Propagate runtime ID across API requests

Propagate Runtime ID Across API Requests

When orchestrating end-to-end integration workflows, an API runner often executes a sequence of requests where downstream calls depend on IDs or values generated by earlier responses.

You are tasked with implementing a function interpolate_request that resolves template placeholders in an HTTP request template using values from a runtime context dictionary.

Requirements

1. Placeholder Syntax:

   • Placeholders are formatted as {{path.to.key}}.
   • Keys are alphanumeric and may contain dots (.) to represent nested lookup in context (e.g., {{customer.default_source}} maps to context["customer"]["default_source"]).

2. Type-Preserving Replacement:

   • If a string field matches exactly a single placeholder (e.g., "{{payment.amount}}"), it must be replaced with the actual runtime value and its original type (e.g., integer 5000, boolean true, or a nested object/list).
   • If a placeholder is embedded within surrounding text (e.g., "/v1/customers/{{customer.id}}/cards"), interpolate the string representation of the resolved value.

3. Nested Data Traversal:

   • The request body can contain arbitrary JSON structures: dictionaries, lists, strings, numbers, booleans, and nulls.
   • You must recursively traverse and interpolate all placeholders within url, headers, and body.

4. Missing Keys:

   • If any placeholder path does not exist in context, raise a KeyError.

Function Signature

def interpolate_request(request: dict, context: dict) -> dict:
    pass

Input and Output Format

• request: A dictionary containing at least "url" (str), and optionally "headers" (dict[str, str]) and "body" (JSON-compatible data: dict, list, primitive, or None).
• context: A nested dictionary containing runtime values produced by previous requests.
• Returns a new dictionary with the exact same structure as request, where all placeholders have been replaced.

Example 1

request = {
    "url": "/v1/charges/{{charge.id}}/refund",
    "headers": {"Idempotency-Key": "refund-{{charge.id}}"},
    "body": {
        "amount": "{{charge.amount}}",
        "metadata": {
            "customer": "{{charge.customer.id}}",
            "full_refund": "{{charge.metadata.is_full}}"
        }
    }
}

context = {
    "charge": {
        "id": "ch_12345",
        "amount": 2500,
        "customer": {"id": "cus_9999"},
        "metadata": {"is_full": True}
    }
}

# Output:
{
    "url": "/v1/charges/ch_12345/refund",
    "headers": {"Idempotency-Key": "refund-ch_12345"},
    "body": {
        "amount": 2500,  # Preserved as int, not "2500"
        "metadata": {
            "customer": "cus_9999",
            "full_refund": True  # Preserved as bool
        }
    }
}

Constraints

• request JSON depth <= 10.
• Context keys are valid identifier strings matching [a-zA-Z0-9_]+.
• String values may contain zero, one, or multiple placeholders.