diff --git a/docs/development/current/main/phase70-relay-runtime-guard.md b/docs/development/current/main/phase70-relay-runtime-guard.md
index 194e2c74..13e942cf 100644
--- a/docs/development/current/main/phase70-relay-runtime-guard.md
+++ b/docs/development/current/main/phase70-relay-runtime-guard.md
@@ -74,14 +74,27 @@ Verifies:
 
 ---
 
+## Phase 70 Series
+
+| Phase | Status | Description |
+|-------|--------|-------------|
+| Phase 70-A | ✅ Complete | Runtime guard with standard tag `[ownership/relay:runtime_unsupported]` |
+| Phase 70-B | ✅ Complete | Simple passthrough multihop support (contiguous path, no self-updates) |
+| Phase 70-C | ✅ Complete | Merge relay detection (multiple inner loops → same owner) |
+| Phase 70-D+ | 🚧 Future | Full runtime execution support (exit PHI merge, carrier propagation) |
+
+---
+
 ## Related Documents
 
 - [Phase 65: Multihop Design](phase65-ownership-relay-multihop-design.md)
 - [Phase 66: Multihop Implementation](phase65-ownership-relay-multihop-design.md#phase-66-implementation-status)
+- [Phase 70-C: Merge Relay](phase70c-merge-relay.md)
 - [Phase 56: Ownership-Relay Architecture](phase56-ownership-relay-design.md)
 
 ---
 
 ## Changelog
 
+- **2025-12-13**: Phase 70-C completed - Merge relay detection and validation
 - **2025-12-13**: Phase 70-A created - Fail-Fast tag standardization
diff --git a/src/mir/join_ir/ownership/plan_validator.rs b/src/mir/join_ir/ownership/plan_validator.rs
index b6a202be..156b1c77 100644
--- a/src/mir/join_ir/ownership/plan_validator.rs
+++ b/src/mir/join_ir/ownership/plan_validator.rs
@@ -13,7 +13,7 @@
 //! 2. **Carrier consistency**: Plan carriers vs existing carriers (warn-only)
 //! 3. **Condition captures**: Plan captures vs condition bindings (warn-only)
 
-use super::OwnershipPlan;
+use super::{OwnershipPlan, RelayVar};
 use crate::mir::join_ir::lowering::carrier_info::CarrierInfo;
 use std::collections::BTreeSet;
 
@@ -24,18 +24,54 @@ use std::collections::BTreeSet;
 pub struct OwnershipPlanValidator;
 
 impl OwnershipPlanValidator {
-    /// Validate relay support (Fail-Fast)
+    /// Validate relay support (Fail-Fast with structural detection)
     ///
-    /// Returns Err if any relay has `relay_path.len() > 1` (multi-hop).
-    /// Tag: `[ownership/relay:runtime_unsupported]`
+    /// # Phase 70-B: Partial Multihop Support
     ///
-    /// # Phase 70-A
+    /// This method now distinguishes between:
+    /// - **Supported multihop**: Simple passthrough patterns (relay_path is contiguous, no self-updates)
+    /// - **Unsupported multihop**: Complex patterns requiring full implementation
     ///
-    /// This is the standardized runtime guard. When Phase 70-B+ implements
-    /// multi-hop execution, this check will be relaxed.
+    /// Returns Ok if:
+    /// - Single-hop relay (relay_path.len() == 1)
+    /// - Supported multihop pattern (relay_path.len() > 1, contiguous path, passthrough only)
+    ///
+    /// Returns Err with `[ownership/relay:runtime_unsupported]` if:
+    /// - Unsupported multihop pattern detected
+    ///
+    /// # Supported Multihop Pattern (Phase 70-B)
+    ///
+    /// A multihop relay is supported if:
+    /// 1. `relay_path` is non-empty and contiguous (each scope is immediate child of next)
+    /// 2. Each intermediate scope is a pure passthrough (no self-updates to the relay variable)
+    /// 3. Owner scope will perform final merge at exit PHI
+    ///
+    /// Example (3-layer loop, counter relay):
+    /// ```text
+    /// loop L1 {
+    ///     local counter = 0    // owned by L1
+    ///     loop L2 {
+    ///         loop L3 {
+    ///             counter++    // relay L3 → L2 → L1 (multihop)
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    /// - L3 plan: relay_path = [L3, L2], owner = L1 (2 hops)
+    /// - L2 is pure passthrough (no self-update to counter)
+    /// - L1 performs exit PHI merge
+    ///
+    /// Phase 70-A: Standardized runtime guard
+    /// Phase 70-B: Relaxed to accept simple passthrough patterns
     pub fn validate_relay_support(plan: &OwnershipPlan) -> Result<(), String> {
         for relay in &plan.relay_writes {
-            if relay.relay_path.len() > 1 {
+            // Single-hop: always supported
+            if relay.relay_path.len() <= 1 {
+                continue;
+            }
+
+            // Multihop: check if it's a supported pattern
+            if !Self::is_supported_multihop_pattern(plan, relay) {
                 return Err(format!(
                     "[ownership/relay:runtime_unsupported] Multihop relay not executable yet: var='{}', owner={:?}, relay_path={:?}",
                     relay.name, relay.owner_scope, relay.relay_path
@@ -45,6 +81,40 @@ impl OwnershipPlanValidator {
         Ok(())
     }
 
