diff --git a/packages/vipeye-internal/src/analyzer/checker.ts b/packages/vipeye-internal/src/analyzer/checker.ts
index d48caaab..36fd9d6e 100644
--- a/packages/vipeye-internal/src/analyzer/checker.ts
+++ b/packages/vipeye-internal/src/analyzer/checker.ts
@@ -36,6 +36,7 @@ import {
     ClassNode,
     ComprehensionIfNode,
     ComprehensionNode,
+    ConstantNode,
     DelNode,
     DictionaryNode,
     ErrorNode,
@@ -89,7 +90,7 @@ import {
 } from '../parser/parseNodes';
 import { ParserOutput } from '../parser/parser';
 import { UnescapeError, UnescapeErrorType, getUnescapedString } from '../parser/stringTokenUtils';
-import { OperatorType, StringTokenFlags, TokenType } from '../parser/tokenizerTypes';
+import { KeywordType, OperatorType, StringTokenFlags, TokenType } from '../parser/tokenizerTypes';
 import { AnalyzerFileInfo } from './analyzerFileInfo';
 import * as AnalyzerNodeInfo from './analyzerNodeInfo';
 import { ConstraintTracker } from './constraintTracker';
@@ -217,6 +218,26 @@ interface TypeVarUsageInfo {
 // functions to be emitted.
 const isPrintCodeComplexityEnabled = false;
 
+// Viper 扩展：extern 函数体允许的语句——只允许 pass 或 None（仅声明原型）。
+function isViperExternAllowedBody(stmt: ParseNode): boolean {
+    if (stmt.nodeType === ParseNodeType.Pass) {
+        return true;
+    }
+    if (stmt.nodeType === ParseNodeType.StatementList) {
+        const smallStatements = (stmt as StatementListNode).d.statements;
+        return smallStatements.every((s) => {
+            if (s.nodeType === ParseNodeType.Pass) {
+                return true;
+            }
+            if (s.nodeType === ParseNodeType.Constant) {
+                return (s as ConstantNode).d.constType === KeywordType.None;
+            }
+            return false;
+        });
+    }
+    return false;
+}
+
 export class Checker extends ParseTreeWalker {
     private readonly _moduleNode: ModuleNode;
     private readonly _fileInfo: AnalyzerFileInfo;
@@ -430,6 +451,22 @@ export class Checker extends ParseTreeWalker {
             );
         }
 
+        // Viper 扩展：extern 声明的函数体必须为 pass 或 None（仅声明原型，不定义实现）。
+        if (
+            node.d.viperModifier === 'extern' &&
+            IsViperDialect(this._fileInfo.executionEnvironment.dialect)
+        ) {
+            const bodyStatements = node.d.suite.d.statements;
+            const invalidStatement = bodyStatements.find((stmt) => !isViperExternAllowedBody(stmt));
+            if (invalidStatement) {
+                this._evaluator.addDiagnostic(
+                    DiagnosticRule.reportGeneralTypeIssues,
+                    LocMessage.viperExternBodyInvalid().format({ name: node.d.name.d.value }),
+                    invalidStatement
+                );
+            }
+        }
+
         if (functionTypeResult) {
             // Track whether we have seen a *args: P.args parameter. Named
             // parameters after this need to be flagged as an error.
@@ -4793,7 +4830,19 @@ export class Checker extends ParseTreeWalker {
                 declaredReturnType !== undefined &&
                 isPureCModifierType(declaredReturnType);
 
