diff --git a/docker/walrus-antithesis/build-test-config-image/docker-compose.yaml b/docker/walrus-antithesis/build-test-config-image/docker-compose.yaml
index 1df37da4e2..fd2ebdd993 100644
--- a/docker/walrus-antithesis/build-test-config-image/docker-compose.yaml
+++ b/docker/walrus-antithesis/build-test-config-image/docker-compose.yaml
@@ -7,6 +7,7 @@
 # - A setup completion service
 # - 4 Walrus nodes for testing
 # - 1 stress client for testing
+# - 1 cross-node invariant observer
 services:
   # Runs a local Sui network with a single validator
   sui-localnet:
@@ -173,6 +174,31 @@ services:
     command: >
       /bin/bash -c "sleep 15 && /root/run-staking.sh"
 
+  # Cross-node invariant observer — scrapes Prometheus metrics from all storage
+  # nodes and verifies data consistency across the cluster. Crashes on violation,
+  # which Antithesis surfaces as a test failure.
+  walrus-observer:
+    depends_on:
+      walrus-deploy:
+        condition: service_completed_successfully
+    networks:
+      testbed-network:
+        ipv4_address: 10.0.0.40
+    image: ${WALRUS_IMAGE_NAME}
+    platform: ${WALRUS_PLATFORM:-linux/amd64}
+    hostname: walrus-observer
+    container_name: walrus-observer
+    environment:
+      - NO_COLOR=1
+      - CHECK_INTERVAL=60
+      - INITIAL_WAIT=180
+      - EVENT_SOURCE_PATIENCE=3
+      - FULLY_STORED_PATIENCE=3
+    volumes:
+      - ./files/run-observer.sh:/root/run-observer.sh
+    command: >
+      /bin/bash -c "/root/run-observer.sh"
+
 # Persistent volumes for sharing data between containers
 volumes:
   sui-bin:
diff --git a/docker/walrus-antithesis/build-test-config-image/files/run-observer.sh b/docker/walrus-antithesis/build-test-config-image/files/run-observer.sh
new file mode 100755
index 0000000000..bb34615da8
--- /dev/null
+++ b/docker/walrus-antithesis/build-test-config-image/files/run-observer.sh
@@ -0,0 +1,240 @@
+#!/bin/bash
+# Copyright (c) Walrus Foundation
+# SPDX-License-Identifier: Apache-2.0
+
+# Cross-node invariant observer for Antithesis testing.
+#
+# Periodically scrapes Prometheus metrics from all storage nodes and verifies
+# cross-node data consistency invariants. Crashes (exits non-zero) when an
+# invariant violation is detected, which Antithesis surfaces as a test failure.
+#
+# Hard invariants (crash on first confirmed violation):
+#   - walrus_blob_info_consistency_check           — same digest per epoch across all nodes
+#   - walrus_per_object_blob_info_consistency_check — same digest per epoch across all nodes
+#
+# Soft invariants (crash after persistent violation):
+#   - walrus_periodic_event_source_for_deterministic_events — same per bucket across all nodes
+#   - walrus_node_blob_data_fully_stored_ratio               — must equal 1 on every node/epoch
+
+set -euo pipefail
+
+# ---------------------------------------------------------------------------
+# Configuration (override via environment variables)
+# ---------------------------------------------------------------------------
+NODES=("10.0.0.10" "10.0.0.11" "10.0.0.12" "10.0.0.13")
+METRICS_PORT="${METRICS_PORT:-9184}"
+CHECK_INTERVAL="${CHECK_INTERVAL:-60}"
+INITIAL_WAIT="${INITIAL_WAIT:-180}"
+# Consecutive rounds a soft-invariant violation must persist before crashing.
+EVENT_SOURCE_PATIENCE="${EVENT_SOURCE_PATIENCE:-3}"
+FULLY_STORED_PATIENCE="${FULLY_STORED_PATIENCE:-3}"
+
+WORK_DIR="/tmp/observer"
+mkdir -p "$WORK_DIR"
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+log()  { echo "[$(date -u '+%Y-%m-%dT%H:%M:%SZ')] [observer] $*"; }
+die()  { log "FATAL: $*" >&2; exit 1; }
+
+# Scrape the /metrics endpoint of a storage node.
+scrape_node() {
+    local ip="$1" out="$2"
+    curl -sf --connect-timeout 5 --max-time 10 \
+        "http://${ip}:${METRICS_PORT}/metrics" > "$out" 2>/dev/null
+}
+
+# Extract (label_value, metric_value) pairs from a Prometheus scrape file.
