Schema Diffing¶

Tessera automatically compares schemas to detect breaking vs non-breaking changes.

How It Works¶

When you publish a new contract, Tessera:

Fetches the current active contract
Compares the schemas using JSON Schema rules
Classifies each change as breaking or non-breaking
Creates a proposal if any breaking changes are detected

Change Types¶

Property Changes¶

Change	Backward	Forward
Add optional property	Safe	Breaking
Add required property	Breaking	Safe
Remove property	Breaking	Safe
Rename property	Breaking	Breaking

Type Changes¶

Change	Backward	Forward
Widen type (int → number)	Safe	Breaking
Narrow type (number → int)	Breaking	Safe
Change type (string → int)	Breaking	Breaking

Required Changes¶

Change	Backward	Forward
Make optional → required	Breaking	Safe
Make required → optional	Safe	Breaking

Enum Changes¶

Change	Backward	Forward
Add enum value	Safe	Breaking
Remove enum value	Breaking	Safe

Examples¶

Safe Change (Backward Compatible)¶

// Before
{
  "properties": {
    "id": {"type": "integer"},
    "name": {"type": "string"}
  }
}

// After - adding optional field
{
  "properties": {
    "id": {"type": "integer"},
    "name": {"type": "string"},
    "email": {"type": "string"}  // New optional field
  }
}

Result: Auto-published as minor version bump.

Breaking Change¶

// Before
{
  "properties": {
    "id": {"type": "integer"},
    "name": {"type": "string"},
    "email": {"type": "string"}
  },
  "required": ["id", "name", "email"]
}

// After - removing required field
{
  "properties": {
    "id": {"type": "integer"},
    "name": {"type": "string"}
  },
  "required": ["id", "name"]
}

Result: Proposal created, consumers must acknowledge.

Compatibility Modes¶

Backward (Default)¶

Consumers using the old schema can read new data.

Breaking changes: - Remove property - Add required property - Narrow type - Remove enum value

Forward¶

Producers using the old schema produce valid new data.

Breaking changes: - Add property (even optional) - Widen type - Add enum value

Full¶

Both backward and forward compatible.

Breaking changes: - Any schema change

None¶

No compatibility checking - all changes are non-breaking.

API Response¶

When publishing, the response indicates what happened:

{
  "action": "published",
  "contract": { ... },
  "changes": []
}

Or for breaking changes:

{
  "action": "proposal_created",
  "proposal": {
    "id": "proposal-uuid",
    "breaking_changes": [
      {
        "path": "$.properties.email",
        "type": "property_removed",
        "description": "Property 'email' was removed"
      }
    ]
  }
}