diff --git a/src/mir/builder/control_flow/joinir/patterns/condition_env_builder.rs b/src/mir/builder/control_flow/joinir/patterns/condition_env_builder.rs
index f0694af6..8a75fefb 100644
--- a/src/mir/builder/control_flow/joinir/patterns/condition_env_builder.rs
+++ b/src/mir/builder/control_flow/joinir/patterns/condition_env_builder.rs
@@ -28,6 +28,7 @@ use crate::ast::ASTNode;
 use crate::mir::join_ir::lowering::condition_env::{ConditionBinding, ConditionEnv};
 use crate::mir::join_ir::lowering::condition_to_joinir::extract_condition_variables;
 use crate::mir::join_ir::lowering::inline_boundary_builder::JoinInlineBoundaryBuilder;
+use crate::mir::join_ir::lowering::join_value_space::JoinValueSpace;
 use crate::mir::loop_pattern_detection::function_scope_capture::CapturedEnv;
 use crate::mir::ValueId;
 use std::collections::BTreeMap;
@@ -125,6 +126,82 @@ impl ConditionEnvBuilder {
         env
     }
 
+    /// Phase 201: Build ConditionEnv using JoinValueSpace (disjoint ValueId regions)
+    ///
+    /// This method uses JoinValueSpace to allocate ValueIds, ensuring that
+    /// param IDs (100+) never collide with local IDs (1000+) used by JoinIR lowering.
+    ///
+    /// # Arguments
+    ///
+    /// * `break_condition` - AST node for the break condition
+    /// * `loop_var_name` - Loop parameter name (excluded from condition-only variables)
+    /// * `variable_map` - HOST function's variable_map (for looking up HOST ValueIds)
+    /// * `loop_var_id` - HOST ValueId for the loop parameter
+    /// * `space` - JoinValueSpace for unified ValueId allocation
+    ///
+    /// # Returns
+    ///
+    /// Tuple of:
+    /// - ConditionEnv: Variable name → JoinIR ValueId mapping
+    /// - Vec<ConditionBinding>: HOST↔JoinIR value mappings for merge
+    /// - loop_var_join_id: The JoinIR ValueId allocated for the loop parameter
+    pub fn build_for_break_condition_v2(
+        break_condition: &ASTNode,
+        loop_var_name: &str,
+        variable_map: &BTreeMap<String, ValueId>,
+        _loop_var_id: ValueId,
+        space: &mut JoinValueSpace,
+    ) -> Result<(ConditionEnv, Vec<ConditionBinding>, ValueId), String> {
+        // Extract all variables used in the condition (excluding loop parameter)
+        let condition_var_names = extract_condition_variables(
+            break_condition,
+            &[loop_var_name.to_string()],
+        );
+
+        let mut env = ConditionEnv::new();
+        let mut bindings = Vec::new();
+
+        // Phase 201: Allocate loop parameter ValueId from JoinValueSpace (Param region)
+        let loop_var_join_id = space.alloc_param();
+        env.insert(loop_var_name.to_string(), loop_var_join_id);
+
+        // For each condition variable, allocate JoinIR-local ValueId and build binding
+        for var_name in &condition_var_names {
+            let host_id = variable_map
+                .get(var_name)
+                .copied()
+                .ok_or_else(|| {
+                    format!(
+                        "Condition variable '{}' not found in variable_map. \
+                         Loop condition references undefined variable.",
+                        var_name
+                    )
+                })?;
+
+            // Phase 201: Allocate from Param region to avoid collision with locals
+            let join_id = space.alloc_param();
+
+            env.insert(var_name.clone(), join_id);
+            bindings.push(ConditionBinding {
+                name: var_name.clone(),
+                host_value: host_id,
+                join_value: join_id,
+            });
+        }
+
+        Ok((env, bindings, loop_var_join_id))
+    }
+
+    /// Phase 201: Build ConditionEnv with loop parameter only using JoinValueSpace
+    ///
+    /// Uses JoinValueSpace to allocate the loop parameter ValueId.
