Supply path optimisation (SPO) is done in eight steps: map your current paths, check ads.txt and sellers.json, read the SupplyChain object, measure fees and duplication, choose preferred paths, use curation and direct deals where they fit, consider agent-negotiated deals with a shared record, and re-audit every quarter. For the definition of SPO and why it exists, see What Is Supply Path Optimisation and Which Platforms Support It?

TL;DR. SPO starts with checking your own log-level data against IAB Tech Lab's ads.txt, sellers.json and SupplyChain object, which show who is authorised to sell, who is an intermediary and how many hops a bid request has taken. The ISBA and PwC study of UK programmatic found that 15% of advertiser spend could not be attributed to any party, so fee measurement sits at the centre of the checklist. Support for SPO is spread across DSPs, SSPs, curation platforms, verification vendors and agentic marketplaces.

Step 1: How do you map your current supply paths?

Mapping starts with log-level data from the DSP: for every impression, the publisher domain, the SSP or exchange that sold it, the seller ID and the price paid. Group the rows by publisher and count the distinct routes to each one. That map is the baseline every later step is measured against.

Expect more routes than you planned for. Jounce Media data cited in The Trade Desk's trader's guide to SPO puts the average number of SSPs integrated by the top 20 publishers at 24.5, and Sincera data in the same guide puts the number of paths to a single impression as high as 600. The ISBA and PwC Programmatic Supply Chain Transparency Study recommends a separate agency seat for each client and readily available log-level data, without which the map is only an estimate.

Step 2: How do you check ads.txt and sellers.json?

Check that every seller ID in the map is authorised by the publisher and identified by the platform that sold it. ads.txt, first released by IAB Tech Lab in June 2017, is a file on the publisher's domain listing the companies authorised to sell its inventory. sellers.json, finalised by the OpenRTB Working Group on 31 July 2019 according to IAB Tech Lab, is published by the SSP or exchange and shows whether each seller is a direct seller or an intermediary.

Exclude any seller ID missing from the publisher's ads.txt. Keep an intermediary only if it adds demand, data or format support that the direct route lacks.

The same question applies when the seller is software. Alkimi's Marketplace, for example, runs AAMP identity checks on buyer and seller agents before they trade, so identity is settled before a negotiation starts rather than inferred from a bid request. More in ads.txt, sellers.json and Agent Identity.

Step 3: How do you read the SupplyChain object?

The SupplyChain object, or schain, travels inside the OpenRTB bid request and lists every party selling or reselling it. According to the IAB Tech Lab specification, each node names the advertising system's domain and the seller ID, and both should match the values in ads.txt. A required flag called complete is set to 1 when the chain runs back to the owner of the site or app, and to 0 when it does not.

Count the nodes, because each hop is a party that can take a fee or lose metadata. Flag every request where complete is 0, and match each node's seller ID against that system's sellers.json. A practical policy is a maximum hop count per format and a preference for complete chains.

Step 4: How do you measure fees and duplication?

Fee measurement asks what share of each pound reaches the publisher. The ISBA and PwC study, published in May 2020, found that publishers received an average of 51% of advertiser spend. It also found an "unknown delta" averaging 15% of advertiser spend, around one third of supply chain costs, that could not be attributed to any party.

The study's data problem is part of the lesson. Of 267 million impressions observed, only 31 million (12%) could be matched end to end, because of low data quality. Expect similar gaps and record them as findings.

Duplication is the second measure: when one impression is offered through several SSPs, a buyer can bid against itself, so a high duplicate rate is a reason to consolidate.

Step 5: How do you choose preferred paths and SSPs?

Preferred paths are the routes that pass Steps 2 to 4: authorised, direct where possible, complete in the SupplyChain object and transparent about fees. Set the best one or two routes to each publisher as preferred in the DSP and consolidate on SSPs that pass every check. Then write fee disclosure and log-level data access into each SSP contract, as the ISBA and PwC study recommends standardised data access terms along the supply chain.

Pruning can go too far: The Trade Desk's guide recommends considering all available paths to an impression by default, noting that cutting paths too aggressively can limit the platform's ability to find efficiencies. Exclude paths that fail the authorisation checks outright and let price and performance decide among the rest.

Step 6: When should you use curation and direct deals?

Curation and direct deals fit where the buyer wants known publishers, audiences or formats rather than whatever the open auction delivers. A curated deal packages supply into a deal ID the buyer activates in its DSP, while private marketplace (PMP) and programmatic guaranteed deals tie the buy to named publishers. The ISBA and PwC study recommends considering maximising PMP over open marketplace volumes where possible.

