feat: Add HTTP caching headers to Agent Card endpoint

[kabir]

Implements A2A specification section 8.6 caching requirements:

• Cache-Control header with configurable max-age
• ETag header derived from Agent Card content hash
• Last-Modified header with initialization timestamp

Implementation:

• Created AgentCardCacheMetadata bean in server-common to compute and cache HTTP headers at initialization
• Enhanced HTTPRestResponse to support additional headers via Map
• Updated RestHandler and JSON-RPC A2AServerRoutes to include caching headers
• Caching headers applied to both REST and JSON-RPC transports (gRPC out of scope per spec)

Configuration:

• Max-age configurable via a2a.agent-card.cache.max-age (default: 3600 seconds)
• ETag calculated as MD5 hash of serialized Agent Card JSON
• Last-Modified set to bean initialization time in RFC 1123 format

Testing:

• Added testAgentCardHeaders() to AbstractA2AServerTest
• Validates all three caching headers are present and correctly formatted
• gRPC test overrides to skip (HTTP-only requirement)

Fixes #749 🦕

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces HTTP caching mechanisms to the Agent Card endpoint, significantly improving performance and reducing server load by allowing clients and intermediaries to cache the Agent Card content. By implementing Cache-Control, ETag, and Last-Modified headers, the system now adheres to the A2A specification's caching requirements, providing a more efficient and compliant way to serve static agent information.

Highlights

• HTTP Caching Headers: Implemented HTTP caching headers (Cache-Control, ETag, Last-Modified) for the Agent Card endpoint, aligning with A2A specification section 8.6.
• New Caching Metadata Bean: Introduced a new AgentCardCacheMetadata bean to compute and store caching header values at application initialization.
• HTTPRestResponse Enhancement: Enhanced the HTTPRestResponse class to support the inclusion of additional HTTP headers.
• Broad Application of Caching: Applied caching headers to both REST and JSON-RPC transports for the Agent Card endpoint.
• New Test Case: Added a new test case (testAgentCardHeaders) to validate the presence and correct formatting of the caching headers.

Changelog

• reference/grpc/src/test/java/io/a2a/server/grpc/quarkus/QuarkusA2AGrpcTest.java
  • Overrode testAgentCardHeaders to skip execution, as gRPC does not utilize HTTP caching headers.
• reference/jsonrpc/src/main/java/io/a2a/server/apps/quarkus/A2AServerRoutes.java
  • Injected AgentCardCacheMetadata to retrieve caching header values.
  • Updated the Javadoc for getAgentCard to document the new caching headers.
  • Modified getAgentCard to add Cache-Control, ETag, and Last-Modified headers to the HTTP response.
• reference/rest/src/main/java/io/a2a/server/rest/quarkus/A2AServerRoutes.java
  • Refactored sendResponse to iterate and add all custom headers provided by HTTPRestResponse to the Vert.x HTTP response.
• server-common/src/main/java/io/a2a/server/AgentCardCacheMetadata.java
  • Added a new @ApplicationScoped bean AgentCardCacheMetadata responsible for calculating and holding the Cache-Control, ETag (MD5 hash of Agent Card JSON), and Last-Modified (initialization timestamp) header values.
• tests/server-common/src/test/java/io/a2a/server/apps/common/AbstractA2AServerTest.java
  • Added testAgentCardHeaders to verify that the Agent Card endpoint correctly returns Cache-Control, ETag, and Last-Modified headers with appropriate formats.
• transport/rest/src/main/java/io/a2a/transport/rest/handler/RestHandler.java
  • Injected AgentCardCacheMetadata into RestHandler.
  • Updated constructors for RestHandler to accept AgentCardCacheMetadata.
  • Modified getAgentCard to construct an HTTPRestResponse with the newly generated caching headers.
  • Extended HTTPRestResponse to include a headers map, along with new constructors and a getter for these headers.
• transport/rest/src/test/java/io/a2a/transport/rest/handler/RestHandlerTest.java
  • Introduced helper methods createCacheMetadata() for AgentCardCacheMetadata instantiation.
  • Updated all RestHandler test instantiations to pass the AgentCardCacheMetadata dependency.

Activity

• No human activity (comments, reviews, etc.) was found for this pull request.

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[gemini-code-assist]

Code Review

This pull request effectively implements HTTP caching for the Agent Card endpoint by adding Cache-Control, ETag, and Last-Modified headers, as specified. The implementation is well-structured, introducing a dedicated AgentCardCacheMetadata bean to compute and cache these headers at startup. The changes are correctly propagated to both JSON-RPC and REST transports, and the new functionality is validated with a new test case.

I have one suggestion to improve the consistency of ETag generation across different environments. Otherwise, the changes look good.

[jmesnil]

@kabir the CI failure looks genuine

[kabir]

@jmesnil Yep, investigating

[kabir]

/gemini review

[gemini-code-assist]

Code Review

This pull request effectively implements HTTP caching for the Agent Card endpoint as per the A2A specification. The changes are well-structured, introducing an AgentCardCacheMetadata bean to encapsulate the caching logic and cleanly integrating it into the REST and JSON-RPC layers. The tests are also comprehensive, including a new test for the caching headers and appropriate overrides for gRPC.

To improve robustness, I recommend failing fast during application startup in AgentCardCacheMetadata if required dependencies are missing, rather than causing a NullPointerException at runtime.

Overall, this is a great addition that improves performance and compliance with the specification.

[kabir]

@jmesnil Should be fine now