fix: Remove unimplemented state_transition_history capability field by madankumarpichamuthu · Pull Request #1396 · a2aproject/A2A

madankumarpichamuthu · 2026-01-21T21:43:44Z

Removes the state_transition_history field from AgentCapabilities message as it has no corresponding API implementation. This capability flag could mislead clients into expecting functionality that doesn't exist in v1.0.

The field can be reintroduced in a future version when a proper implementation is designed, including:

Adding status_history field to Task message
Adding include_status_history parameters to Get/List operations
Defining behavior for history storage and retrieval

Fixes #1228

gemini-code-assist · 2026-01-21T21:43:56Z

Summary of Changes

Hello @madankumarpichamuthu, 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 streamlines the AgentCapabilities gRPC message by removing the state_transition_history field. This field, despite being present in the specification, lacked any corresponding API implementation, potentially misleading clients into expecting functionality that does not exist in the current v1.0 API. The removal ensures the specification accurately reflects available features and avoids confusion.

Highlights

API Specification Cleanup: Removed the state_transition_history field from the AgentCapabilities message in a2a.proto as it was an unimplemented capability.

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Code Review

This pull request removes the unimplemented state_transition_history field from the AgentCapabilities message, which is a good cleanup to prevent client-side confusion. My review includes a suggestion to improve this change by reserving the field number of the deleted field. This is a Protobuf best practice to ensure backward compatibility and prevent future issues, a pattern already present elsewhere in the codebase.

I am having trouble creating individual review comments. Click here to see my feedback.

specification/grpc/a2a.proto (483-484)

Instead of just removing this field, it's better to reserve its field number. This prevents the number from being accidentally reused in the future for a different field, which could cause issues with backward compatibility for clients using older versions of the spec. I see this pattern is already used in the AgentCard message.

  reserved 4;

Reserves field number 4 in the AgentCapabilities message, previously used by the unimplemented state_transition_history capability. Reserving the field number prevents it from being accidentally reused in the future, which could cause backward compatibility issues with clients using older versions of the specification. The state_transition_history capability had no corresponding API implementation, making it misleading for clients. Reserving the field number follows the established pattern used elsewhere in the codebase (AgentCard message reserves fields 3, 14, 15; OAuthFlows message reserves fields 3, 4). Changes: - Added "reserved 4;" to AgentCapabilities message - Added comment documenting previous usage Fixes a2aproject#1228 Co-Authored-By: madankumarpichamuthu <madankumarpichamuthu@users.noreply.github.com>

muscariello · 2026-01-22T09:11:06Z

/vote

git-vote · 2026-01-22T09:11:09Z

Vote created

@muscariello has called for a vote on fix: Remove unimplemented state_transition_history capability field (#1396).

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muscariello · 2026-01-22T10:34:41Z

@amye @darrelmiller do we really need a vote for this one ? It looks like housekeeping.

amye · 2026-01-22T17:16:32Z

I will advocate for +2 TSC and merge, this is housekeeping.

amye · 2026-01-22T17:29:26Z

/cancel-vote

git-vote · 2026-01-22T17:29:29Z

Vote cancelled

@amye has cancelled the vote in progress in this pull request.

