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Creative libraries let buyers manage creatives through AdCP — browsing existing assets, uploading new ones, assigning them to campaigns, and tracking approval status. A creative library is hosted by any agent that declares has_creative_library: true in its capabilities: ad servers (CM360, Flashtalking), creative management platforms (Celtra), or sales agents that also expose the Creative Protocol.

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The model

A creative library is an account-scoped collection of creative resources with stable creative_ids, observable lifecycle state, and assignment relationships that are separate from any one media buy or package. The implementation can be a full ad-server library or a thin view over seller storage, but the protocol commitment is the same: buyers can read the creative resource through list_creatives, update it through sync_creatives, and reference it by creative_id across assignments while the seller retains it. Inline package creatives from a seller that does not advertise a creative library are not library creatives. They are package-scoped creative attachments: the seller accepts and serves them as part of the media buy, but does not advertise independent library management or cross-buy reuse. A creative library organizes assets at three levels:
LevelAdCP equivalentExamples
AccountAccount (via accounts protocol)Advertiser in CM360, brand workspace in Celtra
Conceptconcept_id / concept_nameFlashtalking concept, CM360 creative group, Celtra campaign folder
CreativeCreative item with creative_id, format_id, assetsA 300x250 display ad, a 30s video spot
Concepts group related creatives across sizes and formats. A “Holiday 2026” concept might contain a 300x250 banner, a 728x90 leaderboard, and a 30s video — all expressing the same campaign idea. Use concept_id to filter and manage them as a group.

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Creative state and assignment state are separate

Two things the library tracks independently:
  • Creative state — the review status of the creative itself: processing, pending_review, approved, rejected, or archived. Set by the creative agent’s review workflow. Applies to the creative as a library asset, regardless of where it is used.
  • Assignment state — the relationship between a creative and a package on a specific media buy. Created when the buyer assigns the creative (via sync_creatives, creative_assignments, or inline creatives on create_media_buy). Released when the media buy or package is rejected, canceled, or completed, or when the buyer removes the assignment.
These lifecycles are tracked independently:
  • A creative in the library has zero or more active assignments at any time.
  • Rejecting, canceling, or completing a media buy releases its assignments. It does not change the creative’s review state, remove the creative from the library, or affect the creative’s use in other media buys.
  • When a creative’s review state changes after assignments exist (e.g., a seller revokes an approval, or approves a previously rejected creative), sellers MAY continue or stop in-flight serving based on the new state. Buyers SHOULD re-fetch approval_status per package via get_media_buys after a creative-state change to detect assignment-level impact. See creative review.
Buyer agents reusing a library creative on a new media buy check creative state to know whether the asset is usable, and assignment state to know where it is currently in flight.

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Creatives outlast campaigns

A creative MUST persist in the library independently of the buys that reference it. Buy rejection, cancellation, or completion releases assignments only — the creative remains in the library at its current status and may be reused on subsequent buys. This holds regardless of how the creative entered the library: explicit sync_creatives, library-backed inline creative on create_media_buy, or platform-native upload exposed through AdCP. Sellers whose underlying ad server has no library object distinct from per-buy attachment satisfy this rule by exposing the buyer-synced creative through list_creatives for the buy’s lifetime and continuing to expose its terminal state (including archived) after teardown — the library can be a thin view over per-buy storage; it does not need to be a separate store. Retention beyond active assignments is seller-defined. An agent MAY archive an unassigned creative due to inactivity, post-flight expiry, or storage policy (see creative status lifecycle), MAY transition approved → suspended for recoverable dependency loss such as a published-post authorization expiring, and MAY transition approved → rejected for policy revocation, takedown, or content drift. Whenever a creative’s state changes after it has been synced by a buyer, the seller MUST make the new state observable so the buyer can resync, replace, or stop relying on the asset before reuse:
  • For state changes affecting an active media buy (e.g., approved → suspended or approved → rejected on a creative with live assignments), sellers MUST surface a corresponding impairment on the buy. See media buy health.
  • For state changes on creatives with no active assignments (e.g., seller archives an unassigned creative for inactivity), sellers MUST reflect the new status on the next list_creatives read — that snapshot is the conformant signal today, per the snapshot-and-log contract. A push channel for account-scoped creative state changes is being defined under the creative lifecycle webhooks RFC; when that channel ships, sellers SHOULD additionally fire on it.
Buyers SHOULD confirm availability via list_creatives before reuse on a new buy. A library creative is the bundle of inputs the buyer supplied — uploaded assets, a brief, brand and catalog pointers, or any combination. Retention applies to the bundle; whether the format’s rendered outputs are individually addressable is a format-level concern and is independent of library retention.

