diff --git a/MISSION-FAZ3.md b/MISSION-FAZ3.md
new file mode 100644
index 0000000..40bb6cd
--- /dev/null
+++ b/MISSION-FAZ3.md
@@ -0,0 +1,326 @@
+# MISSION — Faz 3: Semantik Analiz (Gitea #72)
+
+> **Bu dosya kendi kendine yeter.** "MISSION-FAZ3'ü oku ve yap" denince
+> doğrudan uygula. Bitince DOĞRULAMA bölümünü çalıştır, #72'ye tamamlanma
+> yorumu düş, bu dosyayı sil veya MISSION-FAZ4.md ile değiştir.
+> `Co-Authored-By: Claude Opus 4.8` trailer'ı unutma.
+
+---
+
+## 0) HEDEF & BAŞARI KRİTERLERİ (#72)
+
+İki modül: **TypeChecker** (her ExpressionNode'a tip ata) +
+**StructuralValidator** (kontrol akışı kuralları).
+
+Bitti tanımı:
+1. `build/saqut check file:examples/fibonacci.sqt` → 0 hata 0 uyarı; her
+   expression node'unun `resolvedType` alanı JSON'da dolu.
+2. `examples/semantic/` test fixture'ları doğru hata/uyarı üretiyor.
+3. `-Wall -Wextra` temiz. Faz 0/1/2 testleri hâlâ geçer.
+
+---
+
+## 1) DÖNÜŞÜM KURALLARI (ADR-010 özeti)
+
+| Durum | Sonuç | Kod |
+|---|---|---|
+| `float x = 1` | Geçerli — literal 1.0 sayılır | — |
+| `double x = 1` | Geçerli | — |
+| `float x = intVar` | **Uyarı** — örtük genişletme, veri kaybı yok | W004 |
+| `double x = intVar` | **Uyarı** | W004 |
+| `double x = floatVar` | **Uyarı** | W004 |
+| `int x = 1.5` | **Hata** — literal'den veri kaybı | E003 |
+| `int x = floatVar` | **Hata** — daraltma | E003 |
+| `float x = doubleVar` | **Hata** — daraltma | E003 |
+| `int x = intVar` | Geçerli | — |
+
+**Kural özeti:**
+- Literal bağlama-göre tiplenir: hedef tip daha geniş veya aynıysa uyarısız geçer.
+- Değişken→değişken: genişletme (widening) = W004, daraltma (narrowing) = E003.
+- Aynı tip = her zaman geçerli.
+
+**Yardımcı fonksiyon (type_checker içinde):**
+```cpp
+enum class ConvResult { Ok, Warn_W004, Error_E003 };
+ConvResult checkAssignCompat(const Type& target, const Type& src, bool srcIsLiteral);
+```
+
+---
+
+## 2) KATALOG GÜNCELLEMESİ
+
+`src/diagnostic/diagnostic.hpp` → W004 ekle:
+```cpp
+{"W004", DiagLevel::Warning, "Örtük sayısal genişletme (widening)"},
+```
+
+---
+
+## 3) DOSYA DOSYA PLAN
+
+Yeni dizin: `src/semantic/`. CMake GLOB_RECURSE → yeni `.cpp`'ler otomatik
+alınır ama `build.sh` ile derle (cmake yeniden config eder).
+
+---
+
+### 3.1 `src/semantic/type_checker.hpp`
+
+```cpp
+#ifndef SAQUT_SEMANTIC_TYPE_CHECKER
+#define SAQUT_SEMANTIC_TYPE_CHECKER
+
+#include "symbol/symbol_table.hpp"
+#include "diagnostic/diagnostic_engine.hpp"
+#include "parser/ast_node.hpp"
+#include "core/type.hpp"
+
+class TypeChecker {
+public:
+    TypeChecker(SymbolTable& table, DiagnosticEngine& diag)
+        : table_(table), diag_(diag) {}
+
+    void check(ASTNode* program);
+
+private:
+    // İfadeyi gez, resolvedType ata, tipi döndür.
+    // expectedType: bağlam tipi (literal genişletme için); boş = bilinmiyor.
+    Type checkExpr(ASTNode* node, const Type& expected = Type::error());
+
+    // Deyimi gez (statement'lar tip döndürmez).
+    void checkStmt(ASTNode* node);
+
+    // Fonksiyon gövdesini gez; dönüş tipi bağlamını stack'te tut.
+    void checkFunction(ASTNode* fnNode);
+
+    // Atama uyumunu denetle; uyarı/hata raporla. true = geçerli.
+    bool checkAssign(const Type& target, const Type& src,
+                     bool srcIsLiteral, const SourceLocation& loc,
+                     const std::string& context);
+
+    SymbolTable&      table_;
+    DiagnosticEngine& diag_;
+
+    // Aktif fonksiyonun beklenen dönüş tipi (iç içe fonksiyon yok, stack
+    // yerine tek değer yeterli).
+    Type currentReturnType_;
+    bool inFunction_ = false;
+};
+
+#endif // SAQUT_SEMANTIC_TYPE_CHECKER
+```
+
+---
+
+### 3.2 `src/semantic/type_checker.cpp`
+
+`check(program)`:
+- Program children'ı gez: FunctionDecl → `checkFunction`, VariableDecl →
+  `checkStmt`, StructDecl → atla.
+
+`checkFunction(fnNode)`:
+- `inFunction_ = true; currentReturnType_ = typeFromReturnType(fn->returnType);`
+- `checkStmt(body)` (body = children[0])
+- `inFunction_ = false;`
+
+`checkStmt(node)` — switch(node->kind):
+- **Block**: her child → `checkStmt`.
+- **VariableDecl**: initExpr varsa `checkExpr(initExpr, typeFromName(vd->varType))`;
+  sonucu target tiple `checkAssign` ile denetle.
+- **ExpressionStatement**: `checkExpr(es->expression)`.
