diff --git a/schemas/json/layout/expression.schema.v1.json b/schemas/json/layout/expression.schema.v1.json
index 1df3d5044e..efcdf035d7 100644
--- a/schemas/json/layout/expression.schema.v1.json
+++ b/schemas/json/layout/expression.schema.v1.json
@@ -41,7 +41,7 @@
   "definitions": {
     "any": {
       "title": "Any expression",
-      "oneOf": [
+      "anyOf": [
         { "type": "null", "title": "Null/missing value" },
         { "$ref": "#/definitions/strict-string" },
         { "$ref": "#/definitions/strict-boolean" },
@@ -51,7 +51,7 @@
     },
     "string": {
       "title": "Any expression returning string",
-      "oneOf": [
+      "anyOf": [
         { "type": "null", "title": "Null/missing value" },
         { "$ref": "#/definitions/strict-string" },
         { "$ref": "#/definitions/func-if" },
@@ -61,18 +61,23 @@
     },
     "strict-string": {
       "title": "Any expression returning string (strict)",
-      "oneOf": [
+      "anyOf": [
         { "type": "string", "title": "String constant" },
         { "$ref": "#/definitions/func-component" },
         { "$ref": "#/definitions/func-dataModel" },
         { "$ref": "#/definitions/func-instanceContext" },
         { "$ref": "#/definitions/func-frontendSettings" },
-        { "$ref": "#/definitions/func-concat" }
+        { "$ref": "#/definitions/func-concat" },
+        { "$ref": "#/definitions/func-round" },
+        { "$ref": "#/definitions/func-text" },
+        { "$ref": "#/definitions/func-language" },
+        { "$ref": "#/definitions/func-lowerCase" },
+        { "$ref": "#/definitions/func-upperCase" }
       ]
     },
     "boolean": {
       "title": "Any expression returning boolean",
-      "oneOf": [
+      "anyOf": [
         { "type": "null", "title": "Null/missing value" },
         { "$ref": "#/definitions/strict-boolean" },
         { "$ref": "#/definitions/func-if" },
@@ -82,7 +87,7 @@
     },
     "strict-boolean": {
       "title": "Any expression returning boolean (strict)",
-      "oneOf": [
+      "anyOf": [
         { "type": "boolean", "title": "Boolean constant" },
         { "$ref": "#/definitions/func-equals" },
         { "$ref": "#/definitions/func-notEquals" },
@@ -93,12 +98,17 @@
         { "$ref": "#/definitions/func-not" },
         { "$ref": "#/definitions/func-and" },
         { "$ref": "#/definitions/func-or" },
-        { "$ref": "#/definitions/func-authContext" }
+        { "$ref": "#/definitions/func-authContext" },
+        { "$ref": "#/definitions/func-contains" },
+        { "$ref": "#/definitions/func-notContains" },
+        { "$ref": "#/definitions/func-endsWith" },
+        { "$ref": "#/definitions/func-startsWith" },
+        { "$ref": "#/definitions/func-commaContains" }
       ]
     },
     "number": {
       "title": "Any expression returning a number",
-      "oneOf": [
+      "anyOf": [
         { "type": "null", "title": "Null/missing value" },
         { "$ref": "#/definitions/strict-number" },
         { "$ref": "#/definitions/func-if" },
@@ -107,14 +117,15 @@
     },
     "strict-number": {
       "title": "Any expression returning a number (strict)",
-      "oneOf": [
-        { "type": "number", "title": "Numeric constant" }
+      "anyOf": [
+        { "type": "number", "title": "Numeric constant" },
+        { "$ref": "#/definitions/func-stringLength" }
       ]
     },
     "func-if": {
       "title": "If/else conditional expression",
       "description": "This function will evaluate and return the result of either branch. If else is not given, null will be returned instead.",
-      "oneOf": [
+      "anyOf": [
         { "$ref": "#/definitions/func-if-with-else" },
         { "$ref": "#/definitions/func-if-without-else" }
       ]
@@ -293,6 +304,121 @@
         { "$ref": "#/definitions/boolean" }
       ],
       "additionalItems": { "$ref": "#/definitions/boolean" }
+    },
+    "func-round": {
+      "title": "Round function",
+      "description": "This function rounds a number to the nearest integer, or to the specified number of decimals",
+      "type": "array",
+      "items": [
+        { "const":  "round" },
+        { "$ref": "#/definitions/number" },
+        { "$ref": "#/definitions/number" }
+      ],
+      "additionalItems": false
+    },
+    "func-text": {
+      "title": "Text function",
+      "description": "This function retrieves the value of a text resource key, or returns the key if no text resource is found",
+      "type": "array",
+      "items": [
+        { "const":  "text" },
+        { "$ref": "#/definitions/string" }
+      ],
+      "additionalItems": false
+    },
+    "func-language": {
+      "title": "Language function",
+      "description": "This function retrieves the current language (usually 'nb', 'nn' or 'en')",
+      "type": "array",
+      "items": [
+        { "const":  "language" }
+      ],
+      "additionalItems": false
+    },
+    "func-contains": {
+      "title": "Contains function",
+      "description": "This function checks if the first string contains the second string",
+      "type": "array",
+      "items": [
+        { "const":  "contains" },
+        { "$ref": "#/definitions/string" },
+        { "$ref": "#/definitions/string" }
+      ],
+      "additionalItems": false
+    },
+    "func-notContains": {
+      "title": "Not contains function",
+      "description": "This function checks if the first string does not contain the second string",
+      "type": "array",
+      "items": [
+        { "const":  "notContains" },
+        { "$ref": "#/definitions/string" },
+        { "$ref": "#/definitions/string" }
+      ],
+      "additionalItems": false
+    },
+    "func-startsWith": {
+      "title": "Starts with function",
+      "description": "This function checks if the first string starts with the second string",
+      "type": "array",
+      "items": [
+        { "const":  "startsWith" },
+        { "$ref": "#/definitions/string" },
+        { "$ref": "#/definitions/string" }
+      ],
+      "additionalItems": false
+    },
+    "func-endsWith": {
+      "title": "Ends with function",
+      "description": "This function checks if the first string ends with the second string",
+      "type": "array",
+      "items": [
+        { "const":  "endsWith" },
+        { "$ref": "#/definitions/string" },
+        { "$ref": "#/definitions/string" }
+      ],
+      "additionalItems": false
+    },
+    "func-stringLength": {
+      "title": "String length function",
+      "description": "This function returns the length of a string",
+      "type": "array",
+      "items": [
+        { "const":  "stringLength" },
+        { "$ref": "#/definitions/string" }
+      ],
+      "additionalItems": false
+    },
+    "func-commaContains": {
+      "title": "Comma contains function",
+      "description": "This function checks if the first comma-separated string contains the second string",
+      "type": "array",
+      "items": [
+        { "const":  "commaContains" },
+        { "$ref": "#/definitions/string" },
+        { "$ref": "#/definitions/string" }
+      ],
+      "additionalItems": false
+    },
+    "func-lowerCase": {
+      "title": "Lower case function",
+      "description": "This function converts a string to lower case",
+      "type": "array",
+      "items": [
+        { "const":  "lowerCase" },
+        { "$ref": "#/definitions/string" }
+      ],
+      "additionalItems": false
+    },
+    "func-upperCase": {
+      "title": "Upper case function",
+      "description": "This function converts a string to upper case",
+      "type": "array",
+      "items": [
+        { "const":  "upperCase" },
+        { "$ref": "#/definitions/string" }
+      ],
+      "additionalItems": false
     }
   }
 }
diff --git a/src/features/expressions/errors.ts b/src/features/expressions/errors.ts
index 0ff740525c..c5176ce969 100644
--- a/src/features/expressions/errors.ts
+++ b/src/features/expressions/errors.ts
@@ -6,12 +6,6 @@ export class ExprRuntimeError extends Error {
   }
 }
 
