diff --git a/packages/core/src/spec-validator.test.ts b/packages/core/src/spec-validator.test.ts
index e5ee6ca2..9117c97b 100644
--- a/packages/core/src/spec-validator.test.ts
+++ b/packages/core/src/spec-validator.test.ts
@@ -129,6 +129,80 @@ describe("validateSpec", () => {
     expect(watchIssue!.elementKey).toBe("root");
   });
 
+  it("detects action_in_props (legacy pattern)", () => {
+    const spec: Spec = {
+      root: "root",
+      elements: {
+        root: {
+          type: "Button",
+          props: { label: "Submit", action: "submitForm" },
+          children: [],
+        },
+      },
+    };
+    const result = validateSpec(spec);
+    expect(result.valid).toBe(false);
+    expect(result.issues.some((i) => i.code === "action_in_props")).toBe(true);
+  });
+
+  it("detects actionParams_in_props (legacy pattern)", () => {
+    const spec: Spec = {
+      root: "root",
+      elements: {
+        root: {
+          type: "Button",
+          props: {
+            label: "Submit",
+            action: "submitForm",
+            actionParams: { formId: "main" },
+          },
+          children: [],
+        },
+      },
+    };
+    const result = validateSpec(spec);
+    expect(result.valid).toBe(false);
+    expect(result.issues.some((i) => i.code === "actionParams_in_props")).toBe(
+      true,
+    );
+  });
+
+  it("detects children_in_props", () => {
+    const spec: Spec = {
+      root: "root",
+      elements: {
+        root: {
+          type: "Stack",
+          props: { children: ["child1", "child2"] },
+          children: [],
+        },
+        child1: { type: "Text", props: {}, children: [] },
+        child2: { type: "Text", props: {}, children: [] },
+      },
+    };
+    const result = validateSpec(spec);
+    expect(result.valid).toBe(false);
+    expect(result.issues.some((i) => i.code === "children_in_props")).toBe(
+      true,
+    );
+  });
+
+  it("detects state_in_props", () => {
+    const spec: Spec = {
+      root: "root",
+      elements: {
+        root: {
+          type: "Stack",
+          props: { state: { count: 0 } },
+          children: [],
+        },
+      },
+    };
+    const result = validateSpec(spec);
+    expect(result.valid).toBe(false);
+    expect(result.issues.some((i) => i.code === "state_in_props")).toBe(true);
+  });
+
   it("detects orphaned elements when checkOrphans is true", () => {
     const spec: Spec = {
       root: "root",
@@ -247,4 +321,71 @@ describe("autoFixSpec", () => {
     const { fixes } = autoFixSpec(spec);
     expect(fixes).toHaveLength(0);
   });
+
+  it("moves children from props to element level", () => {
+    const spec: Spec = {
+      root: "root",
+      elements: {
+        root: {
+          type: "Stack",
+          props: { children: ["child1", "child2"] },
+          children: [],
+        },
+        child1: { type: "Text", props: {}, children: [] },
+        child2: { type: "Text", props: {}, children: [] },
+      },
+    };
+    const { spec: fixed, fixes } = autoFixSpec(spec);
+    expect(
+      (fixed.elements.root.props as Record<string, unknown>).children,
+    ).toBeUndefined();
+    expect(fixed.elements.root.children).toEqual(["child1", "child2"]);
+    expect(fixes.some((f) => f.includes('"children"'))).toBe(true);
+  });
+
+  it("converts legacy action/actionParams to on.press", () => {
+    const spec: Spec = {
+      root: "root",
+      elements: {
+        root: {
+          type: "Button",
+          props: {
+            label: "Submit",
+            action: "submitForm",
+            actionParams: { formId: "main" },
+          },
+          children: [],
+        },
+      },
+    };
+    const { spec: fixed, fixes } = autoFixSpec(spec);
+    const props = fixed.elements.root.props as Record<string, unknown>;
+    expect(props.action).toBeUndefined();
+    expect(props.actionParams).toBeUndefined();
+    expect(fixed.elements.root.on).toEqual({
+      press: {
+        action: "submitForm",
+        params: { formId: "main" },
+      },
+    });
+    expect(fixes.some((f) => f.includes("legacy"))).toBe(true);
+  });
+
+  it("converts legacy action without params to on.press", () => {
+    const spec: Spec = {
+      root: "root",
+      elements: {
+        root: {
+          type: "Button",
+          props: { label: "OK", action: "confirm" },
+          children: [],
+        },
+      },
+    };
+    const { spec: fixed, fixes } = autoFixSpec(spec);
+    expect(fixed.elements.root.on).toEqual({
+      press: { action: "confirm" },
+    });
+    expect(fixes.length).toBeGreaterThan(0);
+  });
 });
diff --git a/packages/core/src/spec-validator.ts b/packages/core/src/spec-validator.ts
index a04d2f2d..c6f1217b 100644
--- a/packages/core/src/spec-validator.ts
+++ b/packages/core/src/spec-validator.ts
@@ -29,7 +29,11 @@ export interface SpecIssue {
     | "empty_spec"
     | "on_in_props"
     | "repeat_in_props"
-    | "watch_in_props";
+    | "watch_in_props"
+    | "action_in_props"
+    | "actionParams_in_props"
+    | "children_in_props"
+    | "state_in_props";
 }
 