-            if (declaredReturnType && !functionNeverReturns && implicitlyReturnsNone && !isPureModifierReturn) {
+            // Viper 扩展：extern 关键字声明的函数仅声明原型（函数体必须为 pass/None），
+            // 不要求返回语句与声明类型匹配。
+            const isViperExternDecl =
+                IsViperDialect(this._fileInfo.executionEnvironment.dialect) &&
+                node.d.viperModifier === 'extern';
+
+            if (
+                declaredReturnType &&
+                !functionNeverReturns &&
+                implicitlyReturnsNone &&
+                !isPureModifierReturn &&
+                !isViperExternDecl
+            ) {
                 if (isNever(declaredReturnType)) {
                     // If the function consists entirely of "...", assume that it's
                     // an abstract method or a protocol method and don't require that
diff --git a/packages/vipeye-internal/src/localization/localize.ts b/packages/vipeye-internal/src/localization/localize.ts
index 6d29b3a8..63fdb856 100644
--- a/packages/vipeye-internal/src/localization/localize.ts
+++ b/packages/vipeye-internal/src/localization/localize.ts
@@ -1180,6 +1180,8 @@ export namespace Localizer {
         export const unexpectedUnindent = () => getRawString('Diagnostic.unexpectedUnindent');
         export const viperModifierRequiresDef = () =>
             new ParameterizedString<{ name: string }>(getRawString('Diagnostic.viperModifierRequiresDef'));
+        export const viperExternBodyInvalid = () =>
+            new ParameterizedString<{ name: string }>(getRawString('Diagnostic.viperExternBodyInvalid'));
         export const unhashableDictKey = () => getRawString('Diagnostic.unhashableDictKey');
         export const unhashableSetEntry = () => getRawString('Diagnostic.unhashableSetEntry');
         export const unionForwardReferenceNotAllowed = () => getRawString('Diagnostic.unionForwardReferenceNotAllowed');
diff --git a/packages/vipeye-internal/src/localization/package.nls.en-us.json b/packages/vipeye-internal/src/localization/package.nls.en-us.json
index c4a71b1b..b9487649 100644
--- a/packages/vipeye-internal/src/localization/package.nls.en-us.json
+++ b/packages/vipeye-internal/src/localization/package.nls.en-us.json
@@ -1619,6 +1619,7 @@
         "unexpectedIndent": "Unexpected indentation",
         "unexpectedUnindent": "Unindent not expected",
         "viperModifierRequiresDef": "The modifier \"{name}\" must be used with 'def'",
+        "viperExternBodyInvalid": "The extern function \"{name}\" must have a body of only 'pass' or 'None'",
         "unhashableDictKey": "Dictionary key must be hashable",
         "unhashableSetEntry": {
             "message": "Set entry must be hashable",
diff --git a/packages/vipeye-internal/src/localization/package.nls.zh-cn.json b/packages/vipeye-internal/src/localization/package.nls.zh-cn.json
index aa9ac1d7..a64690d7 100644
--- a/packages/vipeye-internal/src/localization/package.nls.zh-cn.json
+++ b/packages/vipeye-internal/src/localization/package.nls.zh-cn.json
@@ -617,6 +617,7 @@
         "unexpectedIndent": "意外缩进",
         "unexpectedUnindent": "不应取消缩进",
         "viperModifierRequiresDef": "修饰关键字 \"{name}\" 必须与 def 搭配使用",
+        "viperExternBodyInvalid": "extern 函数 \"{name}\" 的函数体必须为 pass 或 None",
         "unhashableDictKey": "字典密钥必须可哈希",
         "unhashableSetEntry": "Set 条目必须是可哈希的",
         "uninitializedAbstractVariables": "抽象基类中定义的变量未在 final 类 \"{classType}\" 中初始化",
diff --git a/packages/vipeye-internal/src/parser/parseNodes.ts b/packages/vipeye-internal/src/parser/parseNodes.ts
index b3d79d9a..45acfe1a 100644
--- a/packages/vipeye-internal/src/parser/parseNodes.ts
+++ b/packages/vipeye-internal/src/parser/parseNodes.ts
@@ -471,11 +471,19 @@ export interface FunctionNode extends ParseNodeBase<ParseNodeType.Function> {
         returnAnnotation: ExpressionNode | undefined;
         funcAnnotationComment: FunctionAnnotationNode | undefined;
         suite: SuiteNode;
+        // Viper 扩展：函数声明前的 C 修饰关键字（inline/export/extern），用于语义校验。
+        viperModifier: string | undefined;
     };
 }
 