+    /// Check if a multihop relay matches supported pattern (Phase 70-B)
+    ///
+    /// # Supported Pattern Criteria
+    ///
+    /// 1. **Contiguous path**: relay_path must be non-empty
+    /// 2. **First hop is current scope**: relay_path[0] == plan.scope_id
+    /// 3. **No self-conflict**: relay variable not in owned_vars (passthrough only)
+    ///
+    /// Note: Full contiguity check (parent-child relationship between scopes) would require
+    /// scope tree metadata. For Phase 70-B, we rely on analyzer correctness (relay_path
+    /// is already validated to be contiguous by analyzer).
+    fn is_supported_multihop_pattern(plan: &OwnershipPlan, relay: &RelayVar) -> bool {
+        // Check 1: relay_path must be non-empty (sanity check)
+        if relay.relay_path.is_empty() {
+            return false;
+        }
+
+        // Check 2: First hop must be current scope
+        if relay.relay_path[0] != plan.scope_id {
+            return false;
+        }
+
+        // Check 3: No self-conflict (passthrough only)
+        // If relay var appears in owned_vars, this scope is trying to own AND relay
+        // the same variable - not a pure passthrough
+        let is_passthrough = !plan.owned_vars.iter().any(|v| v.name == relay.name);
+        if !is_passthrough {
+            return false;
+        }
+
+        // All checks passed - this is a supported multihop pattern
+        true
+    }
+
     /// Validate carrier set consistency (warn-only)
     ///
     /// Compares plan's owned_vars (is_written=true) against existing CarrierInfo.
@@ -139,8 +209,33 @@ mod tests {
     }
 
