diff --git a/docs/scratchpads/t_301e4e3b-pr-merge-gitea-empty-uid.md b/docs/scratchpads/t_301e4e3b-pr-merge-gitea-empty-uid.md
new file mode 100644
index 0000000..01540bb
--- /dev/null
+++ b/docs/scratchpads/t_301e4e3b-pr-merge-gitea-empty-uid.md
@@ -0,0 +1,31 @@
+# Scratchpad: t_301e4e3b pr-merge.sh Gitea empty-uid fallback
+
+## Task
+
+Implement a narrow hardening in `packages/mosaic/framework/tools/git/pr-merge.sh` so Gitea merges recover from the known non-interactive `tea pr merge` identity failure: `user does not exist [uid: 0, name: ]`.
+
+## Constraints
+
+- Preserve Mosaic policy gates: squash-only, base branch `main`, queue guard unless explicitly skipped.
+- Preserve the existing authenticated Gitea API fallback when no tea login exists.
+- Do not fallback on arbitrary tea failures.
+- Do not expose tokens or credential-bearing remotes.
+- Scope is limited to the merge wrapper plus focused test/support/scratchpad files.
+
+## External issue
+
+- Gitea issue #520: Harden pr-merge.sh Gitea empty-uid fallback
+
+## Plan
+
+1. Add a focused shell regression harness with mocked `tea` and `curl` proving the known empty uid/name failure must fall back to Gitea API.
+2. Watch the harness fail on current code.
+3. Implement helper functions in `pr-merge.sh` for redacted command display, known failure classification, and authenticated Gitea API merge fallback.
+4. Keep unknown `tea` failures blocking by replaying stderr and exiting non-zero.
+5. Run syntax, shellcheck if available, focused regression, and repo quality gates before push/PR.
+
+## Session log
+
+- 2026-05-22: Read Kanban context, Mosaic global/repo instructions, created isolated branch `fix/t_301e4e3b-pr-merge-gitea-empty-uid`, and opened Gitea issue #520 using the Mosaic issue wrapper/API fallback.
+- 2026-05-22: Added regression harness and watched it fail on current behavior with `user does not exist [uid: 0, name: ]`; implemented narrow fallback and verified known-empty-identity fallback, arbitrary tea failure blocking, and no-tea-login API fallback paths.
+- 2026-05-22: Validation passed for `bash -n`, `shellcheck -x`, focused shell harness, `pnpm typecheck`, `pnpm lint`, `pnpm format:check`, and `pnpm --filter @mosaicstack/mosaic test`. Full `pnpm test` exposed an out-of-scope gateway DB setup failure (`relation "messages" does not exist`) in `apps/gateway/src/__tests__/cross-user-isolation.test.ts`.
diff --git a/packages/mosaic/framework/tools/git/pr-merge.sh b/packages/mosaic/framework/tools/git/pr-merge.sh
index 73a6e99..cc71438 100755
--- a/packages/mosaic/framework/tools/git/pr-merge.sh
+++ b/packages/mosaic/framework/tools/git/pr-merge.sh
@@ -77,6 +77,11 @@ if [[ -z "$PR_NUMBER" ]]; then
     usage
 fi
 
+if [[ ! "$PR_NUMBER" =~ ^[0-9]+$ ]]; then
+    echo "Error: PR number must be numeric." >&2
+    exit 1
+fi
+
 if [[ "$MERGE_METHOD" != "squash" ]]; then
     echo "Error: Mosaic policy enforces squash merge only. Received '$MERGE_METHOD'." >&2
     exit 1
@@ -104,6 +109,7 @@ REPO=$(get_repo_name)
 find_tea_login_for_host() {
     local host="$1"
     local logins_json
+
     command -v tea >/dev/null 2>&1 || return 1
     logins_json=$(tea login list --output json 2>/dev/null) || return 1
     TEA_LOGINS_JSON="$logins_json" python3 - "$host" <<'PY'
@@ -128,9 +134,30 @@ raise SystemExit(1)
 PY
 }
 
+is_known_tea_empty_identity_failure() {
+    local error_file="$1"
+
+    python3 - "$error_file" <<'PY'
+import re