muscariello

LGTM

🤖 I have created a release *beep* *boop* --- ## [1.0.0](v0.3.0...v1.0.0) (2026-03-12) ### ⚠ BREAKING CHANGES * **spec:** Combine `TaskPushNotificationConfig` and `PushNotificationConfig` ([#1500](#1500)) * **spec:** remove duplicated ID from the create task push config request ([#1487](#1487)) * **spec:** pluralize configs in `ListTaskPushNotificationConfigs` ([#1486](#1486)) * **spec:** Add LF prefix to the package. ([#1474](#1474)) * **spec:** Switch to non-complex IDs in requests ([#1389](#1389)) * **spec:** Standardize spelling of "canceled" to use American Spelling throughout ([#1283](#1283)) * **spec:** Align enum format with ADR-001 ProtoJSON specification ([#1384](#1384)) * **spec:** Remove redundant `final` field from `TaskStatusUpdateEvent` ([#1308](#1308)) * **spec:** Move `extendedAgentCard` field to `AgentCapabilities` ([#1307](#1307)) * **spec:** Fixes for the last_updated_after field ([#1358](#1358)) * **spec:** modernize oauth 2.0 flows - remove implicit/password, add device code / pkce ([#1303](#1303)) * **spec:** Make "message" field name consistent between protocol bindings ([#1302](#1302)) * **spec:** Remove deprecated fields from a2a.proto for v1.0 release ([#1301](#1301)) * **spec:** Rename `supportsAuthenticatedExtendedCard` to `supportsExtendedAgentCard` ([#1222](#1222)) * **spec:** Remove v1s from a2a url http bindings * **spec:** Large refactor of specification to separate application protocol definition from mapping to transports ### Features * **spec:** Add `tasks/list` method with filtering and pagination to the specification ([0a9f629](0a9f629)) * **spec:** modernize oauth 2.0 flows - remove implicit/password, add device code / pkce ([#1303](#1303)) ([525ff38](525ff38)) * **spec:** Natively Support Multi-tenancy on gRPC through an additional scope field on the request. ([#1195](#1195)) ([cfbce32](cfbce32)), closes [#1148](#1148) * **spec:** Provide ability for SDKs to be backwards compatible. ([#1401](#1401)) ([227e249](227e249)) * **spec:** Remove v1s from a2a url http bindings ([1bd263f](1bd263f)) ### Bug Fixes * Add missing metadata field to Part message in gRPC specification ([#1019](#1019)) ([b3b266d](b3b266d)), closes [#1005](#1005) * Add name field to FilePart protobuf message ([#983](#983)) ([2b7cb6f](2b7cb6f)), closes [#984](#984) * Clarify blocking calls return on interrupted states ([#1403](#1403)) ([0655ff3](0655ff3)) * **doc:** Makes JSON-RPC SendMessage response clearer ([#1241](#1241)) ([5792804](5792804)) * **docs:** Clearer wording around context id. ([#1588](#1588)) ([dec790a](dec790a)) * **grpc:** Fix inconsistent property name between gRPC and JSON-RPC in Message object ([#1100](#1100)) ([2a1f819](2a1f819)) * **grpc:** missing field in gRPC spec - state_transition_history ([#1138](#1138)) ([a2de798](a2de798)), closes [#1139](#1139) * **grpc:** Update `CreateTaskPushNotificationConfig` endpoint to `/v1/{parent=tasks/*/pushNotificationConfigs}` ([#979](#979)) ([911f9b0](911f9b0)) * **proto:** Add icon_url to a2a.proto ([#986](#986)) ([17e7f62](17e7f62)) * **proto:** Adds metadata field to A2A DataPart proto ([#1004](#1004)) ([a8b45dc](a8b45dc)) * Remove unimplemented state_transition_history capability field ([#1396](#1396)) ([c768a44](c768a44)), closes [#1228](#1228) * Restore CreateTaskPushNotificationConfig method naming ([#1402](#1402)) ([d14f410](d14f410)) * Revert "chore(gRPC): Update a2a.proto to include metadata on GetTaskRequest" ([#1000](#1000)) ([e6b8c65](e6b8c65)) * Simplify Part message structure by flattening FilePart and DataPart ([#1411](#1411)) ([bfae8f7](bfae8f7)) * **spec:** Add LF prefix to the package. ([#1474](#1474)) ([a54e809](a54e809)) * **spec:** add metadata to `CancelTaskRequest` ([#1485](#1485)) ([c441b91](c441b91)), closes [#1484](#1484) * **spec:** Added clarification on timestamps in HTTP query params ([#1425](#1425)) ([6292104](6292104)) * **spec:** Added clarifying text around messages and artifacts ([#1424](#1424)) ([b03d141](b03d141)) * **spec:** Adjust field number for `ListTasksRequest.tenant` to prevent missing number ([#1470](#1470)) ([cd16c52](cd16c52)) * **spec:** Clarify