A seller agent is software, driven by a large language model, that represents a publisher, a sales house or a supply-side platform (SSP) in automated dealings with the software that represents buyers. It reads a campaign brief, matches it against available inventory, quotes prices from the seller's own rules, negotiates within set limits and books the resulting order or deal. Its territory is what programmatic auctions never automated: requests for proposal (RFPs), media kits, email threads and insertion orders. The role exists because AI buying agents need a counterparty that answers in structured form, and because publishers want their own representation once machines negotiate. The Ad Context Protocol (AdCP) calls it a sales agent; IAB Tech Lab, Magnite and VIOOH use Seller Agent as a product name.

Where it sits in the transaction

A seller agent works before the auction rather than inside it. An OpenRTB bid must come back in milliseconds, whereas setup and negotiation can take a day without anyone minding. AdCP's launch specification budgeted around 60 seconds for product discovery, which involves inference and retrieval, against about one second for a list of creative formats read from a database.

According to AdCP's media buy documentation, a transaction follows a fixed sequence. The buyer's agent calls get_products with a brief in plain language, naming the brand and the agency acting for it. The seller agent interprets the brief against its catalogue and returns products in a common schema, each carrying pricing options, a delivery forecast and creative requirements. A refine mode narrows the set, list_creative_formats returns specifications and sync_creatives uploads assets. A create_media_buy call then carries flight dates, a budget per package and a chosen pricing option. The seller approves the buy or sends it to review, and the task can sit as submitted or input-required until a person signs off; only then is a media_buy_id issued. Reporting flows through get_media_buy_delivery, and changes through update_media_buy.

The documentation's worked example shows what buyers are after. A $50,000 quarterly brief sent to three sellers returns connected TV (CTV) pre-roll at a $28 cost per thousand impressions (CPM) against a forecast of 890,000 impressions, display at $12 for 2.1 million and podcast mid-roll at $22 for 340,000. The figures are illustrative rather than market prices.

Behind that interface, the agent writes into the seller's own systems. The open-source Prebid Sales Agent creates orders and line items in Google Ad Manager, with adapters for Kevel, Triton Digital and Broadstreet. IAB Tech Lab's reference Seller Agent, built on OpenDirect 2.1, the Deals API version 1.0 and sellers.json, instead issues deal IDs that buyers target in The Trade Desk, Amazon DSP or Display & Video 360, or pushes deals through PubMatic, Index Exchange and Magnite. It splits work across three tiers: an inventory manager for yield, channel specialists from display to linear TV, and functional agents for pricing, availability, proposal review, upsell and audience. Its default price tiers show identity at work. An anonymous buyer sees only a range; a known seat gets 5% off, an agency 10% and an advertiser 15%, and only the last two may negotiate. As of September 2026 the repository lists 46 Model Context Protocol (MCP) tools and 87 conventional web endpoints.

Authorisation sits outside the conversation. A publisher hosts a file at /.well-known/adagents.json listing each permitted agent's address, what it may sell and which properties it covers, by ID, by tag or by pointing to other publishers' files. Buyer agents are meant to check it before placing an order. According to an issue filed in AdCP's GitHub repository, the file for Raptive's managed network authorises a single agent across about 6,800 publisher domains and runs to 2.6 megabytes.

Publishers can host their own agents or use one supplied by an SSP: Magnite built one into its SpringServe ad server, and PubMatic exposes inventory to agents through AgenticOS. Specialists cover narrower supply, among them VIOOH in digital out-of-home (DOOH) and ITN in local television. Across the table sit buyer agents, run by agencies, demand-side platforms (DSPs) and vendors such as Scope3.

Origin and evolution

Automating direct sales is not new; IAB Tech Lab's OpenDirect specification was written for guaranteed buys long before language models could read a brief. The agentic version dates from autumn 2025. AdCP launched on October 15, 2025with six founding members: Scope3, Yahoo, PubMatic, Swivel, Triton Digital and Optable. Brian O'Kelley, Scope3's chief executive, wrote that month that "AdCP requires publishers to register sales agents publicly", the job adagents.json performs. The protocol itself drew immediate scepticism from publishers and agencies.

