diff --git a/docs/PROTOCOL_NOTES.md b/docs/PROTOCOL_NOTES.md
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diff --git a/tests/reference/RefClient.java b/tests/reference/RefClient.java
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+// Phase 0 spike: reference client for capturing the Informix SQLI wire protocol.
+//
+// Drives the official IBM Informix JDBC driver (com.informix.jdbc.IfxDriver)
+// through controlled scenarios so that tcpdump captures of localhost:9088
+// can be cross-referenced against the decompiled JDBC source. This is the
+// "ground truth" client whose traffic IS by definition spec-correct.
+//
+// Build:
+//   javac -cp build/ifxjdbc.jar tests/reference/RefClient.java -d build/
+// Run:
+//   java -cp build/ifxjdbc.jar:build/ tests.reference.RefClient <scenario>
+//
+// Scenarios: connect-only, select-1, dml-cycle, all
+//
+// Connection params can be overridden via env vars; defaults are the
+// Informix Developer Edition Docker image defaults documented in
+// docs/DECISION_LOG.md.
+
+package tests.reference;
+
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.PreparedStatement;
+import java.sql.ResultSet;
+import java.sql.ResultSetMetaData;
+import java.sql.Statement;
+import java.sql.SQLException;
+
+public class RefClient {
+
+    static final String HOST     = env("IFX_HOST",     "127.0.0.1");
+    static final String PORT     = env("IFX_PORT",     "9088");
+    static final String SERVER   = env("IFX_SERVER",   "informix");
+    static final String DATABASE = env("IFX_DATABASE", "sysmaster");
+    static final String USER     = env("IFX_USER",     "informix");
+    static final String PASSWORD = env("IFX_PASSWORD", "in4mix");
+
+    public static void main(String[] args) throws Exception {
+        Class.forName("com.informix.jdbc.IfxDriver");
+
+        String scenario = args.length > 0 ? args[0] : "connect-only";
+        switch (scenario) {
+            case "connect-only": runConnectOnly(); break;
+            case "select-1":     runSelect1();     break;
+            case "dml-cycle":    runDmlCycle();    break;
+            case "all":
+                runConnectOnly();
+                runSelect1();
+                runDmlCycle();
+                break;
+            default:
+                System.err.println("Unknown scenario: " + scenario);
+                System.err.println("Valid: connect-only | select-1 | dml-cycle | all");
+                System.exit(2);
+        }
+    }
+
+    static String url() {
+        return String.format(
+            "jdbc:informix-sqli://%s:%s/%s:INFORMIXSERVER=%s",
+            HOST, PORT, DATABASE, SERVER
+        );
+    }
+
+    static void log(String fmt, Object... args) {
+        System.out.printf("[ref] " + fmt + "%n", args);
+    }
+
+    static String env(String key, String dflt) {
+        String v = System.getenv(key);
+        return (v == null || v.isEmpty()) ? dflt : v;
+    }
+
+    // -------------------------------------------------------------------
+    // Scenario A: connect + immediate disconnect
+    // -------------------------------------------------------------------
+    static void runConnectOnly() throws SQLException {
+        log("=== connect-only ===");
+        log("URL: %s", url());
+        try (Connection c = DriverManager.getConnection(url(), USER, PASSWORD)) {
+            log("connected; product=%s", c.getMetaData().getDatabaseProductName());
+            // Note: do NOT call getDatabaseProductVersion() — the 4.50.JC10
+            // JDBC driver throws NumberFormatException on Informix 15.0's
+            // version string ("150."). Connection itself works fine.
+        }
+        log("disconnected.");
+    }
+
+    // -------------------------------------------------------------------
+    // Scenario B: connect + SELECT 1 + disconnect
+    // -------------------------------------------------------------------
+    static void runSelect1() throws SQLException {
+        log("=== select-1 ===");
+        try (Connection c = DriverManager.getConnection(url(), USER, PASSWORD);
+             Statement s = c.createStatement();
+             ResultSet rs = s.executeQuery("SELECT 1 FROM systables WHERE tabid = 1")) {
+
+            ResultSetMetaData md = rs.getMetaData();
+            log("columns: %d", md.getColumnCount());
+            for (int i = 1; i <= md.getColumnCount(); i++) {
+                log("  [%d] name=%s type=%d (%s) precision=%d scale=%d",
+                    i, md.getColumnName(i), md.getColumnType(i),
+                    md.getColumnTypeName(i), md.getPrecision(i), md.getScale(i));
+            }
+            int rowNum = 0;
+            while (rs.next()) {
+                rowNum++;
+                log("  row %d: col1=%s", rowNum, rs.getObject(1));
+            }
+            log("rows returned: %d", rowNum);
+        }
+    }
+
+    // -------------------------------------------------------------------
+    // Scenario C: full DML cycle (CREATE / INSERT / SELECT / DROP)
+    // Uses a UUID-suffixed table to be safe to re-run.
+    // -------------------------------------------------------------------
+    static void runDmlCycle() throws SQLException {
+        log("=== dml-cycle ===");
+        String table = "spike_" + Long.toHexString(System.nanoTime());
+        try (Connection c = DriverManager.getConnection(url(), USER, PASSWORD)) {
+            c.setAutoCommit(true);  // sysmaster is unlogged; force autocommit
+
+            try (Statement s = c.createStatement()) {
+                log("CREATE TEMP TABLE %s", table);
+                s.execute("CREATE TEMP TABLE " + table +
+                          " (id INTEGER, name VARCHAR(50), val FLOAT)");
+            }
+
+            try (PreparedStatement ps = c.prepareStatement(
+                    "INSERT INTO " + table + " VALUES (?, ?, ?)")) {
+                ps.setInt(1, 42);
+                ps.setString(2, "hello");
+                ps.setDouble(3, 3.14);
+                int n = ps.executeUpdate();
+                log("INSERT rowcount=%d", n);
+            }
+
+            try (Statement s = c.createStatement();
+                 ResultSet rs = s.executeQuery("SELECT id, name, val FROM " + table)) {
+                ResultSetMetaData md = rs.getMetaData();
+                log("SELECT columns:");
+                for (int i = 1; i <= md.getColumnCount(); i++) {
+                    log("  [%d] %s : %s", i, md.getColumnName(i), md.getColumnTypeName(i));
+                }
+                while (rs.next()) {
+                    log("  row: id=%d name=%s val=%f",
+                        rs.getInt(1), rs.getString(2), rs.getDouble(3));
+                }
+            }
+            // Temp table dropped automatically on disconnect; no DROP needed.
+        }
+    }
+}