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This guide is for sales agents implementing creative format support. Before creating custom format definitions, you should check what formats are already available in the AdCP ecosystem.

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Implementation Philosophy: Check First, Then Extend

Most publishers support some combination of standard formats - whether IAB standard sizes, common video specs, or widely-used native formats. Rather than defining these yourself:
  1. Check the reference creative agent to see if your formats are already defined
  2. Reference the formats if they match your needs
  3. Only create custom formats when you have truly unique requirements
This approach:
  • Reduces maintenance burden - No need to maintain format definitions that already exist
  • Enables creative portability - Buyers can reuse creatives across publishers
  • Improves ecosystem consistency - Everyone uses the same specifications for common formats

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The Reference Creative Agent

URL: https://creative.adcontextprotocol.org Status: Production service - this is a real, working AdCP creative agent The reference creative agent provides authoritative definitions for common creative formats used across the advertising industry:
  • IAB standard display sizes (300x250, 728x90, 320x50, etc.)
  • Standard video formats (15s, 30s, 60s pre-roll, etc.)
  • Audio formats for streaming and podcast insertion
  • DOOH formats for digital out-of-home
  • Native formats for responsive placements
  • Carousel formats for multi-product displays
Before creating custom formats, query the reference creative agent to see if the formats you need already exist.

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Why Use Standard Formats?

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For Sales Agents

  • No maintenance burden: Don’t replicate IAB standard format definitions
  • Ecosystem consistency: Everyone uses the same format specifications
  • Focus on differentiation: Spend time on custom formats unique to your inventory

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For Buyers

  • Portability: One creative works across multiple publishers
  • Predictability: Format requirements are consistent
  • Faster launches: No custom creative production per publisher

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Implementation Steps

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Step 1: Discover What Formats You Need

List the creative formats your inventory accepts. For example:
  • Display: 300x250, 728x90, 320x50
  • Video: 15-second pre-roll, 30-second pre-roll
  • Native: Responsive native format

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Step 2: Check the Reference Creative Agent

Query https://creative.adcontextprotocol.org using list_creative_formats to see which of your formats already exist. The reference agent maintains formats for:
  • All IAB standard display sizes
  • Common video durations and aspect ratios
  • Standard audio formats
  • DOOH specifications
  • Native ad formats

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Step 3: Decide What to Reference vs Define

Reference formats when:
  • The format matches your technical requirements exactly
  • You accept creatives built to standard IAB specifications
  • You want creative portability across publishers
Define custom formats when:
  • You have unique technical requirements (custom dimensions, special asset needs)
  • You need publisher-specific validation or assembly logic
  • You offer premium, differentiated ad experiences

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Step 4: Implement Your Response

When implementing list_creative_formats, include the reference creative agent if you support any standard formats: Most Common: Reference Standard Formats If your inventory accepts standard formats (which most publishers do): 
{
  "formats": [],
  "creative_agents": [
    "https://creative.adcontextprotocol.org"
  ]
}
This tells buyers: “We support all standard formats from the reference creative agent.” With Custom Formats: Combine Both If you have unique formats PLUS standard format support: 
{
  "formats": [
    {
      "format_id": {
        "agent_url": "https://youragent.com",
        "id": "homepage_takeover"
      },
      "name": "Homepage Takeover",
      "type": "rich_media",
      "assets_required": [...]
    }
  ],
  "creative_agents": [
    "https://creative.adcontextprotocol.org"
  ]
}
This tells buyers: “We support our custom homepage takeover format, PLUS all standard formats from the reference creative agent.” Only Custom Formats: Skip the Reference If you ONLY support custom formats with truly unique requirements (rare): 
{
  "formats": [
    {
      "format_id": {
        "agent_url": "https://youragent.com",
        "id": "custom_holographic_display"
      },
      "name": "Holographic Display Format",
      "type": "dooh",
      "assets_required": [...]
    }
  ]
}
Note: This is uncommon. Most publishers accept at least some standard formats (300x250, etc.) and should include the reference creative agent URL.