+# Outputs: label_value<TAB>metric_value  (sorted by label).
+extract_metric() {
+    local file="$1" metric="$2" label="$3"
+    { grep "^${metric}{" "$file" 2>/dev/null || true; } \
+        | sed -n "s/^${metric}{.*${label}=\"\([^\"]*\)\".*} \([^ ]*\).*/\1\t\2/p" \
+        | sort -t"$(printf '\t')" -k1,1
+}
+
+# ---------------------------------------------------------------------------
+# Cross-node comparison for a labelled gauge metric.
+#
+# For every label value present in ALL scraped nodes, assert that the metric
+# value is identical.  Writes per-violation details to a file.
+#
+# Arguments: metric_name  label_name  details_file
+# Echoes the number of mismatched label values (0 = clean).
+# ---------------------------------------------------------------------------
+check_cross_node_metric() {
+    local metric="$1" label="$2" details_file="$3"
+    local num_nodes=${#NODES[@]}
+    local violations=0
+
+    : > "$details_file"
+
+    for i in "${!NODES[@]}"; do
+        extract_metric "$WORK_DIR/raw_${NODES[$i]}.prom" "$metric" "$label" \
+            > "$WORK_DIR/chk_${i}.tsv"
+    done
+
+    # Labels that appear in every node's output.
+    for i in "${!NODES[@]}"; do
+        cut -f1 "$WORK_DIR/chk_${i}.tsv"
+    done | sort | uniq -c | awk -v n="$num_nodes" '$1 == n { print $2 }' \
+        > "$WORK_DIR/chk_common.txt"
+
+    while IFS= read -r lbl; do
+        [ -z "$lbl" ] && continue
+        local ref_val="" mismatch=false
+
+        for i in "${!NODES[@]}"; do
+            local val
+            val=$(awk -F'\t' -v l="$lbl" '$1 == l { print $2 }' "$WORK_DIR/chk_${i}.tsv")
+            if [ -z "$ref_val" ]; then
+                ref_val="$val"
+            elif [ "$val" != "$ref_val" ]; then
+                mismatch=true
+            fi
+        done
+
+        if [ "$mismatch" = true ]; then
+            {
+                echo "  ${metric}{${label}=\"${lbl}\"}:"
+                for i in "${!NODES[@]}"; do
+                    local val
+                    val=$(awk -F'\t' -v l="$lbl" '$1 == l { print $2 }' "$WORK_DIR/chk_${i}.tsv")
+                    echo "    node-${i} (${NODES[$i]}): ${val}"
+                done
+            } >> "$details_file"
+            violations=$((violations + 1))
+        fi
+    done < "$WORK_DIR/chk_common.txt"
+
+    echo "$violations"
+}
+
+# ---------------------------------------------------------------------------
+# Check walrus_node_blob_data_fully_stored_ratio == 1 for all nodes/epochs.
+# Echoes the number of violations.
+# ---------------------------------------------------------------------------
+check_fully_stored_ratio() {
+    local metric="walrus_node_blob_data_fully_stored_ratio"
+    local violations=0
+
+    for i in "${!NODES[@]}"; do
+        while IFS="$(printf '\t')" read -r epoch ratio; do
+            [ -z "$epoch" ] && continue
+            # Numeric comparison: ratio must equal 1.
+            if ! awk -v r="$ratio" 'BEGIN { exit (r == 1) ? 0 : 1 }'; then
+                log "  node-${i} (${NODES[$i]}) epoch=${epoch} ratio=${ratio}" >&2
+                violations=$((violations + 1))
+            fi
+        done < <(extract_metric "$WORK_DIR/raw_${NODES[$i]}.prom" "$metric" "epoch")
+    done
+
+    echo "$violations"
+}
+
+# ---------------------------------------------------------------------------
+# Main loop
+# ---------------------------------------------------------------------------
+log "Cross-node invariant observer starting"
+log "Nodes: ${NODES[*]}, port: ${METRICS_PORT}"
+log "Check interval: ${CHECK_INTERVAL}s, initial wait: ${INITIAL_WAIT}s"
+log "Event source patience: ${EVENT_SOURCE_PATIENCE} rounds"
+log "Fully stored patience: ${FULLY_STORED_PATIENCE} rounds"
+
+log "Waiting ${INITIAL_WAIT}s for cluster stabilization..."