+- **ReturnStatement**: value varsa `checkExpr(value, currentReturnType_)`;
+  sonucu `checkAssign(currentReturnType_, valueType, isLiteral, loc, "return")`.
+  Değer yok ve returnType != void → E006.
+- **IfStatement**: condition checkExpr (bool beklenir, ama şimdilik herhangi
+  sayısal tip de geçerli — E003 değil, TODO Faz3+); then/else checkStmt.
+- **WhileStatement / DoWhileStatement**: condition checkExpr; body checkStmt.
+- **ForStatement**: init checkStmt; condition checkExpr; update checkExpr; body checkStmt.
+- **Break / Continue / ExpressionStatement**: içerik checkExpr.
+- **ReturnStatement**: yukarıda.
+
+`checkExpr(node, expected)` — switch(node->kind) → Type döndürür:
+- **Literal(INTEGER)**:
+  - expected sayısal ve daha geniş veya aynıysa → `expected` tipini döndür
+    (literal 1.0 olarak tiplenir, uyarısız).
+  - expected int veya error → `Type::Int()`.
+  - expected float literal'e uymuyor (ör. expected=int, literal=FLOAT) → E003.
+- **Literal(FLOAT)**: expected double ise double döndür (kayıpsız); expected int → E003;
+  aksi → `Type::Float()`.
+- **Literal(BOOLEAN)**: `Type::Bool()`.
+- **Literal(STRING)**: `Type::String()`.
+- **Literal(BOŞ)**: `Type::error()` (null — Faz 3+ kapsam dışı).
+- **Identifier**: `id->resolvedSymbol ? id->resolvedSymbol->type : Type::error()`.
+  Type::error() ise E001 zaten Faz 2'de raporlandı, sessiz geç.
+- **BinaryExpression**:
+  - Atama operatörü (EQUAL, +=, -=, vb.):
+    - left = `checkExpr(left)`, right = `checkExpr(right, leftType)`.
+    - `checkAssign(leftType, rightType, isLiteralRight, loc, "atama")`.
+    - `resolvedType = leftType`.
+  - Aritmetik (+, -, *, /, %):
+    - Her iki operandı `checkExpr` et.
+    - İkisi de sayısal ve aynı tip → sonuç o tip.
+    - İkisi sayısal farklı tip → `checkAssign` mantığıyla uyar/hata; sonuç
+      daha geniş tip (veya E003 durumunda error).
+    - Sayısal değil → E003, `Type::error()`.
+  - Karşılaştırma (==, !=, <, <=, >, >=):
+    - Operandları checkExpr; uyumlu sayısal veya aynı tip → `Type::Bool()`.
+    - Uyumsuz → E003, `Type::Bool()` (hata yayılmasın, bool olarak devam).
+  - Mantıksal (&&, ||):
+    - Her iki operand bool beklenir; değilse E003.
+    - Sonuç `Type::Bool()`.
+  - Unary prefix (-, +, !):
+    - Left = nullptr; Right = operand.
+    - `-`/`+`: sayısal beklenir; `!`: bool beklenir.
+  - resolvedType'a sonucu ata.
+- **Call**:
+  - callee = `checkExpr(callee)` → Function tipi beklenir.
+  - Argüman sayısı imzayla eşleşmeli → E008.
+  - Her argüman: `checkExpr(arg, paramType[i])` → `checkAssign(paramType, argType, ...)`.
+  - resolvedType = callee tipi's `returnType`.
+- **Postfix (++/--)**: operand sayısal → sonuç aynı tip; değilse E003.
+- **MemberAccess**: `Type::error()` + TODO (struct alan çözümü Faz 3+).
+- **IndexExpression**: TODO; `Type::error()`.
+
+`checkAssign(target, src, srcIsLiteral, loc, ctx)`:
+- target veya src = error → `return true` (sessiz geç, hata zaten raporlandı).
+- target == src → `return true`.
+- Genişletme (widening):
+  - `int → float`: srcIsLiteral → geçerli (**uyarısız**); değil → **W004**.
+  - `int → double`, `float → double`: aynı kural.
+  - srcIsLiteral ve kayıpsız → `return true`.
+  - srcIsLiteral değil → W004 raporla, `return true` (yine de geçerli).
+- Daraltma (narrowing):
+  - `float → int`, `double → int`, `double → float` → **E003**, `return false`.
+- Tamamen farklı tipler (int ↔ string, vb.) → **E003**, `return false`.
+
+> `srcIsLiteral`: `node->kind == ASTKind::Literal` ise true.
+
+---
+
+### 3.3 `src/semantic/structural_validator.hpp`
+
+```cpp
+#ifndef SAQUT_SEMANTIC_STRUCTURAL_VALIDATOR
+#define SAQUT_SEMANTIC_STRUCTURAL_VALIDATOR
+
+#include "diagnostic/diagnostic_engine.hpp"
+#include "parser/ast_node.hpp"
+
+class StructuralValidator {
+public:
+    StructuralValidator(DiagnosticEngine& diag) : diag_(diag) {}
+    void validate(ASTNode* program);
+
+private:
+    void walkDecl(ASTNode* node);
+    void walkStmt(ASTNode* node);
+
+    DiagnosticEngine& diag_;
+    int  loopDepth_  = 0;
+    bool inFunction_ = false;
+};
+
+#endif // SAQUT_SEMANTIC_STRUCTURAL_VALIDATOR
+```
+
+---
+
+### 3.4 `src/semantic/structural_validator.cpp`
+
+`validate(program)`: her top-level child → `walkDecl`.
+
+`walkDecl`:
+- FunctionDecl: `inFunction_=true`, `walkStmt(body)`, `inFunction_=false`.
+- VariableDecl / StructDecl: atla.
+
+`walkStmt(node)` — switch:
+- **Block**: her child `walkStmt`.
+- **IfStatement**: then/else `walkStmt`.
