diff --git a/internal/http/vault_handler_upload.go b/internal/http/vault_handler_upload.go
index 032a3d68..8026f9d3 100644
--- a/internal/http/vault_handler_upload.go
+++ b/internal/http/vault_handler_upload.go
@@ -41,6 +41,24 @@ func (h *VaultHandler) handleUpload(w http.ResponseWriter, r *http.Request) {
 	agentIDStr := r.FormValue("agent_id")
 	teamIDStr := r.FormValue("team_id")
 
+	// Boundary UUID validation — Fix B (Phase 1, H9 mitigation).
+	// validateTeamMembership short-circuits on owner role + lite edition (nil teamAccess),
+	// leaving downstream `mustParseUUID(*doc.TeamID)` as a silent-nil trap. Validate at
+	// the HTTP boundary so bad form input is rejected before any store call or event publish.
+	// See docs/agent-identity-conventions.md.
+	if agentIDStr != "" {
+		if _, err := uuid.Parse(agentIDStr); err != nil {
+			writeJSON(w, http.StatusBadRequest, map[string]string{"error": "invalid agent_id: must be a UUID"})
+			return
+		}
+	}
+	if teamIDStr != "" {
+		if _, err := uuid.Parse(teamIDStr); err != nil {
+			writeJSON(w, http.StatusBadRequest, map[string]string{"error": "invalid team_id: must be a UUID"})
+			return
+		}
+	}
+
 	// Validate team membership if provided.
 	if teamIDStr != "" {
 		if !h.validateTeamMembership(r.Context(), w, teamIDStr) {
diff --git a/internal/http/vault_handler_upload_test.go b/internal/http/vault_handler_upload_test.go
new file mode 100644
index 00000000..83f18188
--- /dev/null
+++ b/internal/http/vault_handler_upload_test.go
@@ -0,0 +1,139 @@
+package http
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"mime/multipart"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// buildUploadRequest constructs a multipart POST request with optional form fields
+// and one minimal file part. It does NOT attempt to reach storage — the handler's
+// boundary UUID validation runs before any store or workspace access.
+func buildUploadRequest(t *testing.T, fields map[string]string) *http.Request {
+	t.Helper()
+	var body bytes.Buffer
+	writer := multipart.NewWriter(&body)
+	for k, v := range fields {
+		if err := writer.WriteField(k, v); err != nil {
+			t.Fatalf("writer.WriteField(%q): %v", k, err)
+		}
+	}
+	// Minimal file part so ParseMultipartForm sees at least one file.
+	part, err := writer.CreateFormFile("files", "note.md")
+	if err != nil {
+		t.Fatalf("CreateFormFile: %v", err)
+	}
+	if _, err := part.Write([]byte("hello")); err != nil {
+		t.Fatalf("part.Write: %v", err)
+	}
+	if err := writer.Close(); err != nil {
+		t.Fatalf("writer.Close: %v", err)
+	}
+
+	req := httptest.NewRequest(http.MethodPost, "/v1/vault/upload", &body)
+	req.Header.Set("Content-Type", writer.FormDataContentType())
+	return req
+}
+
+// assertBadRequest decodes a JSON error response and checks the HTTP status
+// is 400 and the error message contains the expected substring.
+func assertBadRequest(t *testing.T, rr *httptest.ResponseRecorder, wantFragment string) {
+	t.Helper()
+	if rr.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d (body: %s)", rr.Code, http.StatusBadRequest, rr.Body.String())
+	}
+	var payload map[string]string
+	if err := json.Unmarshal(rr.Body.Bytes(), &payload); err != nil {
+		t.Fatalf("decode json: %v (body: %s)", err, rr.Body.String())
+	}
+	msg, ok := payload["error"]
+	if !ok {
+		t.Fatalf("response missing `error` field: %s", rr.Body.String())
+	}
+	if !strings.Contains(msg, wantFragment) {
+		t.Errorf("error = %q, want fragment %q", msg, wantFragment)
+	}
+}
+
+// TestHandleUpload_InvalidAgentIDReturns400 asserts Phase 1 Fix B — bad form
+// `agent_id` is rejected at the HTTP boundary before any store call.
+// Closes the owner/lite edition gap where validateTeamMembership would skip
+// the UUID check.
+func TestHandleUpload_InvalidAgentIDReturns400(t *testing.T) {
+	h := &VaultHandler{} // no store wired — boundary check runs first
+	req := buildUploadRequest(t, map[string]string{
+		"agent_id": "goctech-leader", // agent_key, not UUID
+	})
+	rr := httptest.NewRecorder()
+
+	h.handleUpload(rr, req)
+
+	assertBadRequest(t, rr, "invalid agent_id")
+}
+
+// TestHandleUpload_InvalidTeamIDReturns400 asserts Phase 1 Fix B for team_id.
+// This is the hole validateTeamMembership leaves open: it short-circuits on
+// owner role (line 57) and nil teamAccess (line 60), never parsing the UUID.
+func TestHandleUpload_InvalidTeamIDReturns400(t *testing.T) {
+	h := &VaultHandler{} // lite edition: teamAccess is nil
+	req := buildUploadRequest(t, map[string]string{
+		"team_id": "my-team-key", // team_key, not UUID
+	})
+	rr := httptest.NewRecorder()
+
+	h.handleUpload(rr, req)
+
+	assertBadRequest(t, rr, "invalid team_id")
+}
+
+// TestHandleUpload_InvalidAgentID_OwnerContext asserts the boundary check fires
+// regardless of role. Pre-Fix-B, validateTeamMembership skipped the check for
+// owner role — but agent_id was never validated at all upstream, so the UUID
+// hole also existed for admins. Boundary check closes it for every caller.
+func TestHandleUpload_InvalidAgentID_OwnerContext(t *testing.T) {
+	h := &VaultHandler{}
+	req := buildUploadRequest(t, map[string]string{
+		"agent_id": "definitely-not-a-uuid",
+	})
+	// Simulate owner role context — the boundary check ignores role entirely.
+	ctx := context.WithValue(req.Context(), ownerContextKey{}, true)
+	req = req.WithContext(ctx)
+	rr := httptest.NewRecorder()
+
+	h.handleUpload(rr, req)
+
+	assertBadRequest(t, rr, "invalid agent_id")
+}
+
+// TestHandleUpload_EmptyAgentIDAndTeamID_NoBoundaryError asserts the boundary
+// check skips empty strings (shared-scope uploads). It should proceed to the
+// next validation layer (workspace resolution) — which will fail with a
+// different error since our test VaultHandler has no workspace. The important
+// thing is that the boundary check itself does NOT return 400 with "invalid
+// agent_id" or "invalid team_id" for empty values.
+func TestHandleUpload_EmptyAgentIDAndTeamID_BoundaryCheckSkipped(t *testing.T) {
+	h := &VaultHandler{}
+	req := buildUploadRequest(t, map[string]string{})
+	rr := httptest.NewRecorder()
+
+	h.handleUpload(rr, req)
+
+	// Expect a failure AFTER boundary validation (e.g. workspace missing),
+	// not the boundary UUID validation errors.
+	if rr.Code == http.StatusBadRequest {
+		body := rr.Body.String()
+		if strings.Contains(body, "invalid agent_id") || strings.Contains(body, "invalid team_id") {
+			t.Errorf("boundary check fired on empty input (should skip): body=%s", body)
+		}
+	}
+}
+
+// ownerContextKey is a private test helper for simulating owner role in
+// context. The real context key type lives in internal/store; we only need
+// to ensure the boundary check fires independent of role-related context.
+type ownerContextKey struct{}