-export class LookupNotFound extends ExprRuntimeError {
-  public constructor(context: ExprContext, message: string) {
-    super(context, message);
-  }
-}
-
 export class UnknownTargetType extends ExprRuntimeError {
   public constructor(context: ExprContext, type: string) {
     super(context, `Cannot cast to unknown type '${type}'`);
diff --git a/src/features/expressions/index.ts b/src/features/expressions/index.ts
index 55a838a578..d04bb5804b 100644
--- a/src/features/expressions/index.ts
+++ b/src/features/expressions/index.ts
@@ -3,7 +3,6 @@ import type { Mutable } from 'utility-types';
 
 import {
   ExprRuntimeError,
-  LookupNotFound,
   NodeNotFoundWithoutContext,
   UnexpectedType,
   UnknownSourceType,
@@ -430,7 +429,7 @@ export const ExprFunctions = {
   instanceContext: defineFunc({
     impl(key): string | null {
       if (key === null || instanceContextKeys[key] !== true) {
-        throw new LookupNotFound(this, `Unknown Instance context property ${key}`);
+        throw new ExprRuntimeError(this, `Unknown Instance context property ${key}`);
       }
 
       return (this.dataSources.instanceContext && this.dataSources.instanceContext[key]) || null;
@@ -441,7 +440,7 @@ export const ExprFunctions = {
   frontendSettings: defineFunc({
     impl(key): any {
       if (key === null) {
-        throw new LookupNotFound(this, `Value cannot be null. (Parameter 'key')`);
+        throw new ExprRuntimeError(this, `Value cannot be null. (Parameter 'key')`);
       }
 
       return (this.dataSources.applicationSettings && this.dataSources.applicationSettings[key]) || null;
@@ -452,7 +451,7 @@ export const ExprFunctions = {
   authContext: defineFunc({
     impl(key): boolean | null {
       if (key === null || authContextKeys[key] !== true) {
-        throw new LookupNotFound(this, `Unknown auth context property ${key}`);
+        throw new ExprRuntimeError(this, `Unknown auth context property ${key}`);
       }
 
       return Boolean(this.dataSources.authContext?.[key]);
@@ -463,7 +462,7 @@ export const ExprFunctions = {
   component: defineFunc({
     impl(id): any {
       if (id === null) {
-        throw new LookupNotFound(this, `Cannot lookup component null`);
+        throw new ExprRuntimeError(this, `Cannot lookup component null`);
       }
 
       const node = this.failWithoutNode();
@@ -481,7 +480,7 @@ export const ExprFunctions = {
       // Expressions can technically be used without having all the layouts available, which might lead to unexpected
       // results. We should note this in the error message, so we know the reason we couldn't find the component.
       const hasAllLayouts = node instanceof LayoutPage ? !!node.top : !!node.top.top;
-      throw new LookupNotFound(
+      throw new ExprRuntimeError(
         this,
         hasAllLayouts
           ? `Unable to find component with identifier ${id} or it does not have a simpleBinding`
@@ -494,7 +493,7 @@ export const ExprFunctions = {
   dataModel: defineFunc({
     impl(path): any {
       if (path === null) {
-        throw new LookupNotFound(this, `Cannot lookup dataModel null`);
+        throw new ExprRuntimeError(this, `Cannot lookup dataModel null`);
       }
 
