Sentinel} from './utils/sentinel.js';
 
 export type FieldTextInputValidator = FieldInputValidator<string>;
 
+/**
+ * Class for an editable text field.
+ *
+ * @alias Blockly.FieldTextInput
+ */
 export class FieldTextInput extends FieldInput<string> {
   /**
    * @param opt_value The initial value of the field. Should cast to a string.
@@ -43,6 +48,20 @@ export class FieldTextInput extends FieldInput<string> {
     super(opt_value, opt_validator, opt_config);
   }
 
+  /**
+   * Ensure that the input value casts to a valid string.
+   *
+   * @param opt_newValue The input value.
+   * @returns A valid string, or null if invalid.
+   */
+  protected override doClassValidation_(opt_newValue?: AnyDuringMigration):
+      string|null {
+    if (opt_newValue === undefined) {
+      return null;
+    }
+    return String(opt_newValue);
+  }
+
   /**
    * Construct a FieldTextInput from a JSON arg object,
    * dereferencing any string table references.
diff --git a/core/field_variable.ts b/core/field_variable.ts
index e3b98f4fe..8d2fd276c 100644
--- a/core/field_variable.ts
+++ b/core/field_variable.ts
@@ -306,7 +306,7 @@ export class FieldVariable extends FieldDropdown {
    *
    * @returns Validation function, or null.
    */
-  override getValidator(): Function|null {
+  override getValidator(): FieldVariableValidator|null {
     // Validators shouldn't operate on the initial setValue call.
     // Normally this is achieved by calling setValidator after setValue, but
     // this is not a possibility with variable fields.
@@ -357,7 +357,7 @@ export class FieldVariable extends FieldDropdown {
    *
    * @param newId The value to be saved.
    */
-  protected override doValueUpdate_(newId: AnyDuringMigration) {
+  protected override doValueUpdate_(newId: string) {
     const block = this.getSourceBlock();
     if (!block) {
       throw new UnattachedFieldError();
diff --git a/core/gesture.ts b/core/gesture.ts
index 935aa37d8..065434097 100644
--- a/core/gesture.ts
+++ b/core/gesture.ts
@@ -969,14 +969,14 @@ export class Gesture {
    * @param field The field the gesture started on.
    * @internal
    */
-  setStartField(field: Field) {
+  setStartField<T>(field: Field<T>) {
     if (this.hasStarted_) {
       throw Error(
           'Tried to call gesture.setStartField, ' +
           'but the gesture had already been started.');
     }
     if (!this.startField_) {
-      this.startField_ = field;
+      this.startField_ = field as Field;
     }
   }
 
diff --git a/core/utils/sentinel.ts b/core/utils/sentinel.ts
index f6b8da26d..723c29986 100644
--- a/core/utils/sentinel.ts
+++ b/core/utils/sentinel.ts
@@ -18,4 +18,10 @@ goog.declareModuleId('Blockly.utils.Sentinel');
  *
  * @alias Blockly.utils.Sentinel
  */
-export class Sentinel {}
+export class Sentinel {
+  /**
+   * Provide a unique key so that type guarding properly excludes values like
+   * string.
+   */
+  UNIQUE_KEY?: never;
+}