 export namespace FunctionNode {
-    export function create(defToken: Token, name: NameNode, suite: SuiteNode, typeParams?: TypeParameterListNode) {
+    export function create(
+        defToken: Token,
+        name: NameNode,
+        suite: SuiteNode,
+        typeParams?: TypeParameterListNode,
+        viperModifier?: string
+    ) {
         const node: FunctionNode = {
             start: defToken.start,
             length: defToken.length,
@@ -493,6 +501,7 @@ export namespace FunctionNode {
                 returnAnnotation: undefined,
                 funcAnnotationComment: undefined,
                 suite: suite,
+                viperModifier,
             },
         };
 
diff --git a/packages/vipeye-internal/src/parser/parser.ts b/packages/vipeye-internal/src/parser/parser.ts
index f8aea30b..bf2c9252 100644
--- a/packages/vipeye-internal/src/parser/parser.ts
+++ b/packages/vipeye-internal/src/parser/parser.ts
@@ -430,23 +430,22 @@ export class Parser {
 
         switch (this._peekKeywordType()) {
             case KeywordType.ViperModifier: {
-                // Viper 扩展：inline/export/extern 必须与 def 搭配使用。
+                // Viper 扩展：inline/export/extern 必须与 def（或其别名）搭配使用。
                 // 与 def 搭配时跳过修饰关键字并正常解析函数；单独使用时报错。
                 const modifierToken = this._peekToken() as KeywordToken;
+                const modifierText = this._fileContents!.slice(
+                    modifierToken.start,
+                    modifierToken.start + modifierToken.length
+                );
                 if (
                     this._peekToken(1).type === TokenType.Keyword &&
                     (this._peekToken(1) as KeywordToken).keywordType === KeywordType.Def
                 ) {
                     this._getNextToken();
-                    return this._parseFunctionDef();
+                    return this._parseFunctionDef(/* asyncToken */ undefined, /* decorators */ undefined, modifierText);
                 }
                 this._addSyntaxError(
-                    LocMessage.viperModifierRequiresDef().format({
-                        name: this._fileContents!.slice(
-                            modifierToken.start,
-                            modifierToken.start + modifierToken.length
-                        ),
-                    }),
+                    LocMessage.viperModifierRequiresDef().format({ name: modifierText }),
                     modifierToken
                 );
                 this._getNextToken();
@@ -2050,7 +2049,11 @@ export class Parser {
 
     // funcdef: 'def' NAME parameters ['->' test] ':' suite
     // parameters: '(' [typedargslist] ')'
-    private _parseFunctionDef(asyncToken?: KeywordToken, decorators?: DecoratorNode[]): FunctionNode | ErrorNode {
+    private _parseFunctionDef(
+        asyncToken?: KeywordToken,
+        decorators?: DecoratorNode[],
+        viperModifier?: string
+    ): FunctionNode | ErrorNode {
         const defToken = this._getKeywordToken(KeywordType.Def);
 
         const nameToken = this._getTokenIfIdentifier();
@@ -2119,6 +2122,7 @@ export class Parser {
         this._isInFinallyLoop = wasInFinallyLoop;
 