+    pub fn build_loop_param_only_v2(loop_var_name: &str, space: &mut JoinValueSpace) -> (ConditionEnv, ValueId) {
+        let mut env = ConditionEnv::new();
+        let loop_var_join_id = space.alloc_param();
+        env.insert(loop_var_name.to_string(), loop_var_join_id);
+        (env, loop_var_join_id)
+    }
+
     /// Build ConditionEnv with optional captured variables (Phase 200-B implementation)
     ///
     /// # Phase 200-B Implementation
@@ -351,4 +428,65 @@ mod tests {
             .unwrap_err()
             .contains("undefined_var"));
     }
+
+    /// Phase 201: Test that v2 API uses JoinValueSpace correctly
+    #[test]
+    fn test_build_for_break_condition_v2_uses_param_region() {
+        use crate::mir::join_ir::lowering::join_value_space::JoinValueSpace;
+
+        // Condition: i < max
+        let condition = ASTNode::BinaryOp {
+            operator: crate::ast::BinaryOperator::Less,
+            left: Box::new(ASTNode::Variable {
+                name: "i".to_string(),
+                span: Span::unknown(),
+            }),
+            right: Box::new(ASTNode::Variable {
+                name: "max".to_string(),
+                span: Span::unknown(),
+            }),
+            span: Span::unknown(),
+        };
+
+        let mut variable_map = BTreeMap::new();
+        variable_map.insert("i".to_string(), ValueId(100));
+        variable_map.insert("max".to_string(), ValueId(200));
+
+        let mut space = JoinValueSpace::new();
+        let (env, bindings, loop_var_join_id) = ConditionEnvBuilder::build_for_break_condition_v2(
+            &condition,
+            "i",
+            &variable_map,
+            ValueId(100),
+            &mut space,
+        )
+        .unwrap();
+
+        // Phase 201: Loop param should be in Param region (100+)
+        assert_eq!(loop_var_join_id, ValueId(100)); // First param allocation
+        assert_eq!(env.get("i"), Some(ValueId(100)));
+
+        // Phase 201: Condition variable should also be in Param region
+        assert_eq!(env.get("max"), Some(ValueId(101))); // Second param allocation
+        assert_eq!(bindings.len(), 1);
+        assert_eq!(bindings[0].join_value, ValueId(101));
+
+        // Phase 201: Verify no collision with Local region (1000+)
+        let local_id = space.alloc_local();
+        assert_eq!(local_id, ValueId(1000)); // Locals start at 1000
+        assert_ne!(local_id, loop_var_join_id);
+        assert_ne!(local_id, bindings[0].join_value);
+    }
+
+    #[test]
+    fn test_build_loop_param_only_v2() {
+        use crate::mir::join_ir::lowering::join_value_space::JoinValueSpace;
+
+        let mut space = JoinValueSpace::new();
+        let (env, loop_var_join_id) = ConditionEnvBuilder::build_loop_param_only_v2("i", &mut space);
+
+        // Phase 201: Should use Param region
+        assert_eq!(loop_var_join_id, ValueId(100));
+        assert_eq!(env.get("i"), Some(ValueId(100)));
+    }
 }
diff --git a/src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs b/src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs
index 8661a022..48748ea8 100644
--- a/src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs
+++ b/src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs
@@ -144,6 +144,11 @@ impl MirBuilder {
         // Phase 195: Use unified trace
         trace::trace().varmap("pattern2_start", &self.variable_map);
 
+        // Phase 201: Create JoinValueSpace for unified ValueId allocation
+        // This ensures Param IDs (100+) never collide with Local IDs (1000+)
+        use crate::mir::join_ir::lowering::join_value_space::JoinValueSpace;
+        let mut join_value_space = JoinValueSpace::new();
+
         // Phase 200-C: Integrate capture analysis
         use crate::mir::loop_pattern_detection::function_scope_capture::{analyze_captured_vars_v2, CapturedEnv};
         use super::condition_env_builder::ConditionEnvBuilder;
@@ -170,21 +175,24 @@ impl MirBuilder {
