diff --git a/.circleci/config.yml b/.circleci/config.yml
index 3776c694..19f0d010 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -70,7 +70,8 @@ jobs:
           key: yarn-packages-{{ checksum "yarn.lock" }}
           paths:
             - ~/.cache/yarn
-      - run: # wait until cosm scripts have fully started (this includes time for docker pull)
+      - run:
+          name: Wait for blockchain and REST server to be ready (started in background)
           command: timeout 60 bash -c "until curl -s http://localhost:1317/node_info > /dev/null; do sleep 1; done"
       - run:
           environment:
diff --git a/packages/sdk/src/leb128.spec.ts b/packages/sdk/src/leb128.spec.ts
new file mode 100644
index 00000000..c95f8e38
--- /dev/null
+++ b/packages/sdk/src/leb128.spec.ts
@@ -0,0 +1,39 @@
+import { Encoding } from "@iov/encoding";
+
+const { fromHex } = Encoding;
+
+export function leb128Encode(uint: number): Uint8Array {
+  if (uint < 0) throw new Error("Only non-negative values supported");
+  if (uint > 0x7fffffff) throw new Error("Only values in signed int32 range allowed");
+  const out = new Array<number>();
+  let value = uint;
+  do {
+    // tslint:disable: no-bitwise
+    let byte = value & 0b01111111;
+    value >>= 7;
+
+    // more bytes to come: set high order bit of byte
+    if (value !== 0) byte ^= 0b10000000;
+
+    out.push(byte);
+    // tslint:enable: no-bitwise
+  } while (value !== 0);
+  return new Uint8Array(out);
+}
+
+describe("leb128", () => {
+  describe("leb128Encode", () => {
+    it("works for single byte values", () => {
+      // Values in 7 bit range are encoded as one byte
+      expect(leb128Encode(0)).toEqual(fromHex("00"));
+      expect(leb128Encode(20)).toEqual(fromHex("14"));
+      expect(leb128Encode(127)).toEqual(fromHex("7f"));
+    });
+
+    it("works for multi byte values", () => {
+      // from external souce (wasm-objdump)
+      expect(leb128Encode(145)).toEqual(fromHex("9101"));
+      expect(leb128Encode(1539)).toEqual(fromHex("830c"));
+    });
+  });
+});
diff --git a/packages/sdk/src/restclient.spec.ts b/packages/sdk/src/restclient.spec.ts
index a490139a..aa4cd3f9 100644
--- a/packages/sdk/src/restclient.spec.ts
+++ b/packages/sdk/src/restclient.spec.ts
@@ -4,10 +4,11 @@ import { Encoding } from "@iov/encoding";
 import { HdPaths, Secp256k1HdWallet } from "@iov/keycontrol";
 
 import { encodeSecp256k1Signature, makeSignBytes, marshalTx } from "./encoding";
+import { leb128Encode } from "./leb128.spec";
 import { Log, parseLogs } from "./logs";
 import { RestClient } from "./restclient";
 import contract from "./testdata/contract.json";
-import data from "./testdata/cosmoshub.json";
+import cosmoshub from "./testdata/cosmoshub.json";
 import {
   Coin,
   Msg,
@@ -19,7 +20,7 @@ import {
   StdTx,
 } from "./types";
 
-const { fromBase64 } = Encoding;
+const { fromBase64, toBase64 } = Encoding;
 
 const httpUrl = "http://localhost:1317";
 const defaultNetworkId = "testing";
@@ -49,6 +50,32 @@ function makeSignedTx(firstMsg: Msg, fee: StdFee, memo: string, firstSignature:
   };
 }
 
+function getRandomizedContract(): Uint8Array {
+  const data = fromBase64(contract.data);
+  // The return value of the export function cosmwasm_api_0_6 is unused and
+  // can be randomized for testing.
+  //
+  // Find position of mutable bytes as follows:
+  // $ wasm-objdump -d contract.wasm | grep -F "cosmwasm_api_0_6" -A 1
+  // 00e67c func[149] <cosmwasm_api_0_6>:
+  // 00e67d: 41 83 0c                   | i32.const 1539
+  //
+  // In the last line, the addresses 00e67d-00e67f hold a one byte instruction
+  // (https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md#constants-described-here)
+  // and a two byte value (leb128 encoded 1539)
+
+  // Any unsigned integer from 128 to 16383 is encoded to two leb128 bytes
+  const min = 128;
+  const max = 16383;
+  const random = Math.floor(Math.random() * (max - min)) + min;
+  const bytes = leb128Encode(random);
+
+  data[0x00e67d + 1] = bytes[0];
+  data[0x00e67d + 2] = bytes[1];
+
+  return data;
+}
+
 describe("RestClient", () => {
   it("can be constructed", () => {
     const client = new RestClient(httpUrl);
@@ -78,10 +105,9 @@ describe("RestClient", () => {
   describe("encodeTx", () => {
     it("works for cosmoshub example", async () => {
       pendingWithoutCosmos();
-      // need to convince the compiler we have a valid string for pubkey type
-      const tx: StdTx = data.tx.value as StdTx;
+      const tx = cosmoshub.tx.value;
       const client = new RestClient(httpUrl);
-      expect(await client.encodeTx(tx)).toEqual(fromBase64(data.tx_data));
+      expect(await client.encodeTx(tx)).toEqual(fromBase64(cosmoshub.tx_data));
     });
   });
 