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Connecting to a library

Establish account access before querying: 
{
  "accounts": [{
    "account_id": "acct_acme_2026",
    "account_name": "Acme Corp",
    "credentials": {
      "api_key": "..."
    }
  }]
}
The account setup is the same whether the library is on a standalone creative agent or a sales agent. See accounts protocol for details.

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Browsing creatives

Use list_creatives to browse the library. Filter by concept, format, status, tags, or date range: 
{
  "filters": {
    "concept_ids": ["concept_holiday_2026"],
    "statuses": ["approved"],
    "format_ids": [{
      "agent_url": "https://ads.flashtalking-example.com",
      "id": "display_300x250"
    }]
  },
  "include": {
    "variables": true,
    "assignments": true
  }
}
Each creative in the response includes: 
{
  "creative_id": "ft_88201",
  "name": "Holiday 2026 - Medium Rectangle",
  "format_id": {
    "agent_url": "https://ads.flashtalking-example.com",
    "id": "display_300x250"
  },
  "status": "approved",
  "concept_id": "concept_holiday_2026",
  "concept_name": "Holiday 2026 Campaign",
  "created_date": "2026-10-15T14:00:00Z",
  "updated_date": "2026-11-20T09:30:00Z",
  "tags": ["holiday_2026", "display"],
  "variables": [
    {
      "variable_id": "headline",
      "name": "Headline text",
      "type": "text",
      "default_value": "Holiday Sale — Up to 40% Off"
    }
  ],
  "assignments": [
    { "package_id": "pkg_premium_display", "weight": 100 }
  ]
}
The status field reflects the creative’s current state in the library: processing, pending_review, approved, rejected, or archived. See creative review for how status transitions work.

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Uploading creatives

Use sync_creatives to upload new creatives or update existing ones. The operation uses upsert semantics — if a creative_id already exists, it updates; otherwise it creates. 
{
  "creatives": [
    {
      "creative_id": "acme_video_001",
      "name": "Holiday Sale 30s",
      "format_id": {
        "agent_url": "https://creative.adcontextprotocol.org",
        "id": "video_standard_30s"
      },
      "assets": {
        "video": {
          "url": "https://cdn.acme-example.com/holiday-sale-30s.mp4",
          "width": 1920,
          "height": 1080,
          "duration_ms": 30000
        },
        "click_url": {
          "url": "https://acme-example.com/holiday-sale"
        }
      }
    }
  ]
}
The response tells you what happened to each creative: 
{
  "creatives": [
    {
      "creative_id": "acme_video_001",
      "action": "created"
    }
  ]
}
After upload, the creative enters the library’s review process. Check list_creatives to see when it transitions from pending_review to approved.

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Uploading with assignments

Assign creatives to packages in the same call: 
{
  "creatives": [
    {
      "creative_id": "acme_video_001",
      "name": "Holiday Sale 30s",
      "format_id": { "agent_url": "...", "id": "video_standard_30s" },
      "assets": { "...": "..." }
    }
  ],
  "assignments": [
    {
      "creative_id": "acme_video_001",
      "package_id": "pkg_premium_video"
    }
  ]
}

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Generating tags from library creatives

When you need a serving tag for a creative in the library, use build_creative with creative_id instead of a manifest: 
{
  "creative_id": "ft_88201",
  "concept_id": "concept_holiday_2026",
  "target_format_id": {
    "agent_url": "https://ads.flashtalking-example.com",
    "id": "display_300x250"
  }
}
The creative agent resolves the creative_id from its library and returns a manifest with the serving tag. The tag format depends on the platform:
  • Flashtalking, Celtra: Universal tags that adapt to any environment. No placement context needed.
  • CM360: Placement-level tags that require trafficking context. Pass media_buy_id and package_id:
{
  "creative_id": "cm360_creative_12345",
  "target_format_id": {
    "agent_url": "https://ads.cm360-example.com",
    "id": "display_300x250"
  },
  "media_buy_id": "buy_holiday_q4",
  "package_id": "pkg_premium_display"
}
See tag generation models for the full breakdown.

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Assigning creatives to campaigns

There are two paths for attaching library creatives to a media buy:

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Path 1: Creative assignments on the package

Reference library creatives by ID when creating the media buy: 
{
  "packages": [{
    "product_id": "premium_display",
    "creative_assignments": [
      { "creative_id": "ft_88201", "weight": 60 },
      { "creative_id": "ft_88202", "weight": 40 }
    ]
  }]
}
This works when the creative is already in the agent’s library (via sync_creatives or the platform’s own upload flow).

