Class SubstitutionObserver

Parse inline preview HTML and return tracker URLs from the contract's normative tag/attribute set. URLs that fail WHATWG URL parsing are skipped silently — a downstream binding that expected them surfaces as substitution_binding_missing.

Fetch preview_url under the SSRF policy, validate response shape, and parse the body. Redirects are treated as failure (policy: follow_redirects: false). The body is capped at max_body_bytes bytes — exceeding the cap yields a size_exceeded failure rather than a truncated parse.

Throws PreviewFetchError on any fetch-layer failure so the runner can map the sub-reason to the contract's preview_url_unusable_sub_reasons vocabulary.

See matchBindings.

See assertRfc3986Safe.

See assertUnreservedOnly.

See assertNoNestedExpansion.

See assertSchemePreserved.

See enforceSsrfPolicy.