feat(spec): Provide ability for SDKs to be backwards compatible.

[herczyn]

fixes #1417
for context see #1416

[gemini-code-assist]

Summary of Changes

Hello @herczyn, 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 focuses on enhancing the gRPC API definitions and improving backward compatibility for agents. It streamlines gRPC configurations by removing redundant bindings and introduces explicit protocol versioning for agent interfaces. Additionally, it deprecates older transport-related fields in the AgentCard message, signaling a shift towards newer, more standardized transport mechanisms.

Highlights

• gRPC API Updates: Modified the a2a.proto file to streamline gRPC API definitions by removing redundant additional_bindings in google.api.http options for methods like GetTask, ListTasks, SubscribeToTask, GetTaskPushNotificationConfig, ListTaskPushNotificationConfig, GetExtendedAgentCard, and DeleteTaskPushNotificationConfig.
• Agent Interface Enhancements: Added protocol_versions field to the AgentInterface message in a2a.proto to explicitly define supported A2A protocol versions, ensuring backward compatibility. The field is required and should list the latest supported minor version per major version (e.g., ["0.3", "1.0"]).
• AgentCard Deprecation: Deprecated several fields in the AgentCard message of a2a.proto, including url, preferred_transport, and additional_interfaces. These fields are marked as deprecated and are intended for use only with the 0.3 version of the transport, indicating a move towards newer transport mechanisms.

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[gemini-code-assist]

Code Review

The pull request effectively addresses backward compatibility concerns by refining the A2A protocol's versioning strategy. The relocation of protocol_versions from AgentCard to AgentInterface is a logical improvement, allowing for more granular versioning per transport. The deprecation of older fields in AgentCard with clear comments is a good practice for maintaining backward compatibility while signaling future removals. The changes also include some minor formatting adjustments in the additional_bindings sections, with a suggestion to maintain multi-line formatting for readability.

[herczyn]

/vote

[git-vote]

Vote created

@herczyn has called for a vote on feat(spec): Provide ability for SDKs to be backwards compatible. (#1401).

The members of the following teams have binding votes:

Team
@a2aproject/a2a-tsc

Non-binding votes are also appreciated as a sign of support!

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The vote will be open for 11months 29days 3h 50m 24s. It will pass if at least 51% of the users with binding votes vote In favor 👍. Once it's closed, results will be published here as a new comment.

[git-vote]

Vote status

So far 0.00% of the users with binding vote are in favor and 0.00% are against (passing threshold: 51%).

Summary

In favor	Against	Abstain	Not voted
0	0	0	8

Binding votes (0)

User	Vote	Timestamp
@muscariello	Pending
@darrelmiller	Pending
@geneknit	Pending
@hughesthe1st	Pending
@ToddSegal	Pending
@000-000-000-000-000	Pending
@SivaNSAP	Pending
@spetschulatSFDC	Pending

Non-binding votes (2)

User	Vote	Timestamp
Tehsmash	In favor	2026-01-28 9:05:01.0 +00:00:00
herczyn	In favor	2026-01-28 13:48:56.0 +00:00:00

[muscariello]

LGTM

[herczyn]

LGTM

i assume we still wait for the vote, right?

[muscariello]

@ToddSegal @geneknit @spetschulatSFDC @SivaNSAP @hughesthe1st

[git-vote]

Vote status

So far 75.00% of the users with binding vote are in favor and 0.00% are against (passing threshold: 51%).

Summary

In favor	Against	Abstain	Not voted
6	0	0	2

Binding votes (6)

User	Vote	Timestamp
SivaNSAP	In favor	2026-01-29 19:22:55.0 +00:00:00
ToddSegal	In favor	2026-01-29 19:20:50.0 +00:00:00
darrelmiller	In favor	2026-01-29 0:25:48.0 +00:00:00
geneknit	In favor	2026-01-29 19:04:36.0 +00:00:00
muscariello	In favor	2026-01-29 12:42:57.0 +00:00:00
spetschulatSFDC	In favor	2026-01-29 19:20:21.0 +00:00:00
@hughesthe1st	Pending
@000-000-000-000-000	Pending

Non-binding votes (3)

User	Vote	Timestamp
Tehsmash	In favor	2026-01-28 9:05:01.0 +00:00:00
herczyn	In favor	2026-01-28 13:48:56.0 +00:00:00
holtskinner	In favor	2026-01-29 15:49:14.0 +00:00:00

[git-vote]

Vote closed

The vote passed! 🎉

75.00% of the users with binding vote were in favor and 0.00% were against (passing threshold: 51%).

Summary

In favor	Against	Abstain	Not voted
6	0	0	2

Binding votes (6)

User	Vote	Timestamp
@SivaNSAP	In favor	2026-01-29 19:22:55.0 +00:00:00
@ToddSegal	In favor	2026-01-29 19:20:50.0 +00:00:00
@darrelmiller	In favor	2026-01-29 0:25:48.0 +00:00:00
@geneknit	In favor	2026-01-29 19:04:36.0 +00:00:00
@muscariello	In favor	2026-01-29 12:42:57.0 +00:00:00
@spetschulatSFDC	In favor	2026-01-29 19:20:21.0 +00:00:00

Non-binding votes (3)

User	Vote	Timestamp
@Tehsmash	In favor	2026-01-28 9:05:01.0 +00:00:00
@herczyn	In favor	2026-01-28 13:48:56.0 +00:00:00
@holtskinner	In favor	2026-01-29 15:49:14.0 +00:00:00