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Path 2: Inline creatives on the package

Upload the creative directly with the media buy — no separate sync step: 
{
  "packages": [{
    "product_id": "premium_display",
    "creatives": [{
      "creative_id": "acme_banner_001",
      "name": "Holiday banner",
      "format_id": { "agent_url": "...", "id": "display_300x250" },
      "assets": { "...": "..." }
    }]
  }]
}
For sellers that also advertise creative.has_creative_library: true, the agent adds the creative to its library and assigns it to the package in one operation. For inline-only sellers that do not advertise a creative library, the same creatives payload assigns package-scoped assets without creating a reusable library entry. See inline creative management for details. Library-backed inline creatives follow the same library lifecycle as sync_creatives uploads. When the seller advertises creative.has_creative_library: true, the inline form is a convenience — “sync and assign in one call” — not a separate lifecycle. Once submitted, the creative enters the library with the same review flow, retention, and identifiers it would have under sync_creatives. If the create_media_buy task resolves as pending_manual and the buy never activates, or if the buy is rejected or canceled, only the package assignments are released; the creatives remain in the library and may be referenced by creative_id in a subsequent create_media_buy call. This assignment-release behavior is normative on the media-buy side — see the Media Buy State Transitions rule. For inline-only sellers, buyers should treat the creative body as attached to the media-buy package. The seller may retain enough package-scoped creative state to review, serve, replace, and audit the buy, but it has not advertised reusable library operations such as list_creatives, sync_creatives, or later creative_assignments reuse. Creative review proceeds independently of the media buy outcome. Sellers MUST NOT skip review solely because the buy did not activate, and a buy rejection does not by itself imply rejection of the submitted creatives — a creative rejection MUST be a deliberate review decision with its own rejection_reason, not implicit from the containing buy’s status. Sellers MAY deprioritize review of creatives that currently have no active assignments, provided review completes before any future assignment activates. Capability-flag scope. inline_creative_management: true advertises that the sales agent accepts inline creatives on create_media_buy and update_media_buy; it does not by itself advertise a creative library. The decoupled library lifecycle applies when the seller also advertises creative.has_creative_library: true.

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Multi-seller distribution

When you work with multiple sellers, build creatives once and distribute them:
  1. Build on your creative agent:
{
  "message": "Create a holiday promotion banner",
  "target_format_ids": [
    { "agent_url": "https://creative.adcontextprotocol.org", "id": "display_300x250" },
    { "agent_url": "https://creative.adcontextprotocol.org", "id": "display_728x90" }
  ]
}
  1. Sync to each seller’s library:
{
  "creatives": [
    {
      "creative_id": "holiday_2026_300x250",
      "name": "Holiday 2026 - Medium Rectangle",
      "concept_id": "concept_holiday_2026",
      "format_id": { "agent_url": "https://creative.adcontextprotocol.org", "id": "display_300x250" },
      "assets": { }
    }
  ]
}
Call sync_creatives on each sales agent separately. Use the same creative_id and concept_id across sellers so you can correlate the same creative and campaign concept across your media buys. See Multi-agent creative orchestration for the full pattern.
  1. Track approval per seller — each seller reviews independently. Poll list_creatives on each agent to check status. A creative may be approved on one seller and rejected on another based on their policies.

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Tracking approval status

Creative approval operates at two levels: Library level: The status field on each creative in list_creatives — processing, pending_review, approved, rejected, or archived. Package level: The approval_status on each creative in get_media_buys — pending_review, approved, or rejected with rejection_reason. A creative can be approved in the library but rejected at the package level if it violates placement-specific policies. Poll both after syncing or submitting a media buy with new creatives.

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Dynamic creative optimization (DCO)

Creatives with dynamic content variables appear in the library with a variables array when you request include: { variables: true }. Each variable defines a slot that the ad server fills at serve time: 
{
  "variables": [
    {
      "variable_id": "headline",
      "name": "Headline text",
      "type": "text",
      "default_value": "Holiday Sale — Up to 40% Off"
    },
    {
      "variable_id": "product_image",
      "name": "Product image",
      "type": "image",
      "default_value": "https://cdn.acme-example.com/hero.jpg"
    },
    {
      "variable_id": "cta_color",
      "name": "CTA button color",
      "type": "color",
      "default_value": "#FF6600"
    }
  ]
}
Use has_variables: true in list_creatives filters to find DCO creatives. Variable types match common platform patterns: text, color, image, video, number, boolean. How the ad server uses these variables at serve time (data feeds, targeting rules, optimization algorithms) is outside AdCP’s scope. AdCP models the variable slots, not the optimization logic.

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Next steps