| name | build-seller-agent |
| description | Use when building an AdCP seller agent — a publisher, SSP, or retail media network that sells advertising inventory to buyer agents. |
Build a Seller Agent
A seller agent receives briefs from buyers, returns products, accepts media buys, manages creatives, and reports delivery. The fastest path to a passing agent is to fork a worked adapter and replace its // SWAP: markers with calls to your backend. Each // SWAP: comment marks one line you change to your platform's backend call — change only those lines, run the gate, ship. (examples/CONTRIBUTING.md covers the SWAP-marker convention in detail.) Proxy-shaped sellers use the lighter proxy-seller-snap/ bridge-pattern fork target plus a live-OAuth sandbox runner. This skill tells you which adapter to fork and what cross-cutting rules apply across all of them.
Pick your fork target
Each hello_*_adapter_* entry is a worked, currently-running, three-gate-tested reference adapter. proxy-seller-snap/ is a bridge-pattern fork target for the seed-read problem; pair it with your own live-OAuth sandbox gate before production.
The other sales-* specialisms reuse one of the primary fork targets and apply specialism deltas:
sales-broadcast-tv — forks hello_seller_adapter_guaranteed.ts; deltas around DMA, GRP, daypart, agency estimate number, measurement windows.sales-streaming-tv — forks hello_seller_adapter_guaranteed.ts; deltas around CTV (audience-vs-program targeting, reach/freq forecast). Preview specialism — placeholder storyboard.sales-proposal-mode — forks hello_seller_adapter_proposal_mode.ts; deltas around ProposalManager two-platform composition, ctx.recipes hydration, sole-stateful exemption.sales-exchange — forks hello_seller_adapter_non_guaranteed.ts; preview specialism (placeholder storyboard).
For exact response shapes, error codes, and optional fields, docs/llms.txt is the canonical reference. The fork target stays in sync with the spec because PR #1394's three-gate contract fails CI when it drifts.
When to use this skill
- User wants to sell ad inventory (publisher, SSP, retail media network)
- User mentions
get_products, create_media_buy, or the media buy protocol
Not this skill:
- Building an agency / holdco hub hosting multiple specialisms →
skills/build-holdco-agent/
- Catalog-driven inventory (retail media, restaurants, travel) →
skills/build-retail-media-agent/
- AI ad network coupling generation with sales →
skills/build-generative-seller-agent/
- Buying inventory →
docs/getting-started.md covers the buyer side
Often claimed alongside: audience-sync (walled-garden social + identity provider patterns), signal-marketplace (DSP-side data surface), sales-catalog-driven (retail-media catalog + dynamic-creative). See Common multi-specialism bundles.
Cross-cutting rules
Every sales-* seller hits the cross-cutting rules in ../cross-cutting.md. The high-traffic ones for sellers (deep-linked to the rule):
idempotency_key on every mutating request — create_media_buy, update_media_buy, sync_creatives, sync_audiences, etc.- Resolve-then-authorize — byte-equivalent errors for
media_buy_id cross-tenant lookups
- Authentication —
serve({ authenticate }) baseline
- Account resolution —
createTenantStore for multi-advertiser sellers
- Webhooks — stable
operation_id for IO-approval / delivery-completion notifications
Specialism deltas
The fork target covers the baseline. Specialism subpages cover the deltas — what to add when you claim that specialism on top of the baseline. Each entry below is specialism → when to read it → what's in it.
specialisms/sales-guaranteed.md — read before coding if you claim sales-guaranteed: IO-task envelope, three create_media_buy return shapes (task envelope / pending_creatives / active), TERMS_REJECTED on aggressive measurement_terms. Applying only the task-envelope path fails 5 storyboard create_media_buy steps.specialisms/sales-non-guaranteed.md — read if your inventory clears at request time (programmatic auction, no HITL approval): sync confirmation, bid_price, update_media_buy for in-flight changes.specialisms/sales-broadcast-tv.md — read if you sell linear / broadcast TV: agency_estimate_number, Ad-ID industry_identifiers, measurement_windows (Live/C3/C7).specialisms/sales-streaming-tv.md — read if you sell CTV / OTT streaming inventory (preview): audience-vs-program targeting, reach/freq forecast, frequency caps.specialisms/sales-social.md — read if you're a social platform (Snap, Meta, TikTok, etc.): additive — claim alongside sales-non-guaranteed. Adds sync_audiences, sync_catalogs (DPA), log_event (conversions), get_account_financials.specialisms/sales-proposal-mode.md — read if you negotiate via proposals before line-itemizing: proposals[] with budget_allocations, buying_mode: 'refine'.
sales-catalog-driven and sales-retail-media live in skills/build-retail-media-agent/ because catalog-driven applies beyond retail (restaurants, travel, local commerce).