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What Standard Formats Are Included?

The reference creative agent provides formats across all major channels: Each format includes:
  • Precise technical requirements (dimensions, duration, file types)
  • Required and optional assets with specifications
  • Universal macro support
  • Preview and validation capabilities

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Format Discovery Flow

When buyers discover formats from your sales agent:
  1. Buyer calls list_creative_formats on your sales agent
  2. Your response includes custom formats plus creative_agents: ["https://creative.adcontextprotocol.org"]
  3. Buyer recursively queries the reference agent to discover standard formats
  4. Buyer sees combined list of your custom formats plus all standard formats
The buyer tracks which URLs they’ve queried to avoid infinite loops.

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Best Practices for Sales Agents

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✅ DO Reference Standard Formats

{
  "creative_agents": [
    "https://creative.adcontextprotocol.org"
  ]
}
When: Your inventory accepts standard IAB sizes with no special requirements Why: Reduces maintenance, ensures consistency, buyers already have compatible creatives

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✅ DO Define Custom Formats

{
  "formats": [
    {
      "format_id": {
        "agent_url": "https://youragent.com",
        "id": "native_feed_card"
      },
      "type": "native"
    }
  ]
}
When: You have unique inventory experiences or specific technical requirements Why: Enables differentiation and premium inventory

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❌ DON’T Replicate Standard Formats

{
  "formats": [
    {"format_id": {"agent_url": "https://youragent.com", "id": "display_300x250"}},
    {"format_id": {"agent_url": "https://youragent.com", "id": "display_728x90"}},
    {"format_id": {"agent_url": "https://youragent.com", "id": "display_320x50"}},
    // ... copying 50+ standard formats
  ]
}
Why not: Maintenance burden, version drift, inconsistency across ecosystem Exception: You need custom validation/preview logic for these formats

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✅ DO Use Both When Appropriate

{
  "formats": [
    // Your differentiating formats
  ],
  "creative_agents": [
    "https://creative.adcontextprotocol.org"
  ]
}
Result: Buyers see your custom formats plus all standard formats

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Format ID Namespacing

To prevent conflicts when multiple agents define formats, AdCP uses a namespace pattern for format identifiers.

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Namespace Pattern: {domain}:{format_id}

Structure: 
domain:format_id
Examples:
  • creative.adcontextprotocol.org:display_300x250
  • creative.adcontextprotocol.org:video_30s_hosted
  • youragent.com:homepage_takeover_2024
  • publisher.example:native_feed_card

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Domain Requirements

The domain in a namespaced format_id MUST:
  1. Host a valid agent card at {domain}/.well-known/adagents.json
  2. Declare MCP endpoint in the agent card extension
  3. Declare A2A endpoint in the standard agent card section
Example agent card at https://youragent.com/.well-known/adagents.json: 
{
  "agents": [
    {
      "agent_url": "https://youragent.com",
      "agent_name": "Your Creative Agent",
      "protocols": ["mcp", "a2a"],
      "mcp_endpoint": "https://youragent.com/mcp",
      "a2a_endpoint": "https://youragent.com/a2a",
      "capabilities": ["list_creative_formats", "preview_creative"]
    }
  ]
}
This ensures the domain in the namespace is a valid, discoverable agent that can provide format specifications and validation.

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When to Use Namespaces

Always use namespaced format_ids when defining formats: 
{
  "formats": [
    {
      "format_id": {
        "agent_url": "https://youragent.com",
        "id": "homepage_takeover"
      },
      "name": "Homepage Takeover",
      "type": "rich_media"
    }
  ]
}
Benefits:
  • No collisions - Each agent owns its namespace
  • Clear ownership - Domain identifies the authoritative agent
  • Discoverable - Buyers can query the domain’s agent card
  • Verifiable - Domain must prove ownership via agent card

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Namespace Examples by Agent Type