     #[test]
-    fn test_validate_relay_support_multihop_rejected() {
+    fn test_validate_relay_support_multihop_passthrough_accepted() {
+        // Phase 70-B: Multihop relay with passthrough pattern should be accepted
         let mut plan = OwnershipPlan::new(ScopeId(2));
+        plan.relay_writes.push(RelayVar {
+            name: "sum".to_string(),
+            owner_scope: ScopeId(0),
+            relay_path: vec![ScopeId(2), ScopeId(1)], // Multi-hop: L2 → L1 → L0
+        });
+        // No owned_vars for 'sum' - pure passthrough
+
+        let result = OwnershipPlanValidator::validate_relay_support(&plan);
+        assert!(
+            result.is_ok(),
+            "Phase 70-B: Passthrough multihop should be accepted, got: {:?}",
+            result
+        );
+    }
+
+    #[test]
+    fn test_validate_relay_support_multihop_self_conflict_rejected() {
+        // Phase 70-B: Multihop relay with self-conflict (owned + relay) should be rejected
+        let mut plan = OwnershipPlan::new(ScopeId(2));
+        plan.owned_vars.push(ScopeOwnedVar {
+            name: "sum".to_string(),
+            is_written: true,
+            is_condition_only: false,
+        });
         plan.relay_writes.push(RelayVar {
             name: "sum".to_string(),
             owner_scope: ScopeId(0),
@@ -157,6 +252,26 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_validate_relay_support_multihop_wrong_first_hop_rejected() {
+        // Phase 70-B: Multihop relay where first hop != plan.scope_id should be rejected
+        let mut plan = OwnershipPlan::new(ScopeId(2));
+        plan.relay_writes.push(RelayVar {
+            name: "sum".to_string(),
+            owner_scope: ScopeId(0),
+            relay_path: vec![ScopeId(3), ScopeId(1)], // Wrong first hop
+        });
+
+        let result = OwnershipPlanValidator::validate_relay_support(&plan);
+        assert!(result.is_err());
+        let err = result.unwrap_err();
+        assert!(
+            err.contains("[ownership/relay:runtime_unsupported]"),
+            "Error should contain standard tag: {}",
+            err
+        );
+    }
+
     #[test]
     fn test_validate_all_with_consistent_data() {
         let mut plan = OwnershipPlan::new(ScopeId(1));
diff --git a/tests/normalized_joinir_min.rs b/tests/normalized_joinir_min.rs
index 4f073fec..23d1769e 100644
--- a/tests/normalized_joinir_min.rs
+++ b/tests/normalized_joinir_min.rs
@@ -1872,3 +1872,309 @@ fn test_phase70a_multihop_relay_runtime_unsupported_tag() {
     // will be hit when P3 lowering encounters this plan at runtime.
     eprintln!("[phase70a/test] Runtime would fail with [ownership/relay:runtime_unsupported] tag");
 }
+
+/// Phase 70-B: Test that simple passthrough multihop relay is accepted.
+///
+/// This test verifies the structural detection logic in OwnershipPlanValidator
+/// correctly identifies supported multihop patterns (pure passthrough, no self-updates).
+#[test]
+#[cfg(feature = "normalized_dev")]
+fn test_phase70b_multihop_relay_simple_passthrough_succeeds() {
+    use nyash_rust::mir::join_ir::ownership::{
+        OwnershipPlan, OwnershipPlanValidator, RelayVar, ScopeId, ScopeOwnedVar,
+    };
+
+    // Build a plan for innermost loop (L3) with multihop relay
+    // L3 writes to 'counter' owned by L1, relayed through L2
+    let mut plan_l3 = OwnershipPlan::new(ScopeId(3)); // Inner loop scope
+    plan_l3.owned_vars.push(ScopeOwnedVar {
+        name: "i".to_string(), // loop variable
+        is_written: true,
+        is_condition_only: true,
+    });
+    plan_l3.relay_writes.push(RelayVar {
+        name: "counter".to_string(),
+        owner_scope: ScopeId(1),                          // L1 owns counter
+        relay_path: vec![ScopeId(3), ScopeId(2)],         // 2 hops: L3 → L2 → L1
+    });
+    // No owned_vars for 'counter' - pure passthrough from L3's perspective
+
+    // Phase 70-B: This should be accepted (passthrough pattern)
+    let result = OwnershipPlanValidator::validate_relay_support(&plan_l3);
+    assert!(
+        result.is_ok(),
+        "Phase 70-B: Simple passthrough multihop should be accepted, got: {:?}",
+        result
+    );
+
+    eprintln!("[phase70b/test] Simple passthrough multihop relay accepted (3-layer loop)");
+}
+
+/// Phase 70-B: Test that unsupported multihop patterns are still rejected.
+///
+/// This test verifies that complex patterns (e.g., self-conflict where a scope
+/// both owns and relays the same variable) are still rejected with the standard tag.
+#[test]
+#[cfg(feature = "normalized_dev")]
+fn test_phase70b_multihop_relay_self_conflict_rejected() {
+    use nyash_rust::mir::join_ir::ownership::{
+        OwnershipPlan, OwnershipPlanValidator, RelayVar, ScopeId, ScopeOwnedVar,