                 var.name, var.host_id, var.is_immutable);
         }
 
-        // Phase 200-C: Use existing path and manually add captured variables
-        // TODO Phase 200-D: Refactor to use build_with_captures with boundary builder
-        let (mut env, mut condition_bindings) = ConditionEnvBuilder::build_for_break_condition(
+        // Phase 201: Use v2 API with JoinValueSpace
+        // This allocates loop param and condition vars from Param region (100+)
+        let (mut env, mut condition_bindings, _loop_var_join_id) = ConditionEnvBuilder::build_for_break_condition_v2(
             condition,
             &loop_var_name,
             &self.variable_map,
             loop_var_id,
+            &mut join_value_space,
         )?;
 
-        // Phase 200-C: Manually add captured variables to env for E2E testing
-        // This is a temporary approach until Phase 200-D refactors the boundary creation
+        eprintln!("[pattern2/phase201] Using JoinValueSpace: loop_var '{}' → {:?}",
+            loop_var_name, env.get(&loop_var_name));
+
+        // Phase 201: Add captured variables using JoinValueSpace
         for var in &captured_env.vars {
             if let Some(&host_id) = self.variable_map.get(&var.name) {
-                // Allocate a JoinIR ValueId for this captured variable
-                let join_id = crate::mir::ValueId(env.len() as u32);
+                // Phase 201: Allocate from Param region to avoid collision with locals
+                let join_id = join_value_space.alloc_param();
                 env.insert(var.name.clone(), join_id);
 
                 // Add to condition_bindings for boundary processing
@@ -194,46 +202,34 @@ impl MirBuilder {
                     join_value: join_id,
                 });
 
-                eprintln!("[pattern2/capture] Manually added captured '{}' to env: host={:?}, join={:?}",
+                eprintln!("[pattern2/capture] Phase 201: Added captured '{}': host={:?}, join={:?}",
                     var.name, host_id, join_id);
             }
         }
 
-        // Phase 190-impl-D: Calculate ValueId offset for body-local variables
-        // JoinIR main() params are: [ValueId(0), ValueId(1), ...] for (loop_var, carrier1, carrier2, ...)
-        // Body-local variables must start AFTER all carrier params to avoid collision.
-        // At this point carrier_info.carriers contains all potential carriers (before filtering).
-        // We reserve space for: env.len() (condition vars) + carrier_info.carriers.len() (carriers)
-        let body_local_start_offset = (env.len() + carrier_info.carriers.len()) as u32;
-
-        // Create allocator for body-local variables (starts after reserved param space)
-        let mut body_local_counter = body_local_start_offset;
-        let mut alloc_body_local_value = || {
-            let id = crate::mir::ValueId(body_local_counter);
-            body_local_counter += 1;
-            id
-        };
+        // Phase 201: Allocate carrier ValueIds from Param region
+        // This ensures carriers are also in the safe Param region
+        for carrier in &carrier_info.carriers {
+            let _carrier_join_id = join_value_space.alloc_param();
+            eprintln!("[pattern2/phase201] Allocated carrier '{}' param ID: {:?}",
+                carrier.name, _carrier_join_id);
+        }
 
         // Phase 191: Create empty body-local environment
         // LoopBodyLocalInitLowerer will populate it during init lowering
         use crate::mir::join_ir::lowering::loop_body_local_env::LoopBodyLocalEnv;
         let mut body_local_env = LoopBodyLocalEnv::new();
 
-        eprintln!("[pattern2/body-local] Phase 191: Created empty body-local environment (offset={})",
-            body_local_start_offset);
+        eprintln!("[pattern2/body-local] Phase 201: Created empty body-local environment (param_count={})",
+            join_value_space.param_count());
 
-        // Create allocator for other JoinIR-local ValueIds (Trim pattern, etc.)
