diff --git a/packages/language/src/lint/expression-validation.test.ts b/packages/language/src/lint/expression-validation.test.ts
new file mode 100644
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--- /dev/null
+++ b/packages/language/src/lint/expression-validation.test.ts
@@ -0,0 +1,189 @@
+/*
+ * Copyright (c) 2026, Salesforce, Inc.
+ * All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ * For full license text, see the LICENSE file in the repo root or https://www.apache.org/licenses/LICENSE-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import { parse } from '@agentscript/parser';
+import { Dialect } from '../core/dialect.js';
+import { NamedBlock, NamedCollectionBlock } from '../core/block.js';
+import { ExpressionValue } from '../core/primitives.js';
+import { LintEngine } from '../core/analysis/lint-engine.js';
+import { createSchemaContext } from '../core/analysis/scope.js';
+import {
+  expressionValidationPass,
+  type ExpressionValidationOptions,
+} from './expression-validation.js';
+
+const ExprBlock = NamedBlock('ExprBlock', {
+  value: ExpressionValue.describe('Expression under test'),
+});
+
+const TestSchema = {
+  expr: NamedCollectionBlock(ExprBlock),
+};
+
+const schemaCtx = createSchemaContext({ schema: TestSchema, aliases: {} });
+
+const PASS_CODES = new Set([
+  'unknown-function',
+  'indirect-function-call',
+  'namespace-function-call',
+  'function-argument-count',
+  'malformed-ast',
+  'unsupported-operator',
+  'unsupported-slice-target',
+  'bare-uploaded-files-reference',
+]);
+
+function getDiagnostics(
+  expression: string,
+  options?: ExpressionValidationOptions
+) {
+  const { rootNode: root } = parse(`
+expr one:
+  value: ${expression}
+`);
+  const mappingNode =
+    root.namedChildren.find(n => n.type === 'mapping') ?? root;
+
+  const dialect = new Dialect();
+  const result = dialect.parse(mappingNode, TestSchema);
+
+  const engine = new LintEngine({
+    passes: [expressionValidationPass(options)],
+    source: 'test',
+  });
+  const { diagnostics } = engine.run(result.value, schemaCtx);
+  // Guard against assertions passing on an expression that failed to parse.
+  expect(diagnostics.filter(d => d.code === 'syntax-error')).toHaveLength(0);
+  return diagnostics.filter(d => PASS_CODES.has(d.code as string));
+}
+
+describe('expression-validation lint pass', () => {
+  describe('function calls', () => {
+    it('does not flag a call to a built-in function', () => {
+      expect(getDiagnostics('len(@variables.items)')).toHaveLength(0);
+    });
+
+    it('flags an unknown function and suggests a close match', () => {
+      const diags = getDiagnostics('lenn(@variables.items)');
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('unknown-function');
+      expect(diags[0].message).toContain("'lenn' is not a recognized function");
+      expect(diags[0].data).toEqual({ suggestion: 'len' });
+    });
+
+    it('flags a method call on a member expression', () => {
+      const diags = getDiagnostics('@variables.items.append(1)');
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('namespace-function-call');
+    });
+
+    it('flags an indirect call through a subscript', () => {
+      const diags = getDiagnostics('@variables.fns[0](1)');
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('indirect-function-call');
+    });
+
+    it('flags a call with the wrong number of arguments', () => {
+      const diags = getDiagnostics('len(@variables.a, @variables.b)');
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('function-argument-count');
+      expect(diags[0].message).toBe(
+        "'len' expects exactly 1 argument but received 2"
+      );
+    });
+
+    it('flags a variadic function called with too few arguments', () => {
+      const diags = getDiagnostics('max()');
+      expect(diags).toHaveLength(1);
+      expect(diags[0].message).toBe(
+        "'max' expects at least 1 argument but received 0"
+      );
+    });
+
+    it('accepts any function in a custom name-only function set', () => {
+      const options = { functions: new Set(['custom']) };
+      expect(getDiagnostics('custom()', options)).toHaveLength(0);
+      expect(getDiagnostics('custom(1, 2, 3)', options)).toHaveLength(0);
+      expect(getDiagnostics('len(@variables.items)', options)).toHaveLength(1);
+    });
+  });
+
+  describe('namespaced function calls', () => {
+    const options = { namespacedFunctions: { a2a: new Set(['task']) } };
+
+    it('does not flag an allowed namespaced function', () => {
+      expect(getDiagnostics('a2a.task(@variables.x)', options)).toHaveLength(0);
+    });
+
+    it('flags an unknown function in a known namespace', () => {
+      const diags = getDiagnostics('a2a.tsk(@variables.x)', options);
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('unknown-function');
+      expect(diags[0].message).toContain("in namespace 'a2a'");
+      expect(diags[0].data).toEqual({ suggestion: 'task' });
+    });
+
+    it('flags an unknown namespace', () => {
+      const diags = getDiagnostics('foo.task(@variables.x)', options);
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('unknown-function');
+      expect(diags[0].message).toContain("'foo' is not a recognized function");
+    });
+  });
+
+  describe('operators', () => {
+    it('does not flag supported operators', () => {
+      expect(getDiagnostics('@variables.a + 1 == 2')).toHaveLength(0);
+    });
+
+    it('flags an operator outside the supported set', () => {
+      const diags = getDiagnostics('@variables.a * 2');
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('unsupported-operator');
+      expect(diags[0].message).toBe("Operator '*' is not supported");
+    });
+
+    it('respects a custom supportedOperators set', () => {
+      const options = { supportedOperators: new Set(['*']) };
+      expect(getDiagnostics('@variables.a * 2', options)).toHaveLength(0);
+      expect(getDiagnostics('@variables.a + 2', options)).toHaveLength(1);
+    });
+  });
+
+  describe('@system_variables.uploaded_files', () => {
+    it('flags a bare reference', () => {
+      const diags = getDiagnostics('@system_variables.uploaded_files');
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('bare-uploaded-files-reference');
+    });
+
+    it('flags a bare bracket-form reference', () => {
+      const diags = getDiagnostics('@system_variables["uploaded_files"]');
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('bare-uploaded-files-reference');
+    });
+
+    it('does not flag an indexed, sliced, or len() reference', () => {
+      expect(
+        getDiagnostics('@system_variables.uploaded_files[0]')
+      ).toHaveLength(0);
+      expect(
+        getDiagnostics('@system_variables.uploaded_files[0:2]')
+      ).toHaveLength(0);
+      expect(
+        getDiagnostics('len(@system_variables.uploaded_files)')
+      ).toHaveLength(0);
+    });
+
+    it('flags a slice on @system_variables itself', () => {
+      const diags = getDiagnostics('@system_variables[0:2]');
+      expect(diags).toHaveLength(1);
+      expect(diags[0].code).toBe('unsupported-slice-target');
+    });
+  });
+});