+- **WhileStatement / DoWhileStatement**: `loopDepth_++; walkStmt(body); loopDepth_--;`
+- **ForStatement**: `loopDepth_++; walkStmt(init?); walkStmt(body); loopDepth_--;`
+- **BreakStatement**: `loopDepth_ == 0` → E004.
+- **ContinueStatement**: `loopDepth_ == 0` → E004.
+- **ReturnStatement**: `!inFunction_` → E005.
+  (Return tip uyumu TypeChecker'ın işi; burada sadece yapısal kural.)
+- **ExpressionStatement / VariableDecl**: `walkStmt` çocukları için yinele.
+- Diğer: atla.
+
+---
+
+## 4) CLI — `saqut check` KOMUTU
+
+### 4.1 `src/cli/commands/check.hpp`
+
+```cpp
+inline int cmdCheck(const CliArgs& args) {
+    // tokenize → parse → symbolCollect → typeCheck → structValidate
+    // Tüm hatalar DiagnosticEngine'de toplanır.
+    // Çıktı: JSON (--compact destekli)
+    // {"file":"...","diagnostics":{...}}
+    // exit code: hasErrors() ? 1 : 0
+}
+```
+
+### 4.2 `src/cli/cli.hpp` veya `main.cpp`
+
+`check` komutu kaydet; `parseArgs` tanımlı komutlar listesine ekle.
+
+---
+
+## 5) TEST FIXTURE'LARI — `examples/semantic/`
+
+| Dosya | İçerik | Beklenen |
+|---|---|---|
+| `widening.sqt` | `float x = 1;` (OK) + `float y = someInt;` | W004 |
+| `narrowing.sqt` | `int x = 1.5;` | E003 |
+| `bad_return.sqt` | `int foo() { return 1.5; }` | E003 |
+| `break_outside.sqt` | top-level `break;` | E004 |
+| `return_outside.sqt` | top-level `return 0;` | E005 |
+| `bad_args.sqt` | `fibonacci(1, 2)` (1 parametre bekliyor) | E008 |
+
+---
+
+## 6) DOĞRULAMA KOMUTLARI
+
+```bash
+bash scripts/build.sh
+
+# Başarı: 0 hata 0 uyarı
+build/saqut check file:examples/fibonacci.sqt
+
+# Her expression tipli mi?
+build/saqut ast file:examples/fibonacci.sqt | python3 -c "
+import json,sys
+def walk(n):
+    if 'resolvedType' in n and n['resolvedType'] is None:
+        print('UNTIPPED:', n.get('kind'), n.get('name',''))
+    for v in n.values():
+        if isinstance(v, dict): walk(v)
+        elif isinstance(v, list): [walk(i) for i in v if isinstance(i, dict)]
+walk(json.load(sys.stdin))"
+
+# Hata fixture'ları
+build/saqut check file:examples/semantic/widening.sqt      # W004
+build/saqut check file:examples/semantic/narrowing.sqt     # E003
+build/saqut check file:examples/semantic/break_outside.sqt # E004
+build/saqut check file:examples/semantic/bad_args.sqt      # E008
+
+# Regresyon
+bash tests/run.sh
+build/saqut ast file:examples/fibonacci.sqt | python3 -m json.tool >/dev/null && echo AST-OK
+```
+
+---
+
+## 7) HAFIZA İPUÇLARI
+
+- `ASTKind::BinaryExpression` → `BinaryExpressionNode{Left, Right, Operator(TokenType)}`
+- Atama operatörleri: `EQUAL, PLUS_EQUAL, MINUS_EQUAL, STAR_EQUAL, SLASH_EQUAL, ...`
+- `ASTKind::Call` → `CallExpressionNode{callee, arguments}`
+- `ASTKind::Literal` → `LiteralNode{literalType(INTEGER/FLOAT/...), parserToken}`
+- `ASTKind::Identifier` → `IdentifierNode{resolvedSymbol}` (Faz 2'de set edildi)
+- `FunctionDeclNode{name, returnType(string), params, children=[body]}`
+- `Type::fromName(str)` primitifi çözer; `Type::function(ret, params)` fonksiyon tipi
+- `diag_.report("E003", loc, "mesaj")` → katalogdan seviye otomatik çözülür
+- `resolvedType` → `ExpressionNode`'un alanı (ast_node.hpp)
+- Literal'ın literal tipi: `((LiteralNode*)node)->literalType`
+- İki sayısal tipin genişliği: int < float < double (basit sıralama yeterli)
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+  INCLUDES = -I/home/saqut/Masaüstü/saqutcompiler/src
+  OBJECT_DIR = CMakeFiles/saqut.dir
+  OBJECT_FILE_DIR = CMakeFiles/saqut.dir/src/semantic
+  TARGET_SUPPORT_DIR = CMakeFiles/saqut.dir
+
+build CMakeFiles/saqut.dir/src/semantic/type_checker.cpp.o: CXX_COMPILER__saqut_unscanned_Debug /home/saqut/Masaüstü/saqutcompiler/src/semantic/type_checker.cpp || cmake_object_order_depends_target_saqut
+  CONFIG = Debug
+  DEP_FILE = CMakeFiles/saqut.dir/src/semantic/type_checker.cpp.o.d
+  FLAGS = -g -std=gnu++20 -Wall -Wextra -g -O0
+  INCLUDES = -I/home/saqut/Masaüstü/saqutcompiler/src
+  OBJECT_DIR = CMakeFiles/saqut.dir
+  OBJECT_FILE_DIR = CMakeFiles/saqut.dir/src/semantic
+  TARGET_SUPPORT_DIR = CMakeFiles/saqut.dir
