reducers: propagate tool call action metadata

CHANGELOG.md

@@ -30,6 +30,11 @@ changes accumulate. Track in-flight protocol changes via PRs touching
   point-in-time snapshot. Existing variants (root, session, terminal,
   changeset, annotations) are unchanged.
 
+### Fixed
+
+- Session reducers now apply `_meta` updates from every tool-call-scoped
+  action, not only `session/toolCallStart`.
+
 ## [0.4.0] — Unreleased
 
 Spec version: `0.4.0`

clients/go/CHANGELOG.md

@@ -21,6 +21,11 @@ tag whose matching `## [X.Y.Z]` heading is missing from this file.
   `root` + `recursive` keys (ordered between the existing changeset and
   annotations probes).
 
+### Fixed
+
+- Reducer parity fixtures now require `_meta` updates from every
+  tool-call-scoped action, not only `session/toolCallStart`.
+
 ### Added
 
 - `AnnotationsUpdatedAction` (`annotations/updated`) — partially updates an

clients/go/ahp/reducers.go

@@ -463,6 +463,9 @@ func ApplyActionToSession(state *ahptypes.SessionState, action ahptypes.StateAct
 	case *ahptypes.SessionToolCallContentChangedAction:
 		return updateToolCall(state, a.TurnId, a.ToolCallId, func(tc ahptypes.ToolCallState) ahptypes.ToolCallState {
 			if r, ok := tc.Value.(*ahptypes.ToolCallRunningState); ok {
+				if a.Meta != nil {
+					r.Meta = a.Meta
+				}
 				r.Content = append([]ahptypes.ToolResultContent(nil), a.Content...)
 			}
 			return tc
@@ -760,6 +763,9 @@ func applyToolCallDelta(state *ahptypes.SessionState, a *ahptypes.SessionToolCal
 		}
 		joined := current + a.Content
 		s.PartialInput = &joined
+		if a.Meta != nil {
+			s.Meta = a.Meta
+		}
 		if a.InvocationMessage != nil {
 			im := *a.InvocationMessage
 			s.InvocationMessage = &im
@@ -771,6 +777,9 @@ func applyToolCallDelta(state *ahptypes.SessionState, a *ahptypes.SessionToolCal
 func applyToolCallReady(state *ahptypes.SessionState, a *ahptypes.SessionToolCallReadyAction) ReduceOutcome {
 	return updateToolCall(state, a.TurnId, a.ToolCallId, func(tc ahptypes.ToolCallState) ahptypes.ToolCallState {
 		common := toolCallMeta(tc)
+		if a.Meta != nil {
+			common.meta = a.Meta
+		}
 		switch tc.Value.(type) {
 		case *ahptypes.ToolCallStreamingState, *ahptypes.ToolCallRunningState:
 			if a.Confirmed != nil {
@@ -830,6 +839,10 @@ func applyToolCallConfirmed(state *ahptypes.SessionState, a *ahptypes.SessionToo
 			if a.EditedToolInput != nil {
 				toolInput = a.EditedToolInput
 			}
+			meta := s.Meta
+			if a.Meta != nil {
+				meta = a.Meta
+			}
 			confirmed := ahptypes.ToolCallConfirmationReasonNotNeeded
 			if a.Confirmed != nil {
 				confirmed = *a.Confirmed
@@ -840,7 +853,7 @@ func applyToolCallConfirmed(state *ahptypes.SessionState, a *ahptypes.SessionToo
 				ToolName:          s.ToolName,
 				DisplayName:       s.DisplayName,
 				Contributor:       s.Contributor,
-				Meta:              s.Meta,
+				Meta:              meta,
 				InvocationMessage: s.InvocationMessage,
 				ToolInput:         toolInput,
 				Confirmed:         confirmed,
@@ -851,13 +864,17 @@ func applyToolCallConfirmed(state *ahptypes.SessionState, a *ahptypes.SessionToo
 		if a.Reason != nil {
 			reason = *a.Reason
 		}
+		meta := s.Meta
+		if a.Meta != nil {
+			meta = a.Meta
+		}
 		return ahptypes.ToolCallState{Value: &ahptypes.ToolCallCancelledState{
 			Status:            ahptypes.ToolCallStatusCancelled,
 			ToolCallId:        s.ToolCallId,
 			ToolName:          s.ToolName,
 			DisplayName:       s.DisplayName,
 			Contributor:       s.Contributor,
-			Meta:              s.Meta,
+			Meta:              meta,
 			InvocationMessage: s.InvocationMessage,
 			ToolInput:         s.ToolInput,
 			Reason:            reason,
@@ -871,6 +888,9 @@ func applyToolCallConfirmed(state *ahptypes.SessionState, a *ahptypes.SessionToo
 func applyToolCallComplete(state *ahptypes.SessionState, a *ahptypes.SessionToolCallCompleteAction) ReduceOutcome {
 	return updateToolCall(state, a.TurnId, a.ToolCallId, func(tc ahptypes.ToolCallState) ahptypes.ToolCallState {
 		common := toolCallMeta(tc)
+		if a.Meta != nil {
+			common.meta = a.Meta
+		}
 		var (
 			invocation     ahptypes.StringOrMarkdown
 			toolInput      *string
@@ -936,13 +956,17 @@ func applyToolCallResultConfirmed(state *ahptypes.SessionState, a *ahptypes.Sess
 			return tc
 		}
 		if a.Approved {
+			meta := s.Meta
+			if a.Meta != nil {
+				meta = a.Meta
+			}
 			return ahptypes.ToolCallState{Value: &ahptypes.ToolCallCompletedState{
 				Status:            ahptypes.ToolCallStatusCompleted,
 				ToolCallId:        s.ToolCallId,
 				ToolName:          s.ToolName,
 				DisplayName:       s.DisplayName,
 				Contributor:       s.Contributor,
-				Meta:              s.Meta,
+				Meta:              meta,
 				InvocationMessage: s.InvocationMessage,
 				ToolInput:         s.ToolInput,
 				Success:           s.Success,
@@ -954,13 +978,17 @@ func applyToolCallResultConfirmed(state *ahptypes.SessionState, a *ahptypes.Sess
 				SelectedOption:    s.SelectedOption,
 			}}
 		}
+		meta := s.Meta
+		if a.Meta != nil {
+			meta = a.Meta
+		}
 		return ahptypes.ToolCallState{Value: &ahptypes.ToolCallCancelledState{
 			Status:            ahptypes.ToolCallStatusCancelled,
 			ToolCallId:        s.ToolCallId,
 			ToolName:          s.ToolName,
 			DisplayName:       s.DisplayName,
 			Contributor:       s.Contributor,
-			Meta:              s.Meta,
+			Meta:              meta,
 			InvocationMessage: s.InvocationMessage,
 			ToolInput:         s.ToolInput,
 			Reason:            ahptypes.ToolCallCancellationReasonResultDenied,