       const maybeNode = this.failWithoutNode();
@@ -511,25 +510,19 @@ export const ExprFunctions = {
     returns: ExprVal.Any,
   }),
   round: defineFunc({
-    impl(number, decimalPoints: number | null): number | null {
-      if (number === null) {
-        throw new LookupNotFound(this, `"Value" parameter cannot be null.`);
-      }
-
-      if (decimalPoints !== undefined && decimalPoints !== null) {
-        const factor = 10 ** decimalPoints;
-        return Math.round(number * factor) / factor;
-      }
-
-      return Math.round(number);
+    impl(number, decimalPoints) {
+      const realNumber = number === null ? 0 : number;
+      const realDecimalPoints = decimalPoints === null ? 0 : decimalPoints;
+      return parseFloat(`${realNumber}`).toFixed(realDecimalPoints);
     },
     args: [ExprVal.Number, ExprVal.Number] as const,
-    returns: ExprVal.Number,
+    minArguments: 1,
+    returns: ExprVal.String,
   }),
   text: defineFunc({
-    impl(key): string | null {
+    impl(key) {
       if (key === null) {
-        throw new LookupNotFound(this, `"Key" parameter cannot be null.`);
+        return null;
       }
 
       return getTextResourceByKey(key, this.dataSources.textResources);
@@ -538,20 +531,20 @@ export const ExprFunctions = {
     returns: ExprVal.String,
   }),
   language: defineFunc({
-    impl(): string | null {
+    impl() {
       const selectedLanguage =
         this.dataSources.profile?.selectedAppLanguage ||
         this.dataSources.profile?.profile?.profileSettingPreference?.language;
 
-      return selectedLanguage || null;
+      return selectedLanguage || 'nb';
     },
     args: [] as const,
     returns: ExprVal.String,
   }),
   contains: defineFunc({
-    impl(string: string, stringToContain: string): boolean {
+    impl(string, stringToContain): boolean {
       if (string === null || stringToContain === null) {
-        throw new LookupNotFound(this, `"string" or "stringToContain" parameter cannot be null.`);
+        return false;
       }
 
       return string.includes(stringToContain);
@@ -562,7 +555,7 @@ export const ExprFunctions = {
   notContains: defineFunc({
     impl(string: string, stringToNotContain: string): boolean {
       if (string === null || stringToNotContain === null) {
-        throw new LookupNotFound(this, `"string" or "stringToNotContain" parameter cannot be null.`);
+        return true;
       }
       return !string.includes(stringToNotContain);
     },
@@ -572,7 +565,7 @@ export const ExprFunctions = {
   endsWith: defineFunc({
     impl(string: string, stringToMatch: string): boolean {
       if (string === null || stringToMatch === null) {
-        throw new LookupNotFound(this, `"string" or "stringToMatch" parameter cannot be null.`);
+        return false;
       }
       return string.endsWith(stringToMatch);
     },
@@ -582,29 +575,22 @@ export const ExprFunctions = {
   startsWith: defineFunc({
     impl(string: string, stringToMatch: string): boolean {
       if (string === null || stringToMatch === null) {
-        throw new LookupNotFound(this, `"string" or "stringToMatch" parameter cannot be null.`);
+        return false;
       }
-
       return string.startsWith(stringToMatch);
     },
     args: [ExprVal.String, ExprVal.String] as const,
     returns: ExprVal.Boolean,
   }),
   stringLength: defineFunc({
-    impl(string: string): number {
-      if (string === null) {
-        throw new LookupNotFound(this, `"string" parameter cannot be null.`);
-      }
-
-      return string.length;
-    },
+    impl: (string) => (string === null ? 0 : string.length),
     args: [ExprVal.String] as const,
     returns: ExprVal.Number,
   }),
   commaContains: defineFunc({
-    impl(commaSeparatedString: string, stringToMatch: string): boolean {
+    impl(commaSeparatedString, stringToMatch) {
       if (commaSeparatedString === null || stringToMatch === null) {
-        throw new LookupNotFound(this, `"commaSeparatedString" or "stringToMatch" parameter cannot be null.`);
+        return false;
       }
 
