Unable to trigger pipeline in spinnaker

Question

I am triggering a spinnaker pipeline through the POST request available from Spinnaker using this request JSON to this URL https://spinnaker-gate.prod.hulu.com/webhooks/webhook/nexus-model-serving -

{
    "pipeline_config_id": "720dcdb9-4e14-4ada-bb85-9c2554953c43",
    "app_name": "c7faccbc4ca392a0",
    "deployment_processor": "intel",
    "env": "sandbox",
    "helmChartBranch": "main",
    "helmChartVersion": "latest",
    "model_snapshot_pair": "{\"snapshot_id\": \"0a71ad2f0216011f\", \"model_uri\": \"s3://nexus-platform-prod/prod/model-engineering/model_management/PZN/hero_oswald-impower/0.1.0/26221030/segment-default\", \"predictor\": \"SomePredictor\", \"predictor_file\": \"some_predictor_file\"}",
    "protocol": "http",
    "region": "eu-central-1",
    "runtime": "custom",
    "serve_image": "nexus.docker.artifactory.global.bamgrid.net/nexusmlprojects/serving:hero_oswald_0.1.0_git-c63928a"
}

Although it works from the UI, the pipeline fails to trigger when I hit the request endpoint with this data as part of my code. I get this exception in the UI saying -

model_snapshot_pair_encoded
"${#toBase64(parameters.model_snapshot_pair)}"

Attaching the screenshot here -

Am I missing my anything in the request? Thanks in advance!