diff --git a/src/oracle/oracle-key-format.ts b/src/oracle/oracle-key-format.ts
index a1f3665..d88bd06 100644
--- a/src/oracle/oracle-key-format.ts
+++ b/src/oracle/oracle-key-format.ts
@@ -17,6 +17,9 @@ const RAINFALL_OR_TEMPERATURE_KEY_RE =
   /^(?:rainfall|temperature):(-?\d+(?:\.\d+)?),(-?\d+(?:\.\d+)?):20\d{2}-(?:0[1-9]|1[0-2])$/;
 const FLIGHT_KEY_RE = /^flight:[A-Z0-9]+:20\d{2}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])$/;
 
+/** Maximum oracle key length to prevent DB bloat and oversized-key DoS (#726). */
+export const MAX_ORACLE_KEY_LENGTH = 128;
+
 /**
  * Range-checks the lat/lng embedded in a rainfall/temperature key in addition
  * to the format regex, which only checks that they're signed decimals — same
@@ -24,6 +27,10 @@ const FLIGHT_KEY_RE = /^flight:[A-Z0-9]+:20\d{2}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12
  * src/policy/dto/oracle-key-coordinates.validator.ts.
  */
 export function isValidOracleKeyFormat(key: string): boolean {
+  // Reject oversized keys before regex matching (#726).
+  if (typeof key !== 'string' || key.length > MAX_ORACLE_KEY_LENGTH) {
+    return false;
+  }
   const coordMatch = RAINFALL_OR_TEMPERATURE_KEY_RE.exec(key);
   if (coordMatch) {
     const lat = parseFloat(coordMatch[1]);
diff --git a/src/oracle/oracle.service.ts b/src/oracle/oracle.service.ts
index f8d36b1..18e6b65 100644
--- a/src/oracle/oracle.service.ts
+++ b/src/oracle/oracle.service.ts
@@ -346,7 +346,17 @@ export class OracleService {
   }
 
   /** Get the latest reading for a given oracle key from the database. */
+  // #727 — cache recent reads to avoid repeated identical queries.
+  private readonly readingCache = new Map<string, { value: OracleReading | null; expiresAt: number }>();
+  private static readonly READING_CACHE_TTL_MS = 30_000;
+
   async getLatestReading(key: string): Promise<OracleReading | null> {
+    // Check cache first (#727)
+    const cached = this.readingCache.get(key);
+    if (cached && cached.expiresAt > Date.now()) {
+      return cached.value;
+    }
+
     // #265 — submittedAt alone has no guaranteed tiebreak on ties (plausible
     // under bulk/backfill writes or fast successive submissions), and this
     // result directly drives claim payout decisions. id is a stable secondary
@@ -356,16 +366,24 @@ export class OracleService {
       orderBy: [{ submittedAt: "desc" }, { id: "desc" }],
     });
 
-    if (!record) return null;
+    const result = record
+      ? {
+          dataType: record.dataType,
+          key: record.key,
+          value: record.value.toString(),
+          confidence: record.confidence,
+          timestamp: Math.floor(record.submittedAt.getTime() / 1000),
+          source: record.source,
+        }
+      : null;
 
-    return {
-      dataType: record.dataType,
-      key: record.key,
-      value: record.value.toString(),
-      confidence: record.confidence,
-      timestamp: Math.floor(record.submittedAt.getTime() / 1000),
-      source: record.source,
-    };
+    // Cache the result (#727)
+    this.readingCache.set(key, {
+      value: result,
+      expiresAt: Date.now() + OracleService.READING_CACHE_TTL_MS,
+    });
+
+    return result;
   }
 
   private async executeWithRateLimitBackoff<T>(
diff --git a/src/stellar/stellar.service.ts b/src/stellar/stellar.service.ts
index 2c5a187..bbbed9d 100644
--- a/src/stellar/stellar.service.ts
+++ b/src/stellar/stellar.service.ts
@@ -63,6 +63,24 @@ export class StellarService {
   private readonly _horizonUrl: string;
   private readonly _rpcUrl: string;
 
+  // #728 — per-account transaction serialization. Concurrent invocations from
+  // the same account would race on sequence numbers and fail with
+  // TRANSACTION_BAD_SEQ. A simple promise-chain mutex serializes them.
+  private readonly accountLocks = new Map<string, Promise<void>>();
+
+  private async withAccountLock<T>(publicKey: string, fn: () => Promise<T>): Promise<T> {
+    const prev = this.accountLocks.get(publicKey) ?? Promise.resolve();
+    let release: () => void;
+    const gate = new Promise<void>((resolve) => { release = resolve; });
+    this.accountLocks.set(publicKey, prev.then(() => gate));
+    await prev;
+    try {
+      return await fn();
+    } finally {
+      release!();
+    }
+  }
+
   constructor(private readonly config: ConfigService) {
     const rpcUrl =
       config.get<string>("STELLAR_RPC_URL") ??
@@ -137,6 +155,18 @@ export class StellarService {
       );
     }
     const signer = signerKeypair ?? this.keeperKeypair;
+    // #728 — serialize per-account to prevent sequence number conflicts.
+    return this.withAccountLock(signer.publicKey(), () =>
+      this.doInvokeContract(contractId, method, args, signer),
+    );
+  }
+
+  private async doInvokeContract(
+    contractId: string,
+    method: string,
+    args: xdr.ScVal[],
+    signer: Keypair,
+  ): Promise<string> {
     const contract = new Contract(contractId);
     const MAX_ATTEMPTS = 3;
     let lastError: Error | null = null;