diff --git a/src/mir/loop_canonicalizer/mod.rs b/src/mir/loop_canonicalizer/mod.rs
new file mode 100644
index 00000000..804d5c08
--- /dev/null
+++ b/src/mir/loop_canonicalizer/mod.rs
@@ -0,0 +1,416 @@
+//! Loop Canonicalizer - AST Level Loop Preprocessing
+//!
+//! Phase 1: Type Definitions Only
+//!
+//! ## Purpose
+//!
+//! Decomposes AST-level loops into a normalized "skeleton" representation
+//! to prevent combinatorial explosion in pattern detection and lowering.
+//!
+//! ## Design Principle
+//!
+//! - **Input**: AST (LoopExpr)
+//! - **Output**: LoopSkeleton only (no JoinIR generation)
+//! - **Boundary**: No JoinIR-specific information (BlockId, ValueId, etc.)
+//!
+//! ## Architecture
+//!
+//! ```
+//! AST → LoopSkeleton → Capability Guard → RoutingDecision → Pattern Lowerer
+//! ```
+//!
+//! ## References
+//!
+//! - Design SSOT: `docs/development/current/main/design/loop-canonicalizer.md`
+//! - JoinIR Architecture: `docs/development/current/main/joinir-architecture-overview.md`
+//! - Pattern Space: `docs/development/current/main/loop_pattern_space.md`
+
+use crate::ast::{ASTNode, Span};
+use crate::mir::loop_pattern_detection::LoopPatternKind;
+
+// ============================================================================
+// Core Skeleton Types
+// ============================================================================
+
+/// Loop skeleton - The canonical representation of a loop structure
+///
+/// This is the single output type of the Canonicalizer.
+/// It represents the essential structure of a loop without any
+/// JoinIR-specific information.
+#[derive(Debug, Clone)]
+pub struct LoopSkeleton {
+    /// Sequence of steps (HeaderCond, BodyInit, BreakCheck, Updates, Tail)
+    pub steps: Vec<SkeletonStep>,
+
+    /// Carriers (loop variables with update rules and boundary crossing contracts)
+    pub carriers: Vec<CarrierSlot>,
+
+    /// Exit contract (presence and payload of break/continue/return)
+    pub exits: ExitContract,
+
+    /// Captured variables from outer scope (optional)
+    pub captured: Option<Vec<CapturedSlot>>,
+
+    /// Source location for debugging
+    pub span: Span,
+}
+
+/// Skeleton step - Minimal step kinds for loop structure
+///
+/// Each step represents a fundamental operation in the loop lifecycle.
+#[derive(Debug, Clone)]
+pub enum SkeletonStep {
+    /// Loop continuation condition (the `cond` in `loop(cond)`)
+    HeaderCond {
+        expr: Box<ASTNode>,
+    },
+
+    /// Early exit check (`if cond { break }`)
+    BreakCheck {
+        cond: Box<ASTNode>,
+        has_value: bool,
+    },
+
+    /// Skip check (`if cond { continue }`)
+    ContinueCheck {
+        cond: Box<ASTNode>,
+    },
+
+    /// Carrier update (`i = i + 1`, etc.)
+    Update {
+        carrier_name: String,
+        update_kind: UpdateKind,
+    },
+
+    /// Loop body (all other statements)
+    Body {
+        stmts: Vec<ASTNode>,
+    },
+}
+
+/// Update kind - How a carrier variable is updated
+///
+/// This categorization helps determine which pattern can handle the loop.
+#[derive(Debug, Clone)]
+pub enum UpdateKind {
+    /// Constant step (`i = i + const`)
+    ConstStep {
+        delta: i64,
+    },
+
+    /// Conditional update (`if cond { x = a } else { x = b }`)
+    Conditional {
+        then_value: Box<ASTNode>,
+        else_value: Box<ASTNode>,
+    },
+
+    /// Arbitrary update (everything else)
+    Arbitrary,
+}
+
+/// Exit contract - What kinds of exits the loop has
+///
+/// This determines the exit line architecture needed.
+#[derive(Debug, Clone)]
+pub struct ExitContract {
+    pub has_break: bool,
+    pub has_continue: bool,
+    pub has_return: bool,
+    pub break_has_value: bool,
+}
+
+/// Carrier slot - A loop variable with its role and update rule
+///
+/// Carriers are variables that are updated in each iteration
+/// and need to cross loop boundaries (via PHI nodes in MIR).
+#[derive(Debug, Clone)]
+pub struct CarrierSlot {
+    pub name: String,
+    pub role: CarrierRole,
+    pub update_kind: UpdateKind,
+}
+
+/// Carrier role - The semantic role of a carrier variable
+///
+/// This helps determine the appropriate pattern and PHI structure.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CarrierRole {
+    /// Loop counter (the `i` in `i < n`)
+    Counter,
+
+    /// Accumulator (the `sum` in `sum += x`)
+    Accumulator,
+
+    /// Condition variable (the `is_valid` in `while(is_valid)`)
+    ConditionVar,
+
+    /// Derived value (e.g., `digit_pos` computed from other carriers)
+    Derived,
+}
+
+/// Captured slot - An outer variable used within the loop
+///
+/// These are read-only references to variables defined outside the loop.