       // Split the comma separated string into an array and remove whitespace from each part
@@ -615,9 +601,9 @@ export const ExprFunctions = {
     returns: ExprVal.Boolean,
   }),
   lowerCase: defineFunc({
-    impl(string: string): string {
+    impl(string) {
       if (string === null) {
-        throw new LookupNotFound(this, `"string" parameter cannot be null.`);
+        return null;
       }
       return string.toLowerCase();
     },
@@ -625,9 +611,9 @@ export const ExprFunctions = {
     returns: ExprVal.String,
   }),
   upperCase: defineFunc({
-    impl(string: string): string {
+    impl(string) {
       if (string === null) {
-        throw new LookupNotFound(this, `"string" parameter cannot be null.`);
+        return null;
       }
       return string.toUpperCase();
     },
diff --git a/src/features/expressions/schema.test.ts b/src/features/expressions/schema.test.ts
new file mode 100644
index 0000000000..ef23cd6bc4
--- /dev/null
+++ b/src/features/expressions/schema.test.ts
@@ -0,0 +1,127 @@
+import Ajv from 'ajv';
+import expressionSchema from 'schemas/json/layout/expression.schema.v1.json';
+
+import { ExprFunctions } from 'src/features/expressions/index';
+import { ExprVal } from 'src/features/expressions/types';
+import type { FuncDef } from 'src/features/expressions/types';
+
+type Func = { name: string } & FuncDef<ExprVal[], ExprVal>;
+
+describe('expression schema tests', () => {
+  const functions: Func[] = [];
+  for (const name of Object.keys(ExprFunctions)) {
+    const func = ExprFunctions[name];
+    functions.push({ name, ...func });
+  }
+
+  it.each(functions)(
+    '$name should have a valid func-$name definition',
+    ({ name, args, minArguments, returns, lastArgSpreads }) => {
+      if (name === 'if') {
+        // if is a special case, we'll skip it here
+        return;
+      }
+
+      expect(expressionSchema.definitions[`func-${name}`]).toBeDefined();
+      expect(expressionSchema.definitions[`func-${name}`].type).toBe('array');
+      expect(expressionSchema.definitions[`func-${name}`].items[0]).toEqual({ const: name });
+
+      if (returns === ExprVal.Any) {
+        // At least one of the definitions should be a match
+        const allTypes: any[] = [];
+        for (const type of ['number', 'string', 'boolean']) {
+          allTypes.push(...expressionSchema.definitions[`strict-${type}`].anyOf);
+        }
+        expect(allTypes).toContainEqual({
+          $ref: `#/definitions/func-${name}`,
+        });
+      } else {
+        const returnString = exprValToString(returns);
+        expect(expressionSchema.definitions[`strict-${returnString}`].anyOf).toContainEqual({
+          $ref: `#/definitions/func-${name}`,
+        });
+      }
+
+      if (minArguments === undefined) {
+        expect(expressionSchema.definitions[`func-${name}`].items.length).toBe(args.length + 1);
+      } else {
+        expect(expressionSchema.definitions[`func-${name}`].items.length).toBeGreaterThanOrEqual(minArguments + 1);
+      }
+
+      if (lastArgSpreads) {
+        const lastArg = args[args.length - 1];
+        expect(expressionSchema.definitions[`func-${name}`].additionalItems).toEqual({ $ref: exprValToDef(lastArg) });
+      } else {
+        expect(expressionSchema.definitions[`func-${name}`].additionalItems).toBe(false);
+      }
+    },
+  );
+
+  const ajv = new Ajv({ strict: false });
+  const validate = ajv.compile(expressionSchema);
+
+  it.each(functions)(
+    '$name should validate against generated function calls',
+    ({ name, args, minArguments, lastArgSpreads }) => {
+      if (name === 'if') {
+        // if is a special case, we'll skip it here
+        return;
+      }
+
+      const funcDef = expressionSchema.definitions[`func-${name}`];
+
+      // With exactly the right number of arguments
+      const funcCall = [name, ...args.map(exprValToString)];
+      if (lastArgSpreads) {
+        funcCall.push(...args.map(exprValToString));
+      }
+
+      // Use enum value, if defined in schema
+      for (let i = 0; i < args.length; i++) {
+        const argDef = funcDef.items.length > i + 1 ? funcDef.items[i + 1] : undefined;
+        if (argDef?.enum) {
+          funcCall[i + 1] = argDef.enum[0];
+        }
+      }
+
+      const valid = validate(funcCall);
+      expect(validate.errors).toEqual(null);
+      expect(valid).toBe(true);
+
+      // With too few arguments
+      const funcCallMinArguments = [name];
+      const validMinArguments = validate(funcCallMinArguments);
+
+      // This always validates, because the schema allows for less than the minimum number of arguments. If it didn't,
+      // you wouldn't get autocomplete for functions until you had the minimum number of arguments, which makes for
+      // a bad developer experience. We test this explicitly below, even though is does not seem to be the desired
+      // behavior.
+      expect(validate.errors).toEqual(null);
+      expect(validMinArguments).toBe(true);
+
+      // With too many arguments
+      const funcCallExtra = [name, ...args.map(exprValToString), 'extra'];
+      const validExtra = validate(funcCallExtra);
+      if (lastArgSpreads) {
+        // This always validates, because the last argument spread
+        expect(validate.errors).toEqual(null);
+        expect(validExtra).toBe(true);
+      } else {
+        expect(validExtra).toBe(false);
+      }
+    },
+  );
+
+  it('invalid functions should not validate', () => {
+    const valid = validate(['invalid_function']);
+    expect(valid).toBe(false);
+  });
+});
+
+function exprValToString(val: ExprVal): string {
+  return val.toString().replaceAll('_', '');
+}
+
+function exprValToDef(val: ExprVal): string {
+  return `#/definitions/${exprValToString(val)}`;
+}
diff --git a/src/features/expressions/shared-tests/functions/commaContains/empty-string.json b/src/features/expressions/shared-tests/functions/commaContains/empty-string.json
new file mode 100644
index 0000000000..5beadc3d54
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/commaContains/empty-string.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should not contain an empty string",
+  "expression": ["commaContains", "40, 50, 60", ""],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/commaContains/null.json b/src/features/expressions/shared-tests/functions/commaContains/null.json
new file mode 100644
index 0000000000..30e4156c53
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/commaContains/null.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should not contain a null value",
+  "expression": ["commaContains", "40, 50, 60", null],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/commaContains/null2.json b/src/features/expressions/shared-tests/functions/commaContains/null2.json
new file mode 100644
index 0000000000..009bae2547
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/commaContains/null2.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should not split a null value",
+  "expression": ["commaContains", null, "hello world"],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/contains/empty-string.json b/src/features/expressions/shared-tests/functions/contains/empty-string.json
new file mode 100644
index 0000000000..b4e14894fc
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/contains/empty-string.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should always contain en empty string",
+  "expression": ["contains", "Hello", ""],
+  "expects": true
+}
diff --git a/src/features/expressions/shared-tests/functions/contains/null.json b/src/features/expressions/shared-tests/functions/contains/null.json