-        // Continues from where body_local_counter left off
-        let mut join_value_counter = body_local_counter;
-        let mut alloc_join_value = || {
-            let id = crate::mir::ValueId(join_value_counter);
-            join_value_counter += 1;
-            id
-        };
+        // Phase 201: Create alloc_join_value closure using JoinValueSpace
+        // This ensures all param allocations go through the unified space
+        let mut alloc_join_value = || join_value_space.alloc_param();
 
         // Debug: Log condition bindings
-        eprintln!("[cf_loop/pattern2] Phase 171-172: ConditionEnv contains {} variables:", env.len());
-        eprintln!("  Loop param '{}' → JoinIR ValueId(0)", loop_var_name);
+        eprintln!("[cf_loop/pattern2] Phase 201: ConditionEnv contains {} variables:", env.len());
+        eprintln!("  Loop param '{}' → JoinIR {:?}", loop_var_name, env.get(&loop_var_name));
         if !condition_bindings.is_empty() {
             eprintln!("  {} condition-only bindings:", condition_bindings.len());
             for binding in &condition_bindings {
diff --git a/src/mir/join_ir/lowering/inline_boundary_builder.rs b/src/mir/join_ir/lowering/inline_boundary_builder.rs
index f49da08e..0a3972cd 100644
--- a/src/mir/join_ir/lowering/inline_boundary_builder.rs
+++ b/src/mir/join_ir/lowering/inline_boundary_builder.rs
@@ -27,6 +27,7 @@
 use crate::mir::ValueId;
 use super::inline_boundary::{JoinInlineBoundary, LoopExitBinding};
 use super::condition_to_joinir::ConditionBinding;
+use super::join_value_space::JoinValueSpace;
 
 /// Role of a parameter in JoinIR lowering (Phase 200-A)
 ///
diff --git a/src/mir/join_ir/lowering/join_value_space.rs b/src/mir/join_ir/lowering/join_value_space.rs
new file mode 100644
index 00000000..6be8435a
--- /dev/null
+++ b/src/mir/join_ir/lowering/join_value_space.rs
@@ -0,0 +1,393 @@
+//! Phase 201: JoinValueSpace - Single source of truth for JoinIR ValueId allocation
+//!
+//! This module provides a unified ValueId allocator for JoinIR lowering to prevent
+//! collisions between different allocation contexts (param vs local vs PHI).
+//!
+//! ## Problem Solved
+//!
+//! Before Phase 201, Pattern 2 frontend used `alloc_join_value()` for env variables,
+//! while JoinIR lowering used a separate `alloc_value()` starting from 0. Both could
+//! produce the same ValueId for different purposes, causing PHI corruption after remapping.
+//!
+//! ## ValueId Space Layout
+//!
+//! ```text
+//!  0          100        1000                     u32::MAX
+//!  ├──────────┼──────────┼──────────────────────────┤
+//!  │  PHI     │  Param   │       Local             │
+//!  │  Reserved│  Region  │       Region            │
+//!  └──────────┴──────────┴──────────────────────────┘
+//! ```
+//!
+//! - **PHI Reserved (0-99)**: Pre-reserved for LoopHeader PHI dst
+//! - **Param Region (100-999)**: For ConditionEnv, CarrierInfo.join_id, CapturedEnv
+//! - **Local Region (1000+)**: For Const, BinOp, etc. in pattern lowerers
+//!
+//! ## Usage
+//!
+//! ```ignore
+//! let mut space = JoinValueSpace::new();
+//!
+//! // Pattern frontend allocates param IDs
+//! let i_param = space.alloc_param(); // ValueId(100)
+//! let v_param = space.alloc_param(); // ValueId(101)
+//!
+//! // PHI builder reserves PHI dst
+//! space.reserve_phi(ValueId(0)); // Mark as reserved
+//!
+//! // JoinIR lowerer allocates local IDs
+//! let const_100 = space.alloc_local(); // ValueId(1000)
+//! let binop_result = space.alloc_local(); // ValueId(1001)
+//!
+//! // No collision possible!
+//! ```
+
+use crate::mir::ValueId;
+use std::collections::HashSet;
+
+/// Region boundaries (can be tuned based on actual usage)
+const PHI_MAX: u32 = 99; // PHI dst range: 0-99
+const PARAM_BASE: u32 = 100; // Param range: 100-999
+const LOCAL_BASE: u32 = 1000; // Local range: 1000+
+
+/// Single source of truth for JoinIR ValueId allocation
+///
+/// All JoinIR ValueId allocation should go through this box to ensure
+/// disjoint regions for Param, Local, and PHI dst IDs.