Commercial deployments came quickly. PubMatic launched AgenticOS on January 5, 2026 with WPP Media, Butler/Till and MiQ. A day later Magnite disclosed its December test, with Scope3 as buyer agent, and IAB Tech Lab set out a roadmap promising open-source buyer and seller agents built on OpenRTB, the Advertising Common Object Model (AdCOM) and OpenDirect. On January 29, according to Prebid.org's announcement, AdCP's reference code moved to Prebid as the Prebid Sales Agent, still governed under AdCP. IAB Tech Lab named its programme the Agentic Advertising Management Protocols (AAMP) on February 26, giving seller agents a three-tier hierarchy, and its Agent Registry reached ten entries on March 11. AAMP 2.3, in late July, added a pricing provenance field and moved the Seller Agent's FreeWheel connections onto OAuth.

Why it matters to marketers

For buyers the promise is breadth. A media team can review proposals from 10 to 20 publishers per campaign while hundreds or thousands never see the brief, Barnaby Edwards of IAB Tech Lab wrote in September 2026. For publishers the argument is representation. A seller agent translates audience composition, contextual signals and price floors into formats machines can read, as speakers at a Prebid webinar described in May 2026. Volumes are real but small. VIOOH's agent handled more than 100 curated DOOH deal package requests in the first half of 2026, and PubMatic reported more than 80 autonomous campaigns and 4,000 AI-powered deals by August 6, 2026. Magnite credited its first agentic campaign in Europe, the Middle East and Africa, with Amnet France, with cutting setup time by roughly 70%, a vendor figure from one campaign. The engineering case may be stronger. Jochen Schlosser, Adform's chief technology and product officer, estimated a custom SSP deals integration at 250 to 350 days of work against about 50 on a shared standard.

Limitations and disputes

Money has lagged the software. Joe Root, co-founder of Permutive, said in late 2025 that none of the publishers he spoke to was forecasting a single dollar through AdCP. Michael Barrett, Magnite's chief executive, put the most optimistic 2027 forecasts for agentic spend at $600m to $700m, modest against the wider programmatic market. DataBeat found agentic buyers clearing at a $6.13 CPM against $6.95 for conventional demand, a 13.4% gap publishers absorb as lower revenue per impression.

Negotiation is thinner than the word suggests. Agents, Schlosser said, "can only do rate card fast"; bespoke premium deals still go to people. Invented numbers are a risk across the table too: the AAMP 2.3 field exists to stop buying agents fabricating prices when market data is missing.

Two standards compete for the same job. Shailley Singh, IAB Tech Lab's chief operating officer, listed 13 functions that AAMP and AdCP share, so a publisher selling through both maintains two descriptions of one supply. Authorisation is soft: AdCP's documentation says a missing adagents.json does not mean an agent is unauthorised. "More automation means less transparency," Augustine Fou, a fraud researcher, told The Current in 2025. Neutrality is disputed as well; in the Amnet campaign, Magnite's buyer agent dealt with Magnite's seller agent through Magnite's own coordination layer. The code is young. The Prebid build is labelled alpha, tracks the pre-release AdCP 3.1.1 and calls Google's Gemini for product discovery, while IAB Tech Lab's is configured for Anthropic's Claude models, leaving sellers dependent on a model vendor.

Not the same as

Buyer agent. The counterpart acting for an advertiser or agency. It writes briefs, compares proposals and commits budget; the seller agent answers.

Agentic RTB Framework agents. Containerised services that IAB Tech Lab opened for comment in November 2025and finalised in August 2026. They run inside an exchange or DSP and alter bid requests within the auction's millisecond window, touching impressions rather than packages.

Sales house or authorised seller. A human sales organisation, or a company listed as DIRECT or RESELLER in ads.txt. A seller agent may act for either but is authorised separately; O'Kelley proposed adagents.json as a successor to ads.txt on March 30, 2026.

Seller Assistant. Amazon's agent for marketplace merchants, made autonomous on September 17, 2025, manages retailers' stock, compliance and advertising, not media inventory. In property, a seller's agent is a human estate agent.