new file mode 100644
index 0000000000..f53527bdeb
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/contains/null.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should always return false when the input is null",
+  "expression": ["contains", null, "null"],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/contains/null2.json b/src/features/expressions/shared-tests/functions/contains/null2.json
new file mode 100644
index 0000000000..0302631b7d
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/contains/null2.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should always return false when the input is null",
+  "expression": ["contains", "null", null],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/contains/null3.json b/src/features/expressions/shared-tests/functions/contains/null3.json
new file mode 100644
index 0000000000..b525fb7779
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/contains/null3.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should always return false when the input is null",
+  "expression": ["contains", null, null],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/contains/null4.json b/src/features/expressions/shared-tests/functions/contains/null4.json
new file mode 100644
index 0000000000..286a2e3cfe
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/contains/null4.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should treat stringy nulls as null",
+  "expression": ["contains", "null", "null"],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/endsWith/empty-string.json b/src/features/expressions/shared-tests/functions/endsWith/empty-string.json
new file mode 100644
index 0000000000..1626ffef4c
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/endsWith/empty-string.json
@@ -0,0 +1,5 @@
+{
+  "name": "All strings ends with an empty string",
+  "expression": ["endsWith", "Hello", ""],
+  "expects": true
+}
diff --git a/src/features/expressions/shared-tests/functions/endsWith/ends-with-null.json b/src/features/expressions/shared-tests/functions/endsWith/ends-with-null.json
new file mode 100644
index 0000000000..59e6f17eb2
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/endsWith/ends-with-null.json
@@ -0,0 +1,5 @@
+{
+  "name": "No string ends with a null",
+  "expression": ["endsWith", "Hello", null],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/language/should-return-nb-if-not-set.json b/src/features/expressions/shared-tests/functions/language/should-return-nb-if-not-set.json
new file mode 100644
index 0000000000..d5684add63
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/language/should-return-nb-if-not-set.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should default to nb if no profile is set",
+  "expression": ["language"],
+  "expects": "nb"
+}
diff --git a/src/features/expressions/shared-tests/functions/lowerCase/null.json b/src/features/expressions/shared-tests/functions/lowerCase/null.json
new file mode 100644
index 0000000000..d341f97670
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/lowerCase/null.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should return a null when the input is a null",
+  "expression": ["lowerCase", null],
+  "expects": null
+}
diff --git a/src/features/expressions/shared-tests/functions/notContains/null.json b/src/features/expressions/shared-tests/functions/notContains/null.json
new file mode 100644
index 0000000000..206d1f0583
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/notContains/null.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should always return true if the input is null",
+  "expression": ["notContains", null, null],
+  "expects": true
+}
diff --git a/src/features/expressions/shared-tests/functions/notContains/null2.json b/src/features/expressions/shared-tests/functions/notContains/null2.json
new file mode 100644
index 0000000000..1ff4196b42
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/notContains/null2.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should always return true if the input is null",
+  "expression": ["notContains", "null", null],
+  "expects": true
+}
diff --git a/src/features/expressions/shared-tests/functions/notContains/null3.json b/src/features/expressions/shared-tests/functions/notContains/null3.json
new file mode 100644
index 0000000000..b473ca2a5a
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/notContains/null3.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should always return true if the input is null",
+  "expression": ["notContains", null, "null"],
+  "expects": true
+}
diff --git a/src/features/expressions/shared-tests/functions/notContains/null4.json b/src/features/expressions/shared-tests/functions/notContains/null4.json
new file mode 100644
index 0000000000..f0d906f8e8
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/notContains/null4.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should treat stringy nulls as null",
+  "expression": ["notContains", "null", "null"],
+  "expects": true
+}
diff --git a/src/features/expressions/shared-tests/functions/round/nearest-integer-with-decimal-as-strings.json b/src/features/expressions/shared-tests/functions/round/nearest-integer-with-decimal-as-strings.json
index 796a004f16..0e7d6530ea 100644
--- a/src/features/expressions/shared-tests/functions/round/nearest-integer-with-decimal-as-strings.json
+++ b/src/features/expressions/shared-tests/functions/round/nearest-integer-with-decimal-as-strings.json
@@ -1,5 +1,5 @@
 {
   "name": "Should round a number to the nearest integer with a precision of 2",
   "expression": ["round", "2.2199999", "2"],
-  "expects": 2.22
+  "expects": "2.22"
 }
diff --git a/src/features/expressions/shared-tests/functions/round/nearest-integer-with-decimal.json b/src/features/expressions/shared-tests/functions/round/nearest-integer-with-decimal.json
index 18d3156860..49642cf5a1 100644
--- a/src/features/expressions/shared-tests/functions/round/nearest-integer-with-decimal.json
+++ b/src/features/expressions/shared-tests/functions/round/nearest-integer-with-decimal.json
@@ -1,5 +1,5 @@
 {
   "name": "Should round a number to the nearest integer with a precision of 2",
   "expression": ["round", 3.2199999, 2],
-  "expects": 3.22
+  "expects": "3.22"
 }
diff --git a/src/features/expressions/shared-tests/functions/round/nearest-integer.json b/src/features/expressions/shared-tests/functions/round/nearest-integer.json
index 05ee5aee77..5856e76c8c 100644
--- a/src/features/expressions/shared-tests/functions/round/nearest-integer.json
+++ b/src/features/expressions/shared-tests/functions/round/nearest-integer.json
@@ -1,5 +1,5 @@
 {
   "name": "Should round to nearest integer",
   "expression": ["round", 3.2, null],
-  "expects": 3
+  "expects": "3"
 }
diff --git a/src/features/expressions/shared-tests/functions/round/round-0-decimal-places.json b/src/features/expressions/shared-tests/functions/round/round-0-decimal-places.json
new file mode 100644
index 0000000000..1822cf78ed
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/round/round-0-decimal-places.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should be able to round 0 to 0 decimal places",
+  "expression": ["round", 0, 0],
+  "expects": "0"
+}
diff --git a/src/features/expressions/shared-tests/functions/round/round-0-decimal-places2.json b/src/features/expressions/shared-tests/functions/round/round-0-decimal-places2.json
new file mode 100644
index 0000000000..b79c24939c
