diff --git a/docs/release/7.14.2.md b/docs/release/7.14.2.md
index 6f30acd7f..2f866b12e 100644
--- a/docs/release/7.14.2.md
+++ b/docs/release/7.14.2.md
@@ -166,6 +166,7 @@ landed, so the table below is the current state, not what #440 originally set.
 | TRON `SignTx` | AdvancedMode | `raw_data` still cannot be parsed on this line (#405) |
 | EVM `SignTx` with **no `chain_id`** | **REFUSED** | produced a pre-EIP-155 signature replayable on every EVM chain, and the confirm screen named no network (#445) |
 | 0x `transformERC20` over 1024 bytes | **REFUSED** by default, AdvancedMode to sign | was clear-signed with an undisclosed `transformations[]` tail; now falls through to the raw-hex path (#444) |
+| 0x swaps on **Base / Arbitrum / Avalanche** | **REFUSED** by default, AdvancedMode to sign | the token table's `uint8_t` chain id truncates on these three, so the decoder rendered "Unknown token value" for both amounts — no bound, no disclosure (#455) |
 
 **On the 0x change — this affects real swaps, not an edge case.**
 `EthereumSignTx.data_initial_chunk` holds 1024 bytes and 0x aggregates across liquidity
diff --git a/lib/firmware/ethereum_contracts.c b/lib/firmware/ethereum_contracts.c
index 02cdda856..304a4b6d0 100644
--- a/lib/firmware/ethereum_contracts.c
+++ b/lib/firmware/ethereum_contracts.c
@@ -35,13 +35,36 @@ bool zx_isExchangeProxyChain(uint32_t chain_id) {
      for ZXSWAP_ADDRESS would let the 0x decoder narrate an unrelated contract —
      exactly the confusion the chain scoping exists to prevent. Verified against
      0xProject/protocol packages/contract-addresses/addresses.json. */
+  /* Base (8453), Arbitrum (42161) and Avalanche (43114) are deliberately absent
+     even though 0x deploys the same Exchange Proxy there. The decoder can find
+     the contract on those chains but cannot describe the trade, so allowing
+     them would produce a screen that asserts understanding it does not have.
+
+     TokenType.chain_id and tokenByChainAddress() are uint8_t
+     (include/keepkey/firmware/ethereum_tokens.h:42,54), so those three chain
+     ids truncate to 5, 177 and 106 -- values matching no table entry. The
+     lookup returns UnknownToken and ethereumFormatAmount() short-circuits to
+     the literal "Unknown token value" (lib/firmware/ethereum.c:347) for BOTH
+     operands, so zx_confirmZxTransERC20() renders
+
+         Transform ERC20
+         Input Unknown token value
+         Output Unknown token value
+
+     while the transformations[] body executes. The whole justification for
+     clear-signing transformERC20 is that the input amount and minimum output
+     amount shown on screen bound the outcome; on these chains no amount is
+     shown, so there is no bound and no disclosure -- a blind signature wearing
+     a decoder's title.
+
+     Denying them here routes those swaps to the generic raw-calldata path,
+     which is AdvancedMode-gated and shows the bytes. That is the honest answer
+     until the token table carries a full-width chain id (#455), at which point
+     these three can be restored with their amounts actually rendering. */
   switch (chain_id) {
-    case 1:     /* Ethereum   */
-    case 56:    /* BNB Chain  */
-    case 137:   /* Polygon    */
-    case 8453:  /* Base       */
-    case 42161: /* Arbitrum   */
-    case 43114: /* Avalanche  */
+    case 1:   /* Ethereum  */
+    case 56:  /* BNB Chain */
+    case 137: /* Polygon   */
       return true;
     default:
       /* Including chain_id 0 / absent, which callers treat as unknown. */
diff --git a/unittests/firmware/ethereum.cpp b/unittests/firmware/ethereum.cpp
index 0b09d7199..4755b0770 100644
--- a/unittests/firmware/ethereum.cpp
+++ b/unittests/firmware/ethereum.cpp
@@ -135,12 +135,28 @@ extern "C" {
 // (0xdef1abe32c034e558cdd535791643c58a13acc10), so allowing chain 10 for
 // ZXSWAP_ADDRESS would narrate an unrelated contract.
 TEST(Ethereum, ZxExchangeProxyChainAllowlist) {
-  EXPECT_TRUE(zx_isExchangeProxyChain(1));      // Ethereum
-  EXPECT_TRUE(zx_isExchangeProxyChain(56));     // BNB Chain
-  EXPECT_TRUE(zx_isExchangeProxyChain(137));    // Polygon
-  EXPECT_TRUE(zx_isExchangeProxyChain(8453));   // Base
-  EXPECT_TRUE(zx_isExchangeProxyChain(42161));  // Arbitrum
-  EXPECT_TRUE(zx_isExchangeProxyChain(43114));  // Avalanche
+  // Chains where the token table can actually name the tokens, so the input
+  // and minimum-output amounts that bound the trade are rendered.
+  EXPECT_TRUE(zx_isExchangeProxyChain(1));    // Ethereum
+  EXPECT_TRUE(zx_isExchangeProxyChain(56));   // BNB Chain
+  EXPECT_TRUE(zx_isExchangeProxyChain(137));  // Polygon
+
+  // 0x deploys the same Exchange Proxy on these three, but TokenType.chain_id
+  // and tokenByChainAddress() are uint8_t, so the ids truncate to values that
+  // match no table entry: 8453->5, 42161->177, 43114->106. Both operands then
+  // render "Unknown token value" and the screen shows no bound at all, which
+  // is a blind signature with a decoder's title. They must fall through to the
+  // AdvancedMode raw-calldata path until #455 widens the chain id.
+  EXPECT_FALSE(zx_isExchangeProxyChain(8453))
+      << "Base truncates to 5 and renders no amounts";
+  EXPECT_FALSE(zx_isExchangeProxyChain(42161))
+      << "Arbitrum truncates to 177 and renders no amounts";
+  EXPECT_FALSE(zx_isExchangeProxyChain(43114))
+      << "Avalanche truncates to 106 and renders no amounts";
+
+  // The truncated values themselves must not be a way back in.
+  EXPECT_FALSE(zx_isExchangeProxyChain(177));
+  EXPECT_FALSE(zx_isExchangeProxyChain(106));
 
   EXPECT_FALSE(zx_isExchangeProxyChain(10)) << "Optimism uses a different 0x proxy";