`encodeEventTopics()` (and `watchLogs()`) do not handle anonymous events

[KartikSoneji]

Check existing issues

• I checked there isn't already an issue for the bug I encountered.

Viem Version

2.47.10

Current Behavior

encodeEventTopics() does not handle anonymous events properly.
It incorrectly uses the signature as the first log topic.

Expected Behavior

To correctly encode the log topics.

Steps To Reproduce

const rawEventAbiItem = [
  {
    name: 'RawEvent',
    type: 'event',
    anonymous: true,
    inputs: [
      { name: 'topic0', type: 'bytes32', indexed: true },
      { name: 'topic1', type: 'bytes32', indexed: true },
      { name: 'topic2', type: 'bytes32', indexed: true },
      { name: 'topic3', type: 'bytes32', indexed: true },
    ],
  },
] as const

test('anonymous event: no args', () => {
  expect(encodeEventTopics({ abi: rawEventAbiItem })).toEqual([])
})

test('anonymous event: topic0', () => {
  expect(
    encodeEventTopics({
      abi: rawEventAbiItem,
      args: {
        topic0:
          '0x000000000000000000000000000000000000000000000000000000000000abcd',
      },
    }),
  ).toEqual([
    '0x000000000000000000000000000000000000000000000000000000000000abcd',
    null,
    null,
    null,
  ])
})

test('anonymous event: all topics', () => {
  expect(
    encodeEventTopics({
      abi: rawEventAbiItem,
      args: {
        topic0:
          '0x0000000000000000000000000000000000000000000000000000000000000000',
        topic1:
          '0x0000000000000000000000000000000000000000000000000000000000000001',
        topic2:
          '0x0000000000000000000000000000000000000000000000000000000000000002',
        topic3:
          '0x0000000000000000000000000000000000000000000000000000000000000003',
      },
    }),
  ).toEqual([
    '0x0000000000000000000000000000000000000000000000000000000000000000',
    '0x0000000000000000000000000000000000000000000000000000000000000001',
    '0x0000000000000000000000000000000000000000000000000000000000000002',
    '0x0000000000000000000000000000000000000000000000000000000000000003',
  ])
})

Link to Minimal Reproducible Example

No response

Anything else?

I came across this issue while trying to find a workaround to specify the log topics directly.

• Feature request: Expand event filters to support multiple events #287 (comment)

encodeEventTopics() does not check abiItem.anonymous before setting the sigature as topic0.

viem/src/utils/abi/encodeEventTopics.ts

Line 124 in 14f77b7

return [signature, ...topics]

The logic itself is trivial to fix:

diff --git a/src/utils/abi/encodeEventTopics.ts b/src/utils/abi/encodeEventTopics.ts
index 21415b5..c1c8a29 100644
--- a/src/utils/abi/encodeEventTopics.ts
+++ b/src/utils/abi/encodeEventTopics.ts
@@ -94,9 +94,6 @@ export function encodeEventTopics<
   if (abiItem.type !== 'event')
     throw new AbiEventNotFoundError(undefined, { docsPath })
 
-  const definition = formatAbiItem(abiItem)
-  const signature = toEventSelector(definition as EventDefinition)
-
   let topics: (Hex | Hex[] | null)[] = []
   if (args && 'inputs' in abiItem) {
     const indexedInputs = abiItem.inputs?.filter(
@@ -121,7 +118,15 @@ export function encodeEventTopics<
         }) ?? []
     }
   }
-  return [signature, ...topics]
+
+  // only prepend signature for non-anonymous events
+  if (!abiItem.anonymous) {
+    const definition = formatAbiItem(abiItem)
+    const signature = toEventSelector(definition as EventDefinition)
+    topics.unshift(signature)
+  }
+
+  return topics
 }
 
 export type EncodeArgErrorType =

But a bigger issue is the return type being [Hex, ...(Hex | Hex[] | null)[]], making an empty array fail the typecheck.
Not sure how that should be handled since changing the return type to [] can break existing code.
Maybe it can be inferred based on the type of event?

[Zelys-DFKH]

Confirmed. encodeEventTopics computes and prepends the signature unconditionally — abiItem.anonymous is never checked. The runtime fix is a few lines. The return type ([Hex, ...(Hex | Hex[] | null)[]]) also needs to loosen since anonymous events can return an empty array. Happy to open a PR if that sounds right.

[KartikSoneji]

I think the return type will have to be inferred based on the event definition.
Otherwise, existing code that assumes the return type of encodeEventTopics() will always have at least one value will break.

[Zelys-DFKH]

Good catch on the return type. Updated the PR to infer it from the event definition — non-anonymous events keep [Hex, ...(Hex | Hex[] | null)[]] and anonymous events get (Hex | Hex[] | null)[]. Also added type-level tests to cover both cases.