Validate locally
npm run compliance:fork-matrix -- --test-name-pattern="hello-seller-adapter-guaranteed"
adcp storyboard run http://127.0.0.1:3004/mcp sales_guaranteed \
--bearer "$ADCP_AUTH_TOKEN" --include-bundles --json
The fork-matrix gate is the three-gate contract from docs/guides/EXAMPLE-TEST-CONTRACT.md: tsc strict / storyboard zero-failures / upstream façade. Adopters who fork a hello adapter inherit the gate by extending the test file with their own adapter path and expectedRoutes.
If the gate fails on a storyboard step (not on tsc), re-run the adcp storyboard run ... --json command above for the human-readable 💡 Hint: lines — node:test's assertion formatting compresses them.
For deeper validation (fuzz, request-signing grading, multi-instance, custom invariants): docs/guides/VALIDATE-YOUR-AGENT.md.
Test surfaces — making your agent verifiable without live credentials
Every seller faces the same compliance question: can the runner verify your wire format separately from your live behavior? Wire format gets tested against fixtures. Live behavior needs a real test surface — sandbox credentials, real catalog data, real adapter traffic. The two are independent and produce different reliability claims (see adcp-client#1782 and adcontextprotocol/adcp#4593 for the certification model under discussion).
To pass storyboards, you need the runner's comply_test_controller.seed_product writes to flow into your handler's get_products reads — the seed→read loop has to close. How you close it depends on where your handlers fetch from, not on what kind of seller you are:
-
Handler reads from a store you control (most SSPs, most creative agents — audience-sync adopters with a local audience cache, creative-template adopters with a local format registry). Point comply_test_controller at the same store. Seed writes to your DB, handler reads from your DB, loop closes naturally. No bridge needed — if seeds DO appear in responses but your tsc / type-check fails on seed-merge wiring, you wired it unnecessarily; remove it.
-
Handler reads from a system you don't control (DSPs proxying to Meta/Snap/TikTok, retail-media networks reading retailer catalog APIs, signals agents brokering third-party data marketplaces, walled-garden brokers). A seeded write to your local store is dead — your handler never sees it; if comply_test_controller seeds never appear in your responses, that's the symptom. Wire the TestControllerBridge (docs/guides/VALIDATE-YOUR-AGENT.md § "Platform-proxy sellers"). The real handler still runs first (so a broken upstream call still fails the conformance gate), and the SDK merges seeded fixtures into the response after.
Start from examples/proxy-seller-snap/ for the concrete fork shape: TestControllerBridge<TAccount>, bridgeFromSessionStore, resolved-account session keying, getSeededProducts, getSeededCreatives, and governance list selectors. Keep both resolveAccount (account-bearing tools) and resolveAccountFromAuth (account-less tools) as production trust boundaries; unresolved accounts fail closed. Pair bridge-green storyboards with a live-OAuth sandbox run; bridge passes prove wire conformance, not upstream adapter health.
Either path earns the wire-conformance half of compliance — your storyboards pass, you speak AdCP correctly. The live-integration half requires marker-free passes against a real test surface (sandbox credentials with real upstream traffic), independent of whether the bridge is wired. The _bridge marker the SDK stamps on bridge-merged responses tracks which steps in a storyboard run used fixtures vs real upstream.
Certification names in #1782 are under review; the mechanism here is stable.
Deployment
Single-host HTTP from serve(...) is the default. Multi-host, Express, or stdio transports: deployment.md.
Common shape gotchas
BuildCreativeReturn has 4 valid shapes (framework auto-wraps the bare manifest). VASTAsset requires an embedded delivery_type discriminator. Targeting overlay echo on get_media_buys requires createMediaBuyStore. See SHAPE-GOTCHAS.md — schema validators catch these at runtime; type checkers don't.
Migration notes
- 6.6 → 6.7: Two seller-affecting breaking changes — audit before bumping:
accounts.resolution: 'implicit' now refuses inline {account_id} references (#10), and SalesPlatform split into SalesCorePlatform & SalesIngestionPlatform (#11) — all methods individually optional, self-announcing under tsc --noEmit. See docs/migration-6.6-to-6.7.md for the worked diff plus 15 additive recipes around definePlatform, composeMethod, typed errors, BuyerAgentRegistry.
- 4.x → 5.x:
docs/migration-4.x-to-5.x.md. Full v5 path including createAdcpServer, serve({ authenticate }), and the 5.13 pin to AdCP 3.0.0 GA.