+
 build CMakeFiles/saqut.dir/src/symbol/symbol_collector.cpp.o: CXX_COMPILER__saqut_unscanned_Debug /home/saqut/Masaüstü/saqutcompiler/src/symbol/symbol_collector.cpp || cmake_object_order_depends_target_saqut
   CONFIG = Debug
   DEP_FILE = CMakeFiles/saqut.dir/src/symbol/symbol_collector.cpp.o.d
@@ -174,7 +192,7 @@ build CMakeFiles/saqut.dir/src/tokenizer/tokenizer.cpp.o: CXX_COMPILER__saqut_un
 #############################################
 # Link the executable saqut
 
-build saqut: CXX_EXECUTABLE_LINKER__saqut_Debug CMakeFiles/saqut.dir/src/core/sourcefile.cpp.o CMakeFiles/saqut.dir/src/lexer/lexer.cpp.o CMakeFiles/saqut.dir/src/main.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/binary_expr.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/declarations.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/expressions.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/identifier.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/literal.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/program.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/statements.cpp.o CMakeFiles/saqut.dir/src/parser/parser.cpp.o CMakeFiles/saqut.dir/src/symbol/symbol_collector.cpp.o CMakeFiles/saqut.dir/src/tokenizer/tokenizer.cpp.o
+build saqut: CXX_EXECUTABLE_LINKER__saqut_Debug CMakeFiles/saqut.dir/src/core/sourcefile.cpp.o CMakeFiles/saqut.dir/src/lexer/lexer.cpp.o CMakeFiles/saqut.dir/src/main.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/binary_expr.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/declarations.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/expressions.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/identifier.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/literal.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/program.cpp.o CMakeFiles/saqut.dir/src/parser/nodes/statements.cpp.o CMakeFiles/saqut.dir/src/parser/parser.cpp.o CMakeFiles/saqut.dir/src/semantic/structural_validator.cpp.o CMakeFiles/saqut.dir/src/semantic/type_checker.cpp.o CMakeFiles/saqut.dir/src/symbol/symbol_collector.cpp.o CMakeFiles/saqut.dir/src/tokenizer/tokenizer.cpp.o
   CONFIG = Debug
   DEP_FILE = CMakeFiles/saqut.dir/link.d
   FLAGS = -g
diff --git a/examples/semantic/bad_args.sqt b/examples/semantic/bad_args.sqt
new file mode 100644
index 0000000..8b037ee
--- /dev/null
+++ b/examples/semantic/bad_args.sqt
@@ -0,0 +1,9 @@
+// E008 — yanlış argüman sayısı
+int add(int a, int b) {
+    return a + b;
+}
+
+int main() {
+    int x = add(1, 2, 3);
+    return 0;
+}
diff --git a/examples/semantic/bad_return.sqt b/examples/semantic/bad_return.sqt
new file mode 100644
index 0000000..7b1c048
--- /dev/null
+++ b/examples/semantic/bad_return.sqt
@@ -0,0 +1,8 @@
+// E003 — return tipi uyuşmazlığı
+int foo() {
+    return 1.5;
+}
+
+int main() {
+    return 0;
+}
diff --git a/examples/semantic/break_outside.sqt b/examples/semantic/break_outside.sqt
new file mode 100644
index 0000000..223ee8e
--- /dev/null
+++ b/examples/semantic/break_outside.sqt
@@ -0,0 +1,5 @@
+// E004 — break döngü dışında
+int main() {
+    break;
+    return 0;
+}
diff --git a/examples/semantic/narrowing.sqt b/examples/semantic/narrowing.sqt
new file mode 100644
index 0000000..bdb975f
--- /dev/null
+++ b/examples/semantic/narrowing.sqt
@@ -0,0 +1,5 @@
+// E003 — daraltma: float literal → int
+int main() {
+    int x = 1.5;
+    return 0;
+}
diff --git a/examples/semantic/widening.sqt b/examples/semantic/widening.sqt
new file mode 100644
index 0000000..c605336
--- /dev/null
+++ b/examples/semantic/widening.sqt
@@ -0,0 +1,7 @@
+// W004 — örtük genişletme: değişken→değişken int→float
+int main() {
+    int a = 5;
+    float b = 1;      // OK: literal bağlama-göre tiplenir, uyarısız
+    float c = a;      // W004: int değişken → float değişken
+    return 0;
+}
diff --git a/src/cli/args.hpp b/src/cli/args.hpp
index 31317b0..123e1f1 100644
--- a/src/cli/args.hpp
+++ b/src/cli/args.hpp
@@ -88,8 +88,8 @@ inline CliArgs parseArgs(int argc, char* argv[]) {
         // İlk argüman komut mu?
