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/docs/scratchpads/t_5aab9cc8-pr-merge-eval-injection.md b/docs/scratchpads/t_5aab9cc8-pr-merge-eval-injection.md
new file mode 100644
index 0000000..23ce55c
--- /dev/null
+++ b/docs/scratchpads/t_5aab9cc8-pr-merge-eval-injection.md
@@ -0,0 +1,48 @@
+# t_5aab9cc8 — pr-merge.sh eval injection remediation
+
+## Objective
+
+Remediate PR #521 review blocker: `packages/mosaic/framework/tools/git/pr-merge.sh` must reject non-numeric PR numbers before metadata lookup/merge and must not use `eval` for GitHub merge execution.
+
+## Scope
+
+- Shell wrapper only: `packages/mosaic/framework/tools/git/pr-merge.sh`
+- Focused regression harness: `packages/mosaic/framework/tools/git/test-pr-merge-gitea-empty-uid.sh`
+- No API/frontend/infra surfaces.
+
+## Acceptance Criteria
+
+- AC1: `PR_NUMBER` is validated as digits-only immediately after required-argument parsing, before metadata lookup.
+- AC2: GitHub merge path uses a quoted argv array, not command-string construction plus `eval`.
+- AC3: Focused tests prove PR-number metacharacters are rejected and cannot execute injected shell commands on GitHub path.
+- AC4: Focused tests prove PR-number metacharacters are rejected on Gitea path before tea/curl merge calls.
+- AC5: Existing Gitea empty-uid fallback behavior remains green.
+- AC6: Syntax, shellcheck where available, focused harness, and relevant repo gates are rerun or absence documented.
+
+## Plan
+
+1. Add failing regression tests for GitHub eval injection and Gitea invalid PR rejection.
+2. Implement fail-closed PR number validation before metadata lookup.
+3. Replace GitHub `eval` command with argv array execution.
+4. Run required validation and update this scratchpad with evidence.
+5. Commit, queue-guard, push branch, update PR #521.
+
+## TDD Log
+
+- RED: `AGENT_WORK_ROOT="$HERMES_KANBAN_WORKSPACE/work" bash packages/mosaic/framework/tools/git/test-pr-merge-gitea-empty-uid.sh` failed on vulnerable code with `Expected GitHub metacharacter PR number to be rejected` and showed the injected PR number reached the GitHub merge path.
+- GREEN: Added digits-only validation before metadata lookup and replaced GitHub `eval` with an argv array. The focused harness now passes and verifies invalid PR numbers are rejected before GitHub `gh` calls and before Gitea `tea`/`curl` calls.
+
+## Validation Evidence
+
+- PASS: `AGENT_WORK_ROOT="$HERMES_KANBAN_WORKSPACE/work" bash -n packages/mosaic/framework/tools/git/pr-merge.sh packages/mosaic/framework/tools/git/test-pr-merge-gitea-empty-uid.sh`
+- PASS: `shellcheck -x packages/mosaic/framework/tools/git/pr-merge.sh packages/mosaic/framework/tools/git/test-pr-merge-gitea-empty-uid.sh`
+- PASS: `AGENT_WORK_ROOT="$HERMES_KANBAN_WORKSPACE/work" bash packages/mosaic/framework/tools/git/test-pr-merge-gitea-empty-uid.sh`
+- PASS: `pnpm --filter @mosaicstack/mosaic... build`
+- PASS: `pnpm --filter @mosaicstack/mosaic lint`
+- PASS: `pnpm --filter @mosaicstack/mosaic typecheck`
+- PASS: `pnpm --filter @mosaicstack/mosaic test` — 32 files / 291 tests passed.
+- REVIEW: `/home/hermes/.config/mosaic/tools/codex/codex-code-review.sh --uncommitted` could not run due Codex 401 Unauthorized. Independent delegate review completed read-only with PASS / no blockers; non-blocking suggestion to assert GitHub mock log remains empty was applied.
+
+## Risks / Blockers
+
+- No active blockers.
diff --git a/packages/mosaic/framework/tools/git/pr-merge.sh b/packages/mosaic/framework/tools/git/pr-merge.sh
index ad8c318..dd94d14 100755
--- a/packages/mosaic/framework/tools/git/pr-merge.sh
+++ b/packages/mosaic/framework/tools/git/pr-merge.sh
@@ -2,9 +2,10 @@
 # pr-merge.sh - Merge pull requests on Gitea or GitHub
 # Usage: pr-merge.sh -n PR_NUMBER [-m squash] [-d] [--skip-queue-guard]
 
