fix(sandbox): cap the Go daemon's inline dispatch request body size

packages/sandbox/daemon-go/internal/dispatch/dispatch.go

@@ -3,6 +3,7 @@ package dispatch
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"io"
 	"log/slog"
 	"net/http"
@@ -18,6 +19,12 @@ import (
 
 const tombstoneTTL = 60 * time.Second
 
+// maxDispatchBodyBytes bounds the inline `/dispatch` request body. Matches
+// maxOffloadBytes: a run whose messages are actually this large is expected to
+// come in via messagesRef, not inline, so this cap just stops an unbounded
+// read from parking the pod's memory on one request.
+const maxDispatchBodyBytes = maxOffloadBytes
+
 // Terminal code for a run this pod could not finish (shutdown / dropped
 // connection) as opposed to one that was cancelled on purpose. Studio maps it
 // to `SandboxUnreachableError` and continues the turn elsewhere — the literal
@@ -282,8 +289,13 @@ func (reg *Registry) HandleDispatch(w http.ResponseWriter, r *http.Request, deps
 		jsonError(w, 401, map[string]string{"error": "unauthorized"})
 		return
 	}
-	body, err := io.ReadAll(r.Body)
+	body, err := io.ReadAll(http.MaxBytesReader(w, r.Body, maxDispatchBodyBytes))
 	if err != nil {
+		var tooLarge *http.MaxBytesError
+		if errors.As(err, &tooLarge) {
+			jsonError(w, 413, map[string]string{"error": "body_too_large"})
+			return
+		}
 		jsonError(w, 400, map[string]string{"error": "bad_json"})
 		return
 	}

packages/sandbox/daemon-go/internal/dispatch/dispatch_test.go

@@ -354,3 +354,18 @@ func TestUnauthorizedCancelLeavesNoTombstone(t *testing.T) {
 		t.Fatal("a rejected cancel must not tombstone the run")
 	}
 }
+
+// An oversized inline dispatch body must be rejected before it is buffered in
+// full — the offload path already caps at this size, and the inline path had
+// no cap at all.
+func TestDispatchRejectsOversizedBody(t *testing.T) {
+	const token = "tkn"
+	body := strings.NewReader(strings.Repeat("a", maxDispatchBodyBytes+1))
+	req := httptest.NewRequest("POST", "/dispatch", body)
+	req.Header.Set("Authorization", "Bearer "+token)
+	rec := httptest.NewRecorder()
+	NewRegistry().HandleDispatch(rec, req, Deps{DaemonToken: func() string { return token }})
+	if rec.Code != 413 {
+		t.Fatalf("dispatch with oversized body returned %d, want 413", rec.Code)
+	}
+}