         const functionNode = FunctionNode.create(defToken, NameNode.create(nameToken), suite, typeParameters);
+        functionNode.d.viperModifier = viperModifier;
         if (asyncToken) {
             functionNode.d.isAsync = true;
             extendRange(functionNode, asyncToken);
diff --git a/packages/vipeye-internal/src/parser/tokenizer.ts b/packages/vipeye-internal/src/parser/tokenizer.ts
index 077adfcf..c4c09030 100644
--- a/packages/vipeye-internal/src/parser/tokenizer.ts
+++ b/packages/vipeye-internal/src/parser/tokenizer.ts
@@ -644,35 +644,44 @@ enum MagicsKind {
 // Viper 扩展：C 风格修饰/声明关键字处理。
 // Viper 支持将 C 修饰/声明语义写成语句前缀关键字：
 //   inline/export/extern def f(...)         —— 修饰关键字，必须与 def 搭配
+//   fn/fun/func/function f(...)             —— def 的别名
 //   enum/rnum class|struct Foo              —— 等价 t.CEnum / t.REnum
 //   enum Foo: / rnum Foo:                   —— 单独使用（等价 class Foo:）
 //   struct Foo:                             —— 独立的 struct 关键字（等价 class）
 // 返回 'skip'：跳过该关键字 token（后续 def/class 正常解析，位置与缩进不变）；
 // 返回 'class'：把 enum/rnum/struct 当作 class 关键字；
-// 返回 'viperModifier'：inline/export/extern 单独使用（未与 def 搭配），由 parser 报错；
+// 返回 'def'：把 fn/fun/func/function 当作 def 关键字；
+// 返回 'viperModifier'：inline/export/extern 生成修饰关键字 token，由 parser
+//   判断是否与 def 搭配（搭配则跳过并记录，单独使用则报错）；
 // undefined：按普通标识符处理（变量名、成员名、导入名、`state:` 注解等）。
 function getViperStatementKeywordAction(
     text: string,
     identifierStart: number,
     identifierEnd: number,
-    name: string
-): 'skip' | 'class' | 'viperModifier' | undefined {
+    name: string,
+    precededByViperModifier: boolean
+): 'skip' | 'class' | 'def' | 'viperModifier' | undefined {
     const nextWord = peekViperKeywordWord(text, identifierEnd);
     if (name === 'inline' || name === 'export' || name === 'extern') {
         // 修饰关键字仅在语句起始处生效，避免误伤 `self.inline` 等成员名。
         if (!isViperAtStatementStart(text, identifierStart)) {
             return undefined;
         }
-        if (nextWord === 'def') {
-            return 'skip';
-        }
         // 后跟 ':'（注解，如 `state: t.CArray[...]` 风格的字段声明）按普通标识符处理。
         if (isViperNextCharColon(text, identifierEnd)) {
             return undefined;
         }
-        // 单独使用（未与 def 搭配）→ 关键字 token，由 parser 报错。
+        // 一律生成修饰关键字 token，由 parser 判断是否与 def 搭配。
         return 'viperModifier';
     }
+    if (name === 'fn' || name === 'fun' || name === 'func' || name === 'function') {
+        // def 别名：语句起始（或紧跟在修饰关键字后，如 `inline fn f()`）且后跟
+        // 标识符（函数名）时按 def 关键字处理。
+        if (!isViperAtStatementStart(text, identifierStart) && !precededByViperModifier) {
+            return undefined;
+        }
+        return nextWord.length > 0 && isIdentifierStartChar(nextWord.charCodeAt(0)) ? 'def' : undefined;
+    }
     if (name === 'enum' || name === 'rnum') {
         if (nextWord === 'class' || nextWord === 'struct') {
             return 'skip';
@@ -1468,12 +1477,20 @@ export class Tokenizer {
             } else {
                 const value = this._internIdentifier(text, start, end, length);
                 // Viper 扩展：C 风格修饰/声明关键字。
-                // inline/export/extern 作为 def 前缀修饰、enum/rnum 作为 class/struct
-                // 前缀时跳过该 token（保留原始位置与缩进；着色由 TextMate 规则处理）；
+                // inline/export/extern 生成修饰关键字 token（parser 判断是否配 def）；
                 // enum/rnum/struct 在语句起始且后跟类名时按 class 关键字处理；
-                // inline/export/extern 单独使用（未与 def 搭配）时生成修饰关键字 token，
-                // 由 parser 报错。
-                const viperAction = getViperStatementKeywordAction(text, start, end, value);
+                // fn/fun/func/function 在语句起始（或紧跟修饰关键字）时按 def 关键字处理。
+                const precededByViperModifier =
+                    this._tokens.length > 0 &&
+                    this._tokens[this._tokens.length - 1].type === TokenType.Keyword &&
+                    (this._tokens[this._tokens.length - 1] as KeywordToken).keywordType === KeywordType.ViperModifier;
+                const viperAction = getViperStatementKeywordAction(
+                    text,
+                    start,
+                    end,
+                    value,
+                    precededByViperModifier
+                );
                 if (viperAction === 'skip') {
                     return true;
                 }
@@ -1481,6 +1498,10 @@ export class Tokenizer {
                     this._tokens.push(KeywordToken.create(start, length, KeywordType.Class, this._getComments()));
                     return true;
                 }
+                if (viperAction === 'def') {
+                    this._tokens.push(KeywordToken.create(start, length, KeywordType.Def, this._getComments()));
+                    return true;
+                }
                 if (viperAction === 'viperModifier') {
                     this._tokens.push(
                         KeywordToken.create(start, length, KeywordType.ViperModifier, this._getComments())
diff --git a/packages/vipeye-internal/src/tests/viperGenericTypeVarCall.test.ts b/packages/vipeye-internal/src/tests/viperGenericTypeVarCall.test.ts
index 6c9448dc..86906f9e 100644
--- a/packages/vipeye-internal/src/tests/viperGenericTypeVarCall.test.ts
+++ b/packages/vipeye-internal/src/tests/viperGenericTypeVarCall.test.ts
@@ -340,7 +340,7 @@ export def double_export(x: t.CInt) -> t.CInt:
     return x * 2
 
