feat: add support for optional custom error messages by using ajv-errors (#44)

Author: gfreitash

README.md

@@ -34,6 +34,57 @@ jobs:
           files: .github/workflows/**.yml
 ```
 
+### Example with Custom Error Messages
+
+When `custom-errors` is enabled, you can use the `errorMessage` keyword in your
+JSON schema to provide more user-friendly error messages. This is powered by the
+[ajv-errors](https://github.com/ajv-validator/ajv-errors) library.
+
+```yaml
+jobs:
+  validate-config:
+    name: Validate configuration files
+    runs-on: ubuntu-latest
+    steps:
+      - name: Validate config with custom errors
+        uses: dsanders11/[email protected]
+        with:
+          schema: ./config.schema.json
+          files: ./config/*.yml
+          custom-errors: true
+```
+
+Example schema with custom error messages:
+
+```json
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "type": "object",
+  "properties": {
+    "name": {
+      "type": "string",
+      "minLength": 1
+    },
+    "version": {
+      "type": "string",
+      "pattern": "^\\d+\\.\\d+\\.\\d+$"
+    }
+  },
+  "required": ["name", "version"],
+  "errorMessage": {
+    "type": "Configuration must be an object",
+    "required": {
+      "name": "Configuration must have a 'name' property",
+      "version": "Configuration must have a 'version' property"
+    },
+    "properties": {
+      "name": "Name must be a non-empty string",
+      "version": "Version must follow semantic versioning (e.g., '1.0.0')"
+    }
+  }
+}
+```
+
 ### Validating Schema
 
 Schemas can be validated by setting the `schema` input to the string literal
@@ -56,6 +107,8 @@ simply set a URL fragment (e.g. `#bust-cache`) on the schema URL.
   `true`)
 - `all-errors` - Whether to report all errors or stop after the first (default:
   `false`)
+- `custom-errors` - Enable support for custom error messages using ajv-errors
+  (default: `false`)
 
 ### Outputs
 

__tests__/main.test.ts

@@ -595,4 +595,106 @@ describe('action', () => {
       );
     });
   });
+
+  describe('custom error messages', () => {
+    it('forces allErrors to true when custom-errors is enabled', async () => {
+      mockGetBooleanInput({ 'custom-errors': true, 'all-errors': false });
+      mockGetInput({ schema });
+      mockGetMultilineInput({ files });
+
+      vi.mocked(fs.readFile)
+        .mockResolvedValueOnce(schemaContents)
+        .mockResolvedValueOnce('invalid content');
+      mockGlobGenerator(['/foo/bar/baz/config.yml']);
+
+      await main.run();
+      expect(runSpy).toHaveReturned();
+      expect(process.exitCode).not.toBeDefined();
+
+      // Should report multiple errors even though all-errors was false
+      expect(core.error).toHaveBeenCalledTimes(4);
+      expect(core.setOutput).toHaveBeenCalledTimes(1);
+      expect(core.setOutput).toHaveBeenLastCalledWith('valid', false);
+    });
+
+    it('provides custom error messages when validation fails', async () => {
+      mockGetBooleanInput({ 'custom-errors': true, 'fail-on-invalid': true });
+      mockGetInput({ schema });
+      mockGetMultilineInput({ files });
+
+      // Create a schema with custom error messages
+      const customErrorSchemaContents = JSON.stringify({
+        title: 'Test schema with custom errors',
+        $schema: 'http://json-schema.org/draft-07/schema#',
+        type: 'object',
+        properties: {
+          name: { type: 'string', minLength: 1 }
+        },
+        required: ['name'],
+        errorMessage: {
+          properties: {
+            name: 'Name must be a non-empty string'
+          }
+        }
+      });
+
+      const invalidInstanceContents = JSON.stringify({ name: '' });
+
+      vi.mocked(fs.readFile)
+        .mockResolvedValueOnce(customErrorSchemaContents)
+        .mockResolvedValueOnce(invalidInstanceContents);
+      mockGlobGenerator(['/foo/bar/baz/config.yml']);
+
+      await main.run();
+      expect(runSpy).toHaveReturned();
+      expect(process.exitCode).toEqual(1);
+
+      expect(core.error).toHaveBeenCalledWith(
+        'Error while validating file: /foo/bar/baz/config.yml'
+      );
+
+      // Check that we get our custom error message in the JSON output
+      const errorCalls = vi.mocked(core.error).mock.calls;
+      const hasCustomMessage = errorCalls.some(
+        call =>
+          typeof call[0] === 'string' &&
+          call[0].includes('Name must be a non-empty string')
+      );
+      expect(hasCustomMessage).toBe(true);
+
+      expect(core.setOutput).toHaveBeenCalledTimes(1);
+      expect(core.setOutput).toHaveBeenLastCalledWith('valid', false);
+    });
+
+    it('works without custom-errors when disabled', async () => {
+      mockGetBooleanInput({ 'custom-errors': false, 'fail-on-invalid': false });
+      mockGetInput({ schema });
+      mockGetMultilineInput({ files });
+
+      vi.mocked(fs.readFile)
+        .mockResolvedValueOnce(schemaContents)
+        .mockResolvedValueOnce('invalid content');
+      mockGlobGenerator(['/foo/bar/baz/config.yml']);
+
+      await main.run();
+      expect(runSpy).toHaveReturned();
+      expect(process.exitCode).not.toBeDefined();
+
+      expect(core.error).toHaveBeenCalledWith(
+        'Error while validating file: /foo/bar/baz/config.yml'
+      );
+
+      // Should NOT have any custom error messages (no errorMessage keyword)
+      const errorCalls = vi.mocked(core.error).mock.calls;
+      const hasCustomErrors = errorCalls.some(
+        call =>
+          typeof call[0] === 'string' &&
+          call[0].includes('"keyword":"errorMessage"')
+      );
+      expect(hasCustomErrors).toBe(false);
+
+      expect(core.setOutput).toHaveBeenCalledTimes(1);
+      expect(core.setOutput).toHaveBeenLastCalledWith('valid', false);
+    });
+  });
 });

action.yml

@@ -25,6 +25,10 @@ inputs:
     description: Report all errors instead of stopping at the first
     required: false
     default: false
+  custom-errors:
+    description: Enable support for custom error messages using ajv-errors
+    required: false
+    default: false
 
 outputs:
   valid: