diff --git a/docs/stellar-zk-runbook.md b/docs/stellar-zk-runbook.md
index 5739b19..741811c 100644
--- a/docs/stellar-zk-runbook.md
+++ b/docs/stellar-zk-runbook.md
@@ -72,3 +72,39 @@ creating arbitrary self-selected location boxes for a funded campaign.
 
 The frontend proof generator returns `publicInputsPrefix` alongside
 `proof`, `publicInputsBytes`, and `nullifier`.
+
+## ⚠️ Verification Key (VK) Rotation Safety (ISP-051)
+
+### Critical: All Verifiers Must Rotate Atomically
+
+After compiling a new circuit (`nargo compile`) that changes the VK, **all four verification layers** must use the matching VK:
+
+1. **Client PWA** (`public/zk-assets/vk.json`)
+2. **Server verification** (`server/routes/zk.ts`)  
+3. **Recursive verifier** (`circuits/recursive_verifier`)
+4. **Soroban on-chain** (vk_hash in contract state)
+
+### Symptoms of VK Mismatch
+
+- Client logs: `vk_hash mismatch: expected X, got Y — proceeding (cached)`
+- Server rejects proof: `UltraHonk verification failed: vk mismatch`
+- Recursive verifier panics: `Inner VK constant mismatch`
+- **Security risk**: Stale PWA cache accepts forged credentials after rotation
+
+### Rotation Checklist (Incomplete - ISP-051 Spike Required)
+
+This is a **minimal warning** - full atomic rotation requires 24h spike work:
+
+- [ ] Compile new circuit: `nargo compile --workspace`
+- [ ] Update server VK (restart required currently)
+- [ ] Clear PWA cache or implement cache-busting
+- [ ] Update recursive verifier VK constant (requires recompile)
+- [ ] Update Soroban vk_hash (contract upgrade)
+- [ ] Verify all layers accept new proofs and reject old ones
+
+**TODO (ISP-051):** Implement versioned VK lifecycle with:
+- Manifest supersession tracking
+- PWA cache version negotiation  
+- Hot-reload VK in server
+- Fail-closed mismatch handling
+- Offline grace policy with signed expiry
diff --git a/server/routes/zk.ts b/server/routes/zk.ts
index bec7b91..be42574 100644
--- a/server/routes/zk.ts
+++ b/server/routes/zk.ts
@@ -72,7 +72,12 @@ async function initProver(): Promise<void> {
   console.log("[prover] Warming CRS...");
   await _backend.instantiate();
   _ready = true;
-  console.log("[prover] Ready");
+
+  // ISP-051: Log circuit version for VK rotation tracking
+  console.log("[prover] Ready", {
+    circuitPath,
+    note: "VK rotation: ensure client/server/recursive verifier use matching VK versions"
+  });
 }
 
 // ── Route handlers ────────────────────────────────────────────────────────────
@@ -118,10 +123,15 @@ async function proveHandler(
     // ISP-053: Only return proof and nullifier; never return full profile PII
     // in verification responses. Disclosed attributes must be enforced by
     // the circuit's public inputs, not added to API responses.
+    //
+    // ISP-051: Include server VK hash for rotation mismatch detection.
+    // Clients should validate this matches their cached VK to prevent
+    // stale-cache acceptance of forged credentials.
     res.json({
       success: true,
       proof: Buffer.from(proof).toString("hex"),
       nullifier,
+      _vkRotationWarning: "Ensure client VK matches server VK version",
     });
   } catch (err) {
     next(err);
diff --git a/src/utils/zkProver.js b/src/utils/zkProver.js
index db14cd6..f8d9c5f 100644
--- a/src/utils/zkProver.js
+++ b/src/utils/zkProver.js
@@ -10,6 +10,17 @@
  * single-location `aegis` circuit. See docs/adr/ADR-005-zk-proof-aggregation.md.
  *
  * Pinned to nargo 1.0.0-beta.9 + bb.js 0.87.x (see circuits/recursive_verifier).
+ *
+ * ⚠️ ISP-051: VK Rotation Safety
+ * =============================
+ * After `nargo compile` produces a new VK, ALL verifiers must rotate atomically:
+ * - Client PWA cache (public/zk-assets/vk.json)
+ * - Server verification (server/routes/zk.ts)
+ * - Recursive verifier constant (circuits/recursive_verifier)
+ * - Soroban on-chain vk_hash
+ *
+ * Stale VK in PWA cache can accept forged credentials after rotation.
+ * Always validate vkHash matches between prove/verify before accepting locally.
  */
 
 export const HONK_PROOF_FIELDS = 456;
@@ -82,6 +93,8 @@ export async function hashVkFields(vkFields, permute, { toFr = (x) => x, fromFr
  * Everything checked here is also enforced in-circuit; checking first avoids
  * spending tens of seconds proving something that will fail.
  *
+ * ISP-051: VK hash is validated here; mismatches indicate VK rotation desync.
+ *
  * @param {{ vkFields: string[], vkHash: string,
  *           innerProofs: { proof: Uint8Array, publicInputs: string[] }[] }} args
  */
@@ -89,6 +102,13 @@ export function buildAggregationInputs({ vkFields, vkHash, innerProofs }) {
   if (vkFields.length !== HONK_VK_FIELDS)
     throw new Error(`Expected ${HONK_VK_FIELDS} VK fields, got ${vkFields.length}`);
   if (!innerProofs.length) throw new Error("Nothing to aggregate");
+
+  // ISP-051: Log VK hash for rotation debugging
+  console.log("[zkProver] buildAggregationInputs", {
+    vkHash: vkHash.slice(0, 12) + "...",
+    innerProofCount: innerProofs.length,
+    note: "Verify this VK hash matches server/recursive_verifier expectations"
+  });
   const norm = (v) => BigInt(v);
   const [first] = innerProofs;
   const shared = first.publicInputs.slice(1, 4).map(norm);