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  5. A2A v1 Prompt Decorator

A2A v1 Prompt Decorator Policy

Policy Name

A2A v1 Prompt Decorator

Summary

Decorates A2A v1 prompts with additional context information

Category

A2A

First Omni Gateway version available

v1.13.0

Returned Status Codes

No return codes exist for this policy.
This policy supports Agent2Agent Protocol (A2A) version v0.3.0. To learn more about A2A versions, see A2A Releases.

Summary

The A2A v1 Prompt Decorator policy injects additional content into agent prompts before they’re sent to the upstream agent. This policy supports A2A v1.0 SendMessage and SendStreamingMessage requests across all transport protocols: JSON-RPC, HTTP+JSON, and gRPC.

You can decorate prompts with either text content or file references. Text decorators add plain text to the beginning of the message, while file decorators add external resources via URL or inline base64-encoded content. All decorators support conditional evaluation using DataWeave expressions, enabling you to control when specific decorations are applied based on request attributes, authentication context, or message parameters.

The policy injects content to the beginning of the user message’s parts[] array before the request reaches the upstream agent, ensuring the additional context appears before the user’s original message. The A2A v1 Prompt Decorator policy decorates A2A v1 prompts with additional context information. You can add text instructions or file references (via URL or base64-encoded content) to the beginning of messages before they reach the upstream agent. This policy supports A2A v1.0 traffic across all transport protocols: JSON-RPC, HTTP+JSON, and gRPC.

This policy supports A2A v1.0 traffic only. For pre-v1.0 A2A traffic, use the A2A Prompt Decorator Policy.

Configuring Policy Parameters

Omni Gateway Local Mode

The A2A v1 Prompt Decorator policy isn’t supported in Local Mode.

Managed Omni Gateway and Omni Gateway Connected Mode

When you apply the policy from the UI, the following parameters are displayed:

Element Description Required Default

Text Decorators

Array of text decorations to add to prompts. Each decorator can include a text expression and an optional condition.

For more information, see Text Decorators.

No

Empty array

File Decorators

Array of file-type decorations to add to prompts. Each decorator specifies file content (URL or base64), file type, and optional metadata like filename and MIME type.

For more information, see File Decorators.

No

Empty array

Text Decorators

Text decorators add plain text content to the beginning of the message. Each text decorator has the following properties:

Property Description Required Default

Prompt Text

DataWeave expression that generates the prompt text injected into the request. The result is added as a text part with media type text/plain.

The expression can access:

  • attributes: Request attributes (headers, query parameters, etc.)

  • authentication: Authentication context from upstream authentication policies

  • vars.params: The SendMessageParams object containing the message and conversation ID

For example: #['Use imperial metric units']

Yes

None

Condition

Optional DataWeave expression that filters which prompts to decorate. If the expression evaluates to true, the decorator is applied; otherwise, it is skipped.

For example: #[attributes.headers['x-environment']=='production']

No

Always applied

File Decorators

File decorators add file references or inline file content to the beginning of the message. Each file decorator has the following properties:

Property Description Required Default

Content

DataWeave expression containing either a URL to an external file (when File Source is Uri) or base64-encoded file content (when File Source is Base64).

The expression can access attributes, authentication, and vars.params.

Base64 content is validated against the pattern ^[A-Za-z0-9+/]*={0,2}$. Invalid content causes the decorator to be skipped with a warning logged.

For example, URI: #['https://example.com/schema.json']

For example, Base64: #['QmFzZTY0IGVuY29kZWQgZmlsZSBjb250ZW50']

Yes

None

File Source

Specifies whether the Content field contains a URL or base64-encoded data. Supported values:

  • Uri: Content is a URL to an external file

  • Base64: Content is base64-encoded file data

Yes

Base64

File Name

Optional DataWeave expression that generates the filename for the injected file part.

For example: #['api-schema.json']

No

None

File Mime Type

Optional DataWeave expression that generates the MIME type for the injected file part.

If not specified, defaults to application/octet-stream.

For example: #['application/json']

No

application/octet-stream

Condition

Optional DataWeave expression that filters which prompts to decorate. If the expression evaluates to true, the decorator is applied; otherwise, it is skipped.

For example: #[attributes.headers['x-include-schema']=='true']

No

Always applied

Examples

Text Decoration

This example adds a text instruction to all prompts, directing the agent to use Imperial units:

apiVersion: gateway.mulesoft.com/v1alpha1
kind: PolicyBinding
metadata:
  name: a2a-v1-prompt-decorator-text
spec:
  targetRef:
    kind: ApiInstance
    name: my-a2a-v1-agent
  policyRef:
    name: a-two-a-v1-prompt-decorator-flex
  config:
    textDecorators:
      - text: "#['Use Imperial metric units for all measurements.']"

Conditional Text Decoration

This example adds different text decorations based on the request environment header:

apiVersion: gateway.mulesoft.com/v1alpha1
kind: PolicyBinding
metadata:
  name: a2a-v1-prompt-decorator-conditional
spec:
  targetRef:
    kind: ApiInstance
    name: my-a2a-v1-agent
  policyRef:
    name: a-two-a-v1-prompt-decorator-flex
  config:
    textDecorators:
      - text: "#['You are operating in PRODUCTION mode. Be extra careful.']"
        condition: "#[attributes.headers['x-environment']=='production']"
      - text: "#['You are operating in TEST mode. Feel free to experiment.']"
        condition: "#[attributes.headers['x-environment']=='test']"

File Decoration with URI

This example adds an external schema file to all prompts:

apiVersion: gateway.mulesoft.com/v1alpha1
kind: PolicyBinding
metadata:
  name: a2a-v1-prompt-decorator-file-uri
spec:
  targetRef:
    kind: ApiInstance
    name: my-a2a-v1-agent
  policyRef:
    name: a-two-a-v1-prompt-decorator-flex
  config:
    fileDecorators:
      - file: "#['https://example.com/api-schema.json']"
        fileType: Uri
        fileName: "#['api-schema.json']"
        fileMimeType: "#['application/json']"

File Decoration with Base64 Content

This example adds inline base64-encoded content as a file:

apiVersion: gateway.mulesoft.com/v1alpha1
kind: PolicyBinding
metadata:
  name: a2a-v1-prompt-decorator-file-base64
spec:
  targetRef:
    kind: ApiInstance
    name: my-a2a-v1-agent
  policyRef:
    name: a-two-a-v1-prompt-decorator-flex
  config:
    fileDecorators:
      - file: "#['SGVsbG8sIFdvcmxkIQ==']"
        fileType: Base64
        fileName: "#['greeting.txt']"
        fileMimeType: "#['text/plain']"

Combined Text and File Decorations

The following example combines both text and file decorations:

apiVersion: gateway.mulesoft.com/v1alpha1
kind: PolicyBinding
metadata:
  name: a2a-v1-prompt-decorator-combined
spec:
  targetRef:
    kind: ApiInstance
    name: my-a2a-v1-agent
  policyRef:
    name: a-two-a-v1-prompt-decorator-flex
  config:
    textDecorators:
      - text: "#['Use Imperial metric units.']"
      - text: "#['Follow the API schema provided in the attached file.']"
    fileDecorators:
      - file: "#['https://example.com/schema.json']"
        fileType: Uri
        fileName: "#['api-schema.json']"
        fileMimeType: "#['application/json']"