diff --git a/workspace-server/internal/handlers/a2a_proxy_helpers.go b/workspace-server/internal/handlers/a2a_proxy_helpers.go
index d9d9c8ec..afe8a69c 100644
--- a/workspace-server/internal/handlers/a2a_proxy_helpers.go
+++ b/workspace-server/internal/handlers/a2a_proxy_helpers.go
@@ -437,8 +437,26 @@ func (h *WorkspaceHandler) logA2ASuccess(ctx context.Context, workspaceID, calle
 	}
 }
 
+// nilIfEmpty returns nil for empty OR system-caller strings. The second
+// branch matters: a system-caller prefix like "system:restart-context"
+// is a non-UUID string (it's a routing marker, not a real workspace id).
+// Persisting it to activity_logs.source_id would poison the column —
+// downstream joins (e.g. activity.go:443 LEFT JOIN workspaces w ON w.id =
+// activity_logs.source_id) and UUID-cast lookups would return NULL or
+// error. Normalizing system callers to NULL source_id here keeps the
+// "who-did-what" association intact (the callerID is still in
+// activity_logs.caller_id as a free-form field if needed) without
+// poisoning the source_id FK contract.
+//
+// This is the production-code root cause of #2680 (post-restart wedge):
+// restart_context.go:296 calls ProxyA2ARequest with callerID =
+// "system:restart-context" → nilIfEmpty("system:restart-context")
+// returned a non-nil pointer to the system prefix → LogActivity
+// persisted it to activity_logs.source_id as the literal string →
+// downstream lookups failed → wedge-detector side-effects → workspace
+// stayed degraded instead of online. See #2693.
 func nilIfEmpty(s string) *string {
-	if s == "" {
+	if s == "" || isSystemCaller(s) {
 		return nil
 	}
 	return &s
diff --git a/workspace-server/internal/handlers/a2a_proxy_helpers_test.go b/workspace-server/internal/handlers/a2a_proxy_helpers_test.go
index b3677cc1..cc6085e5 100644
--- a/workspace-server/internal/handlers/a2a_proxy_helpers_test.go
+++ b/workspace-server/internal/handlers/a2a_proxy_helpers_test.go
@@ -27,6 +27,54 @@ func TestNilIfEmpty_NonEmptyString(t *testing.T) {
 	}
 }
 
+// System-caller prefixes (webhook:, system:, test:, channel:) are
+// non-UUID routing markers, not real workspace ids. Persisting them
+// to activity_logs.source_id would poison the column — downstream
+// joins (e.g. activity.go:443 LEFT JOIN workspaces w ON w.id =
+// activity_logs.source_id) and UUID-cast lookups would return NULL
+// or error. nilIfEmpty must normalize all system-caller prefixes to
+// nil. This is the production-code root cause of #2680 (post-restart
+// wedge): restart_context.go:296 calls ProxyA2ARequest with callerID
+// = "system:restart-context" → nilIfEmpty("system:restart-context")
+// returned a non-nil pointer to the system prefix → LogActivity
+// persisted it to activity_logs.source_id as the literal string →
+// downstream lookups failed → wedge-detector side-effects → workspace
+// stayed degraded instead of online. See #2693.
+
+func TestNilIfEmpty_SystemCallerPrefixes(t *testing.T) {
+	// The four systemCallerPrefixes from a2a_proxy.go:82-84. All must
+	// return nil from nilIfEmpty (the post-#2680 contract). isSystemCaller
+	// uses strings.HasPrefix, so the prefix must be at the start.
+	systemPrefixes := []string{
+		"webhook:github",
+		"system:restart-context", // the actual offender
+		"system:other-svc",
+		"test:integration-1",
+		"channel:discord",
+	}
+	for _, p := range systemPrefixes {
+		got := nilIfEmpty(p)
+		if got != nil {
+			t.Errorf("system-caller %q: got non-nil pointer (would poison activity_logs.source_id), want nil", p)
+		}
+	}
+}
+
+func TestNilIfEmpty_RealWorkspaceUUIDStillPreserved(t *testing.T) {
+	// The fix must NOT regress the canonical case: a real workspace
+	// UUID (no system prefix) must STILL be passed through to
+	// activity_logs.source_id as a non-nil pointer. Otherwise we'd
+	// hide real-workspace attribution.
+	realUUID := "9a40df22-ba4b-3fc0-75c1-66dd6869ff25" // a real UUID-shaped string
+	got := nilIfEmpty(realUUID)
+	if got == nil {
+		t.Fatal("real workspace UUID: got nil, want non-nil pointer")
+	}
+	if *got != realUUID {
+		t.Errorf("real workspace UUID: got %q, want %q", *got, realUUID)
+	}
+}
+
 // ─────────────────────────────────────────────────────────────────────────────
 // extractToolTrace tests
 // ─────────────────────────────────────────────────────────────────────────────