diff --git a/tools/smokes/v2/lib/output_validator.sh b/tools/smokes/v2/lib/output_validator.sh
new file mode 100644
index 00000000..8a9f6aca
--- /dev/null
+++ b/tools/smokes/v2/lib/output_validator.sh
@@ -0,0 +1,76 @@
+#!/bin/bash
+# Output Validator SSOT
+# Single Source of Truth for smoke test output validation
+#
+# Box-First principle: Separation of concerns
+# - Extract: Pattern-based extraction (numeric lines, etc.)
+# - Assert: Comparison and failure reporting
+#
+# Fail-Fast principle: Explicit errors with clear messages
+
+set -e
+
+# Extract numeric lines from output
+# Usage: extract_numeric_lines N < output.txt
+# Returns: First N lines that match numeric pattern (including negative numbers)
+extract_numeric_lines() {
+  local limit="$1"
+  if [ -z "$limit" ]; then
+    echo "[ERROR] extract_numeric_lines: limit parameter required" >&2
+    return 1
+  fi
+
+  # Pattern: optional minus, followed by digits
+  # Use head to limit output
+  grep -E '^-?[0-9]+$' | head -n "$limit" | tr -d '\r'
+}
+
+# Assert multiline string equality
+# Usage: assert_equals_multiline EXPECTED ACTUAL
+# Returns: 0 if equal, 1 if different (with error message)
+assert_equals_multiline() {
+  local expected="$1"
+  local actual="$2"
+
+  if [ -z "$expected" ]; then
+    echo "[ERROR] assert_equals_multiline: expected parameter required" >&2
+    return 1
+  fi
+
+  if [ "$actual" = "$expected" ]; then
+    return 0
+  else
+    echo "[FAIL] Output mismatch" >&2
+    echo "[EXPECTED]:" >&2
+    echo "$expected" >&2
+    echo "[ACTUAL]:" >&2
+    echo "$actual" >&2
+    return 1
+  fi
+}
+
+# Validate numeric output (extract + assert in one step)
+# Usage: validate_numeric_output EXPECTED_LINES EXPECTED_VALUE OUTPUT
+# Returns: 0 if valid, 1 if invalid
+validate_numeric_output() {
+  local expected_lines="$1"
+  local expected_value="$2"
+  local output="$3"
+
+  if [ -z "$expected_lines" ] || [ -z "$expected_value" ]; then
+    echo "[ERROR] validate_numeric_output: expected_lines and expected_value required" >&2
+    return 1
+  fi
+
+  local clean
+  clean=$(printf "%s\n" "$output" | extract_numeric_lines "$expected_lines")
+
+  assert_equals_multiline "$expected_value" "$clean"
+}
+
+# Box-First design notes:
+# 1. extract_numeric_lines: Single Responsibility - extraction only
+# 2. assert_equals_multiline: Single Responsibility - comparison only
+# 3. validate_numeric_output: Composition box - combines extract + assert
+# 4. All functions follow Fail-Fast principle - explicit parameter checks
+# 5. Clear separation of concerns - easy to extend with new patterns
diff --git a/tools/smokes/v2/profiles/integration/apps/phase103_if_only_early_return_vm.sh b/tools/smokes/v2/profiles/integration/apps/phase103_if_only_early_return_vm.sh
index 444349f4..03eb346e 100644
--- a/tools/smokes/v2/profiles/integration/apps/phase103_if_only_early_return_vm.sh
+++ b/tools/smokes/v2/profiles/integration/apps/phase103_if_only_early_return_vm.sh
@@ -2,6 +2,7 @@
 # Phase 103 P1: if-only early return regression (VM)
 
 source "$(dirname "$0")/../../../lib/test_runner.sh"
+source "$(dirname "$0")/../../../lib/output_validator.sh"
 export SMOKES_USE_PYVM=0
 require_env || exit 2
 
@@ -26,12 +27,10 @@ if [ "$EXIT_CODE" -eq 124 ]; then
   FAIL_COUNT=$((FAIL_COUNT + 1))
 elif [ "$EXIT_CODE" -eq 0 ]; then
   EXPECTED=$'7\n2'
-  CLEAN=$(printf "%s\n" "$OUTPUT" | grep -E '^-?[0-9]+$' | head -n 2 | paste -sd '\n' - | tr -d '\r')
-  if [ "$CLEAN" = "$EXPECTED" ]; then
+  if validate_numeric_output 2 "$EXPECTED" "$OUTPUT"; then
     echo "[PASS] Output verified: 7 then 2"
     PASS_COUNT=$((PASS_COUNT + 1))
   else
-    echo "[FAIL] Unexpected output (expected lines: 7 then 2)"
     echo "[INFO] output (tail):"
     echo "$OUTPUT" | tail -n 50 || true
     FAIL_COUNT=$((FAIL_COUNT + 1))
diff --git a/tools/smokes/v2/profiles/integration/apps/phase103_if_only_vm.sh b/tools/smokes/v2/profiles/integration/apps/phase103_if_only_vm.sh
index 20b1a2f9..24c01dd6 100644
--- a/tools/smokes/v2/profiles/integration/apps/phase103_if_only_vm.sh
+++ b/tools/smokes/v2/profiles/integration/apps/phase103_if_only_vm.sh
@@ -4,6 +4,7 @@
 # Verifies that if-only lowering (nested if + merge) is stable in VM backend.
 