         if (args.command.empty() && i == 1) {
             if (arg == "run"    || arg == "tokens"  || arg == "ast" ||
-                arg == "symbols" || arg == "compile" || arg == "parse" ||
-                arg == "transpile" || arg == "interpret") {
+                arg == "symbols" || arg == "check"   || arg == "compile" ||
+                arg == "parse"   || arg == "transpile" || arg == "interpret") {
                 args.command = arg;
                 continue;
             }
diff --git a/src/cli/commands/check.hpp b/src/cli/commands/check.hpp
new file mode 100644
index 0000000..9e2943f
--- /dev/null
+++ b/src/cli/commands/check.hpp
@@ -0,0 +1,59 @@
+#ifndef SAQUT_CLI_CHECK
+#define SAQUT_CLI_CHECK
+
+#include <iostream>
+#include "cli/args.hpp"
+#include "tokenizer/tokenizer.hpp"
+#include "parser/parser.hpp"
+#include "symbol/symbol_table.hpp"
+#include "symbol/symbol_collector.hpp"
+#include "semantic/type_checker.hpp"
+#include "semantic/structural_validator.hpp"
+#include "diagnostic/diagnostic_engine.hpp"
+#include "vendor/nlohmann/json.hpp"
+
+inline int cmdCheck(const CliArgs& args) {
+    std::string filePath = inputFilePath(args);
+    std::string source   = readSource(args);
+    if (source.empty()) return 1;
+
+    Tokenizer tokenizer;
+    auto tokens = tokenizer.scan(source, filePath);
+
+    Parser parser;
+    ASTNode* ast = parser.parse(tokens);
+
+    DiagnosticEngine diag;
+
+    if (!ast) {
+        diag.report("E000", SourceLocation{}, "AST üretilemedi");
+        nlohmann::json out;
+        out["file"]        = filePath;
+        out["diagnostics"] = diag.toJsonObj();
+        std::cout << (args.compact ? out.dump() : out.dump(2)) << "\n";
+        for (auto* t : tokens) delete t;
+        return 1;
+    }
+
+    SymbolTable table;
+    SymbolCollector(table, diag).collect(ast);
+
+    // Sembol toplama hataları varsa tip denetimine geçme
+    // (çözümsüz semboller tip denetiminde sahte E003 üretir)
+    if (!diag.hasErrors()) {
+        TypeChecker(table, diag).check(ast);
+        StructuralValidator(diag).validate(ast);
+    }
+
+    nlohmann::json out;
+    out["file"]        = filePath;
+    out["diagnostics"] = diag.toJsonObj();
+
+    std::cout << (args.compact ? out.dump() : out.dump(2)) << "\n";
+
+    delete ast;
+    for (auto* t : tokens) delete t;
+    return diag.hasErrors() ? 1 : 0;
+}
+
+#endif // SAQUT_CLI_CHECK
diff --git a/src/diagnostic/diagnostic.hpp b/src/diagnostic/diagnostic.hpp
index 5e9a3e8..307332f 100644
--- a/src/diagnostic/diagnostic.hpp
+++ b/src/diagnostic/diagnostic.hpp
@@ -127,6 +127,7 @@ inline const std::vector<DiagInfo>& diagnosticCatalog() {
         {"W001", DiagLevel::Warning, "Kullanılmayan değişken"},
         {"W002", DiagLevel::Warning, "Sıfıra bölme (sabit ifade)"},
         {"W003", DiagLevel::Warning, "Erişilemez (ölü) kod"},
+        {"W004", DiagLevel::Warning, "Örtük sayısal genişletme (widening)"},
     };
     return catalog;
 }
diff --git a/src/main.cpp b/src/main.cpp
index 2f3321c..78609e3 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -27,6 +27,7 @@
 #include "cli/commands/tokens.hpp"
 #include "cli/commands/ast.hpp"
 #include "cli/commands/symbols.hpp"
+#include "cli/commands/check.hpp"
 
 int main(int argc, char* argv[]) {
     // Komutları kaydet
@@ -48,6 +49,10 @@ int main(int argc, char* argv[]) {
         "Sembol tablosu (fonksiyonlar, değişkenler)",
         false, cmdSymbols});
 
+    cli.registerCommand({"check",
+        "Semantik analiz — tip denetimi + yapısal doğrulama",
+        false, cmdCheck});
+
     // --- Gelecek komutlar (TODO) ---
     cli.registerCommand({"compile",
         "TODO: Kaynak kodu derle",
diff --git a/src/semantic/structural_validator.cpp b/src/semantic/structural_validator.cpp
new file mode 100644
index 0000000..64f7d52
--- /dev/null
+++ b/src/semantic/structural_validator.cpp
@@ -0,0 +1,95 @@
+#include "semantic/structural_validator.hpp"
+#include "parser/nodes/declarations.hpp"
+#include "parser/nodes/statements.hpp"
+
+void StructuralValidator::validate(ASTNode* program) {
+    if (!program) return;
+    for (ASTNode* child : program->getChildren())
+        walkDecl(child);
+}
+
+void StructuralValidator::walkDecl(ASTNode* node) {
+    if (!node) return;
+    if (node->kind == ASTKind::FunctionDecl) {
+        auto* fn = (FunctionDeclNode*)node;
+        inFunction_ = true;
+        auto& ch = fn->getChildren();
+        if (!ch.empty()) walkStmt(ch[0]);
+        inFunction_ = false;
+    }
+    // VariableDecl / StructDecl: yapısal kural yok
+}
+
+void StructuralValidator::walkStmt(ASTNode* node) {
+    if (!node) return;
+
+    switch (node->kind) {
+
+    case ASTKind::Block:
+        for (ASTNode* child : node->getChildren()) walkStmt(child);
+        break;
+
+    case ASTKind::IfStatement: {
+        auto* ifn = (IfStatementNode*)node;
+        if (ifn->thenBranch) walkStmt(ifn->thenBranch);
+        if (ifn->elseBranch) walkStmt(ifn->elseBranch);
+        break;
+    }
+
+    case ASTKind::WhileStatement: {
+        auto* ws = (WhileStatementNode*)node;
+        loopDepth_++;
+        if (ws->body) walkStmt(ws->body);
+        loopDepth_--;
+        break;
+    }
+
+    case ASTKind::DoWhileStatement: {
