Function frame_commitment 

pub fn frame_commitment(provider_id: &str, frame: &ContextFrame) -> [u8; 32]

The commitment binding one frame’s identity to its provenance chain (SPEC.md §6.5.2) — the preimage a single-frame attestation signs.

SHA256(
  domain::FRAME ‖ enc(provider_id) ‖ enc(frame.id)
               ‖ enc_opt(frame.content_digest) ‖ chain_head
)

content_digest is included so that, when the frame declares one, the signature covers the frame’s bytes and not merely its name.

When the frame declares none, it does not. enc_opt writes a single 0x00 presence byte, so the preimage records the absence honestly rather than substituting a placeholder — but what gets signed is then identity and provenance alone, and a provider can re-serve entirely different content under the same frame id with that signature still checking out. The encoding is doing its job; the guarantee is simply narrower than the presence of a signature suggests.

Two things follow, and both are load-bearing (#128):

• SPEC.md §6.5.2 requires a provider that signs a frame to populate content_digest. A digest-less frame remains conformant; signing one is not. This function still computes the commitment for such a frame, because a verifier has to be able to check signatures produced before that rule, or by an implementation that ignores it.
• verify_frame_attestation returns AttestationVerdict::ValidIdentityOnly rather than AttestationVerdict::Valid for exactly that case, so no caller can mistake the narrower guarantee for the wider one.

A frame that declares no digest and carries no attestation is a different thing again: unverifiable by design (docs/context-reuse.md §4), and no rule here applies to it.