 /**
@@ -162,6 +166,46 @@ export function validateSpec(
         code: "watch_in_props",
       });
     }
+
+    // 3f. `action` inside props (legacy pattern, should use `on` field)
+    if (props && "action" in props && props.action !== undefined) {
+      issues.push({
+        severity: "error",
+        message: `Element "${key}" has "action" inside "props". Use the "on" field instead: { "on": { "press": { "action": "...", "params": {...} } } }`,
+        elementKey: key,
+        code: "action_in_props",
+      });
+    }
+
+    // 3g. `actionParams` inside props (legacy pattern, should use `on` field)
+    if (props && "actionParams" in props && props.actionParams !== undefined) {
+      issues.push({
+        severity: "error",
+        message: `Element "${key}" has "actionParams" inside "props". Use the "on" field instead: { "on": { "press": { "action": "...", "params": {...} } } }`,
+        elementKey: key,
+        code: "actionParams_in_props",
+      });
+    }
+
+    // 3h. `children` inside props (should be a top-level field)
+    if (props && "children" in props && props.children !== undefined) {
+      issues.push({
+        severity: "error",
+        message: `Element "${key}" has "children" inside "props". It should be a top-level field on the element (sibling of type/props).`,
+        elementKey: key,
+        code: "children_in_props",
+      });
+    }
+
+    // 3i. `state` inside props (should be at spec level or use $state/$bindState)
+    if (props && "state" in props && props.state !== undefined) {
+      issues.push({
+        severity: "error",
+        message: `Element "${key}" has "state" inside "props". State should be defined at spec.state level, not inside element props. Use { "$state": "/path" } or { "$bindState": "/path" } for state references.`,
+        elementKey: key,
+        code: "state_in_props",
+      });
+    }
   }
 
   // 4. Orphaned elements (optional)
@@ -274,6 +318,53 @@ export function autoFixSpec(spec: Spec): {
       fixes.push(`Moved "watch" from props to element level on "${key}".`);
     }
 
+    // Fix children inside props → move to element level
+    currentProps = fixed.props as Record<string, unknown> | undefined;
+    if (
+      currentProps &&
+      "children" in currentProps &&
+      currentProps.children !== undefined
+    ) {
+      const { children, ...restProps } = currentProps;
+      // Only move if it's an array of strings (element keys)
+      if (
+        Array.isArray(children) &&
+        children.every((c) => typeof c === "string")
+      ) {
+        fixed = {
+          ...fixed,
+          props: restProps,
+          children: children as string[],
+        };
+        fixes.push(`Moved "children" from props to element level on "${key}".`);
+      }
+    }
+
+    // Fix legacy action/actionParams → convert to on.press
+    currentProps = fixed.props as Record<string, unknown> | undefined;
+    if (
+      currentProps &&
+      "action" in currentProps &&
+      currentProps.action !== undefined
+    ) {
+      const { action, actionParams, ...restProps } = currentProps;
+      const actionBinding = {
+        action: action as string,
+        ...(actionParams ? { params: actionParams as Record<string, unknown> } : {}),
+      };
+      fixed = {
+        ...fixed,
+        props: restProps,
+        on: {
+          ...(fixed.on ?? {}),
+          press: actionBinding,
+        },
+      };
+      fixes.push(
+        `Converted legacy "action"/"actionParams" to "on.press" on "${key}".`,
+      );
+    }
+
     fixedElements[key] = fixed;
   }