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+# SPIKE INDEX 500–549
+
+This index tracks the first batch of proposed cross-boundary integration spikes for PHASE.
+
+Status: seed/index placeholder, not implemented in this branch.
+
+## Notes
+
+- The second batch intentionally does not duplicate issues already captured in this sequence.
+- New spike work should be recorded here first, then implemented behind feature flags and documented in PROJECT_ARCHITECTURE.md and docs/TECHNICAL.md as required.
+
+## Batch scope
+
+- Security and distributor invariants
+- Retry-safe write paths
+- Gateway/resilience hardening
+- Contract drift and migration safety
+
+See the second-batch index for the next proposed set: docs/spikes/SPIKE_INDEX_550_599.md