+import sys
+
+with open(sys.argv[1], encoding="utf-8", errors="replace") as handle:
+    error = handle.read()
+
+known_empty_identity = re.search(
+    r"user does not exist.*\[.*uid:\s*0,\s*name:\s*\]",
+    error,
+    flags=re.IGNORECASE | re.DOTALL,
+)
+raise SystemExit(0 if known_empty_identity else 1)
+PY
+}
+
 merge_gitea_with_api() {
     local host="$1" api_url token basic_auth body_file raw_code payload
-    body_file=$(mktemp)
+    api_url="https://${host}/api/v1/repos/${OWNER}/${REPO}/pulls/${PR_NUMBER}/merge"
+    mkdir -p "${AGENT_WORK_ROOT:-/home/hermes/agent-work}"
+    body_file=$(mktemp "${AGENT_WORK_ROOT:-/home/hermes/agent-work}/pr-merge-api-response.XXXXXX")
     payload='{"Do":"squash"}'
 
     token=$(get_gitea_token "$host" || true)
@@ -166,10 +193,15 @@ import json
 import sys
 code, path = sys.argv[1], sys.argv[2]
 try:
-    data = json.load(open(path, encoding="utf-8"))
-    message = data.get("message") or data.get("error") or "unknown API error"
+    with open(path, encoding="utf-8", errors="replace") as handle:
+        raw = handle.read(500)
+    data = json.loads(raw) if raw else {}
+    message = data.get("message") or data.get("error") or raw or "empty response"
 except Exception:
-    message = open(path, encoding="utf-8", errors="replace").read()[:200] or "empty response"
+    try:
+        message = open(path, encoding="utf-8", errors="replace").read(500) or "empty response"
+    except Exception:
+        message = "unreadable response"
 print(f"Error: Gitea API merge failed with HTTP {code}: {message}")
 PY
     rm -f "$body_file"
@@ -206,11 +238,25 @@ case "$PLATFORM" in
             exit 1
         }
         TEA_LOGIN="${GITEA_LOGIN:-$(find_tea_login_for_host "$HOST" || true)}"
+
         if [[ -n "$TEA_LOGIN" ]]; then
-            tea pr merge "$PR_NUMBER" --style squash --repo "$OWNER/$REPO" --login "$TEA_LOGIN"
+            mkdir -p "${AGENT_WORK_ROOT:-/home/hermes/agent-work}"
+            TEA_ERROR_FILE=$(mktemp "${AGENT_WORK_ROOT:-/home/hermes/agent-work}/pr-merge-tea-error.XXXXXX")
+            if tea pr merge "$PR_NUMBER" --style squash --repo "$OWNER/$REPO" --login "$TEA_LOGIN" 2> "$TEA_ERROR_FILE"; then
+                rm -f "$TEA_ERROR_FILE"
+            elif is_known_tea_empty_identity_failure "$TEA_ERROR_FILE"; then
+                cat "$TEA_ERROR_FILE" >&2
+                echo "Known tea empty identity failure detected; using authenticated Gitea API merge fallback." >&2
+                rm -f "$TEA_ERROR_FILE"
+                merge_gitea_with_api "$HOST"
+            else
+                cat "$TEA_ERROR_FILE" >&2
+                rm -f "$TEA_ERROR_FILE"
+                exit 1
+            fi
         else
             echo "No tea login configured for $HOST; using authenticated Gitea API merge fallback." >&2
-            merge_gitea_with_api "$HOST" "https://${HOST}/api/v1/repos/${OWNER}/${REPO}/pulls/${PR_NUMBER}/merge"
+            merge_gitea_with_api "$HOST"
         fi
 
         # Delete branch after merge if requested
diff --git a/packages/mosaic/framework/tools/git/test-pr-merge-gitea-empty-uid.sh b/packages/mosaic/framework/tools/git/test-pr-merge-gitea-empty-uid.sh
new file mode 100755
index 0000000..9a3e8a9
--- /dev/null
+++ b/packages/mosaic/framework/tools/git/test-pr-merge-gitea-empty-uid.sh
@@ -0,0 +1,145 @@
+#!/bin/bash
+# Regression harness for pr-merge.sh Gitea non-interactive tea empty identity fallback.
