diff --git a/workspace-server/internal/handlers/activity.go b/workspace-server/internal/handlers/activity.go
index 4f7cf98e..468f743e 100644
--- a/workspace-server/internal/handlers/activity.go
+++ b/workspace-server/internal/handlers/activity.go
@@ -15,6 +15,7 @@ import (
 	"github.com/Molecule-AI/molecule-monorepo/platform/internal/db"
 	"github.com/Molecule-AI/molecule-monorepo/platform/internal/events"
 	"github.com/gin-gonic/gin"
+	"github.com/google/uuid"
 )
 
 type ActivityHandler struct {
@@ -55,10 +56,25 @@ func (h *ActivityHandler) List(c *gin.Context) {
 	workspaceID := c.Param("id")
 	activityType := c.Query("type")
 	source := c.Query("source") // "canvas" = source_id IS NULL, "agent" = source_id IS NOT NULL
+	peerID := c.Query("peer_id") // optional UUID — restrict to rows where this peer is sender OR target
 	limitStr := c.DefaultQuery("limit", "100")
 	sinceSecsStr := c.Query("since_secs")
 	sinceID := c.Query("since_id")
 
+	// Validate peer_id as a UUID at the trust boundary so a malformed
+	// caller (the agent or a downstream MCP tool) can't smuggle SQL
+	// fragments into the WHERE clause via the parameter, even though
+	// args are bound. UUID-shape rejection is also the cleanest 400
+	// signal for the wheel-side chat_history MCP tool — clearer than a
+	// generic "no rows" empty list when the agent passed an obviously
+	// wrong id.
+	if peerID != "" {
+		if _, err := uuid.Parse(peerID); err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{"error": "peer_id must be a UUID"})
+			return
+		}
+	}
+
 	limit := 100
 	if n, err := strconv.Atoi(limitStr); err == nil && n > 0 {
 		limit = n
@@ -135,6 +151,22 @@ func (h *ActivityHandler) List(c *gin.Context) {
 		c.JSON(http.StatusBadRequest, gin.H{"error": "source must be 'canvas' or 'agent'"})
 		return
 	}
+	if peerID != "" {
+		// Restrict to rows where this peer is either the sender (source_id)
+		// or the recipient (target_id) of an A2A turn. This is the
+		// "conversation history with peer X" view the wheel-side
+		// chat_history MCP tool surfaces — agent receives a peer_agent
+		// push, wants to see the prior 20 turns with that workspace
+		// without paging through every other peer's traffic.
+		//
+		// Bound as a single arg, matched twice — keeps argIdx accurate
+		// and avoids duplicate parameter binding (some drivers reject the
+		// same arg slot reused, ours is fine but the explicit form is
+		// clearer to read and matches the rest of the builder.)
+		query += fmt.Sprintf(" AND (source_id = $%d OR target_id = $%d)", argIdx, argIdx)
+		args = append(args, peerID)
+		argIdx++
+	}
 	if sinceSecs > 0 {
 		// Use a parameterized interval so the value is bound, not
 		// interpolated into the SQL string. `make_interval(secs => $N)`
diff --git a/workspace-server/internal/handlers/activity_test.go b/workspace-server/internal/handlers/activity_test.go
index ec53a3f2..b3aafd23 100644
--- a/workspace-server/internal/handlers/activity_test.go
+++ b/workspace-server/internal/handlers/activity_test.go
@@ -167,6 +167,119 @@ func TestActivityList_SourceWithType(t *testing.T) {
 	}
 }
 