contextId behavior when message is sent with taskId but without contextId ([#1309](#1309)) ([a336a5a](a336a5a)) * **spec:** Clarify versioning strategy and client responsibilities in protocol specification ([#1259](#1259)) ([a4afeea](a4afeea)) * **spec:** Fix/1251 clarify authentication scheme ([#1256](#1256)) ([3e6c7db](3e6c7db)) * **spec:** Fixes for the last_updated_after field ([#1358](#1358)) ([0e204bf](0e204bf)) * **spec:** Make "message" field name consistent between protocol bindings ([#1302](#1302)) ([1e5f462](1e5f462)), closes [#1230](#1230) * **spec:** make `history_length` optional ([#1071](#1071)) ([0572953](0572953)) * **spec:** pluralize configs in `ListTaskPushNotificationConfigs` ([#1486](#1486)) ([cf735cb](cf735cb)) * **spec:** Remove config from binding. ([#1587](#1587)) ([010b9cc](010b9cc)) * **spec:** Remove deprecated fields from a2a.proto for v1.0 release ([#1301](#1301)) ([60f83c3](60f83c3)), closes [#1227](#1227) * **spec:** remove duplicated ID from the create task push config request ([#1487](#1487)) ([393898d](393898d)) * **spec:** Remove metadata field from ListTasksRequest ([#1235](#1235)) ([b6ef9ee](b6ef9ee)) * **spec:** Remove reserved and fix tags ordering ([#1494](#1494)) ([1997c9d](1997c9d)) * **spec:** Rename `supportsAuthenticatedExtendedCard` to `supportsExtendedAgentCard` ([#1222](#1222)) ([c196824](c196824)), closes [#1215](#1215) * **spec:** Standardize spelling of "canceled" to use American Spelling throughout ([#1283](#1283)) ([4dd980f](4dd980f)) * **spec:** Suggest Unique Identifier fields to be UUID ([#966](#966)) ([00cf76e](00cf76e)) * **spec:** Switch to non-complex IDs in requests ([#1389](#1389)) ([2596c1c](2596c1c)), closes [#1390](#1390) * **spec:** Update security schemes example ([#1364](#1364)) ([f9a8f5b](f9a8f5b)) * Update the Java tutorials and descriptions ([#1181](#1181)) ([202aa06](202aa06)) ### Documentation * **spec:** Align enum format with ADR-001 ProtoJSON specification ([#1384](#1384)) ([810eaa1](810eaa1)), closes [#1344](#1344) ### Code Refactoring * **spec:** Combine `TaskPushNotificationConfig` and `PushNotificationConfig` ([#1500](#1500)) ([d1ed0da](d1ed0da)) * **spec:** Large refactor of specification to separate application protocol definition from mapping to transports ([b078419](b078419)) * **spec:** Move `extendedAgentCard` field to `AgentCapabilities` ([#1307](#1307)) ([40d6286](40d6286)) * **spec:** Remove redundant `final` field from `TaskStatusUpdateEvent` ([#1308](#1308)) ([5b101cc](5b101cc)) --- This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please). Co-authored-by: Amye Scavarda Perrin <amye@amye.org> Co-authored-by: Holt Skinner <13262395+holtskinner@users.noreply.github.com>

madankumarpichamuthu requested a review from a team as a code owner January 21, 2026 21:43

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madankumarpichamuthu mentioned this pull request Jan 21, 2026

[Bug]: Evaluate removing state_transition_history capability from AgentCapabilities #1228

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madankumarpichamuthu force-pushed the fix/1228-remove-state-transition-history branch from bee9ebc to cb39fc5 Compare January 21, 2026 21:55

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Merge branch 'main' into fix/1228-remove-state-transition-history

115cf45

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darrelmiller merged commit c768a44 into a2aproject:main Jan 25, 2026
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chore(main): release 1.0.0 #1221

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fix: Remove unimplemented state_transition_history capability field#1396

fix: Remove unimplemented state_transition_history capability field#1396
darrelmiller merged 2 commits intoa2aproject:mainfrom
madankumarpichamuthu:fix/1228-remove-state-transition-history

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madankumarpichamuthu commented Jan 21, 2026

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specification/grpc/a2a.proto (483-484)

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