+#[derive(Debug, Clone)]
+pub struct JoinValueSpace {
+    /// Next available param ID (starts at PARAM_BASE)
+    next_param: u32,
+    /// Next available local ID (starts at LOCAL_BASE)
+    next_local: u32,
+    /// Reserved PHI dst IDs (debug verification only)
+    reserved_phi: HashSet<u32>,
+}
+
+/// Region classification for ValueIds
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Region {
+    /// PHI Reserved region (0-99)
+    PhiReserved,
+    /// Param region (100-999)
+    Param,
+    /// Local region (1000+)
+    Local,
+    /// Unknown/invalid region
+    Unknown,
+}
+
+impl JoinValueSpace {
+    /// Create a new JoinValueSpace with default regions
+    pub fn new() -> Self {
+        Self {
+            next_param: PARAM_BASE,
+            next_local: LOCAL_BASE,
+            reserved_phi: HashSet::new(),
+        }
+    }
+
+    /// Allocate a parameter ValueId (for ConditionEnv, CarrierInfo, etc.)
+    ///
+    /// Returns ValueId in Param Region (100-999).
+    /// Panics in debug mode if param region overflows.
+    pub fn alloc_param(&mut self) -> ValueId {
+        let id = self.next_param;
+        debug_assert!(
+            id < LOCAL_BASE,
+            "Param region overflow: {} >= {}",
+            id,
+            LOCAL_BASE
+        );
+        self.next_param += 1;
+        ValueId(id)
+    }
+
+    /// Allocate a local ValueId (for Const, BinOp, etc. in lowerers)
+    ///
+    /// Returns ValueId in Local Region (1000+).
+    pub fn alloc_local(&mut self) -> ValueId {
+        let id = self.next_local;
+        self.next_local += 1;
+        ValueId(id)
+    }
+
+    /// Reserve a PHI dst ValueId (called by PHI builder before allocation)
+    ///
+    /// No allocation - just marks the ID as reserved for PHI use.
+    /// This is for debug verification only; the actual PHI dst comes from
+    /// MirBuilder (host side), not JoinValueSpace.
+    pub fn reserve_phi(&mut self, id: ValueId) {
+        debug_assert!(
+            id.0 <= PHI_MAX,
+            "PHI reservation out of range: {} > {}",
+            id.0,
+            PHI_MAX
+        );
+        self.reserved_phi.insert(id.0);
+    }
+
+    /// Check if a ValueId is reserved as PHI dst
+    pub fn is_phi_reserved(&self, id: ValueId) -> bool {
+        self.reserved_phi.contains(&id.0)
+    }
+
+    /// Determine which region a ValueId belongs to
+    pub fn region_of(&self, id: ValueId) -> Region {
+        if id.0 <= PHI_MAX {
+            Region::PhiReserved
+        } else if id.0 < LOCAL_BASE {
+            Region::Param
+        } else {
+            Region::Local
+        }
+    }
+
+    /// Get the current param counter (for debugging)
+    pub fn param_count(&self) -> u32 {
+        self.next_param - PARAM_BASE
+    }
+
+    /// Get the current local counter (for debugging)
+    pub fn local_count(&self) -> u32 {
+        self.next_local - LOCAL_BASE
+    }
+
+    /// Get the number of reserved PHI IDs (for debugging)
+    pub fn phi_reserved_count(&self) -> usize {
+        self.reserved_phi.len()
+    }
+
+    /// Verify no overlap between regions (debug assertion)
+    ///
+    /// This checks that:
+    /// 1. Param region hasn't overflowed into Local region
+    /// 2. Reserved PHI IDs are within PHI region
+    ///
+    /// Returns Ok(()) if valid, Err(message) if invalid.