Reference Creative Agent: 
{
  "format_id": {
    "agent_url": "https://creative.adcontextprotocol.org",
    "id": "display_300x250"
  }
}
Publisher Sales Agent: 
{
  "format_id": {
    "agent_url": "https://youragent.com",
    "id": "custom_format"
  }
}
DCO Platform: 
{
  "format_id": {
    "agent_url": "https://dco.example",
    "id": "dynamic_creative_v2"
  }
}

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Conflict Resolution

With namespaced format_ids, conflicts cannot occur - each domain controls its own namespace. No conflict: 
// Two different formats, both valid
{
  "format_id": {
    "agent_url": "https://publisher-a.com",
    "id": "video_30s"
  }
}
{
  "format_id": {
    "agent_url": "https://publisher-b.com",
    "id": "video_30s"
  }
}
If a buyer encounters the same namespaced format_id from multiple sources, they are the same format - the namespace guarantees identity.

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Validation Rules

  1. Domain MUST match agent_url domain: 
    // ✅ Valid - domain matches
{
  "format_id": {
    "agent_url": "https://youragent.com",
    "id": "format_x"
  }
}

// ❌ Invalid - domain mismatch
{
  "format_id": {
    "agent_url": "https://otheragent.com",
    "id": "format_x"
  }
}
  2. Domain MUST have valid agent card:
    • Agent card must exist at {domain}/.well-known/adagents.json
    • Must declare MCP and/or A2A endpoints
    • Endpoints must be functional
  3. Format_id MUST follow pattern:
    • {domain}:{format_id} structure
    • Domain is valid DNS hostname
    • Format_id is alphanumeric with underscores/hyphens

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Migration from Unnamespaced IDs

If you previously used simple IDs like display_300x250, migrate to namespaced versions: Before: 
{
  "format_id": {
    "agent_url": "https://youragent.com",
    "id": "display_300x250_old"
  }
}
After: 
{
  "format_id": {
    "agent_url": "https://youragent.com",
    "id": "display_300x250"
  }
}
Support both during transition if needed, but new implementations should use namespaced IDs from the start.

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Reference Agent as Format Authority

Each format includes a format_id field with an agent_url indicating its authoritative source: 
{
  "format_id": {
    "agent_url": "https://creative.adcontextprotocol.org",
    "id": "display_300x250"
  },
  "name": "Medium Rectangle",
  "type": "display"
}
The creative agent at that URL is the definitive source for:
  • Complete format specifications
  • Asset requirements and validation rules
  • Preview generation
  • Rendering instructions

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When NOT to Use Standard Formats

Define your own formats when:
  1. Unique technical requirements - Your platform needs different specs than IAB standards
  2. Custom validation - You require additional creative review or approval
  3. Proprietary assembly - Your rendering pipeline has special requirements
  4. Premium experiences - Differentiated ad products not covered by standard formats
Even in these cases, you can reference standard formats for basic inventory while defining custom formats for premium placements.

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Implementation Notes

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Format Authority Pattern

The agent_url field enables a distributed format authority model:
  • Reference agent is authoritative for IAB standards
  • Each publisher is authoritative for their custom formats
  • Buyers can discover and validate against the correct authority

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Version Management

The reference creative agent maintains format versions and compatibility:
  • Format definitions evolve with industry standards
  • Backward compatibility is maintained
  • Buyers can rely on stable format_id values

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What Makes a Format “Standard”

Standard formats are those defined by the reference creative agent at https://creative.adcontextprotocol.org based on:
  1. Industry specifications - IAB standards, VAST/VPAID specs, common ad unit sizes
  2. Cross-platform compatibility - Work across multiple publishers without customization
  3. Stable definitions - Versioned and maintained for consistency across the ecosystem
Protocol perspective: At the protocol level, standard formats are just formats like any other - there’s no special API treatment. The agent_url field identifies the reference agent as the authoritative source, just as it would for any custom format. Ecosystem perspective: Standard formats enable portability. A buyer can build one creative and use it across many publishers who reference the same format definitions.