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The JSON API provides an HTTP/REST interface to the Canton Ledger API. In Canton 3.x, it is integrated directly into the participant node and translates JSON HTTP requests into gRPC Ledger API calls.

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Quick Reference

PropertyValue
ProtocolHTTP/1.1 + JSON
Default port7575
Underlying APILedger API v2 (gRPC)
AuthenticationJWT Bearer tokens

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When to Use the JSON API

The JSON API is the recommended access method when:
  • You’re building browser-based frontends or web applications
  • Your language/framework works better with HTTP/JSON than gRPC
  • You want simpler tooling for development and debugging (curl, Postman, etc.)
  • You’re using the TypeScript/JavaScript Wallet SDK
Use gRPC directly when:
  • You need maximum performance for high-throughput streaming
  • You’re using Java and want the native gRPC experience
  • You need access to gRPC-specific features not exposed via JSON

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Key Endpoints

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Version Check

curl http://localhost:7575/v2/version

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Command Submission

curl localhost:7575/v2/commands/submit-and-wait \
  -H "Content-Type: application/json" \
  -d '{
    "commands": [
      {
        "CreateCommand": {
          "createArguments": {
            "issuer": "<partyId>",
            "owner": "<partyId>",
            "name": "Example Asset Name"
          },
          "templateId": "#json-tests:Main:Asset"
        }
      }
    ],
    "userId": "ledger-api-user",
    "commandId": "example-app-create-1234",
    "actAs": ["<partyId>"],
    "readAs": ["<partyId>"]
  }'
Other submission endpoints include /v2/commands/submit-and-wait-for-transaction (returns the full transaction) and /v2/commands/async/submit (returns immediately).

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Active Contract Queries

curl localhost:7575/v2/state/active-contracts \
  -H "Content-Type: application/json" \
  -d '{
    "eventFormat": {
      "filtersByParty": {},
      "filtersForAnyParty": {
        "cumulative": [
          {
            "identifierFilter": {
              "WildcardFilter": {
                "value": {
                  "includeCreatedEventBlob": true
                }
              }
            }
          }
        ]
      },
      "verbose": false
    },
    "verbose": false,
    "activeAtOffset": 20
  }'

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Transaction Streaming

The JSON API supports both HTTP POST and WebSocket connections for streaming updates. Use /v2/updates or /v2/updates/flats for flat transaction streams, and /v2/updates/trees for transaction trees. WebSocket connections use the daml.ws.auth subprotocol.

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Ledger End

GET /v2/state/ledger-end returns the latest absolute offset on the participant. Use it as the activeAtOffset value when querying the ACS to get the most recent state, or pass an older non-pruned offset to read state at a historical point.

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Interactive Submission

External-party command submission goes through two endpoints rather than the standard submit-and-wait flow:
  • POST /v2/interactive-submission/prepare — produces a prepared transaction the external party signs off-participant.
  • POST /v2/interactive-submission/execute — submits the signed prepared transaction back to the participant for ledger execution.
See Validator API for the surrounding external-signing flow.

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Configuration

The JSON API is configured via the http-ledger-api section of the participant node’s configuration. In cn-quickstart LocalNet deployments, it is pre-configured and available at http://localhost:7575. The OpenAPI and AsyncAPI specifications are available at runtime: 
curl http://localhost:7575/docs/openapi
curl http://localhost:7575/docs/asyncapi

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Authentication

Pass a JWT Bearer token in the Authorization header: 
curl -H "Authorization: Bearer <token>" http://localhost:7575/v2/...
The token must be valid for the OIDC provider configured for your deployment.