diff --git a/__tests__/lib/codec.test.ts b/__tests__/lib/codec.test.ts
index 25f4663..751b3de 100644
--- a/__tests__/lib/codec.test.ts
+++ b/__tests__/lib/codec.test.ts
@@ -11,9 +11,35 @@ import {
   decodeArgLegacy,
   decodeAllArgs,
   decomposeArgHex,
+  normalizeFieldName,
 } from "../../lib/codec";
 import { createMockDedotClient } from "../helpers/mock-client";
 
+// ---------------------------------------------------------------------------
+// normalizeFieldName
+// ---------------------------------------------------------------------------
+describe("normalizeFieldName", () => {
+  it("converts snake_case to camelCase", () => {
+    expect(normalizeFieldName("ref_time")).toBe("refTime");
+    expect(normalizeFieldName("proof_size")).toBe("proofSize");
+    expect(normalizeFieldName("storage_deposit_limit")).toBe("storageDepositLimit");
+  });
+
+  it("preserves already camelCase names", () => {
+    expect(normalizeFieldName("refTime")).toBe("refTime");
+    expect(normalizeFieldName("proofSize")).toBe("proofSize");
+  });
+
+  it("handles hash prefix replacement", () => {
+    expect(normalizeFieldName("#field_name")).toBe("fieldName");
+  });
+
+  it("handles single word names", () => {
+    expect(normalizeFieldName("value")).toBe("value");
+    expect(normalizeFieldName("dest")).toBe("dest");
+  });
+});
+
 // ---------------------------------------------------------------------------
 // encodeArg
 // ---------------------------------------------------------------------------
@@ -827,6 +853,82 @@ describe("decomposeArgHex", () => {
     }
   });
 
+  it("normalizes snake_case metadata field names to camelCase for value lookup", () => {
+    // This is the exact scenario from gas estimation auto-fill:
+    // Metadata has snake_case fields (ref_time, proof_size) but the auto-filled
+    // value object uses camelCase (refTime, proofSize) matching Dedot's convention.
+    const tryEncode = jest.fn().mockReturnValue(new Uint8Array([0x01]));
+    const client = createMockDedotClient({
+      registry: {
+        findCodec: jest.fn().mockReturnValue({
+          tryEncode,
+          tryDecode: jest.fn(),
+        }),
+        findType: jest.fn().mockReturnValue({
+          typeDef: {
+            type: "Struct",
+            value: {
+              fields: [
+                { name: "ref_time", typeId: 10 },
+                { name: "proof_size", typeId: 20 },
+              ],
+            },
+          },
+        }),
+      },
+    });
+    // Value uses camelCase keys (as produced by gas estimation auto-fill)
+    const result = decomposeArgHex(client, 1, {
+      refTime: "117234441",
+      proofSize: "135850",
+    });
+    expect(result.kind).toBe("compound");
+    if (result.kind === "compound") {
+      expect(result.compoundType).toBe("Struct");
+      expect(result.children).toHaveLength(2);
+      // Labels should be normalized to camelCase
+      expect(result.children[0].label).toBe("refTime");
+      expect(result.children[1].label).toBe("proofSize");
+      // Encoder should have been called with coerced values (BigInt), not undefined
+      expect(tryEncode).toHaveBeenCalledWith(BigInt("117234441"));
+      expect(tryEncode).toHaveBeenCalledWith(BigInt("135850"));
+    }
+  });
+
+  it("handles already-camelCase metadata field names without double-normalizing", () => {
+    const tryEncode = jest.fn().mockReturnValue(new Uint8Array([0x01]));
+    const client = createMockDedotClient({
+      registry: {
+        findCodec: jest.fn().mockReturnValue({
+          tryEncode,
+          tryDecode: jest.fn(),
+        }),
+        findType: jest.fn().mockReturnValue({
+          typeDef: {
+            type: "Struct",
+            value: {
+              fields: [
+                { name: "refTime", typeId: 10 },
+                { name: "proofSize", typeId: 20 },
+              ],
+            },
+          },
+        }),
+      },
+    });
+    const result = decomposeArgHex(client, 1, {
+      refTime: "100",
+      proofSize: "200",
+    });
+    expect(result.kind).toBe("compound");
+    if (result.kind === "compound") {
+      expect(result.children[0].label).toBe("refTime");
+      expect(result.children[1].label).toBe("proofSize");
+      expect(tryEncode).toHaveBeenCalledWith(BigInt("100"));
+      expect(tryEncode).toHaveBeenCalledWith(BigInt("200"));
+    }
+  });
+
   it("decomposes Enum single-field variant", () => {
     const tryEncode = jest.fn().mockReturnValue(new Uint8Array([0x01]));
     const client = createMockDedotClient({
diff --git a/components/builder/extrinsic-builder.tsx b/components/builder/extrinsic-builder.tsx
index 247f05d..344a74e 100644
--- a/components/builder/extrinsic-builder.tsx
+++ b/components/builder/extrinsic-builder.tsx
@@ -110,29 +110,11 @@ const ExtrinsicBuilder: React.FC<ExtrinsicBuilderProps> = ({
   useEffect(() => {
     if (!isReviveInstantiate || !gasEstimation.weightRequired || !tx) return;
 