A curated deal is still a path, so run it through Steps 2 to 4 like any other route. The trade-off with agent negotiation is set out in Curation or Agent Negotiation.

Step 7: Should you consider agent-negotiated direct deals?

Agent-negotiated deals shorten the path further for selected publishers. A buyer's AI agent negotiates terms directly with a publisher's seller agent, usually through a marketplace, so the path becomes buyer, seller and marketplace. This suits relationships where the spend justifies negotiated terms, while open auction buying stays in the conventional stack.

AAMP, IAB Tech Lab's Agentic Advertising Management Protocols, announced in early 2026, adapts existing ad tech standards for agents. AdCP, the Ad Context Protocol, is published by AgenticAdvertising.org as an open protocol for agents to discover, plan, buy, sell and measure media.

The SPO question for agent deals is the record of what was agreed. In the Agree. Transact. Verify. simulation, 95.3% of deals had mismatched records when each side kept its own, against 0.19% with a shared record. These are simulated results, not live market data, but they point to the check to add: a deal record both parties hold and either can retrieve independently.

Step 8: How often should you re-audit supply paths?

Re-audit quarterly. Publishers edit ads.txt, SSPs add resellers, and a route that passed one quarter can fail the next; as The Trade Desk's guide puts it, "SPO is never one and done."

Each review re-runs Steps 1 to 4 and tracks four numbers against the baseline: the share of spend reaching publishers, the share on complete supply chains, the share bought from authorised direct sellers, and active routes per publisher. Between reviews, set alerts for new unauthorised seller IDs and reconcile agent-negotiated deals against the shared record.

The printable SPO checklist

Pull log-level data from every DSP seat, with a separate seat for each client.
Map spend by publisher, SSP, seller ID and route.
Exclude seller IDs missing from the publisher's ads.txt.
Record each seller's sellers.json status: direct seller or intermediary.
Read the SupplyChain object: count hops, flag incomplete chains, match nodes to sellers.json.
Calculate the share of spend reaching the publisher on each path.
Measure duplicate routes to the same placement.
Set preferred paths and consolidate SSPs.
Write fee disclosure and log-level data access into SSP contracts.
Audit curated, PMP and programmatic guaranteed paths too.
For agent-negotiated deals, require a deal record held by both parties.
Re-audit quarterly, with alerts in between.

Which kinds of platforms support SPO?

SPO support is spread across categories, and most programmes use several.

Demand-side platforms (DSPs). DSPs are where most SPO decisions execute: preferred paths, reseller exclusions and SupplyChain filtering. The Trade Desk's OpenPath, for example, lets buyers purchase directly from more than 300 publishers integrated with the platform, according to its guide.

Supply-side platforms (SSPs). SSPs publish sellers.json, pass the SupplyChain object and offer direct routes to buyers who consolidate spend with them.

Curation platforms. Curators package publishers, audiences and routes into deals buyers activate in their DSP.

Verification vendors. These measure invalid traffic, viewability and brand safety, and reporting by path shows which routes deliver what they claim.

Agentic marketplaces. Agentic marketplaces let buyer and seller agents negotiate terms directly, with agent identity checks and a deal record both sides can retrieve.

Where does an agentic marketplace fit in an SPO programme?

Agent negotiation is usually the last step a buyer reaches, and only for selected publishers. Alkimi is an enterprise platform for agentic advertising built for that step: its Marketplace is neutral, works within IAB Tech Lab's AAMP framework today and is being built to be AdCP compatible, and humans keep approval rights at the deal level. Its DealSheet is written at agreement, timestamped and held by both parties, and its Gym lets buyers and sellers build their own agents, set the rules and spar against counterparty agents before any real money moves.

The simulation figures in Step 7 come from the company's own research.

Frequently asked questions

What is the first step in supply path optimisation?

Map your current paths from log-level DSP data, by publisher, SSP, seller ID and route. Every later step is measured against that map.

What is the difference between ads.txt and sellers.json?

ads.txt is published by the publisher and lists who is authorised to sell its inventory. sellers.json is published by the SSP or exchange and shows whether each seller is a direct seller or an intermediary.

How many SSPs should a buyer keep?

There is no standard number, and cutting paths too aggressively can remove useful efficiencies. Keep the SSPs that pass the authorisation and fee checks, then let price and performance decide.

Does SPO apply to agent-negotiated deals?

Yes, because a shorter path still has to be verified. The buyer needs to confirm the seller agent's identity and hold a deal record both sides share.

Supply Path Optimisation in 8 Steps: A Practical Checklist