 extern def double_extern(x: t.CInt) -> t.CInt:
-    return x * 2
+    pass
 
 # state 不是关键字，仍可作字段名/成员名
 state: t.CInt = 0
@@ -471,4 +471,89 @@ inline def ok(x: t.CInt) -> t.CInt:
         fs.rmSync(testDir, { recursive: true, force: true });
     }
     serviceProvider.dispose();
+});
+
+test('Viper def 别名 fn/fun/func/function 与 extern 函数体校验', () => {
+    const tempFile = new RealTempFile();
+    const serviceProvider = createServiceProvider(createFromRealFileSystem(tempFile), new NullConsole(), tempFile);
+    const projectRoot = UriEx.file(TRANSPYC_INCLUDES);
+    const configOptions = new ConfigOptions(projectRoot);
+    configOptions.internalTestMode = true;
+    configOptions.initializeTypeCheckingMode('standard');
+
+    const execEnv = new ExecutionEnvironment(
+        'python',
+        projectRoot,
+        configOptions.diagnosticRuleSet,
+        configOptions.defaultPythonVersion,
+        configOptions.defaultPythonPlatform,
+        [projectRoot],
+        /* skipNativeLibraries */ false,
+        /* defaultDialect */ 'viper'
+    );
+    configOptions.executionEnvironments = [execEnv];
+
+    const code = `import t
+
+# def 别名
+fn f1(x: t.CInt) -> t.CInt:
+    return x
+
+fun f2(x: t.CInt) -> t.CInt:
+    return x
+
+func f3(x: t.CInt) -> t.CInt:
+    return x
+
+function f4(x: t.CInt) -> t.CInt:
+    return x
+
+# extern：函数体必须为 pass 或 None
+extern def e_ok1(x: t.CInt) -> t.CInt:
+    pass
+
+extern def e_ok2(x: t.CInt) -> t.CInt:
+    None
+
+extern def e_bad(x: t.CInt) -> t.CInt:
+    return x
+
+def use():
+    a: t.CInt = f1(1) + f2(2) + f3(3) + f4(4)
+`;
+    const testDir = path.join(__dirname, '__viper_alias_extern_test__');
+    const fs = require('fs');
+    fs.mkdirSync(testDir, { recursive: true });
+    const testFile = path.join(testDir, 'alias_extern.vp');
+    fs.writeFileSync(testFile, code, 'utf8');
+
+    try {
+        const importResolver = new ImportResolver(serviceProvider, configOptions, new FullAccessHost(serviceProvider));
+        const program = new Program(importResolver, configOptions, serviceProvider);
+        const uri = UriEx.file(testFile);
+        program.setTrackedFiles([uri]);
+        while (program.analyze()) {
+            // 循环
+        }
+
+        const diagnostics = getDiagnostics(program, configOptions, uri);
+        console.log('=== alias_extern.vp diagnostics ===');
+        diagnostics.forEach((d) => console.log(`  ${d}`));
+
+        // 期望恰好 1 个 extern 函数体错误（e_bad），fn/fun/func/function 别名与
+        // extern pass/None 函数体都不应报错。
+        const externErrors = diagnostics.filter(
+            (d) => d.includes('必须为 pass 或 None') || d.includes("body of only 'pass' or 'None'")