 source "$(dirname "$0")/../../../lib/test_runner.sh"
+source "$(dirname "$0")/../../../lib/output_validator.sh"
 export SMOKES_USE_PYVM=0
 require_env || exit 2
 
@@ -28,12 +29,10 @@ if [ "$EXIT_CODE" -eq 124 ]; then
   FAIL_COUNT=$((FAIL_COUNT + 1))
 elif [ "$EXIT_CODE" -eq 0 ]; then
   EXPECTED="2"
-  CLEAN=$(printf "%s\n" "$OUTPUT" | grep -E '^-?[0-9]+$' | head -n 1 | tr -d '\r')
-  if [ "$CLEAN" = "$EXPECTED" ]; then
+  if validate_numeric_output 1 "$EXPECTED" "$OUTPUT"; then
     echo "[PASS] Output verified: 2"
     PASS_COUNT=$((PASS_COUNT + 1))
   else
-    echo "[FAIL] Unexpected output (expected: 2)"
     echo "[INFO] output (tail):"
     echo "$OUTPUT" | tail -n 50 || true
     FAIL_COUNT=$((FAIL_COUNT + 1))
diff --git a/tools/smokes/v2/profiles/integration/apps/phase113_if_only_partial_assign_vm.sh b/tools/smokes/v2/profiles/integration/apps/phase113_if_only_partial_assign_vm.sh
index c4a17d87..558630ed 100644
--- a/tools/smokes/v2/profiles/integration/apps/phase113_if_only_partial_assign_vm.sh
+++ b/tools/smokes/v2/profiles/integration/apps/phase113_if_only_partial_assign_vm.sh
@@ -4,6 +4,7 @@
 # Verifies that if-only lowering with partial assignment preserves original value on else side.
 
 source "$(dirname "$0")/../../../lib/test_runner.sh"
+source "$(dirname "$0")/../../../lib/output_validator.sh"
 export SMOKES_USE_PYVM=0
 require_env || exit 2
 
@@ -28,12 +29,10 @@ if [ "$EXIT_CODE" -eq 124 ]; then
   FAIL_COUNT=$((FAIL_COUNT + 1))
 elif [ "$EXIT_CODE" -eq 0 ]; then
   EXPECTED=$'1\n2'
-  CLEAN=$(printf "%s\n" "$OUTPUT" | grep -E '^-?[0-9]+$' | head -n 2 | tr -d '\r')
-  if [ "$CLEAN" = "$EXPECTED" ]; then
+  if validate_numeric_output 2 "$EXPECTED" "$OUTPUT"; then
     echo "[PASS] Output verified: 1\\n2"
     PASS_COUNT=$((PASS_COUNT + 1))
   else
-    echo "[FAIL] Unexpected output (expected: 1\\n2)"
     echo "[INFO] output (tail):"
     echo "$OUTPUT" | tail -n 50 || true
     FAIL_COUNT=$((FAIL_COUNT + 1))
diff --git a/tools/smokes/v2/profiles/integration/apps/phase114_if_only_return_then_post_vm.sh b/tools/smokes/v2/profiles/integration/apps/phase114_if_only_return_then_post_vm.sh
index aad52152..acdde57f 100644
--- a/tools/smokes/v2/profiles/integration/apps/phase114_if_only_return_then_post_vm.sh
+++ b/tools/smokes/v2/profiles/integration/apps/phase114_if_only_return_then_post_vm.sh
@@ -4,6 +4,7 @@
 # Verifies that if-only with early return and post-if statements works correctly.
 
 source "$(dirname "$0")/../../../lib/test_runner.sh"
+source "$(dirname "$0")/../../../lib/output_validator.sh"
 export SMOKES_USE_PYVM=0
 require_env || exit 2
 
@@ -28,12 +29,10 @@ if [ "$EXIT_CODE" -eq 124 ]; then
   FAIL_COUNT=$((FAIL_COUNT + 1))
 elif [ "$EXIT_CODE" -eq 0 ]; then
   EXPECTED=$'7\n2'
-  CLEAN=$(printf "%s\n" "$OUTPUT" | grep -E '^-?[0-9]+$' | head -n 2 | tr -d '\r')
-  if [ "$CLEAN" = "$EXPECTED" ]; then
+  if validate_numeric_output 2 "$EXPECTED" "$OUTPUT"; then
     echo "[PASS] Output verified: 7\\n2"
     PASS_COUNT=$((PASS_COUNT + 1))
   else
-    echo "[FAIL] Unexpected output (expected: 7\\n2)"
     echo "[INFO] output (tail):"
     echo "$OUTPUT" | tail -n 50 || true
     FAIL_COUNT=$((FAIL_COUNT + 1))