diff --git a/workspace-server/internal/handlers/a2a_proxy_helpers.go b/workspace-server/internal/handlers/a2a_proxy_helpers.go
index 4932de31..c7f83249 100644
--- a/workspace-server/internal/handlers/a2a_proxy_helpers.go
+++ b/workspace-server/internal/handlers/a2a_proxy_helpers.go
@@ -67,6 +67,35 @@ func (h *WorkspaceHandler) handleA2ADispatchError(ctx context.Context, workspace
 	// with 202 status here was the original cycle 53 bug — callers saw
 	// proxyErr != nil and logged "delegation failed: proxy a2a error".
 	if isUpstreamBusyError(err) {
+		// Capability primitive #5 — see project memory
+		// `project_runtime_native_pluggable.md`. When the target workspace's
+		// adapter has declared provides_native_session=True, the SDK
+		// owns its own queue/session state (claude-agent-sdk's streaming
+		// session, hermes-agent's in-container event log, etc.). Adding
+		// the platform's a2a_queue layer on top would double-buffer the
+		// same in-flight state — and worse, the platform queue's drain
+		// timing has no relationship to the SDK's actual readiness, so
+		// the queued request might dispatch while the SDK is STILL busy.
+		//
+		// For native_session targets, return 503 + Retry-After directly.
+		// The caller's adapter handles retry on its own schedule, and
+		// the SDK's own queue absorbs the in-flight request when it does.
+		// Observability is preserved: logA2AFailure already ran above;
+		// activity_logs records the busy event; the broadcaster fires.
+		if runtimeOverrides.HasCapability(workspaceID, "session") {
+			log.Printf("ProxyA2A: target %s busy and declares native_session — skip enqueue, return 503", workspaceID)
+			return 0, nil, &proxyA2AError{
+				Status:  http.StatusServiceUnavailable,
+				Headers: map[string]string{"Retry-After": strconv.Itoa(busyRetryAfterSeconds)},
+				Response: gin.H{
+					"error":           "workspace agent busy — adapter handles retry (native_session)",
+					"busy":            true,
+					"retry_after":     busyRetryAfterSeconds,
+					"native_session":  true,
+				},
+			}
+		}
+
 		idempotencyKey := extractIdempotencyKey(body)
 		if qid, depth, qerr := EnqueueA2A(
 			ctx, workspaceID, callerID, PriorityTask, body, a2aMethod, idempotencyKey,
diff --git a/workspace-server/internal/handlers/native_session_test.go b/workspace-server/internal/handlers/native_session_test.go
new file mode 100644
index 00000000..2f70d882
--- /dev/null
+++ b/workspace-server/internal/handlers/native_session_test.go
@@ -0,0 +1,99 @@
+package handlers
+
+import (
+	"context"
+	"net/http"
+	"testing"
+)
+
+// TestHandleA2ADispatchError_NativeSession_SkipsEnqueue validates capability
+// primitive #5: when the target workspace has declared
+// provides_native_session=True, a busy-shaped dispatch error MUST short-
+// circuit straight to 503 + Retry-After. The platform's a2a_queue is
+// skipped because the SDK owns its own queue/session state — double-
+// buffering would cause spurious dispatches when the SDK is still busy.
+//
+// Pin via sqlmock: we deliberately do NOT expect any INSERT INTO a2a_queue.
+// If a future refactor re-introduces enqueueing under native_session,
+// sqlmock fails the test on the unexpected query.
+func TestHandleA2ADispatchError_NativeSession_SkipsEnqueue(t *testing.T) {
+	setupTestDB(t)
+	setupTestRedis(t)
+	handler := NewWorkspaceHandler(newTestBroadcaster(), nil, "http://localhost:8080", t.TempDir())
+
+	// Pre-populate the cache: ws-native owns its session natively.
+	runtimeOverrides.SetCapabilities("ws-native", map[string]bool{"session": true})
+	defer runtimeOverrides.Reset()
+
+	// DeadlineExceeded triggers isUpstreamBusyError. Without the native
+	// gate, this would fire EnqueueA2A → INSERT INTO a2a_queue. With
+	// the gate, it short-circuits to 503. We expect ZERO queue queries;
+	// sqlmock's ExpectationsWereMet implicitly enforces that on teardown.
+	_, _, perr := handler.handleA2ADispatchError(
+		context.Background(), "ws-native", "", []byte("{}"), "message/send",
+		context.DeadlineExceeded, 1, false,
+	)
+	if perr == nil {
+		t.Fatal("expected proxy error, got nil")
+	}
+	if perr.Status != http.StatusServiceUnavailable {
+		t.Errorf("got status %d, want 503 (native_session bypasses queue but still 503s)", perr.Status)
+	}
+	if perr.Headers["Retry-After"] == "" {
+		t.Error("expected Retry-After header on native-session 503")
+	}
+	// Pin the marker so callers' adapters can distinguish this from a
+	// queue-failure 503: the body has native_session=true.
+	if got, _ := perr.Response["native_session"].(bool); !got {
+		t.Errorf("expected native_session=true in response body; got %+v", perr.Response)
+	}
+	// And busy=true stays so existing busy-handling code paths still trigger.
+	if got, _ := perr.Response["busy"].(bool); !got {
+		t.Errorf("expected busy=true in response body; got %+v", perr.Response)
+	}
+}
+
+// TestHandleA2ADispatchError_NoNativeSession_StillEnqueues is the negative
+// pin: a workspace WITHOUT the capability flag falls through to the
+// existing EnqueueA2A path (and 503 if that fails). Same shape as
+// TestHandleA2ADispatchError_ContextDeadline; we duplicate it here so
+// the native_session gate change is bracketed by both positive and
+// negative tests in the same file.
+func TestHandleA2ADispatchError_NoNativeSession_StillEnqueues(t *testing.T) {
+	mock := setupTestDB(t)
+	setupTestRedis(t)
+	handler := NewWorkspaceHandler(newTestBroadcaster(), nil, "http://localhost:8080", t.TempDir())
+
+	// Cache is empty for this workspace → falls through to EnqueueA2A.
+	runtimeOverrides.Reset()
+	defer runtimeOverrides.Reset()
+
+	mock.ExpectQuery(`INSERT INTO a2a_queue`).
+		WithArgs("ws-platform-queue", nil, PriorityTask, "{}", "message/send", nil).
+		WillReturnError(errTestQueueUnavailable)
+
+	_, _, perr := handler.handleA2ADispatchError(
+		context.Background(), "ws-platform-queue", "", []byte("{}"), "message/send",
+		context.DeadlineExceeded, 1, false,
+	)
+	if perr == nil {
+		t.Fatal("expected proxy error, got nil")
+	}
+	// Queue insert failed → falls through to legacy 503 (without
+	// native_session marker).
+	if perr.Status != http.StatusServiceUnavailable {
+		t.Errorf("got status %d, want 503", perr.Status)
+	}
+	if got, _ := perr.Response["native_session"].(bool); got {
+		t.Errorf("non-native workspace should NOT carry native_session=true in response; got %+v", perr.Response)
+	}
+}
+
+// errTestQueueUnavailable is reused in this file's tests to simulate a
+// transient queue-insert failure without dragging in fmt.Errorf at every
+// call site.
+var errTestQueueUnavailable = &queueUnavailableErr{}
+
+type queueUnavailableErr struct{}
+
+func (e *queueUnavailableErr) Error() string { return "test: queue unavailable" }