+        );
+        assert.strictEqual(
+            externErrors.length,
+            1,
+            `expected 1 extern body error, got: ${JSON.stringify(diagnostics)}`
+        );
+
+        program.dispose();
+    } finally {
+        fs.rmSync(testDir, { recursive: true, force: true });
+    }
+    serviceProvider.dispose();
 });
\ No newline at end of file
diff --git a/packages/vscode-vipeye/language-configuration.json b/packages/vscode-vipeye/language-configuration.json
index bb59706e..7e67bec8 100644
--- a/packages/vscode-vipeye/language-configuration.json
+++ b/packages/vscode-vipeye/language-configuration.json
@@ -28,7 +28,7 @@
     ],
     "onEnterRules": [
         {
-            "beforeText": "^\\s*(?:(?:inline|export|extern|enum|rnum)\\s+)?(?:def|class|for|if|elif|else|while|try|finally|with|except|async|struct)\\b.*?:\\s*$",
+            "beforeText": "^\\s*(?:(?:inline|export|extern|enum|rnum)\\s+)?(?:def|fn|fun|func|function|class|for|if|elif|else|while|try|finally|with|except|async|struct)\\b.*?:\\s*$",
             "action": { "indent": "indent" }
         },
         {
@@ -45,7 +45,7 @@
     },
     "wordPattern": "(?x)\n  \\b\\w*\\b  \n  |  (?<=\\.)\\w+\n  | (?<=@)\\w+\n  | \\p{Lu}[\\p{Ll}_\\p{Lu}\\p{Nd}]*\n  | [[:alpha:]_][[:alpha:]_\\p{Nd}]*\n  | \\p{Lu}[\\p{Ll}_\\p{Lu}\\p{Nd}]*f?\\b",
     "indentationRules": {
-        "increaseIndentPattern": "^\\s*(?:(?:inline|export|extern|enum|rnum)\\s+)?(?:class|def|elif|else|except|finally|for|if|try|while|with|struct|async\\s+def|async\\s+for|async\\s+with)\\b.*?:\\s*$",
+        "increaseIndentPattern": "^\\s*(?:(?:inline|export|extern|enum|rnum)\\s+)?(?:class|def|fn|fun|func|function|elif|else|except|finally|for|if|try|while|with|struct|async\\s+def|async\\s+for|async\\s+with)\\b.*?:\\s*$",
         "decreaseIndentPattern": "^\\s*(elif|else|except|finally)\\b.*?:\\s*$"
     }
 }
diff --git a/packages/vscode-vipeye/syntaxes/viper.tmLanguage.json b/packages/vscode-vipeye/syntaxes/viper.tmLanguage.json
index 186cb352..37994255 100644
--- a/packages/vscode-vipeye/syntaxes/viper.tmLanguage.json
+++ b/packages/vscode-vipeye/syntaxes/viper.tmLanguage.json
@@ -37,6 +37,10 @@
                     "name": "storage.type.function.viper",
                     "match": "\\bdef\\b"
                 },
+                {
+                    "name": "storage.type.function.viper",
+                    "match": "\\b(fn|fun|func|function)\\b(?=\\s+[A-Za-z_][A-Za-z0-9_]*)"
+                },
                 {
                     "name": "storage.type.class.viper",
                     "match": "\\bclass\\b"