-set -e
+set -euo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# shellcheck source=packages/mosaic/framework/tools/git/detect-platform.sh
 source "$SCRIPT_DIR/detect-platform.sh"
 
 # Default values
@@ -69,6 +70,11 @@ if [[ -z "$PR_NUMBER" ]]; then
     usage
 fi
 
+if [[ ! "$PR_NUMBER" =~ ^[0-9]+$ ]]; then
+    echo "Error: Invalid PR number '$PR_NUMBER'. PR number must contain digits only." >&2
+    exit 1
+fi
+
 if [[ "$MERGE_METHOD" != "squash" ]]; then
     echo "Error: Mosaic policy enforces squash merge only. Received '$MERGE_METHOD'." >&2
     exit 1
@@ -92,21 +98,138 @@ PLATFORM=$(detect_platform)
 OWNER=$(get_repo_owner)
 REPO=$(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'
+import json
+import os
+import sys
+
+host = sys.argv[1]
+try:
+    logins = json.loads(os.environ.get("TEA_LOGINS_JSON", "[]"))
+except Exception:
+    raise SystemExit(1)
+
+for login in logins if isinstance(logins, list) else []:
+    url = str(login.get("url") or login.get("URL") or "")
+    name = str(login.get("name") or login.get("Name") or "")
+    if url.rstrip("/").endswith(host) and name:
+        print(name)
+        raise SystemExit(0)
+
+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="$1"
+    local api_url="https://${host}/api/v1/repos/${OWNER}/${REPO}/pulls/${PR_NUMBER}/merge"
+    local token body_file payload
+
+    token=$(get_gitea_token "$host" || true)
+    if [[ -z "$token" ]]; then
+        echo "Error: No Gitea API token available for authenticated merge fallback on $host." >&2
+        return 1
+    fi
+
+    mkdir -p "${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"}'
+
+    if curl -fsS \
+        -X POST \
+        -H "Authorization: token $token" \
+        -H 'Content-Type: application/json' \
+        -d "$payload" \
+        "$api_url" > "$body_file"; then
+        rm -f "$body_file"
+        return 0
+    fi
+
+    python3 - "$body_file" <<'PY' >&2
+import json
+import sys
+
+path = sys.argv[1]
+try:
+    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:
+    try:
+        with open(path, encoding="utf-8", errors="replace") as handle:
+            message = handle.read(500) or "empty response"
+    except Exception:
+        message = "unreadable response"
+
+print(f"Error: Gitea API merge fallback failed: {message}")
+PY
+    rm -f "$body_file"
+    return 1
+}
+
 case "$PLATFORM" in
     github)
-        CMD="gh pr merge $PR_NUMBER --squash"
-        [[ "$DELETE_BRANCH" == true ]] && CMD="$CMD --delete-branch"
-        eval "$CMD"
+        cmd=(gh pr merge "$PR_NUMBER" --squash)
+        [[ "$DELETE_BRANCH" == true ]] && cmd+=(--delete-branch)
+        "${cmd[@]}"
         ;;
     gitea)
-        CMD="tea pr merge $PR_NUMBER --style squash --repo $OWNER/$REPO --login ${GITEA_LOGIN:-mosaicstack}"
+        HOST=$(get_remote_host) || {
+            echo "Error: Cannot determine host from origin remote URL" >&2
+            exit 1
+        }
+        TEA_LOGIN="${GITEA_LOGIN:-$(find_tea_login_for_host "$HOST" || true)}"
 