-    // Resolve weight field names from metadata
-    const weightField = tx.meta?.fields?.find((f) => f.name === "weight_limit");
-    if (weightField) {
-      try {
-        const weightType = client.registry.findType(weightField.typeId);
-        const { typeDef } = weightType;
-        if (typeDef.type === "Struct" && typeDef.value.fields.length >= 2) {
-          const [field0, field1] = typeDef.value.fields;
-          const name0 = String(field0.name);
-          const name1 = String(field1.name);
-          builderForm.setValue("weight_limit", {
-            [name0]: String(gasEstimation.weightRequired.refTime),
-            [name1]: String(gasEstimation.weightRequired.proofSize),
-          });
-        }
-      } catch {
-        // Fallback: use camelCase names
-        builderForm.setValue("weight_limit", {
-          refTime: String(gasEstimation.weightRequired.refTime),
-          proofSize: String(gasEstimation.weightRequired.proofSize),
-        });
-      }
-    }
+    // Auto-fill weight_limit with camelCase keys (Dedot codec convention)
+    builderForm.setValue("weight_limit", {
+      refTime: String(gasEstimation.weightRequired.refTime),
+      proofSize: String(gasEstimation.weightRequired.proofSize),
+    });
 
     // Auto-fill storage deposit only for Charge
     if (gasEstimation.storageDeposit?.type === "Charge") {
diff --git a/lib/codec.ts b/lib/codec.ts
index 010ef53..a2989b7 100644
--- a/lib/codec.ts
+++ b/lib/codec.ts
@@ -1,5 +1,13 @@
 import { DedotClient } from "dedot";
-import { u8aToHex, hexToU8a, hexStripPrefix, hexAddPrefix, decodeAddress } from "dedot/utils";
+import { u8aToHex, hexToU8a, hexStripPrefix, hexAddPrefix, decodeAddress, stringCamelCase } from "dedot/utils";
+
+/**
+ * Normalize a metadata field name to camelCase, matching Dedot's internal convention.
+ * Metadata uses snake_case (ref_time, proof_size), but Dedot codecs expect camelCase.
+ */
+export function normalizeFieldName(name: string): string {
+  return stringCamelCase(name.replace("#", "_"));
+}
 
 /**
  * Result type for encoding operations
@@ -541,7 +549,7 @@ export function decomposeArgHex(
       if (fields.length > 0 && typeof value === "object" && value !== null && !Array.isArray(value)) {
         const obj = value as Record<string, unknown>;
         const children: HexChildItem[] = fields.map((field) => {
-          const fieldName = field.name || "";
+          const fieldName = normalizeFieldName(field.name || "");
           const fieldValue = obj[fieldName];
           const result = encodeArg(client, field.typeId, fieldValue);
           return {
@@ -577,7 +585,7 @@ export function decomposeArgHex(
           if (typeof enumObj.value === "object" && enumObj.value !== null) {
             const obj = enumObj.value as Record<string, unknown>;
             const children: HexChildItem[] = variant.fields.map((field) => {
-              const fieldName = field.name || "";
+              const fieldName = normalizeFieldName(field.name || "");
               const fieldValue = obj[fieldName];
               const result = encodeArg(client, field.typeId, fieldValue);
               return {