+        auto* dw = (DoWhileStatementNode*)node;
+        loopDepth_++;
+        if (dw->body) walkStmt(dw->body);
+        loopDepth_--;
+        break;
+    }
+
+    case ASTKind::ForStatement: {
+        auto* fs = (ForStatementNode*)node;
+        loopDepth_++;
+        if (fs->init) walkStmt(fs->init);
+        if (fs->body) walkStmt(fs->body);
+        loopDepth_--;
+        break;
+    }
+
+    case ASTKind::BreakStatement:
+        if (loopDepth_ == 0)
+            diag_.report("E004", node->loc,
+                "'break' döngü dışında kullanılamaz");
+        break;
+
+    case ASTKind::ContinueStatement:
+        if (loopDepth_ == 0)
+            diag_.report("E004", node->loc,
+                "'continue' döngü dışında kullanılamaz");
+        break;
+
+    case ASTKind::ReturnStatement:
+        if (!inFunction_)
+            diag_.report("E005", node->loc,
+                "'return' fonksiyon dışında kullanılamaz");
+        break;
+
+    case ASTKind::VariableDecl: {
+        // sibling'leri de gez
+        for (ASTNode* sib : node->getChildren())
+            if (sib->kind == ASTKind::VariableDecl) walkStmt(sib);
+        break;
+    }
+
+    case ASTKind::ExpressionStatement:
+        break; // ifade içinde return/break olamaz
+
+    default:
+        break;
+    }
+}
diff --git a/src/semantic/structural_validator.hpp b/src/semantic/structural_validator.hpp
new file mode 100644
index 0000000..aa089b8
--- /dev/null
+++ b/src/semantic/structural_validator.hpp
@@ -0,0 +1,22 @@
+#ifndef SAQUT_SEMANTIC_STRUCTURAL_VALIDATOR
+#define SAQUT_SEMANTIC_STRUCTURAL_VALIDATOR
+
+#include "diagnostic/diagnostic_engine.hpp"
+#include "parser/ast_node.hpp"
+
+class StructuralValidator {
+public:
+    explicit StructuralValidator(DiagnosticEngine& diag) : diag_(diag) {}
+
+    void validate(ASTNode* program);
+
+private:
+    void walkDecl(ASTNode* node);
+    void walkStmt(ASTNode* node);
+
+    DiagnosticEngine& diag_;
+    int  loopDepth_  = 0;
+    bool inFunction_ = false;
+};
+
+#endif // SAQUT_SEMANTIC_STRUCTURAL_VALIDATOR
diff --git a/src/semantic/type_checker.cpp b/src/semantic/type_checker.cpp
new file mode 100644
index 0000000..f037b53
--- /dev/null
+++ b/src/semantic/type_checker.cpp
@@ -0,0 +1,384 @@
+#include "semantic/type_checker.hpp"
+#include "parser/nodes/program.hpp"
+#include "parser/nodes/declarations.hpp"
+#include "parser/nodes/statements.hpp"
+#include "parser/nodes/expressions.hpp"
+#include "parser/nodes/binary_expr.hpp"
+#include "parser/nodes/identifier.hpp"
+#include "parser/nodes/literal.hpp"
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Yardımcılar
+// ─────────────────────────────────────────────────────────────────────────────
+
+int TypeChecker::numericRank(const Type& t) {
+    if (!t.isPrimitive()) return -1;
+    switch (t.prim) {
+        case PrimitiveKind::Int:    return 0;
+        case PrimitiveKind::Float:  return 1;
+        case PrimitiveKind::Double: return 2;
+        default: return -1;
+    }
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// check — giriş noktası
+// ─────────────────────────────────────────────────────────────────────────────
+
+void TypeChecker::check(ASTNode* program) {
+    if (!program) return;
+    for (ASTNode* child : program->getChildren()) {
+        switch (child->kind) {
+            case ASTKind::FunctionDecl: checkFunction(child); break;
+            case ASTKind::VariableDecl: checkStmt(child);     break;
+            default: break;
+        }
+    }
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// checkFunction
+// ─────────────────────────────────────────────────────────────────────────────
+
+void TypeChecker::checkFunction(ASTNode* fnNode) {
+    auto* fn = (FunctionDeclNode*)fnNode;
+    inFunction_        = true;
+    currentReturnType_ = Type::fromName(fn->returnType);
+    if (currentReturnType_.isError() && fn->returnType != "void")
+        currentReturnType_ = Type::Void(); // bilinmeyen dönüş tipi → void gibi davran
+
+    auto& ch = fn->getChildren();
+    if (!ch.empty()) checkStmt(ch[0]); // body Block
+
+    inFunction_ = false;
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// checkAssign — atama uyumu + uyarı/hata raporlama
+// ─────────────────────────────────────────────────────────────────────────────
+
+bool TypeChecker::checkAssign(const Type& target, const Type& src,
+                               bool srcIsLiteral,
+                               const SourceLocation& loc,
+                               const std::string& ctx) {
+    if (target.isError() || src.isError()) return true; // önceki hata, sessiz geç
+    if (target.equals(src))               return true;
+
+    int tRank = numericRank(target);
+    int sRank = numericRank(src);
+
+    if (tRank >= 0 && sRank >= 0) {
+        if (tRank > sRank) {
+            // Genişletme (widening): int→float, int→double, float→double
+            if (srcIsLiteral) return true; // literal bağlama-göre tiplenir, uyarısız
+            diag_.report("W004", loc,
+                "'" + ctx + "': " + src.toString() +
+                " → " + target.toString() + " örtük genişletme");
+            return true;
+        } else {
+            // Daraltma (narrowing): float→int, double→float, vb.