         # Delete branch after merge if requested
         if [[ "$DELETE_BRANCH" == true ]]; then
             echo "Note: Branch deletion after merge may need to be done separately with tea" >&2
         fi
 
-        eval "$CMD"
+        if [[ -n "$TEA_LOGIN" ]]; then
+            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"
+        fi
         ;;
     *)
         echo "Error: Could not detect git platform" >&2
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..d64aa0e
--- /dev/null
+++ b/packages/mosaic/framework/tools/git/test-pr-merge-gitea-empty-uid.sh
@@ -0,0 +1,216 @@
+#!/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' "$LOG_FILE" >&2
+    exit 1
+fi
+
+SENTINEL="$SANDBOX/injected-sentinel"
+INJECTION="123; touch $SENTINEL #"
+
+cat > "$MOCK_BIN/gh" <<'EOF'
+#!/bin/bash
+set -euo pipefail
+printf 'gh %q ' "$@" >> "$PR_MERGE_TEST_LOG"
+printf '\n' >> "$PR_MERGE_TEST_LOG"
+if [[ "$*" == *"pr view"* ]]; then
+    cat <<'JSON'
+{"number":123,"title":"mock","baseRefName":"main","headRefName":"feature/mock"}
+JSON
+    exit 0
+fi
+if [[ "$*" == *"pr merge"* ]]; then
+    exit 0
+fi
+echo "unexpected gh invocation: $*" >&2
+exit 98
+EOF
+chmod +x "$MOCK_BIN/gh"
+
+cd "$REPO_DIR"
+git remote set-url origin https://github.com/mosaicstack/stack.git
+: > "$LOG_FILE"
+rm -f "$SENTINEL"
+if "$SCRIPT_DIR/pr-merge.sh" -n "$INJECTION" -m squash --skip-queue-guard > "$OUTPUT" 2>&1; then
+    echo "Expected GitHub metacharacter PR number to be rejected." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$OUTPUT" >&2
+    exit 1
+fi
+if [[ -e "$SENTINEL" ]]; then
+    echo "GitHub metacharacter PR number executed injected shell command." >&2
+    exit 1
+fi
+if [[ -s "$LOG_FILE" ]]; then
+    echo "GitHub metacharacter PR number should be rejected before gh calls." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$LOG_FILE" >&2
+    exit 1
+fi
+if ! grep -q 'Invalid PR number' "$OUTPUT"; then
+    echo "Expected invalid PR number error for GitHub metacharacter input." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$OUTPUT" >&2
+    exit 1
+fi
+
+cd "$REPO_DIR"
+git remote set-url origin https://git.mosaicstack.dev/mosaicstack/stack.git
+export GITEA_LOGIN="git.mosaicstack.dev"
+: > "$LOG_FILE"
+rm -f "$SENTINEL"
+if "$SCRIPT_DIR/pr-merge.sh" -n "$INJECTION" -m squash --skip-queue-guard > "$OUTPUT" 2>&1; then
+    echo "Expected Gitea metacharacter PR number to be rejected." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$OUTPUT" >&2
+    exit 1
+fi
+if [[ -e "$SENTINEL" ]]; then
+    echo "Gitea metacharacter PR number executed injected shell command." >&2
+    exit 1
+fi
+if [[ -s "$LOG_FILE" ]]; then
+    echo "Gitea metacharacter PR number should be rejected before tea/curl calls." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$LOG_FILE" >&2
+    exit 1
+fi
+if ! grep -q 'Invalid PR number' "$OUTPUT"; then
+    echo "Expected invalid PR number error for Gitea metacharacter input." >&2
+    sed 's/redacted-test-token/***REDACTED***/g' "$OUTPUT" >&2
+    exit 1
+fi
+
+echo "pr-merge.sh Gitea fallback regression passed"