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+WORK_ROOT="${AGENT_WORK_ROOT:-/home/hermes/agent-work}"
+SANDBOX="$WORK_ROOT/pr-merge-empty-uid-test-$$"
+MOCK_BIN="$SANDBOX/bin"
+REPO_DIR="$SANDBOX/repo"
+LOG_FILE="$SANDBOX/mock.log"
+
+cleanup() {
+    rm -rf "$SANDBOX"
+}
+trap cleanup EXIT
+
+mkdir -p "$MOCK_BIN" "$REPO_DIR"
+: > "$LOG_FILE"
+
+cat > "$MOCK_BIN/tea" <<'EOF'
+#!/bin/bash
+set -euo pipefail
+printf 'tea %q ' "$@" >> "$PR_MERGE_TEST_LOG"
+printf '\n' >> "$PR_MERGE_TEST_LOG"
+if [[ "$*" == *"pr merge"* ]]; then
+    echo 'user does not exist [uid: 0, name: ]' >&2
+    exit 1
+fi
+exit 0
+EOF
+chmod +x "$MOCK_BIN/tea"
+
+cat > "$MOCK_BIN/curl" <<'EOF'
+#!/bin/bash
+set -euo pipefail
+printf 'curl %q ' "$@" >> "$PR_MERGE_TEST_LOG"
+printf '\n' >> "$PR_MERGE_TEST_LOG"
+args=" $* "
+if [[ "$args" == *"/api/v1/repos/mosaicstack/stack/pulls/123"* && "$args" != *"/api/v1/repos/mosaicstack/stack/pulls/123/merge"* ]]; then
+    cat <<'JSON'
+{"number":123,"title":"mock","state":"open","user":{"login":"tester"},"head":{"ref":"feature/mock"},"base":{"ref":"main"},"labels":[],"assignees":[],"html_url":"https://git.mosaicstack.dev/mosaicstack/stack/pulls/123","mergeable":true}
+JSON
+    exit 0
+fi
+if [[ "$args" == *"-X POST"* && "$args" == *"/api/v1/repos/mosaicstack/stack/pulls/123/merge"* ]]; then
+    cat <<'JSON'
+{"merged":true,"message":"mock merge complete"}
+JSON
+    exit 0
+fi
+echo "unexpected curl invocation: $*" >&2
+exit 97
+EOF
+chmod +x "$MOCK_BIN/curl"
+
+cd "$REPO_DIR"
+git init -q
+git remote add origin https://git.mosaicstack.dev/mosaicstack/stack.git
+
+export PATH="$MOCK_BIN:$PATH"
+export PR_MERGE_TEST_LOG="$LOG_FILE"
+export GITEA_LOGIN="git.mosaicstack.dev"
+export GITEA_TOKEN="redacted-test-token"
+
+OUTPUT="$SANDBOX/output.log"
+if ! "$SCRIPT_DIR/pr-merge.sh" -n 123 -m squash --skip-queue-guard > "$OUTPUT" 2>&1; then
+    echo "Expected pr-merge.sh to recover via Gitea API fallback." >&2
+    echo "--- output ---" >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$OUTPUT" >&2
+    echo "--- mock log ---" >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$LOG_FILE" >&2
+    exit 1
+fi
+
+if ! grep -q '/api/v1/repos/mosaicstack/stack/pulls/123/merge' "$LOG_FILE"; then
+    echo "Expected authenticated Gitea merge API endpoint to be called." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$LOG_FILE" >&2
+    exit 1
+fi
+
+if grep -q 'redacted-test-token' "$OUTPUT"; then
+    echo "Token leaked to pr-merge.sh output." >&2
+    exit 1
+fi
+
+cat > "$MOCK_BIN/tea" <<'EOF'
+#!/bin/bash
+set -euo pipefail
+printf 'tea %q ' "$@" >> "$PR_MERGE_TEST_LOG"
+printf '\n' >> "$PR_MERGE_TEST_LOG"
+if [[ "$*" == *"pr merge"* ]]; then
+    echo 'tea network timeout' >&2
+    exit 2
+fi
+exit 0
+EOF
+chmod +x "$MOCK_BIN/tea"
+: > "$LOG_FILE"
+if "$SCRIPT_DIR/pr-merge.sh" -n 123 -m squash --skip-queue-guard > "$OUTPUT" 2>&1; then
+    echo "Expected arbitrary tea failure to remain blocking." >&2
+    exit 1
+fi
+if grep -q '/api/v1/repos/mosaicstack/stack/pulls/123/merge' "$LOG_FILE"; then
+    echo "Arbitrary tea failure unexpectedly used Gitea API merge fallback." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$LOG_FILE" >&2
+    exit 1
+fi
+if ! grep -q 'tea network timeout' "$OUTPUT"; then
+    echo "Expected arbitrary tea error to be preserved in output." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$OUTPUT" >&2
+    exit 1
+fi
+
+cat > "$MOCK_BIN/tea" <<'EOF'
+#!/bin/bash
+set -euo pipefail
+printf 'tea %q ' "$@" >> "$PR_MERGE_TEST_LOG"
+printf '\n' >> "$PR_MERGE_TEST_LOG"
+if [[ "$*" == *"login list"* ]]; then
+    echo '[]'
+    exit 0
+fi
+if [[ "$*" == *"pr merge"* ]]; then
+    echo 'tea merge should not run without a configured host login' >&2
+    exit 99
+fi
+exit 0
+EOF
+chmod +x "$MOCK_BIN/tea"
+unset GITEA_LOGIN
+: > "$LOG_FILE"
+if ! "$SCRIPT_DIR/pr-merge.sh" -n 123 -m squash --skip-queue-guard > "$OUTPUT" 2>&1; then
+    echo "Expected missing tea login to use authenticated Gitea API fallback." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$OUTPUT" >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$LOG_FILE" >&2
+    exit 1
+fi
+if ! grep -q '/api/v1/repos/mosaicstack/stack/pulls/123/merge' "$LOG_FILE"; then
+    echo "Expected missing tea login path to call Gitea API merge endpoint." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$LOG_FILE" >&2
+    exit 1
+fi
+
+echo "pr-merge.sh Gitea fallback regression passed"