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/round/round-0-decimal-places2.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should be able to round 0 to 0 decimal places",
+  "expression": ["round", 2.9999, 0],
+  "expects": "3"
+}
diff --git a/src/features/expressions/shared-tests/functions/round/round-negative-number.json b/src/features/expressions/shared-tests/functions/round/round-negative-number.json
index c5e464a7c2..63b9e5edd6 100644
--- a/src/features/expressions/shared-tests/functions/round/round-negative-number.json
+++ b/src/features/expressions/shared-tests/functions/round/round-negative-number.json
@@ -1,5 +1,5 @@
 {
   "name": "Should round negative number",
   "expression": ["round", -2.999, null],
-  "expects": -3
+  "expects": "-3"
 }
diff --git a/src/features/expressions/shared-tests/functions/round/round-null.json b/src/features/expressions/shared-tests/functions/round/round-null.json
new file mode 100644
index 0000000000..dc1a2c6664
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/round/round-null.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should be able to round a null",
+  "expression": ["round", null],
+  "expects": "0"
+}
diff --git a/src/features/expressions/shared-tests/functions/round/round-null2.json b/src/features/expressions/shared-tests/functions/round/round-null2.json
new file mode 100644
index 0000000000..bebf3af4a2
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/round/round-null2.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should be able to round a null",
+  "expression": ["round", null, 2],
+  "expects": "0.00"
+}
diff --git a/src/features/expressions/shared-tests/functions/round/round-strings.json b/src/features/expressions/shared-tests/functions/round/round-strings.json
index 5f4c373ee8..b067d72536 100644
--- a/src/features/expressions/shared-tests/functions/round/round-strings.json
+++ b/src/features/expressions/shared-tests/functions/round/round-strings.json
@@ -1,5 +1,5 @@
 {
   "name": "Should be able to round a number to the nearest integer even if it is a string",
   "expression": ["round", "3.99", null],
-  "expects": 4
+  "expects": "4"
 }
diff --git a/src/features/expressions/shared-tests/functions/round/round-with-too-many-args.json b/src/features/expressions/shared-tests/functions/round/round-with-too-many-args.json
new file mode 100644
index 0000000000..603931c73f
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/round/round-with-too-many-args.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should fail when given too many arguments",
+  "expression": ["round", 3.99, 2, 3],
+  "expectsFailure": "Expected 1-2 argument(s), got 3"
+}
diff --git a/src/features/expressions/shared-tests/functions/round/round-without-decimalCount.json b/src/features/expressions/shared-tests/functions/round/round-without-decimalCount.json
new file mode 100644
index 0000000000..0bdcbef2b7
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/round/round-without-decimalCount.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should be able to round a number without the optional second argument",
+  "expression": ["round", "3.99"],
+  "expects": "4"
+}
diff --git a/src/features/expressions/shared-tests/functions/round/round-without-decimalCount2.json b/src/features/expressions/shared-tests/functions/round/round-without-decimalCount2.json
new file mode 100644
index 0000000000..01b5fa64d4
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/round/round-without-decimalCount2.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should be able to round a number without the optional second argument",
+  "expression": ["round", 3.99],
+  "expects": "4"
+}
diff --git a/src/features/expressions/shared-tests/functions/startsWith/empty-string.json b/src/features/expressions/shared-tests/functions/startsWith/empty-string.json
new file mode 100644
index 0000000000..5da24b3da3
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/startsWith/empty-string.json
@@ -0,0 +1,5 @@
+{
+  "name": "All strings start with an empty string",
+  "expression": ["startsWith", "Hello world", ""],
+  "expects": true
+}
diff --git a/src/features/expressions/shared-tests/functions/startsWith/null.json b/src/features/expressions/shared-tests/functions/startsWith/null.json
new file mode 100644
index 0000000000..b088b4f16e
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/startsWith/null.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should always be false when given a null",
+  "expression": ["startsWith", null, "null"],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/startsWith/null2.json b/src/features/expressions/shared-tests/functions/startsWith/null2.json
new file mode 100644
index 0000000000..92656df76b
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/startsWith/null2.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should always be false when given a null",
+  "expression": ["startsWith", null, null],
+  "expects": false
+}
diff --git a/src/features/expressions/shared-tests/functions/stringLength/null.json b/src/features/expressions/shared-tests/functions/stringLength/null.json
new file mode 100644
index 0000000000..c4b7e2ec81
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/stringLength/null.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should return 0 when the input is a null",
+  "expression": ["stringLength", null],
+  "expects": 0
+}
diff --git a/src/features/expressions/shared-tests/functions/text/null.json b/src/features/expressions/shared-tests/functions/text/null.json
new file mode 100644
index 0000000000..fec35efc08
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/text/null.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should return null when the input is null",
+  "expression": ["text", null],
+  "expects": null
+}
diff --git a/src/features/expressions/shared-tests/functions/text/should-return-text-resource-with-variable-in-rep-group-no-index-markers.json b/src/features/expressions/shared-tests/functions/text/should-return-text-resource-with-variable-in-rep-group-no-index-markers.json
new file mode 100644
index 0000000000..c6cf039f17
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/text/should-return-text-resource-with-variable-in-rep-group-no-index-markers.json
@@ -0,0 +1,114 @@
+{
+  "name": "Should text resource with resolved variable inside a repeating group, without index markers",
+  "disabledFrontend": true,
+  "expression": ["text", "found.key"],
+  "expects": "Hello world Arne",
+  "context": {
+    "component": "myndig",
+    "rowIndices": [1, 0],
+    "currentLayout": "Page2"
+  },
+  "textResources": [
+    {
+      "id": "found.key",
+      "value": "Hello world {0}",
+      "variables": [
+        {
+          "key": "Bedrifter.Ansatte.Navn",
+          "dataSource": "dataModel.default"
+        }
+      ]
+    }
+  ],
+  "layouts": {
+    "Page1": {
+      "$schema": "https://altinncdn.no/schemas/json/layout/layout.schema.v1.json",
+      "data": {
+        "layout": []
+      }
+    },
+    "Page2": {
+      "$schema": "https://altinncdn.no/schemas/json/layout/layout.schema.v1.json",
+      "data": {
+        "layout": [
+          {
+            "id": "bedrifter",
+            "type": "Group",
+            "maxCount": 99,
+            "dataModelBindings": {
+              "group": "Bedrifter"
+            },
+            "children": ["bedriftsNavn", "ansatte"]
+          },
+          {
+            "id": "bedriftsNavn",
+            "type": "Input",
+            "dataModelBindings": {
+              "simpleBinding": "Bedrifter.Navn"
+            }
+          },
+          {
+            "id": "ansatte",