+/// (Write access would make them carriers instead.)
+#[derive(Debug, Clone)]
+pub struct CapturedSlot {
+    pub name: String,
+    pub is_mutable: bool,
+}
+
+// ============================================================================
+// Capability Guard - Fail-Fast Reasons
+// ============================================================================
+
+/// Routing decision - The result of pattern selection
+///
+/// This contains both the chosen pattern (if any) and detailed
+/// diagnostic information about why other patterns were rejected.
+#[derive(Debug, Clone)]
+pub struct RoutingDecision {
+    /// Selected pattern (None = Fail-Fast)
+    pub chosen: Option<LoopPatternKind>,
+
+    /// Missing capabilities that prevented other patterns
+    pub missing_caps: Vec<&'static str>,
+
+    /// Selection reasoning (for debugging)
+    pub notes: Vec<String>,
+
+    /// Error tags for contract_checks integration
+    pub error_tags: Vec<String>,
+}
+
+/// Capability tags - Standardized vocabulary for Fail-Fast reasons
+///
+/// These constants define the capabilities required by different patterns.
+/// When a loop lacks a required capability, it uses the corresponding tag
+/// to explain why it cannot be lowered by that pattern.
+pub mod capability_tags {
+    /// Requires: Carrier update is constant step (`i = i + const`)
+    pub const CAP_MISSING_CONST_STEP: &str = "CAP_MISSING_CONST_STEP";
+
+    /// Requires: Single break point only
+    pub const CAP_MISSING_SINGLE_BREAK: &str = "CAP_MISSING_SINGLE_BREAK";
+
+    /// Requires: Single continue point only
+    pub const CAP_MISSING_SINGLE_CONTINUE: &str = "CAP_MISSING_SINGLE_CONTINUE";
+
+    /// Requires: Loop header condition has no side effects
+    pub const CAP_MISSING_PURE_HEADER: &str = "CAP_MISSING_PURE_HEADER";
+
+    /// Requires: Condition variable defined in outer local scope
+    pub const CAP_MISSING_OUTER_LOCAL_COND: &str = "CAP_MISSING_OUTER_LOCAL_COND";
+
+    /// Requires: All exit bindings are complete (no missing values)
+    pub const CAP_MISSING_EXIT_BINDINGS: &str = "CAP_MISSING_EXIT_BINDINGS";
+
+    /// Requires: LoopBodyLocal can be promoted to carrier
+    pub const CAP_MISSING_CARRIER_PROMOTION: &str = "CAP_MISSING_CARRIER_PROMOTION";
+
+    /// Requires: Break value types are consistent across all break points
+    pub const CAP_MISSING_BREAK_VALUE_TYPE: &str = "CAP_MISSING_BREAK_VALUE_TYPE";
+}
+
+// ============================================================================
+// Implementation Helpers
+// ============================================================================
+
+impl LoopSkeleton {
+    /// Create a new empty skeleton
+    pub fn new(span: Span) -> Self {
+        Self {
+            steps: Vec::new(),
+            carriers: Vec::new(),
+            exits: ExitContract::default(),
+            captured: None,
+            span,
+        }
+    }
+
+    /// Count the number of break checks in this skeleton
+    pub fn count_break_checks(&self) -> usize {
+        self.steps
+            .iter()
+            .filter(|s| matches!(s, SkeletonStep::BreakCheck { .. }))
+            .count()
+    }
+
+    /// Count the number of continue checks in this skeleton
+    pub fn count_continue_checks(&self) -> usize {
+        self.steps
+            .iter()
+            .filter(|s| matches!(s, SkeletonStep::ContinueCheck { .. }))
+            .count()
+    }
+
+    /// Get all carrier names
+    pub fn carrier_names(&self) -> Vec<&str> {
+        self.carriers.iter().map(|c| c.name.as_str()).collect()
+    }
+}
+
+impl ExitContract {
+    /// Create a contract with no exits
+    pub fn none() -> Self {
+        Self {
+            has_break: false,
+            has_continue: false,
+            has_return: false,
+            break_has_value: false,
+        }
+    }
+
+    /// Check if any exit exists
+    pub fn has_any_exit(&self) -> bool {
+        self.has_break || self.has_continue || self.has_return
+    }
+}
+
+impl Default for ExitContract {
+    fn default() -> Self {
+        Self::none()
+    }
+}
+
+impl RoutingDecision {
+    /// Create a successful routing decision
+    pub fn success(pattern: LoopPatternKind) -> Self {
+        Self {
+            chosen: Some(pattern),
+            missing_caps: Vec::new(),
+            notes: Vec::new(),
+            error_tags: Vec::new(),
+        }
+    }
+
+    /// Create a failed routing decision (Fail-Fast)
+    pub fn fail_fast(missing_caps: Vec<&'static str>, reason: String) -> Self {
