diff --git a/tests/ralph.bats b/tests/ralph.bats
new file mode 100644
index 00000000..8b0a6248
--- /dev/null
+++ b/tests/ralph.bats
@@ -0,0 +1,180 @@
+#!/usr/bin/env bats
+# Tests for ralph.sh
+# Run with: bats tests/ralph.bats
+
+setup() {
+    TEST_DIR="$(mktemp -d)"
+    cp ralph.sh "$TEST_DIR/"
+    cd "$TEST_DIR"
+
+    cat > prd.json << 'EOF'
+{
+  "project": "TestProject",
+  "branchName": "ralph/test-feature",
+  "description": "Test feature",
+  "userStories": [
+    {
+      "id": "US-001",
+      "title": "Test story",
+      "description": "Test description",
+      "acceptanceCriteria": ["Test passes"],
+      "priority": 1,
+      "passes": false,
+      "notes": ""
+    }
+  ]
+}
+EOF
+
+    echo "Mock prompt" > prompt.md
+    echo "Mock claude prompt" > CLAUDE.md
+}
+
+teardown() {
+    cd /
+    rm -rf "$TEST_DIR"
+}
+
+# =============================================================================
+# Argument Parsing Tests
+# =============================================================================
+
+@test "--tool amp is accepted" {
+    run bash -c '
+        TOOL="amp"
+        args=(--tool amp)
+        while [[ ${#args[@]} -gt 0 ]]; do
+            case ${args[0]} in
+                --tool) TOOL="${args[1]}"; args=("${args[@]:2}") ;;
+                *) args=("${args[@]:1}") ;;
+            esac
+        done
+        [[ "$TOOL" == "amp" ]] && echo "PASS"
+    '
+    [[ "$output" == "PASS" ]]
+}
+
+@test "--tool claude is accepted" {
+    run bash -c '
+        TOOL="amp"
+        args=(--tool claude)
+        while [[ ${#args[@]} -gt 0 ]]; do
+            case ${args[0]} in
+                --tool) TOOL="${args[1]}"; args=("${args[@]:2}") ;;
+                *) args=("${args[@]:1}") ;;
+            esac
+        done
+        [[ "$TOOL" == "claude" ]] && echo "PASS"
+    '
+    [[ "$output" == "PASS" ]]
+}
+
+@test "--tool=claude syntax is accepted" {
+    run bash -c '
+        TOOL="amp"
+        args=(--tool=claude)
+        while [[ ${#args[@]} -gt 0 ]]; do
+            case ${args[0]} in
+                --tool=*) TOOL="${args[0]#*=}"; args=("${args[@]:1}") ;;
+                *) args=("${args[@]:1}") ;;
+            esac
+        done
+        [[ "$TOOL" == "claude" ]] && echo "PASS"
+    '
+    [[ "$output" == "PASS" ]]
+}
+
+@test "numeric argument sets max iterations" {
+    run bash -c '
+        MAX_ITERATIONS=10
+        args=(5)
+        while [[ ${#args[@]} -gt 0 ]]; do
+            if [[ "${args[0]}" =~ ^[0-9]+$ ]]; then
+                MAX_ITERATIONS="${args[0]}"
+            fi
+            args=("${args[@]:1}")
+        done
+        [[ "$MAX_ITERATIONS" == "5" ]] && echo "PASS"
+    '
+    [[ "$output" == "PASS" ]]
+}
+
+# =============================================================================
+# Tool Validation Tests
+# =============================================================================
+
+@test "invalid tool is rejected" {
+    run bash -c '
+        TOOL="invalid"
+        if [[ "$TOOL" != "amp" && "$TOOL" != "claude" ]]; then
+            echo "Error: Invalid tool"
+            exit 1
+        fi
+    '
+    [[ "$status" -eq 1 ]]
+    [[ "$output" == *"Invalid tool"* ]]
+}
+
+# =============================================================================
+# PRD JSON Tests
+# =============================================================================
+
+@test "prd.json branchName is extracted correctly" {
+    run bash -c "jq -r '.branchName' prd.json"
+    [[ "$output" == "ralph/test-feature" ]]
+}
+
+@test "prd.json userStories exist" {
+    run bash -c "jq '.userStories | length' prd.json"
+    [[ "$output" == "1" ]]
+}
+
+@test "prd.json story has required fields" {
+    run bash -c "jq '.userStories[0] | has(\"id\", \"title\", \"passes\")' prd.json"
+    [[ "$output" == "true" ]]
+}
+
+# =============================================================================
+# Archive Logic Tests
+# =============================================================================
+
+@test "branch change is detected" {
+    echo "ralph/old-feature" > .last-branch
+    CURRENT_BRANCH=$(jq -r '.branchName // empty' prd.json)
+    LAST_BRANCH=$(cat .last-branch)
+    [[ "$CURRENT_BRANCH" != "$LAST_BRANCH" ]]
+}
+
+@test "last branch file is updated" {
+    CURRENT_BRANCH=$(jq -r '.branchName' prd.json)
+    echo "$CURRENT_BRANCH" > .last-branch
+    [[ "$(cat .last-branch)" == "ralph/test-feature" ]]
+}
+
+# =============================================================================
+# Progress File Tests
+# =============================================================================
+
+@test "progress file header format" {
+    echo "# Ralph Progress Log" > progress.txt
+    echo "Started: $(date)" >> progress.txt
+    echo "---" >> progress.txt
+    run head -1 progress.txt
+    [[ "$output" == "# Ralph Progress Log" ]]
+}
+
+# =============================================================================
+# Completion Detection Tests
+# =============================================================================
+
+@test "completion signal is detected" {
+    OUTPUT="Some output <promise>COMPLETE</promise> more output"
+    run bash -c "echo '$OUTPUT' | grep -q '<promise>COMPLETE</promise>' && echo 'DETECTED'"
+    [[ "$output" == "DETECTED" ]]
+}
+
+@test "incomplete output does not trigger completion" {
+    OUTPUT="Some output without completion signal"
+    run bash -c "echo '$OUTPUT' | grep -q '<promise>COMPLETE</promise>' || echo 'NOT_COMPLETE'"
+    [[ "$output" == "NOT_COMPLETE" ]]
+}