+            "type": "Group",
+            "maxCount": 99,
+            "dataModelBindings": {
+              "group": "Bedrifter.Ansatte"
+            },
+            "children": ["navn", "alder", "myndig"]
+          },
+          {
+            "id": "navn",
+            "type": "Input",
+            "dataModelBindings": {
+              "simpleBinding": "Bedrifter.Ansatte.Navn"
+            }
+          },
+          {
+            "id": "alder",
+            "type": "Input",
+            "dataModelBindings": {
+              "simpleBinding": "Bedrifter.Ansatte.Alder"
+            }
+          },
+          {
+            "id": "myndig",
+            "type": "Paragraph",
+            "textResourceBindings": {
+              "title": "Hurra, den ansatte er myndig!"
+            }
+          }
+        ]
+      }
+    }
+  },
+  "dataModel": {
+    "Bedrifter": [
+      {
+        "Navn": "Hell og lykke AS",
+        "Ansatte": [
+          {
+            "Navn": "Kaare",
+            "Alder": 24
+          },
+          {
+            "Navn": "Per",
+            "Alder": 24
+          }
+        ]
+      },
+      {
+        "Navn": "Nedtur og motgang AS",
+        "Ansatte": [
+          {
+            "Navn": "Arne",
+            "Alder": 24
+          },
+          {
+            "Navn": "Vidar",
+            "Alder": 14
+          }
+        ]
+      }
+    ]
+  }
+}
diff --git a/src/features/expressions/shared-tests/functions/text/should-return-text-resource-with-variable-in-rep-group.json b/src/features/expressions/shared-tests/functions/text/should-return-text-resource-with-variable-in-rep-group.json
new file mode 100644
index 0000000000..c6e2f17569
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/text/should-return-text-resource-with-variable-in-rep-group.json
@@ -0,0 +1,114 @@
+{
+  "name": "Should text resource with resolved variable inside a repeating group",
+  "disabledFrontend": true,
+  "expression": ["text", "found.key"],
+  "expects": "Hello world Vidar",
+  "context": {
+    "component": "myndig",
+    "rowIndices": [1, 1],
+    "currentLayout": "Page2"
+  },
+  "textResources": [
+    {
+      "id": "found.key",
+      "value": "Hello world {0}",
+      "variables": [
+        {
+          "key": "Bedrifter[{0}].Ansatte[{1}].Navn",
+          "dataSource": "dataModel.default"
+        }
+      ]
+    }
+  ],
+  "layouts": {
+    "Page1": {
+      "$schema": "https://altinncdn.no/schemas/json/layout/layout.schema.v1.json",
+      "data": {
+        "layout": []
+      }
+    },
+    "Page2": {
+      "$schema": "https://altinncdn.no/schemas/json/layout/layout.schema.v1.json",
+      "data": {
+        "layout": [
+          {
+            "id": "bedrifter",
+            "type": "Group",
+            "maxCount": 99,
+            "dataModelBindings": {
+              "group": "Bedrifter"
+            },
+            "children": ["bedriftsNavn", "ansatte"]
+          },
+          {
+            "id": "bedriftsNavn",
+            "type": "Input",
+            "dataModelBindings": {
+              "simpleBinding": "Bedrifter.Navn"
+            }
+          },
+          {
+            "id": "ansatte",
+            "type": "Group",
+            "maxCount": 99,
+            "dataModelBindings": {
+              "group": "Bedrifter.Ansatte"
+            },
+            "children": ["navn", "alder", "myndig"]
+          },
+          {
+            "id": "navn",
+            "type": "Input",
+            "dataModelBindings": {
+              "simpleBinding": "Bedrifter.Ansatte.Navn"
+            }
+          },
+          {
+            "id": "alder",
+            "type": "Input",
+            "dataModelBindings": {
+              "simpleBinding": "Bedrifter.Ansatte.Alder"
+            }
+          },
+          {
+            "id": "myndig",
+            "type": "Paragraph",
+            "textResourceBindings": {
+              "title": "Hurra, den ansatte er myndig!"
+            }
+          }
+        ]
+      }
+    }
+  },
+  "dataModel": {
+    "Bedrifter": [
+      {
+        "Navn": "Hell og lykke AS",
+        "Ansatte": [
+          {
+            "Navn": "Kaare",
+            "Alder": 24
+          },
+          {
+            "Navn": "Per",
+            "Alder": 24
+          }
+        ]
+      },
+      {
+        "Navn": "Nedtur og motgang AS",
+        "Ansatte": [
+          {
+            "Navn": "Arne",
+            "Alder": 24
+          },
+          {
+            "Navn": "Vidar",
+            "Alder": 14
+          }
+        ]
+      }
+    ]
+  }
+}
diff --git a/src/features/expressions/shared-tests/functions/text/should-return-text-resource-with-variable.json b/src/features/expressions/shared-tests/functions/text/should-return-text-resource-with-variable.json
new file mode 100644
index 0000000000..19b5dbaa7d
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/text/should-return-text-resource-with-variable.json
@@ -0,0 +1,25 @@
+{
+  "name": "Should text resource with resolved variable",
+  "disabledFrontend": true,
+  "expression": ["text", "found.key"],
+  "expects": "Hello world foo bar",
+  "textResources": [
+    {
+      "id": "found.key",
+      "value": "Hello world {0}",
+      "variables": [
+        {
+          "key": "My.Model.Value",
+          "dataSource": "dataModel.default"
+        }
+      ]
+    }
+  ],
+  "dataModel": {
+    "My": {
+      "Model": {
+        "Value": "foo bar"
+      }
+    }
+  }
+}
diff --git a/src/features/expressions/shared-tests/functions/text/should-return-text-resource.json b/src/features/expressions/shared-tests/functions/text/should-return-text-resource.json
new file mode 100644
index 0000000000..06fa4cce9e
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/text/should-return-text-resource.json
@@ -0,0 +1,11 @@
+{
+  "name": "Should return text resource value",
+  "expression": ["text", "found.key"],
+  "expects": "Hello world",
+  "textResources": [
+    {
+      "id": "found.key",
+      "value": "Hello world"
+    }
+  ]
+}
diff --git a/src/features/expressions/shared-tests/functions/upperCase/null.json b/src/features/expressions/shared-tests/functions/upperCase/null.json
new file mode 100644
index 0000000000..ba45196545
--- /dev/null
+++ b/src/features/expressions/shared-tests/functions/upperCase/null.json
@@ -0,0 +1,5 @@
+{
+  "name": "Should return a null when the input is a null",
+  "expression": ["upperCase", null],
+  "expects": null
+}
diff --git a/src/features/expressions/shared.test.ts b/src/features/expressions/shared.test.ts
index 8fde380f31..843201b501 100644
--- a/src/features/expressions/shared.test.ts
+++ b/src/features/expressions/shared.test.ts
@@ -53,6 +53,7 @@ describe('Expressions shared function tests', () => {
     it.each(folder.content)(
       '$name',
       ({
+        disabledFrontend,
         expression,
         expects,
         expectsFailure,
@@ -65,6 +66,11 @@ describe('Expressions shared function tests', () => {
         textResources,
         profile,
       }) => {
+        if (disabledFrontend) {
+          // Skipped tests usually means that the frontend does not support the feature yet
+          return;
+        }
+
         const dataSources: HierarchyDataSources = {
           ...getHierarchyDataSourcesMock(),
           formData: dataModel ? dot.dot(dataModel) : {},
diff --git a/src/features/expressions/shared.ts b/src/features/expressions/shared.ts
index 4adbe7c2f0..998cacedca 100644
--- a/src/features/expressions/shared.ts
+++ b/src/features/expressions/shared.ts
@@ -19,6 +19,7 @@ export interface Layouts {
 
