diff --git a/ralph_loop.sh b/ralph_loop.sh
index c243052..683d654 100755
--- a/ralph_loop.sh
+++ b/ralph_loop.sh
@@ -1147,9 +1147,11 @@ execute_claude_code() {
         # Use stdbuf to disable buffering for real-time output
         # Use portable_timeout for consistent timeout protection (Issue: missing timeout)
         # Capture all pipeline exit codes for proper error handling
+        # stdin must be redirected from /dev/null because newer Claude CLI versions
+        # read from stdin even in -p (print) mode, causing the process to hang
         set -o pipefail
         portable_timeout ${timeout_seconds}s stdbuf -oL "${LIVE_CMD_ARGS[@]}" \
-            2>&1 | stdbuf -oL tee "$output_file" | stdbuf -oL jq --unbuffered -j "$jq_filter" 2>/dev/null | tee "$LIVE_LOG_FILE"
+            < /dev/null 2>&1 | stdbuf -oL tee "$output_file" | stdbuf -oL jq --unbuffered -j "$jq_filter" 2>/dev/null | tee "$LIVE_LOG_FILE"
 
         # Capture exit codes from pipeline
         local -a pipe_status=("${PIPESTATUS[@]}")
@@ -1206,7 +1208,10 @@ execute_claude_code() {
         if [[ "$use_modern_cli" == "true" ]]; then
             # Modern execution with command array (shell-injection safe)
             # Execute array directly without bash -c to prevent shell metacharacter interpretation
-            if portable_timeout ${timeout_seconds}s "${CLAUDE_CMD_ARGS[@]}" > "$output_file" 2>&1 &
+            # stdin must be redirected from /dev/null because newer Claude CLI versions
+            # read from stdin even in -p (print) mode, causing SIGTTIN suspension
+            # when the process is backgrounded
+            if portable_timeout ${timeout_seconds}s "${CLAUDE_CMD_ARGS[@]}" < /dev/null > "$output_file" 2>&1 &
             then
                 :  # Continue to wait loop
             else
diff --git a/tests/unit/test_cli_modern.bats b/tests/unit/test_cli_modern.bats
index 6bdbfe7..28dbfbe 100644
--- a/tests/unit/test_cli_modern.bats
+++ b/tests/unit/test_cli_modern.bats
@@ -642,6 +642,71 @@ EOF
     [[ "$found_prompt" == "true" ]]
 }
 
+# =============================================================================
+# BACKGROUND EXECUTION STDIN REDIRECT TESTS
+# Newer Claude CLI reads stdin even in -p mode, causing SIGTTIN suspension
+# when the process is backgrounded. Verify /dev/null redirect is present.
+# =============================================================================
+
+@test "modern CLI background execution redirects stdin from /dev/null" {
+    # Verify the implementation in ralph_loop.sh redirects stdin from /dev/null
+    # to prevent SIGTTIN suspension when claude is backgrounded.
+    # Without this, newer Claude CLI versions hang indefinitely.
+
+    run grep 'portable_timeout.*CLAUDE_CMD_ARGS.*< /dev/null.*&' "${BATS_TEST_DIRNAME}/../../ralph_loop.sh"
+
+    assert_success
+    [[ "$output" == *'< /dev/null'* ]]
+}
+
+@test "live mode execution redirects stdin from /dev/null" {
+    # Verify the live (streaming) mode also redirects stdin from /dev/null.
+    # This path is used by ralph --monitor (which adds --live).
+    # The live mode splits across two lines (line continuation with \),
+    # so we check the continuation line that has < /dev/null.
+
+    local script="${BATS_TEST_DIRNAME}/../../ralph_loop.sh"
+
+    # The live mode has LIVE_CMD_ARGS on one line and < /dev/null on the next
+    run grep '< /dev/null 2>&1 |' "$script"
+
+    assert_success
+    [[ "$output" == *'< /dev/null'* ]]
+}
+
+@test "all claude execution paths redirect stdin" {
+    # Verify that ALL portable_timeout invocations of claude redirect stdin,
+    # to prevent regressions. There are 3 paths: modern background, live, legacy.
+    # Legacy uses < "$PROMPT_FILE", the other two must use < /dev/null.
+    # We check that no portable_timeout line invoking claude lacks a stdin redirect
+    # (either on the same line or a continuation line).
+
+    local script="${BATS_TEST_DIRNAME}/../../ralph_loop.sh"
+
+    # All 3 portable_timeout lines that invoke claude should have < somewhere nearby
+    # Modern background: has < /dev/null on same line
+    run grep 'portable_timeout.*CLAUDE_CMD_ARGS.*< /dev/null' "$script"
+    assert_success
+
+    # Live mode: has < /dev/null on continuation line
+    run grep '< /dev/null 2>&1 |' "$script"
+    assert_success
+
+    # Legacy mode: has < "$PROMPT_FILE" on same line
+    run grep 'portable_timeout.*CLAUDE_CODE_CMD.*< ' "$script"
+    assert_success
+}
+
+@test "modern CLI background execution has comment explaining stdin redirect" {
+    # Verify the fix is documented with context about why /dev/null is needed
+
+    run grep -c 'stdin must be redirected' "${BATS_TEST_DIRNAME}/../../ralph_loop.sh"
+
+    assert_success
+    # Should appear in both background and live mode sections
+    [[ "$output" == "2" ]]
+}
+
 # =============================================================================
 # .RALPHRC CONFIGURATION LOADING TESTS
 # Tests for the environment variable precedence fix