diff --git a/modules/sdk-coin-stx/src/lib/abstractContractBuilder.ts b/modules/sdk-coin-stx/src/lib/abstractContractBuilder.ts
index 86067227d1c..922ba13f941 100644
--- a/modules/sdk-coin-stx/src/lib/abstractContractBuilder.ts
+++ b/modules/sdk-coin-stx/src/lib/abstractContractBuilder.ts
@@ -42,12 +42,43 @@ export abstract class AbstractContractBuilder extends TransactionBuilder {
       this._contractName = txData.payload.contractName;
       this._functionName = txData.payload.functionName;
       this._functionArgs = txData.payload.functionArgs;
+      // Restore post-conditions from the deserialized tx — `toJson()` does not carry them, so
+      // without this a re-build (e.g. the fee re-build via `factory.from(hex)`) would drop them and
+      // a `Deny`-mode contract call would abort on-chain (SIP-005 post-condition check).
+      if (tx.stxTransaction) {
+        this._postConditionMode = tx.stxTransaction.postConditionMode;
+        this._postConditions = tx.stxTransaction.postConditions.values as PostCondition[];
+      }
       super.initBuilder(tx);
     } else {
       throw new BuildTransactionError('Transaction should be contract call');
     }
   }
 
+  /**
+   * Set the post-condition mode (`Allow` / `Deny`) for this contract call.
+   *
+   * @obligation callers building a contract call that moves assets MUST set a mode AND matching
+   *   post-conditions, or leave this unset and let the caller attach explicit post-conditions —
+   *   consequence: an unset mode defaults to `Deny` in `@stacks/transactions`, so any asset movement
+   *   not covered by a post-condition aborts the tx on-chain
+   * @returns {AbstractContractBuilder} this builder
+   */
+  postConditionMode(mode: PostConditionMode): this {
+    this._postConditionMode = mode;
+    return this;
+  }
+
+  /**
+   * Set the post-conditions for this contract call.
+   *
+   * @returns {AbstractContractBuilder} this builder
+   */
+  postConditions(postConditions: PostCondition[]): this {
+    this._postConditions = postConditions;
+    return this;
+  }
+
   /** @inheritdoc */
   protected async buildImplementation(): Promise<Transaction> {
     this._options = this.buildContractCallOptions();
diff --git a/modules/sdk-coin-stx/test/unit/transactionBuilder/pox5Builder.ts b/modules/sdk-coin-stx/test/unit/transactionBuilder/pox5Builder.ts
index 67ddba11bae..8cda4695ce7 100644
--- a/modules/sdk-coin-stx/test/unit/transactionBuilder/pox5Builder.ts
+++ b/modules/sdk-coin-stx/test/unit/transactionBuilder/pox5Builder.ts
@@ -6,6 +6,9 @@ import {
   cvToString,
   cvToValue,
   deserializeCV,
+  FungibleConditionCode,
+  makeStandardSTXPostCondition,
+  PostConditionMode,
 } from '@stacks/transactions';
 import { coins } from '@bitgo/statics';
 import should from 'should';
@@ -115,6 +118,34 @@ describe('Stacks: PoX-5 Builder', function () {
     should.equal((await rebuilt.build()).toBroadcastFormat(), tx.toBroadcastFormat());
   });
 
+  it('carries post-condition mode/conditions and preserves them across the fee re-build (SI-1936)', async () => {
+    const functionArgs = pox5RegisterForBondArgsHex.map((hex) =>
+      deserializeCV(Buffer.from(hex.replace(/^0x/, ''), 'hex'))
+    );
+    const postCondition = makeStandardSTXPostCondition(
+      testData.TX_SENDER.address,
+      FungibleConditionCode.LessEqual,
+      100000000
+    );
+    const builder = configure(factory.getPox5Builder());
+    builder.functionName('register-for-bond');
+    builder.functionArgs(functionArgs);
+    builder.postConditionMode(PostConditionMode.Deny);
+    builder.postConditions([postCondition]);
+    const tx = (await builder.build()) as StxLib.Transaction;
+
+    should.equal(tx.stxTransaction.postConditionMode, PostConditionMode.Deny);
+    should.equal(tx.stxTransaction.postConditions.values.length, 1);
+
+    // `buildTransactionWithFee` re-builds via `factory.from(hex)`; the post-conditions must survive
+    // (initBuilder restores them from the deserialized tx).
+    const rebuilt = factory.from(tx.toBroadcastFormat());
+    rebuilt.fromPubKey(testData.TX_SENDER.pub);
+    const rebuiltTx = (await rebuilt.build()) as StxLib.Transaction;
+    should.equal(rebuiltTx.stxTransaction.postConditionMode, PostConditionMode.Deny);
+    should.equal(rebuiltTx.stxTransaction.postConditions.values.length, 1);
+  });
+
   it('validates the PoX-5 contract address during factory routing', async () => {
     const builder = configure(factory.getPox5Builder());
     builder.registerForBond({