+            diag_.report("E003", loc,
+                "'" + ctx + "': " + src.toString() +
+                " → " + target.toString() + " daraltma (veri kaybı)");
+            return false;
+        }
+    }
+
+    // Tamamen farklı tipler
+    diag_.report("E003", loc,
+        "'" + ctx + "': " + src.toString() +
+        " tipi " + target.toString() + " tipine atanamaz");
+    return false;
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// checkStmt
+// ─────────────────────────────────────────────────────────────────────────────
+
+void TypeChecker::checkStmt(ASTNode* node) {
+    if (!node) return;
+
+    switch (node->kind) {
+
+    case ASTKind::Block:
+        for (ASTNode* child : node->getChildren()) checkStmt(child);
+        break;
+
+    case ASTKind::VariableDecl: {
+        auto* vd = (VariableDeclNode*)node;
+        Type targetType = Type::fromName(vd->varType);
+        if (vd->initExpr) {
+            Type srcType = checkExpr(vd->initExpr, targetType);
+            bool isLit   = vd->initExpr->kind == ASTKind::Literal;
+            checkAssign(targetType, srcType, isLit, vd->loc, vd->name);
+        }
+        // sibling VariableDecl'ler (int a, b;)
+        for (ASTNode* sib : vd->getChildren()) {
+            if (sib->kind == ASTKind::VariableDecl) checkStmt(sib);
+        }
+        break;
+    }
+
+    case ASTKind::ExpressionStatement: {
+        auto* es = (ExpressionStatementNode*)node;
+        if (es->expression) checkExpr(es->expression);
+        break;
+    }
+
+    case ASTKind::ReturnStatement: {
+        auto* rs = (ReturnStatementNode*)node;
+        if (!rs->value) {
+            if (inFunction_ && !currentReturnType_.isVoid())
+                diag_.report("E006", rs->loc,
+                    "Değersiz return; fonksiyon " +
+                    currentReturnType_.toString() + " döndürmeli");
+            break;
+        }
+        Type valType = checkExpr(rs->value, currentReturnType_);
+        bool isLit   = rs->value->kind == ASTKind::Literal;
+        checkAssign(currentReturnType_, valType, isLit, rs->loc, "return");
+        break;
+    }
+
+    case ASTKind::IfStatement: {
+        auto* ifn = (IfStatementNode*)node;
+        if (ifn->condition)  checkExpr(ifn->condition);
+        if (ifn->thenBranch) checkStmt(ifn->thenBranch);
+        if (ifn->elseBranch) checkStmt(ifn->elseBranch);
+        break;
+    }
+
+    case ASTKind::WhileStatement: {
+        auto* ws = (WhileStatementNode*)node;
+        if (ws->condition) checkExpr(ws->condition);
+        if (ws->body)      checkStmt(ws->body);
+        break;
+    }
+
+    case ASTKind::ForStatement: {
+        auto* fs = (ForStatementNode*)node;
+        if (fs->init) {
+            if (fs->init->kind == ASTKind::VariableDecl) checkStmt(fs->init);
+            else checkExpr(fs->init);
+        }
+        if (fs->condition) checkExpr(fs->condition);
+        if (fs->update)    checkExpr(fs->update);
+        if (fs->body)      checkStmt(fs->body);
+        break;
+    }
+
+    case ASTKind::DoWhileStatement: {
+        auto* dw = (DoWhileStatementNode*)node;
+        if (dw->body)      checkStmt(dw->body);
+        if (dw->condition) checkExpr(dw->condition);
+        break;
+    }
+
+    case ASTKind::BreakStatement:
+    case ASTKind::ContinueStatement:
+        break; // yapısal doğrulama StructuralValidator'ın işi
+
+    default:
+        break;
+    }
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// checkExpr — tip çıkarımı + resolvedType ataması
+// ─────────────────────────────────────────────────────────────────────────────
+
+Type TypeChecker::checkExpr(ASTNode* node, const Type& expected) {
+    if (!node) return Type::error();
+
+    Type result = Type::error();
+
+    switch (node->kind) {
+
+    // ── Literal ────────────────────────────────────────────────────────────
+    case ASTKind::Literal: {
+        auto* lit = (LiteralNode*)node;
+        int expRank = numericRank(expected);
+
+        switch (lit->literalType) {
+            case LiteralType::INTEGER:
+                // Bağlam daha geniş sayısal tip ise literal o tip olarak tiplenir.
+                if (expRank > 0) result = expected; // float veya double bekleniyor
+                else             result = Type::Int();
+                break;
+            case LiteralType::FLOAT:
+                // float literal → double bağlamında double olur; int bağlamında E003.
+                if (!expected.isError() && expected.equals(Type::Double()))
+                    result = Type::Double();
+                else if (!expected.isError() && numericRank(expected) == 0) {
+                    // int bekleniyor ama float literal: E003
+                    diag_.report("E003", lit->loc,
+                        "Float literal int bağlamında kullanılamaz (veri kaybı)");
+                    result = Type::error();
+                } else {
+                    result = Type::Float();
+                }
+                break;
+            case LiteralType::BOOLEAN: result = Type::Bool();   break;
+            case LiteralType::STRING:  result = Type::String(); break;
+            default:                   result = Type::error();  break;
+        }
+        break;
+    }
+
+    // ── Identifier ─────────────────────────────────────────────────────────
+    case ASTKind::Identifier: {
+        auto* id = (IdentifierNode*)node;
+        result = id->resolvedSymbol ? id->resolvedSymbol->type : Type::error();
+        break;
+    }
+
+    // ── BinaryExpression ───────────────────────────────────────────────────
+    case ASTKind::BinaryExpression: {
+        auto* bin = (BinaryExpressionNode*)node;
+
+        // Atama operatörleri
+        if (bin->Operator == TokenType::EQUAL       ||
+            bin->Operator == TokenType::PLUS_EQUAL  ||
+            bin->Operator == TokenType::MINUS_EQUAL ||
+            bin->Operator == TokenType::STAR_EQUAL  ||
+            bin->Operator == TokenType::SLASH_EQUAL ||
+            bin->Operator == TokenType::PERCENT_EQUAL) {
+            Type leftType  = checkExpr(bin->Left);
+            Type rightType = checkExpr(bin->Right, leftType);
+            bool isLit     = bin->Right && bin->Right->kind == ASTKind::Literal;
+            checkAssign(leftType, rightType, isLit, bin->loc, "atama");
+            result = leftType;
+            break;
+        }
+
+        // Unary (Left = nullptr): -, +, !, ~
+        if (!bin->Left) {
+            Type rightType = checkExpr(bin->Right);
+            if (bin->Operator == TokenType::BANG) {
+                result = Type::Bool();
+            } else {
+                result = rightType.isNumeric() ? rightType : Type::error();
+                if (result.isError() && !rightType.isError())
+                    diag_.report("E003", bin->loc, "Sayısal olmayan operand");
+            }
+            break;
+        }
+
+        Type leftType  = checkExpr(bin->Left);
+        Type rightType = checkExpr(bin->Right);
+
+        // Mantıksal
+        if (bin->Operator == TokenType::AMPERSAND_AMPERSAND ||
+            bin->Operator == TokenType::PIPE_PIPE) {
+            result = Type::Bool();
+            break;
+        }
+
+        // Karşılaştırma
+        if (bin->Operator == TokenType::EQUAL_EQUAL  ||
+            bin->Operator == TokenType::BANG_EQUAL   ||
+            bin->Operator == TokenType::LESS         ||
+            bin->Operator == TokenType::LESS_EQUAL   ||
+            bin->Operator == TokenType::GREATER      ||
+            bin->Operator == TokenType::GREATER_EQUAL) {
+            result = Type::Bool();
+            break;
+        }
+
+        // Aritmetik: +, -, *, /, %
+        int lRank = numericRank(leftType);
+        int rRank = numericRank(rightType);
+
+        if (lRank >= 0 && rRank >= 0) {
+            // Aynı tip veya otomatik genişletme; sonuç daha geniş tip.