+    #[cfg(debug_assertions)]
+    pub fn verify_no_overlap(&self) -> Result<(), String> {
+        // Check param region hasn't overflowed
+        if self.next_param >= LOCAL_BASE {
+            return Err(format!(
+                "Param region overflow: next_param={} >= LOCAL_BASE={}",
+                self.next_param, LOCAL_BASE
+            ));
+        }
+
+        // Check all reserved PHI IDs are in PHI region
+        for &phi_id in &self.reserved_phi {
+            if phi_id > PHI_MAX {
+                return Err(format!(
+                    "PHI ID {} is out of PHI region (max={})",
+                    phi_id, PHI_MAX
+                ));
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Create an allocator closure for local IDs
+    ///
+    /// This is a convenience method to create a closure compatible with
+    /// existing lowerer signatures that expect `FnMut() -> ValueId`.
+    ///
+    /// # Example
+    ///
+    /// ```ignore
+    /// let mut space = JoinValueSpace::new();
+    /// let mut alloc_local = space.local_allocator();
+    /// let id1 = alloc_local(); // ValueId(1000)
+    /// let id2 = alloc_local(); // ValueId(1001)
+    /// ```
+    pub fn local_allocator(&mut self) -> impl FnMut() -> ValueId + '_ {
+        move || self.alloc_local()
+    }
+
+    /// Create an allocator closure for param IDs
+    ///
+    /// Similar to local_allocator(), but for param region.
+    pub fn param_allocator(&mut self) -> impl FnMut() -> ValueId + '_ {
+        move || self.alloc_param()
+    }
+}
+
+impl Default for JoinValueSpace {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_new_space_has_correct_initial_values() {
+        let space = JoinValueSpace::new();
+        assert_eq!(space.next_param, PARAM_BASE);
+        assert_eq!(space.next_local, LOCAL_BASE);
+        assert!(space.reserved_phi.is_empty());
+    }
+
+    #[test]
+    fn test_alloc_param_returns_correct_ids() {
+        let mut space = JoinValueSpace::new();
+        let id1 = space.alloc_param();
+        let id2 = space.alloc_param();
+        let id3 = space.alloc_param();
+
+        assert_eq!(id1, ValueId(100));
+        assert_eq!(id2, ValueId(101));
+        assert_eq!(id3, ValueId(102));
+    }
+
+    #[test]
+    fn test_alloc_local_returns_correct_ids() {
+        let mut space = JoinValueSpace::new();
+        let id1 = space.alloc_local();
+        let id2 = space.alloc_local();
+        let id3 = space.alloc_local();
+
+        assert_eq!(id1, ValueId(1000));
+        assert_eq!(id2, ValueId(1001));
+        assert_eq!(id3, ValueId(1002));
+    }
+
+    #[test]
+    fn test_param_and_local_do_not_overlap() {
+        let mut space = JoinValueSpace::new();
+
+        // Allocate many params
+        for _ in 0..100 {
+            space.alloc_param();
+        }
+
+        // Allocate many locals
+        for _ in 0..100 {
+            space.alloc_local();
+        }
+
+        // Param should be in range [100, 200)
+        assert_eq!(space.next_param, 200);
+        // Local should be in range [1000, 1100)
+        assert_eq!(space.next_local, 1100);
+
+        // No overlap possible
+        assert!(space.next_param < LOCAL_BASE);
+    }
+
+    #[test]
+    fn test_reserve_phi() {
+        let mut space = JoinValueSpace::new();
+        space.reserve_phi(ValueId(0));
+        space.reserve_phi(ValueId(5));
+        space.reserve_phi(ValueId(10));
+
+        assert!(space.is_phi_reserved(ValueId(0)));
+        assert!(space.is_phi_reserved(ValueId(5)));
+        assert!(space.is_phi_reserved(ValueId(10)));
+        assert!(!space.is_phi_reserved(ValueId(1)));
+        assert!(!space.is_phi_reserved(ValueId(100)));
+    }
+
+    #[test]
+    fn test_region_of() {
+        let space = JoinValueSpace::new();
+
+        // PHI region