clients/kotlin/CHANGELOG.md

@@ -22,6 +22,11 @@ versions (`*-SNAPSHOT`) are explicitly rejected by the publish pipeline; bump
   channel decodes via the existing `SnapshotStateSerializer` shape probe
   (required `root` + `recursive` keys).
 
+### Fixed
+
+- `sessionReducer` now applies `_meta` (`meta`) updates from every
+  tool-call-scoped action, not only `session/toolCallStart`.
+
 ### Added
 
 - `AnnotationsUpdatedAction` (`annotations/updated`) — partially updates an

clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt

@@ -269,7 +269,9 @@ private data class ToolCallBase(
     val displayName: String,
     val contributor: ToolCallContributor?,
     val meta: Map<String, JsonElement>?,
-)
+) {
+    fun withMeta(meta: Map<String, JsonElement>?): ToolCallBase = copy(meta = meta ?: this.meta)
+}
 
 private fun toolCallBase(tc: ToolCallState): ToolCallBase = when (tc) {
     is ToolCallStateStreaming -> tc.value.let {
@@ -672,6 +674,7 @@ public fun sessionReducer(state: SessionState, action: StateAction): SessionStat
             if (tc !is ToolCallStateStreaming) tc else {
                 ToolCallStateStreaming(
                     tc.value.copy(
+                        meta = a.meta ?: tc.value.meta,
                         partialInput = (tc.value.partialInput ?: "") + a.content,
                         invocationMessage = a.invocationMessage ?: tc.value.invocationMessage,
                     ),
@@ -687,7 +690,7 @@ public fun sessionReducer(state: SessionState, action: StateAction): SessionStat
                 if (tc !is ToolCallStateStreaming && tc !is ToolCallStateRunning) {
                     tc
                 } else {
-                    val base = toolCallBase(tc)
+                    val base = toolCallBase(tc).withMeta(a.meta)
                     if (a.confirmed != null) {
                         ToolCallStateRunning(
                             ToolCallRunningState(
@@ -730,7 +733,7 @@ public fun sessionReducer(state: SessionState, action: StateAction): SessionStat
         refreshSummaryStatus(
             updateToolCallInParts(state, a.turnId, a.toolCallId) { tc ->
                 if (tc !is ToolCallStatePendingConfirmation) tc else {
-                    val base = toolCallBase(tc)
+                    val base = toolCallBase(tc).withMeta(a.meta)
                     val selectedOption = resolveSelectedOption(tc.value.options, a.selectedOptionId)
                     if (a.approved) {
                         ToolCallStateRunning(
@@ -791,7 +794,7 @@ public fun sessionReducer(state: SessionState, action: StateAction): SessionStat
                     )
                     else -> return@updateToolCallInParts tc
                 }
-                val base = toolCallBase(tc)
+                val base = toolCallBase(tc).withMeta(a.meta)
                 if (a.requiresResultConfirmation == true) {
                     ToolCallStatePendingResultConfirmation(
                         ToolCallPendingResultConfirmationState(
@@ -842,7 +845,7 @@ public fun sessionReducer(state: SessionState, action: StateAction): SessionStat
         refreshSummaryStatus(
             updateToolCallInParts(state, a.turnId, a.toolCallId) { tc ->
                 if (tc !is ToolCallStatePendingResultConfirmation) tc else {
-                    val base = toolCallBase(tc)
+                    val base = toolCallBase(tc).withMeta(a.meta)
                     if (a.approved) {
                         ToolCallStateCompleted(
                             ToolCallCompletedState(
@@ -888,7 +891,7 @@ public fun sessionReducer(state: SessionState, action: StateAction): SessionStat
         val a = action.value
         updateToolCallInParts(state, a.turnId, a.toolCallId) { tc ->
             if (tc !is ToolCallStateRunning) tc else {
-                ToolCallStateRunning(tc.value.copy(content = a.content))
+                ToolCallStateRunning(tc.value.copy(meta = a.meta ?: tc.value.meta, content = a.content))
             }
         }
     }