Recent developments

ITN's seller agent entered Magnite Orchestration on September 10, with full integration due in the fourth quarter, and six days later Magnite extended Orchestration to linear television, smart TV home screens, pause ads, audio and display, naming Samsung Ads, iHeartMedia and DIRECTV Advertising without disclosing volumes or fees. On September 22 IAB Tech Lab released AAMP 3.0 and its OpenProposal specification, a proposal object of 71 fields under 11 headers, of which only 3 need a round of negotiation. Comments close on October 22, 2026, and the draft warns against building production systems on it. A day later Schlosser told ExchangeWire that very little money passes through either protocol, and set out three endings: one dies, the two merge or both survive side by side.

Timeline

  • October 15, 2025: The Ad Context Protocol launches with six founding members and a defined sales agent role
  • October 27, 2025: Brian O'Kelley sets out adagents.json as AdCP's public register of authorised sales agents
  • November 13, 2025: IAB Tech Lab opens the Agentic RTB Framework 1.0 for public comment
  • November 16, 2025: Swivel launches direct transactions between buyer and seller agents outside the bid stream
  • December 2025: Magnite tests a seller agent in SpringServe with Scope3 acting as buyer agent
  • January 5, 2026: PubMatic launches AgenticOS with WPP Media, Butler/Till and MiQ
  • January 6, 2026: Magnite discloses its seller agent; IAB Tech Lab publishes its agentic roadmap with buyer and seller agent kits
  • January 29, 2026: Prebid.org takes stewardship of AdCP's sales agent code as the Prebid Sales Agent
  • February 26, 2026: IAB Tech Lab names its programme the Agentic Advertising Management Protocols (AAMP)
  • March 11, 2026: The IAB Tech Lab Agent Registry reaches ten entries
  • March 30, 2026: O'Kelley proposes adagents.json as a successor to ads.txt
  • April 2026: AAMP 2.0 is released and programmatic guaranteed joins its agentic transaction types
  • May 14, 2026: Prebid holds a public webinar on connecting seller and buyer agents
  • June 11, 2026: Magnite opens Orchestration in beta above its Seller Agent and Buyer Agent
  • June 26, 2026: VIOOH reports more than 100 agent-handled DOOH deal package requests in the first half
  • July 30, 2026: AAMP 2.3 adds a pricing provenance field and OAuth for the Seller Agent's FreeWheel connections
  • August 2026: The Agentic RTB Framework 1.0 is finalised
  • August 6, 2026: PubMatic reports more than 80 autonomous campaigns and 4,000 AI-powered deals since launch
  • August 20, 2026: IAB Tech Lab counts 13 functions shared by AAMP and AdCP
  • September 8, 2026: Magnite discloses its first agentic campaign in Europe, the Middle East and Africa, with Amnet France
  • September 10, 2026: ITN's seller agent enters Magnite Orchestration
  • September 16, 2026: Magnite extends Orchestration to four further inventory categories
  • September 22, 2026: IAB Tech Lab releases AAMP 3.0 and OpenProposal for public comment
  • September 23, 2026: Adform's Jochen Schlosser says very little money runs through either protocol
  • October 22, 2026: The AAMP 3.0 comment period is scheduled to close

Summary

Who. Publishers, sales houses and SSPs deploy seller agents, either their own or ones supplied by Magnite, PubMatic, Prebid.org or specialists such as VIOOH and ITN. Buyer agents run by agencies, DSPs and vendors such as Scope3 are the counterparties. AgenticAdvertising.org governs AdCP; IAB Tech Lab maintains AAMP and its reference Seller Agent.

What. Software driven by a language model that turns a buyer's brief into priced products, negotiates within the seller's rules and books orders or deal IDs in the seller's ad server, under authorisation declared in adagents.json or an agent registry.

When. The role took shape with AdCP's launch on October 15, 2025. Commercial agents followed in December 2025 and January 2026, Prebid took over the open-source code on January 29, 2026, and IAB Tech Lab opened AAMP 3.0 for comment on September 22, 2026.

Where. In the planning and negotiation layer of direct, guaranteed and private marketplace deals across CTV, display, audio, DOOH and linear TV, connected over MCP and the Agent2Agent (A2A) protocol rather than the OpenRTB bid stream.

Why. Buyers want to query far more sellers than a human team can reach, and publishers want representation when machines negotiate. Spend remains small, pricing evidence is mixed and two competing standards have yet to converge.