 export interface SharedTest {
   name: string;
+  disabledFrontend?: boolean;
   layouts?: Layouts;
   dataModel?: any;
   instance?: IInstance;
diff --git a/src/features/expressions/validation.ts b/src/features/expressions/validation.ts
index f2783b3c95..fa2e2d1a9a 100644
--- a/src/features/expressions/validation.ts
+++ b/src/features/expressions/validation.ts
@@ -115,14 +115,16 @@ function validateFunctionArgLength(
     return;
   }
 
-  if (actual.length !== minExpected) {
-    addError(
-      ctx,
-      path,
-      ValidationErrorMessage.ArgsWrongNum,
-      `${minExpected}${canSpread ? '+' : ''}`,
-      `${actual.length}`,
-    );
+  const maxExpected = ExprFunctions[func]?.args.length;
+  if (actual.length < minExpected || actual.length > maxExpected) {
+    let expected = `${minExpected}`;
+    if (canSpread) {
+      expected += '+';
+    } else if (maxExpected !== minExpected) {
+      expected += `-${maxExpected}`;
+    }
+
+    addError(ctx, path, ValidationErrorMessage.ArgsWrongNum, `${expected}`, `${actual.length}`);
   }
 }
 
diff --git a/src/utils/layout/schema.test.ts b/src/utils/layout/schema.test.ts
index 1d81a6e2ee..8378d651e0 100644
--- a/src/utils/layout/schema.test.ts
+++ b/src/utils/layout/schema.test.ts
@@ -62,12 +62,8 @@ describe('Layout schema', () => {
           // Ignore errors about id not matching pattern. This is common, and we don't care that much about it.
           return false;
         }
-        if (error.message?.startsWith("must have required property 'size'")) {
-          // Fairly common for Header components. Maybe we should make this one optional?
-          return false;
-        }
 
-        return true;
+        return !error.message?.startsWith("must have required property 'size'");
       });
 
     for (const { appName, setName, entireFiles } of allLayoutSets) {