clients/rust/CHANGELOG.md

@@ -24,6 +24,11 @@ matching `## [X.Y.Z]` heading is missing from this file.
   terminal / changeset / annotations slots. `reset_host` / `reset` clear the
   new slot.
 
+### Fixed
+
+- Session reducers now apply `_meta` (`meta`) updates from every
+  tool-call-scoped action, not only `session/toolCallStart`.
+
 ### Added
 
 - `AnnotationsUpdatedAction` (`annotations/updated`) — partially updates an

clients/rust/crates/ahp/src/reducers.rs

@@ -875,6 +875,9 @@ fn apply_tool_call_delta(
         ToolCallState::Streaming(mut s) => {
             let current = s.partial_input.unwrap_or_default();
             s.partial_input = Some(current + &a.content);
+            if let Some(meta) = &a.meta {
+                s.meta = Some(meta.clone());
+            }
             if let Some(im) = &a.invocation_message {
                 s.invocation_message = Some(im.clone());
             }
@@ -890,6 +893,7 @@ fn apply_tool_call_ready(
 ) -> ReduceOutcome {
     update_tool_call(state, &a.turn_id, &a.tool_call_id, |tc| {
         let (tool_call_id, tool_name, display_name, contributor, meta) = tool_call_meta(&tc);
+        let meta = a.meta.clone().or(meta);
         match tc {
             ToolCallState::Streaming(_) | ToolCallState::Running(_) => {
                 if let Some(confirmed) = a.confirmed {
@@ -949,7 +953,7 @@ fn apply_tool_call_confirmed(
         let tool_name = s.tool_name;
         let display_name = s.display_name;
         let contributor = s.contributor;
-        let meta = s.meta;
+        let meta = a.meta.clone().or(s.meta);
         let invocation_message = s.invocation_message;
         let tool_input = s.tool_input;
         if a.approved {
@@ -989,6 +993,7 @@ fn apply_tool_call_complete(
 ) -> ReduceOutcome {
     update_tool_call(state, &a.turn_id, &a.tool_call_id, |tc| {
         let (tool_call_id, tool_name, display_name, contributor, meta) = tool_call_meta(&tc);
+        let meta = a.meta.clone().or(meta);
         let (invocation_message, tool_input, confirmed, selected_option) = match tc {
             ToolCallState::Running(s) => (
                 s.invocation_message,
@@ -1056,7 +1061,7 @@ fn apply_tool_call_result_confirmed(
                 tool_name: s.tool_name,
                 display_name: s.display_name,
                 contributor: s.contributor,
-                meta: s.meta,
+                meta: a.meta.clone().or(s.meta),
                 invocation_message: s.invocation_message,
                 tool_input: s.tool_input,
                 success: s.success,
@@ -1073,7 +1078,7 @@ fn apply_tool_call_result_confirmed(
                 tool_name: s.tool_name,
                 display_name: s.display_name,
                 contributor: s.contributor,
-                meta: s.meta,
+                meta: a.meta.clone().or(s.meta),
                 invocation_message: s.invocation_message,
                 tool_input: s.tool_input,
                 reason: ToolCallCancellationReason::ResultDenied,
@@ -1091,6 +1096,9 @@ fn apply_tool_call_content_changed(
 ) -> ReduceOutcome {
     update_tool_call(state, &a.turn_id, &a.tool_call_id, |tc| match tc {
         ToolCallState::Running(mut s) => {
+            if let Some(meta) = &a.meta {
+                s.meta = Some(meta.clone());
+            }
             s.content = Some(a.content.clone());
             ToolCallState::Running(s)
         }