+        assert_eq!(space.region_of(ValueId(0)), Region::PhiReserved);
+        assert_eq!(space.region_of(ValueId(50)), Region::PhiReserved);
+        assert_eq!(space.region_of(ValueId(99)), Region::PhiReserved);
+
+        // Param region
+        assert_eq!(space.region_of(ValueId(100)), Region::Param);
+        assert_eq!(space.region_of(ValueId(500)), Region::Param);
+        assert_eq!(space.region_of(ValueId(999)), Region::Param);
+
+        // Local region
+        assert_eq!(space.region_of(ValueId(1000)), Region::Local);
+        assert_eq!(space.region_of(ValueId(5000)), Region::Local);
+        assert_eq!(space.region_of(ValueId(u32::MAX)), Region::Local);
+    }
+
+    #[test]
+    fn test_counters() {
+        let mut space = JoinValueSpace::new();
+
+        assert_eq!(space.param_count(), 0);
+        assert_eq!(space.local_count(), 0);
+        assert_eq!(space.phi_reserved_count(), 0);
+
+        space.alloc_param();
+        space.alloc_param();
+        space.alloc_local();
+        space.reserve_phi(ValueId(0));
+
+        assert_eq!(space.param_count(), 2);
+        assert_eq!(space.local_count(), 1);
+        assert_eq!(space.phi_reserved_count(), 1);
+    }
+
+    #[cfg(debug_assertions)]
+    #[test]
+    fn test_verify_no_overlap_success() {
+        let mut space = JoinValueSpace::new();
+        space.alloc_param();
+        space.alloc_local();
+        space.reserve_phi(ValueId(0));
+
+        assert!(space.verify_no_overlap().is_ok());
+    }
+
+    #[test]
+    fn test_local_allocator_closure() {
+        let mut space = JoinValueSpace::new();
+        let id1;
+        let id2;
+        {
+            let mut alloc = space.local_allocator();
+            id1 = alloc();
+            id2 = alloc();
+        }
+
+        assert_eq!(id1, ValueId(1000));
+        assert_eq!(id2, ValueId(1001));
+    }
+
+    #[test]
+    fn test_param_allocator_closure() {
+        let mut space = JoinValueSpace::new();
+        let id1;
+        let id2;
+        {
+            let mut alloc = space.param_allocator();
+            id1 = alloc();
+            id2 = alloc();
+        }
+
+        assert_eq!(id1, ValueId(100));
+        assert_eq!(id2, ValueId(101));
+    }
+
+    /// Phase 201-A scenario: Verify that the bug case is impossible
+    ///
+    /// Previously: env['v'] = ValueId(7), const 100 dst = ValueId(7) -> collision
+    /// Now: env['v'] = alloc_param() -> ValueId(100+), const 100 = alloc_local() -> ValueId(1000+)
+    #[test]
+    fn test_phase201a_scenario_no_collision() {
+        let mut space = JoinValueSpace::new();
+
+        // Pattern 2 frontend allocates param for carrier 'v'
+        let v_param = space.alloc_param(); // ValueId(100)
+
+        // JoinIR lowering allocates local for const 100
+        let const_100 = space.alloc_local(); // ValueId(1000)
+
+        // They are in different regions - no collision!
+        assert_ne!(v_param, const_100);
+        assert_eq!(space.region_of(v_param), Region::Param);
+        assert_eq!(space.region_of(const_100), Region::Local);
+    }
+}
diff --git a/src/mir/join_ir/lowering/mod.rs b/src/mir/join_ir/lowering/mod.rs
index ed7589af..ff5f37db 100644
--- a/src/mir/join_ir/lowering/mod.rs
+++ b/src/mir/join_ir/lowering/mod.rs
@@ -47,6 +47,7 @@ pub mod if_phi_spec; // Phase 61-2
 pub(crate) mod if_select; // Phase 33: Internal If/Select lowering
 pub mod inline_boundary; // Phase 188-Impl-3: JoinIR→Host boundary
 pub mod inline_boundary_builder; // Phase 200-2: Builder pattern for JoinInlineBoundary
+pub mod join_value_space; // Phase 201: Unified JoinIR ValueId allocation
 pub(crate) mod loop_form_intake; // Internal loop form intake
 pub(crate) mod loop_pattern_router; // Phase 33-12: Loop pattern routing (re-exported)
 pub(crate) mod loop_pattern_validator; // Phase 33-23: Loop structure validation