400 POST https://<REDACTED_URL>: Could not find field "%24alt" in the type "google.cloud.aiplatform.v1beta1.FindNeighborsRequest".

[vigneshkumarravisankar]

Expected behavior:
The request should succeed without failing on "%24alt" parameter.

Actual behavior:
The API rejects the request, complaining that "%24alt" is not a valid field in FindNeighborsRequest.

[ericgribkoff]

The error message says that the request message is malformed with an invalid field specified (that's what the error about "%24alt" means). This would need to be fixed in the calling code.

[vigneshkumarravisankar]

I don't think so its fixable in the calling code, I have attached the images of the code above!

[ericgribkoff]

I suggest starting with the example code at https://cloud.google.com/vertex-ai/docs/vector-search/query-index-public-endpoint#query_public_endpoint-python_vertex_ai_sdk and seeing where your argument construction differs.

[taimo3810]

I've investigated this issue and found that the root cause lies in how the requests library handles the $ character in the $alt query parameter.

Root Cause

The requests library encodes query parameters by default, converting $ to %24.
The SDK's REST transport (google/cloud/aiplatform_v1beta1/services/match_service/transports/rest_base.py) adds the $alt parameter:

# The SDK sets this:
query_params["$alt"] = "json;enum-encoding=int"

# But `requests` sends this:
# ?%24alt=json%3Benum-encoding%3Dint

This impacts users who:

• Explicitly set api_transport="rest".
• Connect to Public Endpoints (*.vdb.vertexai.goog), which default to REST.

Reproduction

Minimal Case:

import requests
# requests encodes '$' -> '%24'
print(requests.Request('GET', 'http://a.b', params={'$alt': 'json'}).prepare().url)

SDK Reproduction:

from google.cloud import aiplatform

aiplatform.init(api_transport="rest")
# ... setup endpoint ...
# Fails because of encoded $alt parameter
endpoint.find_neighbors(...) 

Workaround

You can use the gRPC client (MatchServiceClient) directly to bypass the REST transport layer:

from google.cloud.aiplatform_v1beta1 import MatchServiceClient
from google.api_core.client_options import ClientOptions

client = MatchServiceClient(
    client_options=ClientOptions(api_endpoint=public_domain)
)
response = client.find_neighbors(request=request)

(This appears to be related to #5467)

[taimo3810]

Following up on the root cause analysis above: since this issue originates in the code generation layer, I've filed an issue and submitted a fix to the upstream generator:

• Issue: Bug: REST transport URL-encodes $alt as %24alt, causing API errors google-cloud-python#16206
• PR: fix: prevent URL-encoding of $alt query parameter in REST transport gapic-generator-python#2515

The fix modifies the REST transport template to build the query string manually with urlencode(safe='$'), preventing $alt from being encoded as %24alt.

Once merged, regenerating the SDK should resolve this issue.