+sleep "$INITIAL_WAIT"
+
+event_source_streak=0
+fully_stored_streak=0
+round=0
+
+while true; do
+    round=$((round + 1))
+    log "=== Round ${round} ==="
+
+    # ------------------------------------------------------------------
+    # Scrape all nodes; skip the round if any node is unreachable.
+    # ------------------------------------------------------------------
+    all_ok=true
+    for i in "${!NODES[@]}"; do
+        if scrape_node "${NODES[$i]}" "$WORK_DIR/raw_${NODES[$i]}.prom"; then
+            log "Scraped node-${i} (${NODES[$i]})"
+        else
+            log "Cannot reach node-${i} (${NODES[$i]}), skipping round"
+            all_ok=false
+            break
+        fi
+    done
+    if [ "$all_ok" = false ]; then
+        sleep "$CHECK_INTERVAL"
+        continue
+    fi
+
+    # ------------------------------------------------------------------
+    # Hard invariant 1: certified blob digest must match across nodes.
+    # ------------------------------------------------------------------
+    v=$(check_cross_node_metric \
+        "walrus_blob_info_consistency_check" "epoch" \
+        "$WORK_DIR/details_blob_info.txt")
+    if [ "$v" -gt 0 ]; then
+        log "INVARIANT VIOLATION — blob_info_consistency_check (${v} epoch(s)):"
+        cat "$WORK_DIR/details_blob_info.txt"
+        die "blob_info_consistency_check: mismatched digests across nodes"
+    fi
+    log "walrus_blob_info_consistency_check: OK"
+
+    # ------------------------------------------------------------------
+    # Hard invariant 2: per-object blob digest must match across nodes.
+    # ------------------------------------------------------------------
+    v=$(check_cross_node_metric \
+        "walrus_per_object_blob_info_consistency_check" "epoch" \
+        "$WORK_DIR/details_per_object.txt")
+    if [ "$v" -gt 0 ]; then
+        log "INVARIANT VIOLATION — per_object_blob_info_consistency_check (${v} epoch(s)):"
+        cat "$WORK_DIR/details_per_object.txt"
+        die "per_object_blob_info_consistency_check: mismatched digests across nodes"
+    fi
+    log "walrus_per_object_blob_info_consistency_check: OK"
+
+    # ------------------------------------------------------------------
+    # Soft invariant 1: event source consistency (tolerate transient lag).
+    # Nodes may be at different event-processing positions, so bucket
+    # values can temporarily diverge. Crash only after a persistent
+    # mismatch across EVENT_SOURCE_PATIENCE consecutive rounds.
+    # ------------------------------------------------------------------
+    v=$(check_cross_node_metric \
+        "walrus_periodic_event_source_for_deterministic_events" "bucket" \
+        "$WORK_DIR/details_event_source.txt")
+    if [ "$v" -gt 0 ]; then
+        event_source_streak=$((event_source_streak + 1))
+        log "Event source mismatch (streak: ${event_source_streak}/${EVENT_SOURCE_PATIENCE}):"
+        cat "$WORK_DIR/details_event_source.txt"
+        if [ "$event_source_streak" -ge "$EVENT_SOURCE_PATIENCE" ]; then
+            die "periodic_event_source: persistent mismatch for ${EVENT_SOURCE_PATIENCE} consecutive rounds"
+        fi
+    else
+        event_source_streak=0
+        log "walrus_periodic_event_source_for_deterministic_events: OK"
+    fi
+
+    # ------------------------------------------------------------------
+    # Soft invariant 2: all blobs fully stored (tolerate recovery lag).
+    # A node that just recovered may briefly report < 1 while syncing
+    # completes.  Crash only after FULLY_STORED_PATIENCE rounds.
+    # ------------------------------------------------------------------
+    v=$(check_fully_stored_ratio)
+    if [ "$v" -gt 0 ]; then
+        fully_stored_streak=$((fully_stored_streak + 1))
+        log "Fully-stored-ratio violation (streak: ${fully_stored_streak}/${FULLY_STORED_PATIENCE})"
+        if [ "$fully_stored_streak" -ge "$FULLY_STORED_PATIENCE" ]; then
+            die "node_blob_data_fully_stored_ratio: persistent violation for ${FULLY_STORED_PATIENCE} consecutive rounds"
+        fi
+    else
+        fully_stored_streak=0
+        log "walrus_node_blob_data_fully_stored_ratio: OK"
+    fi
+
+    log "All checks passed for round ${round}"
+    sleep "$CHECK_INTERVAL"
+done