A signed AdCP v3.2.3 create_media_buy can succeed while the VAST duration is wrong
AdCP v3.2.3 (Oct 7) enforces request signing on create_media_buy but does not compare VAST Duration to duration_ms, a dev.to post says. A signed, schema-valid buy can complete with the wrong creative behind the URL.
The Engineer · Build desk
Three figures from the post's example mismatch: the SDK schedules sixty-five seconds; the manifest's duration_ms is 30000, which satisfies a 60-second format capability bound.
| Figure | Value | Note | Claim |
|---|---|---|---|
| SDK schedules, in the post's example mismatch | 65 seconds | 2 | |
| Manifest duration_ms | 30,000 ms | 1 | |
| Capability bound | 60 seconds | 6 |
What happened
- A successful create_media_buy means the seller accepted the terms, returned a manifest and echoed the negotiated adcp_version.
- For video, the manifest's VAST asset can be a URL, so the XML itself was never part of the MCP payload.
- Since AdCP 3.1, format capabilities can declare duration_ms_range or duration_ms_exact, so buyers pick slots with explicit bounds.
- The CTV player, the SSAI stitcher and the verification SDK read XML and follow wrapper hops until they find an InLine.
Why it matters
- exposure The failure surfaces at playback and in measurement, after the buy returned success: the SDK schedules the longer spot, quartiles fire on the wrong clock, and some stacks reject the break.
- decision Teams running buyer or seller agents need their own step that follows the wrapper chain to the InLine and compares its Duration to duration_ms, because signing does not supply one.
- constraint A green conformance run shows the server speaks the protocol. It cannot gate creative readiness.
The duration_ms field in the manifest is optional. The schema marks it with the note "if known" [11]. Since nothing fetches the tag [8], the value, when present, is a claim. The post cites a companion write-up on declared versus verified assets that says as much: "every field on the VAST asset is a claim about a document nobody parsed during the buy" [4].
Wrappers widen the gap. The manifest stores one URL, and hop one can be a Wrapper with no Duration, because duration sits on the terminal InLine [15]. QA that stops at the first response has approved a redirect graph, the post says. Identity fields drift the same way. The JSON may reference a creative id while the tag omits UniversalAdId or ships a registry value the SSAI path cannot resolve [16].
The post's numbers show how each layer can pass on its own. A 30,000 ms manifest sits inside a 60-second capability bound, and the InLine reads 00:01:05 [1]. That is 65 seconds. It is 35 seconds longer than the manifest [5] and 5 seconds over the bound [6]. The example is constructed. The post does not report a live incident.
We think leaving the fetch out of the signed call is a defensible boundary. A tool that downloaded and parsed a wrapper chain would put third-party ad servers inside the latency of a spend commitment. The cost moves elsewhere: someone has to open the XML before the flight starts.
The post's remedy is a linter. It names vastlint, which it says owns the creative envelope the video buy referenced [3]. All of this rests on that one dev.to post.
What to watch
- Whether AdCP adds a duration cross-check, or a rule that sellers resolve the tag, to the spec or the conformance storyboards.
- Whether duration_ms moves from optional to required on the VAST asset, or sellers start populating it by default.
- A reported case of a signed buy serving a duration different from its manifest.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence35
- Adoption
- Insufficient
- Hype gap+10
- Incentives60
- Confidence50
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
In the post's example mismatch, a manifest duration_ms of 30000 satisfies a sixty-second format capability bound, while the InLine's <Duration> reads 00:01:05, sixty-five seconds.
- [2]
In that example the SDK schedules sixty-five seconds, quartile tracking fires on the wrong clock, and some stacks reject the break outright.
- [3]
The post says vastlint owns the creative envelope the video buy referenced.
- [4]
A companion 'declared versus verified asset' post states that every field on the VAST asset is a claim about a document nobody parsed during the buy.
- [5]
The example InLine duration exceeds the manifest duration by 35 seconds.
- [6]
The example InLine duration exceeds the 60-second capability bound by 5 seconds.
- [7]
On October 7, 2026 AdCP shipped v3.2.3 with a security fix that enforces request signing on spend-committing calls such as create_media_buy.
- [8]
The v3.2.3 fix does not fetch the tag behind a VAST asset URL or compare <Duration> to the duration_ms in the manifest.
- [9]
A buy can be signed, schema-valid, and complete while the InLine the stitcher needs is still wrong.
- [10]
A buyer agent discovers inventory with get_products, checks get_adcp_capabilities, and commits with create_media_buy; success means the seller accepted the terms, returned a manifest, and echoed the negotiated adcp_version.
- [11]
The AdCP schema marks duration_ms on the VAST asset as optional ('if known').
- [12]
With delivery_type: url on a VAST asset, the VAST bytes were not part of the MCP payload.
- [13]
Format capabilities from AdCP 3.1 can declare duration_ms_range or duration_ms_exact so buyers select slots with explicit bounds.
- [14]
The CTV player, the SSAI stitcher and the verification SDK read XML and follow wrapper hops until they find an InLine or fail.
- [15]
The manifest stores one URL; hop one can be a <Wrapper> with no <Duration>, and duration lives on the terminal InLine. QA that stops at the first response approves a redirect graph, not a servable creative.
- [16]
AdCP may reference a creative id in JSON while the tag omits UniversalAdId or ships a registry value the SSAI path cannot resolve.
- [17]
AdCP conformance storyboards grade whether a server speaks the protocol: auth headers, idempotency, schema on each response, and on 3.2.x signatures where the trust model requires them; passing is not a substitute for opening the tag.
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Sources
1 independent publisher whose own reporting we read for this story.
- dev.toAdCP Video Buy vs VAST InLine: The MCP Task Can Finish Before the Player Has a Spot
1 article · October 13, 2026
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Topics
- VAST video ad servingFollow
- Model Context ProtocolFollow
- Agentic ad buyingFollow
- Connected TV advertisingFollow