@@ -134,6 +160,8 @@ describe("RestClient", () => {
       const signer = await wallet.createIdentity("abc" as ChainId, faucetPath);
       const client = new RestClient(httpUrl);
 
+      let codeId: number;
+
       // upload
       {
         const memo = "My first contract on chain";
@@ -141,7 +169,7 @@ describe("RestClient", () => {
           type: "wasm/store-code",
           value: {
             sender: faucetAddress,
-            wasm_byte_code: contract.data,
+            wasm_byte_code: toBase64(getRandomizedContract()),
             source: "https://github.com/confio/cosmwasm/raw/0.7/lib/vm/testdata/contract_0.6.wasm",
             builder: "cosmwasm-opt:0.6.2",
           },
@@ -166,7 +194,10 @@ describe("RestClient", () => {
         expect(result.code).toBeFalsy();
         const [firstLog] = parseSuccess(result.raw_log);
         const codeIdAttr = firstLog.events[0].attributes.find(attr => attr.key === "code_id");
-        expect(codeIdAttr).toEqual({ key: "code_id", value: "1" });
+        if (!codeIdAttr) throw new Error("Could not find code_id attribute");
+        codeId = Number.parseInt(codeIdAttr.value, 10);
+        expect(codeId).toBeGreaterThanOrEqual(1);
+        expect(codeId).toBeLessThanOrEqual(200);
       }
 
       let contractAddress: string;
@@ -188,7 +219,7 @@ describe("RestClient", () => {
           type: "wasm/instantiate",
           value: {
             sender: faucetAddress,
-            code_id: "1",
+            code_id: codeId.toString(),
             init_msg: {
               verifier: "cosmos1ltkhnmdcqemmd2tkhnx7qx66tq7e0wykw2j85k",
               beneficiary: "cosmos1ltkhnmdcqemmd2tkhnx7qx66tq7e0wykw2j85k",
diff --git a/scripts/cosm/start.sh b/scripts/cosm/start.sh
index 3722d815..2c0b285f 100755
--- a/scripts/cosm/start.sh
+++ b/scripts/cosm/start.sh
@@ -15,10 +15,6 @@ echo "Using temporary dir $TMP_DIR"
 WASMD_LOGFILE="$TMP_DIR/wasmd.log"
 REST_SERVER_LOGFILE="$TMP_DIR/rest-server.log"
 
-# pull the newest copy of the docker image
-# this is important as the sleep timeout below will fail on first run (downloading entire docker stack usually > 10s)
-docker pull "$REPOSITORY:$VERSION"
-
 # This starts up wasmd
 docker volume rm -f wasmd_data
 docker run --rm \
@@ -35,12 +31,10 @@ docker run --rm \
 echo "wasmd running and logging into $WASMD_LOGFILE"
 
 # Debug chain start
-sleep 3
-cat "$WASMD_LOGFILE"
+# sleep 3 && cat "$WASMD_LOGFILE"
 
-sleep 10
-
-if [ "$(docker inspect -f '{{.State.Running}}' "$CONTAINER_NAME")" != "true" ]; then
+# Use a large timeout because of potentially long image download in `docker run`
+if ! timeout 120 bash -c "until docker inspect -f '{{.State.Running}}' '$CONTAINER_NAME' &> /dev/null; do sleep 0.5; done"; then
   echo "Container named '$CONTAINER_NAME' not running. We cannot continue." \
     "This can happen when 'docker run' needs too long to download and start." \
     "It might be worth retrying this step once the image is in the local docker cache."
@@ -57,6 +51,10 @@ docker exec "$CONTAINER_NAME" \
 
 echo "rest server running on http://localhost:1317 and logging into $REST_SERVER_LOGFILE"
 
+# Give REST server some time to come alive. No idea why this helps. Needed for CI.
+if [ -n "${CI:-}" ]; then
+  sleep 0.5
+fi
+
 # Debug rest server start
-sleep 3
-cat "$REST_SERVER_LOGFILE"
+# sleep 3 && cat "$REST_SERVER_LOGFILE"