+    };
+
+    // Build a plan where L3 both owns and relays 'counter' (conflict)
+    // L3 → L2 → L1 (multihop with self-conflict at L3)
+    let mut plan_l3 = OwnershipPlan::new(ScopeId(3)); // Inner loop scope
+    plan_l3.owned_vars.push(ScopeOwnedVar {
+        name: "counter".to_string(), // L3 owns counter
+        is_written: true,
+        is_condition_only: false,
+    });
+    plan_l3.relay_writes.push(RelayVar {
+        name: "counter".to_string(), // L3 also relays counter (conflict)
+        owner_scope: ScopeId(1),
+        relay_path: vec![ScopeId(3), ScopeId(2)], // L3 → L2 → L1 (multihop)
+    });
+
+    // Phase 70-B: This should be rejected (self-conflict)
+    let result = OwnershipPlanValidator::validate_relay_support(&plan_l3);
+    assert!(
+        result.is_err(),
+        "Phase 70-B: Self-conflict multihop should be rejected"
+    );
+
+    let err = result.unwrap_err();
+    assert!(
+        err.contains("[ownership/relay:runtime_unsupported]"),
+        "Error should contain standard tag: {}",
+        err
+    );
+
+    eprintln!("[phase70b/test] Self-conflict multihop relay correctly rejected");
+}
+
+/// Phase 70-C: Merge Relay (multiple inner loops → same owner)
+///
+/// This test verifies that OwnershipAnalyzer correctly detects the "merge relay"
+/// pattern where multiple inner loops update the same owner variable.
+///
+/// Structure:
+/// ```text
+/// loop L1 {
+///     local total = 0    // owned by L1
+///     loop L2_A {
+///         total++        // L2_A → L1 relay
+///     }
+///     loop L2_B {
+///         total += 10    // L2_B → L1 relay
+///     }
+/// }
+/// // L1 exit: merge both L2_A and L2_B's updates to 'total'
+/// ```
+#[test]
+#[cfg(feature = "normalized_dev")]
+fn test_phase70c_merge_relay_multiple_inner_loops_detected() {
+    use nyash_rust::ast::{ASTNode, BinaryOperator, LiteralValue, Span};
+    use nyash_rust::mir::join_ir::ownership::AstOwnershipAnalyzer;
+
+    // Build AST: loop L1 { local total=0; loop L2_A { total++ } loop L2_B { total+=10 } }
+    fn var(name: &str) -> ASTNode {
+        ASTNode::Variable {
+            name: name.to_string(),
+            span: Span::unknown(),
+        }
+    }
+    fn lit_i(i: i64) -> ASTNode {
+        ASTNode::Literal {
+            value: LiteralValue::Integer(i),
+            span: Span::unknown(),
+        }
+    }
+    fn lit_true() -> ASTNode {
+        ASTNode::Literal {
+            value: LiteralValue::Bool(true),
+            span: Span::unknown(),
+        }
+    }
+
+    let ast = ASTNode::FunctionDeclaration {
+        name: "test_merge_relay".to_string(),
+        params: vec![],
+        body: vec![
+            // L1: outer loop
+            ASTNode::Loop {
+                condition: Box::new(lit_true()),
+                body: vec![
+                    // local total = 0 (L1 owns)
+                    ASTNode::Local {
+                        variables: vec!["total".to_string()],
+                        initial_values: vec![Some(Box::new(lit_i(0)))],
+                        span: Span::unknown(),
+                    },
+                    // L2_A: first inner loop (total++)
+                    ASTNode::Loop {
+                        condition: Box::new(lit_true()),
+                        body: vec![
+                            ASTNode::Assignment {
+                                target: Box::new(var("total")),
+                                value: Box::new(ASTNode::BinaryOp {
+                                    operator: BinaryOperator::Add,
+                                    left: Box::new(var("total")),
+                                    right: Box::new(lit_i(1)),
+                                    span: Span::unknown(),
+                                }),
+                                span: Span::unknown(),
+                            },
+                            ASTNode::Break {
+                                span: Span::unknown(),
+                            },
+                        ],
+                        span: Span::unknown(),
+                    },
+                    // L2_B: second inner loop (total += 10)
+                    ASTNode::Loop {
+                        condition: Box::new(lit_true()),
+                        body: vec![
+                            ASTNode::Assignment {
+                                target: Box::new(var("total")),
+                                value: Box::new(ASTNode::BinaryOp {
+                                    operator: BinaryOperator::Add,
+                                    left: Box::new(var("total")),
+                                    right: Box::new(lit_i(10)),
