diff --git a/packages/blockly/core/block.ts b/packages/blockly/core/block.ts
index 4c8f80842..b74e32102 100644
--- a/packages/blockly/core/block.ts
+++ b/packages/blockly/core/block.ts
@@ -1721,8 +1721,8 @@ export class Block {
 
     // Validate that each arg has a corresponding message
     let n = 0;
-    while (json['args' + n]) {
-      if (json['message' + n] === undefined) {
+    while (json[`args${n}`]) {
+      if (json[`message${n}`] === undefined) {
         throw Error(
           warningPrefix +
             `args${n} must have a corresponding message (message${n}).`,
@@ -1732,14 +1732,13 @@ export class Block {
     }
 
     // Set basic properties of block.
-    // Makes styles backward compatible with old way of defining hat style.
-    if (json['style'] && json['style'].hat) {
-      this.hat = json['style'].hat;
+    // Handle legacy style object format for backwards compatibility
+    if (json['style'] && typeof json['style'] === 'object') {
+      this.hat = (json['style'] as {hat?: string}).hat;
       // Must set to null so it doesn't error when checking for style and
       // colour.
       json['style'] = null;
     }
-
     if (json['style'] && json['colour']) {
       throw Error(warningPrefix + 'Must not have both a colour and a style.');
     } else if (json['style']) {
@@ -1750,12 +1749,12 @@ export class Block {
 
     // Interpolate the message blocks.
     let i = 0;
-    while (json['message' + i] !== undefined) {
+    while (json[`message${i}`] !== undefined) {
       this.interpolate(
-        json['message' + i],
-        json['args' + i] || [],
+        json[`message${i}`] || '',
+        json[`args${i}`] || [],
         // Backwards compatibility: lastDummyAlign aliases implicitAlign.
-        json['implicitAlign' + i] || json['lastDummyAlign' + i],
+        json[`implicitAlign${i}`] || (json as any)[`lastDummyAlign${i}`],
         warningPrefix,
       );
       i++;
diff --git a/packages/blockly/core/interfaces/i_json_block_definition.ts b/packages/blockly/core/interfaces/i_json_block_definition.ts
new file mode 100644
index 000000000..0b3d57676
--- /dev/null
+++ b/packages/blockly/core/interfaces/i_json_block_definition.ts
@@ -0,0 +1,126 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {FieldCheckboxFromJsonConfig} from '../field_checkbox.js';
+import {FieldDropdownFromJsonConfig} from '../field_dropdown';
+import {FieldImageFromJsonConfig} from '../field_image';
+import {FieldNumberFromJsonConfig} from '../field_number';
+import {FieldTextInputFromJsonConfig} from '../field_textinput';
+import {FieldVariableFromJsonConfig} from '../field_variable';
+import {Align} from '../inputs/align.js';
+
+/**
+ * Defines the JSON structure for a block definition.
+ *
+ * @example
+ * ```typescript
+ * const blockDef:  JsonBlockDefinition = {
+ *   type: 'custom_block',
+ *   message0: 'move %1 steps',
+ *   args0: [
+ *     {
+ *       'type': 'field_number',
+ *       'name': 'INPUT',
+ *     },
+ *   ],
+ *   previousStatement: null,
+ *   nextStatement: null,
+ * };
+ * ```
+ */
+export interface JsonBlockDefinition {
+  type: string;
+  style?: string | null;
+  colour?: string | number;
+  output?: string | string[] | null;
+  previousStatement?: string | string[] | null;
+  nextStatement?: string | string[] | null;
+  outputShape?: number;
+  inputsInline?: boolean;
+  tooltip?: string;
+  helpUrl?: string;
+  extensions?: string[];
+  mutator?: string;
+  enableContextMenu?: boolean;
+  suppressPrefixSuffix?: boolean;
+
+  [key: `message${number}`]: string | undefined;
+  [key: `args${number}`]: JsonBlockArg[] | undefined;
+  [key: `implicitAlign${number}`]: string | undefined;
+}
+
+export type JsonBlockArg =
+  | InputValueArg
+  | InputStatementArg
+  | InputDummyArg
+  | InputEndRowArg
+  | FieldInputArg
+  | FieldNumberArg
+  | FieldDropdownArg
+  | FieldCheckboxArg
+  | FieldImageArg
+  | FieldVariableArg
+  | UnknownArg;
+
+interface UnknownArg {
+  type: string;
+  [key: string]: unknown;
+}
+
+/** Input args */
+interface InputValueArg {
+  type: 'input_value';
+  name?: string;
+  check?: string | string[];
+  align?: Align;
+}
+
+interface InputStatementArg {
+  type: 'input_statement';
+  name?: string;
+  check?: string | string[];
+}
+
+interface InputDummyArg {
+  type: 'input_dummy';
+  name?: string;
+}
+
+interface InputEndRowArg {
+  type: 'input_end_row';
+  name?: string;
+}
+
+/** Field args */
+interface FieldInputArg extends FieldTextInputFromJsonConfig {
+  type: 'field_input';
+  name?: string;
+}
+
+interface FieldNumberArg extends FieldNumberFromJsonConfig {
+  type: 'field_number';
+  name?: string;
+}
+
+interface FieldDropdownArg extends FieldDropdownFromJsonConfig {
+  type: 'field_dropdown';
+  name?: string;
+}
+
+interface FieldCheckboxArg extends FieldCheckboxFromJsonConfig {
+  type: 'field_checkbox';
+  name?: string;
+}
+
+interface FieldImageArg extends FieldImageFromJsonConfig {
+  type: 'field_image';
+  name?: string;
+}
+
+interface FieldVariableArg extends FieldVariableFromJsonConfig {
+  type: 'field_variable';
+  name?: string;
+}
diff --git a/packages/blockly/tests/typescript/src/field/json_block_custom_args.ts b/packages/blockly/tests/typescript/src/field/json_block_custom_args.ts
new file mode 100644
index 000000000..9cafdbdef
--- /dev/null
+++ b/packages/blockly/tests/typescript/src/field/json_block_custom_args.ts
@@ -0,0 +1,26 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {defineBlocksWithJsonArray} from 'blockly-test/core';
+import type {JsonBlockDefinition} from 'blockly-test/core/interfaces/i_json_block_definition';
+
+import './different_user_input';
+
+const mitosisBlockDefinition: JsonBlockDefinition = {
+  type: 'mitosis_block',
+  message0: 'split cell %1',
+  args0: [
+    {
+      type: 'field_mitosis',
+      name: 'CELL',
+      cellId: 'cell-A',
+    },
+  ],
+  previousStatement: null,
+  nextStatement: null,
+};
+
+defineBlocksWithJsonArray([mitosisBlockDefinition]);