+// ---------- Activity List peer_id filter ----------
+//
+// peer_id surfaces the conversation history with one specific peer
+// for the wheel-side chat_history MCP tool. The filter joins
+// (source_id = $X OR target_id = $X) so both inbound (where this
+// peer was the sender) and outbound (where this peer was the
+// recipient) turns appear in the same view, ordered by created_at.
+
+const testPeerUUID = "11111111-2222-3333-4444-555555555555"
+
+func TestActivityList_PeerIDFilter(t *testing.T) {
+	mock := setupTestDB(t)
+	broadcaster := newTestBroadcaster()
+	handler := NewActivityHandler(broadcaster)
+
+	// peer_id binds twice in the query (source_id OR target_id) but is
+	// added to args once — sqlmock matches positional args, so the
+	// binding shape is what matters.
+	mock.ExpectQuery(
+		`SELECT .+ FROM activity_logs WHERE workspace_id = .+ AND \(source_id = .+ OR target_id = .+\)`,
+	).
+		WithArgs("ws-1", testPeerUUID, 100).
+		WillReturnRows(sqlmock.NewRows([]string{
+			"id", "workspace_id", "activity_type", "source_id", "target_id",
+			"method", "summary", "request_body", "response_body",
+			"tool_trace", "duration_ms", "status", "error_detail", "created_at",
+		}))
+
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "ws-1"}}
+	c.Request = httptest.NewRequest(
+		"GET", "/workspaces/ws-1/activity?peer_id="+testPeerUUID, nil,
+	)
+	handler.List(c)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("expected 200, got %d: %s", w.Code, w.Body.String())
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("unmet expectations: %v", err)
+	}
+}
+
+func TestActivityList_PeerIDComposesWithType(t *testing.T) {
+	// peer_id + type + source must compose into a single AND-chain so
+	// the wheel can fetch e.g. "all peer_agent inbound from peer X" in
+	// one round-trip. Pin both args + arg order so a future refactor
+	// of the builder can't silently rearrange placeholders.
+	mock := setupTestDB(t)
+	broadcaster := newTestBroadcaster()
+	handler := NewActivityHandler(broadcaster)
+
+	mock.ExpectQuery(
+		`SELECT .+ FROM activity_logs WHERE workspace_id = .+ AND activity_type = .+ AND source_id IS NOT NULL AND \(source_id = .+ OR target_id = .+\)`,
+	).
+		WithArgs("ws-1", "a2a_receive", testPeerUUID, 100).
+		WillReturnRows(sqlmock.NewRows([]string{
+			"id", "workspace_id", "activity_type", "source_id", "target_id",
+			"method", "summary", "request_body", "response_body",
+			"tool_trace", "duration_ms", "status", "error_detail", "created_at",
+		}))
+
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "ws-1"}}
+	c.Request = httptest.NewRequest(
+		"GET",
+		"/workspaces/ws-1/activity?type=a2a_receive&source=agent&peer_id="+testPeerUUID,
+		nil,
+	)
+	handler.List(c)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("expected 200, got %d: %s", w.Code, w.Body.String())
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("unmet expectations: %v", err)
+	}
+}
+
+func TestActivityList_PeerIDRejectsNonUUID(t *testing.T) {
+	// Trust-boundary check: a malformed peer_id must 400 before any
+	// query is built. Defends against caller bugs (typoed UUID,
+	// leading whitespace) and against any future code path that might
+	// otherwise interpolate the value into the URL or another query.
+	gin.SetMode(gin.TestMode)
+	broadcaster := newTestBroadcaster()
+	handler := NewActivityHandler(broadcaster)
+
+	for _, bad := range []string{
+		"not-a-uuid",
+		"%27%20OR%201%3D1%20--",                          // URL-encoded ' OR 1=1 --
+		"11111111-2222-3333-4444",                        // truncated
+		"11111111-2222-3333-4444-555555555555-extra",     // overlong
+		"11111111-2222-3333-4444-55555555555G",           // non-hex
+	} {
+		w := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(w)
+		c.Params = gin.Params{{Key: "id", Value: "ws-1"}}
+		c.Request = httptest.NewRequest(
+			"GET", "/workspaces/ws-1/activity?peer_id="+bad, nil,
+		)
+		handler.List(c)
+
+		if w.Code != http.StatusBadRequest {
+			t.Errorf("peer_id=%q: expected 400, got %d (%s)", bad, w.Code, w.Body.String())
+		}
+	}
+}
+
 // ---------- Activity type allowlist (#125: memory_write added) ----------
 
 func TestActivityReport_AcceptsMemoryWriteType(t *testing.T) {