+        Self {
+            chosen: None,
+            missing_caps,
+            notes: vec![reason.clone()],
+            error_tags: vec![format!("[loop_canonicalizer/fail_fast] {}", reason)],
+        }
+    }
+
+    /// Add a diagnostic note
+    pub fn add_note(&mut self, note: String) {
+        self.notes.push(note);
+    }
+
+    /// Check if routing succeeded
+    pub fn is_success(&self) -> bool {
+        self.chosen.is_some()
+    }
+
+    /// Check if routing failed
+    pub fn is_fail_fast(&self) -> bool {
+        self.chosen.is_none()
+    }
+}
+
+// ============================================================================
+// Display Implementations
+// ============================================================================
+
+impl std::fmt::Display for CarrierRole {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            CarrierRole::Counter => write!(f, "Counter"),
+            CarrierRole::Accumulator => write!(f, "Accumulator"),
+            CarrierRole::ConditionVar => write!(f, "ConditionVar"),
+            CarrierRole::Derived => write!(f, "Derived"),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_skeleton_creation() {
+        let skeleton = LoopSkeleton::new(Span::unknown());
+        assert_eq!(skeleton.steps.len(), 0);
+        assert_eq!(skeleton.carriers.len(), 0);
+        assert!(!skeleton.exits.has_any_exit());
+    }
+
+    #[test]
+    fn test_exit_contract() {
+        let mut contract = ExitContract::none();
+        assert!(!contract.has_any_exit());
+
+        contract.has_break = true;
+        assert!(contract.has_any_exit());
+    }
+
+    #[test]
+    fn test_routing_decision() {
+        let success = RoutingDecision::success(LoopPatternKind::Pattern1SimpleWhile);
+        assert!(success.is_success());
+        assert!(!success.is_fail_fast());
+
+        let fail = RoutingDecision::fail_fast(
+            vec![capability_tags::CAP_MISSING_CONST_STEP],
+            "Test failure".to_string(),
+        );
+        assert!(!fail.is_success());
+        assert!(fail.is_fail_fast());
+        assert_eq!(fail.missing_caps.len(), 1);
+    }
+
+    #[test]
+    fn test_carrier_role_display() {
+        assert_eq!(CarrierRole::Counter.to_string(), "Counter");
+        assert_eq!(CarrierRole::Accumulator.to_string(), "Accumulator");
+        assert_eq!(CarrierRole::ConditionVar.to_string(), "ConditionVar");
+        assert_eq!(CarrierRole::Derived.to_string(), "Derived");
+    }
+
+    #[test]
+    fn test_skeleton_count_helpers() {
+        use crate::ast::LiteralValue;
+
+        let mut skeleton = LoopSkeleton::new(Span::unknown());
+
+        skeleton.steps.push(SkeletonStep::BreakCheck {
+            cond: Box::new(ASTNode::Literal {
+                value: LiteralValue::Bool(true),
+                span: Span::unknown(),
+            }),
+            has_value: false,
+        });
+
+        skeleton.steps.push(SkeletonStep::ContinueCheck {
+            cond: Box::new(ASTNode::Literal {
+                value: LiteralValue::Bool(true),
+                span: Span::unknown(),
+            }),
+        });
+
+        assert_eq!(skeleton.count_break_checks(), 1);
+        assert_eq!(skeleton.count_continue_checks(), 1);
+    }
+
+    #[test]
+    fn test_skeleton_carrier_names() {
+        let mut skeleton = LoopSkeleton::new(Span::unknown());
+
+        skeleton.carriers.push(CarrierSlot {
+            name: "i".to_string(),
+            role: CarrierRole::Counter,
+            update_kind: UpdateKind::ConstStep { delta: 1 },
+        });
+
+        skeleton.carriers.push(CarrierSlot {
+            name: "sum".to_string(),
+            role: CarrierRole::Accumulator,
+            update_kind: UpdateKind::Arbitrary,
+        });
+
+        let names = skeleton.carrier_names();
+        assert_eq!(names, vec!["i", "sum"]);
+    }
+}
diff --git a/src/mir/mod.rs b/src/mir/mod.rs
index d5c449a0..3ca35b2f 100644
--- a/src/mir/mod.rs
+++ b/src/mir/mod.rs
@@ -18,6 +18,7 @@ pub mod instruction;
 pub mod instruction_introspection; // Introspection helpers for tests (instruction names)
 pub mod instruction_kinds; // small kind-specific metadata (Const/BinOp)
 pub mod loop_api; // Minimal LoopBuilder facade (adapter-ready)
+pub mod loop_canonicalizer; // Phase 1: Loop skeleton canonicalization (AST preprocessing)
 pub mod naming; // Static box / entry naming rules（NamingBox）
 pub mod optimizer;
 pub mod ssot; // Shared helpers (SSOT) for instruction lowering