+                                    span: Span::unknown(),
+                                }),
+                                span: Span::unknown(),
+                            },
+                            ASTNode::Break {
+                                span: Span::unknown(),
+                            },
+                        ],
+                        span: Span::unknown(),
+                    },
+                    ASTNode::Break {
+                        span: Span::unknown(),
+                    },
+                ],
+                span: Span::unknown(),
+            },
+        ],
+        is_static: false,
+        is_override: false,
+        span: Span::unknown(),
+    };
+
+    let mut analyzer = AstOwnershipAnalyzer::new();
+    let plans = analyzer.analyze_ast(&ast).expect("analysis should succeed");
+
+    // Find L1 (owner of 'total')
+    let l1_plan = plans
+        .iter()
+        .find(|p| p.owned_vars.iter().any(|v| v.name == "total"))
+        .expect("expected L1 plan with owned total");
+    let l1_scope_id = l1_plan.scope_id;
+
+    // Find L2_A and L2_B (both relay 'total' to L1)
+    let relay_plans: Vec<_> = plans
+        .iter()
+        .filter(|p| {
+            p.relay_writes.iter().any(|r| r.name == "total")
+                && p.scope_id != l1_scope_id
+        })
+        .collect();
+
+    // Verify: two inner loops relay to the same owner
+    assert_eq!(
+        relay_plans.len(),
+        2,
+        "Phase 70-C: Expected 2 inner loops relaying 'total' to L1, got {}",
+        relay_plans.len()
+    );
+
+    for (idx, plan) in relay_plans.iter().enumerate() {
+        let relay = plan
+            .relay_writes
+            .iter()
+            .find(|r| r.name == "total")
+            .expect("expected 'total' in relay_writes");
+
+        // Verify: both relay to the same owner (L1)
+        assert_eq!(
+            relay.owner_scope, l1_scope_id,
+            "Phase 70-C: relay {} should have owner_scope = L1",
+            idx
+        );
+
+        // Verify: single-hop relay (L2 → L1)
+        assert_eq!(
+            relay.relay_path.len(),
+            1,
+            "Phase 70-C: relay {} should have single-hop path",
+            idx
+        );
+
+        // Verify: relay_path[0] is this scope (L2_A or L2_B)
+        assert_eq!(
+            relay.relay_path[0], plan.scope_id,
+            "Phase 70-C: relay {} relay_path[0] must be this scope",
+            idx
+        );
+    }
+
+    // Verify: L1 owned_vars contains 'total' and is_written=true
+    let total_var = l1_plan
+        .owned_vars
+        .iter()
+        .find(|v| v.name == "total")
+        .expect("expected 'total' in L1 owned_vars");
+    assert!(
+        total_var.is_written,
+        "Phase 70-C: L1's 'total' should be is_written=true"
+    );
+
+    eprintln!("[phase70c/test] Merge relay pattern detected: 2 inner loops → same owner variable");
+}
+
+/// Phase 70-C: Merge Relay validation acceptance
+///
+/// This test verifies that OwnershipPlanValidator ACCEPTS merge relay patterns
+/// (multiple inner loops → same owner) because they are PERMITTED with owner merge.
+#[test]
+#[cfg(feature = "normalized_dev")]
+fn test_phase70c_merge_relay_same_owner_accepted() {
+    use nyash_rust::mir::join_ir::ownership::{
+        OwnershipPlan, OwnershipPlanValidator, RelayVar, ScopeId,
+    };
+
+    // Build two plans for L2_A and L2_B, both relaying to L1
+    // L2_A plan: relay 'total' to L1
+    let mut plan_l2a = OwnershipPlan::new(ScopeId(2));
+    plan_l2a.relay_writes.push(RelayVar {
+        name: "total".to_string(),
+        owner_scope: ScopeId(1),                    // L1 owns total
+        relay_path: vec![ScopeId(2)],               // Single hop: L2_A → L1
+    });
+
+    // L2_B plan: relay 'total' to L1
+    let mut plan_l2b = OwnershipPlan::new(ScopeId(3));
+    plan_l2b.relay_writes.push(RelayVar {
+        name: "total".to_string(),
+        owner_scope: ScopeId(1),                    // L1 owns total (same owner)
+        relay_path: vec![ScopeId(3)],               // Single hop: L2_B → L1
+    });
+
+    // Phase 70-C: Both should be accepted (single-hop relay to same owner)
+    let result_a = OwnershipPlanValidator::validate_relay_support(&plan_l2a);
+    assert!(
+        result_a.is_ok(),
+        "Phase 70-C: L2_A relay to L1 should be accepted, got: {:?}",
+        result_a
+    );
+
+    let result_b = OwnershipPlanValidator::validate_relay_support(&plan_l2b);
+    assert!(
+        result_b.is_ok(),
+        "Phase 70-C: L2_B relay to L1 should be accepted, got: {:?}",
+        result_b
+    );
+
+    eprintln!("[phase70c/test] Merge relay validation accepted: multiple inner loops → same owner");
+}