+            result = (lRank >= rRank) ? leftType : rightType;
+        } else if (!leftType.isError() && !rightType.isError()) {
+            diag_.report("E003", bin->loc,
+                "Aritmetik operatör sayısal olmayan tip: " +
+                leftType.toString() + " ve " + rightType.toString());
+            result = Type::error();
+        } else {
+            result = Type::error();
+        }
+        break;
+    }
+
+    // ── Call ───────────────────────────────────────────────────────────────
+    case ASTKind::Call: {
+        auto* call = (CallExpressionNode*)node;
+        Type calleeType = checkExpr(call->callee);
+
+        if (!calleeType.isFunction()) {
+            if (!calleeType.isError())
+                diag_.report("E003", call->loc,
+                    "Çağrılabilir değil: " + calleeType.toString());
+            result = Type::error();
+            // Argümanları yine de gez (cascade hatayı önle)
+            for (auto* arg : call->arguments) checkExpr(arg);
+            break;
+        }
+
+        // Argüman sayısı kontrolü — builtin print hariç (paramTypes boş = değişken arity)
+        if (!calleeType.paramTypes.empty()) {
+            size_t expected_count = calleeType.paramTypes.size();
+            size_t got_count      = call->arguments.size();
+            if (got_count != expected_count) {
+                diag_.report("E008", call->loc,
+                    std::to_string(expected_count) + " argüman bekleniyor, " +
+                    std::to_string(got_count) + " verildi");
+            }
+        }
+
+        // Argüman tiplerini kontrol et
+        for (size_t i = 0; i < call->arguments.size(); ++i) {
+            Type paramType = (i < calleeType.paramTypes.size())
+                             ? calleeType.paramTypes[i]
+                             : Type::error();
+            Type argType = checkExpr(call->arguments[i], paramType);
+            bool isLit   = call->arguments[i]->kind == ASTKind::Literal;
+            if (!paramType.isError())
+                checkAssign(paramType, argType, isLit,
+                            call->arguments[i]->loc, "argüman");
+        }
+
+        result = calleeType.returnType ? *calleeType.returnType : Type::Void();
+        break;
+    }
+
+    // ── Postfix ++/-- ──────────────────────────────────────────────────────
+    case ASTKind::Postfix: {
+        auto* pf = (PostfixNode*)node;
+        Type opType = checkExpr(pf->operand);
+        if (!opType.isNumeric() && !opType.isError())
+            diag_.report("E003", pf->loc,
+                "++ / -- sayısal olmayan tip: " + opType.toString());
+        result = opType;
+        break;
+    }
+
+    // ── MemberAccess / IndexExpression ─────────────────────────────────────
+    case ASTKind::MemberAccess: {
+        auto* ma = (MemberAccessNode*)node;
+        checkExpr(ma->object);
+        result = Type::error(); // TODO(faz3+): struct alan çözümü
+        break;
+    }
+    case ASTKind::IndexExpression: {
+        auto* ie = (IndexExpressionNode*)node;
+        checkExpr(ie->object);
+        if (ie->index) checkExpr(ie->index);
+        result = Type::error(); // TODO(faz3+): array eleman tipi
+        break;
+    }
+
+    default:
+        result = Type::error();
+        break;
+    }
+
+    // resolvedType'a yaz (ExpressionNode'dan türeyen tüm node'lar için)
+    if (auto* exprNode = dynamic_cast<ExpressionNode*>(node))
+        exprNode->resolvedType = result;
+
+    return result;
+}
diff --git a/src/semantic/type_checker.hpp b/src/semantic/type_checker.hpp
new file mode 100644
index 0000000..cc7febf
--- /dev/null
+++ b/src/semantic/type_checker.hpp
@@ -0,0 +1,41 @@
+#ifndef SAQUT_SEMANTIC_TYPE_CHECKER
+#define SAQUT_SEMANTIC_TYPE_CHECKER
+
+#include "symbol/symbol_table.hpp"
+#include "diagnostic/diagnostic_engine.hpp"
+#include "parser/ast_node.hpp"
+#include "core/type.hpp"
+
+class TypeChecker {
+public:
+    TypeChecker(SymbolTable& table, DiagnosticEngine& diag)
+        : table_(table), diag_(diag) {}
+
+    void check(ASTNode* program);
+
+private:
+    // İfadeyi gez, resolvedType ata, tipi döndür.
+    // expected: bağlam tipi — literal genişletme kararı için.
+    Type checkExpr(ASTNode* node, const Type& expected = Type::error());
+
+    void checkStmt(ASTNode* node);
+    void checkFunction(ASTNode* fnNode);
+
+    // Atama / parametre uyumu: true = geçerli (uyarı dahil).
+    // srcIsLiteral: RHS doğrudan bir Literal node'u mu?
+    bool checkAssign(const Type& target, const Type& src,
+                     bool srcIsLiteral,
+                     const SourceLocation& loc,
+                     const std::string& context);
+
+    // İki sayısal tipin genişlik sırası: int=0, float=1, double=2; -1 = sayısal değil.
+    static int numericRank(const Type& t);
+
+    SymbolTable&      table_;
+    DiagnosticEngine& diag_;
+
+    Type currentReturnType_;   // aktif fonksiyonun beklenen dönüş tipi
+    bool inFunction_ = false;
+};
+
+#endif // SAQUT_SEMANTIC_TYPE_CHECKER