diff --git a/README.md b/README.md
index 307988c..05ee16f 100644
--- a/README.md
+++ b/README.md
@@ -69,6 +69,21 @@ on real hardware, and the comments in `control.py` say which.
 
 ## Compatibility
 
+**Check the serial number, not the model name.** Characters 6-8 of the serial are
+the platform tag:
+
+| tag | platform | verdict |
+|---|---|---|
+| `ESU EMU ESA BPS BPU EMJ IJL` | 105 — ES/EM/BP, AA55-era meter bus | **supported** |
+| `SPB SPN` | 745 — SBP **G2** (`GW…-SBP-20`) | no |
+| `ESN ESC` | 745 — ES **G2** (`GW…-ES-20`) | no |
+| `ETU EHU BTU …` | 205/745/753 — ET/EH/BT hybrids | no |
+| `SDT DST MSU NSU …` | PV-only inverters — no battery to control | no |
+
+A "-20" or "G2" suffix means a different platform with a different meter
+protocol. The tags come from the `goodwe` library's `PLATFORM_105_MODELS`, which
+is also what Home Assistant's own integration uses to pick a protocol.
+
 - GoodWe **ES / BP family** inverters (AA55 / RS485 meter-bus generation)
 - The protocol is **reverse-engineered**. There is no vendor contract, and a
   firmware change on GoodWe's side could break every installation at once. Say
diff --git a/docs/GoodWe-RS485-Field-Guide.pdf b/docs/GoodWe-RS485-Field-Guide.pdf
index a4e1155..42f4264 100644
Binary files a/docs/GoodWe-RS485-Field-Guide.pdf and b/docs/GoodWe-RS485-Field-Guide.pdf differ
diff --git a/docs/build_guide.py b/docs/build_guide.py
index 1a8e93a..82f80a8 100644
--- a/docs/build_guide.py
+++ b/docs/build_guide.py
@@ -146,6 +146,26 @@ figcaption { font-size:9pt; color:#5b6169; margin-top:1.2mm; }
 <!-- ============ PAGE 2 ============ -->
 <div class="page" id="p2">
   <h2>Step 0 — Before you travel</h2>
+
+  <div class="crit">
+    <h3>Compatibility gate — 60 seconds, do this first</h3>
+    <p>Read the inverter's <b>serial number</b> (label, app, or portal) and take
+    <b>characters 6–8</b>. That three-letter tag is the platform.</p>
+    <table style="margin-bottom:2mm">
+      <tr><th style="width:34mm">Tag</th><th>Platform</th><th style="width:30mm">Verdict</th></tr>
+      <tr><td><code>ESU EMU ESA BPS<br>BPU EMJ IJL</code></td>
+          <td><b>Platform 105</b> — ES / EM / BP, AA55-era meter bus</td>
+          <td><b>SUPPORTED</b></td></tr>
+      <tr><td><code>SPB SPN</code></td><td>SBP <b>G2</b> (GW…-SBP-20) — platform 745</td><td><b>NO</b></td></tr>
+      <tr><td><code>ESN ESC</code></td><td>ES <b>G2</b> (GW…-ES-20) — platform 745</td><td><b>NO</b></td></tr>
+      <tr><td><code>ETU EHU BTU …</code></td><td>ET / EH / BT hybrids — platform 205/745/753</td><td><b>NO</b></td></tr>
+      <tr><td><code>SDT DST MSU NSU …</code></td><td>PV-only inverters — <b>no battery to control</b></td><td><b>NO</b></td></tr>
+    </table>
+    <p style="margin:0"><small>Reference unit: <code>95000BPS225W0290</code> → <code>BPS</code> → supported.
+    A “-20” or “G2” suffix in the model name means a different platform with a different meter
+    protocol. <b>If the tag is not on the supported list, stop — do not quote the job.</b></small></p>
+  </div>
+
   <div class="cols">
     <div class="box">
       <h3>Confirm with the client</h3>
diff --git a/docs/field-guide.html b/docs/field-guide.html
index a42d071..ddbb264 100644
--- a/docs/field-guide.html
+++ b/docs/field-guide.html
@@ -111,6 +111,26 @@ figcaption { font-size:9pt; color:#5b6169; margin-top:1.2mm; }
 <!-- ============ PAGE 2 ============ -->
 <div class="page" id="p2">
   <h2>Step 0 — Before you travel</h2>
+
+  <div class="crit">
+    <h3>Compatibility gate — 60 seconds, do this first</h3>
+    <p>Read the inverter's <b>serial number</b> (label, app, or portal) and take
+    <b>characters 6–8</b>. That three-letter tag is the platform.</p>
+    <table style="margin-bottom:2mm">
+      <tr><th style="width:34mm">Tag</th><th>Platform</th><th style="width:30mm">Verdict</th></tr>
+      <tr><td><code>ESU EMU ESA BPS<br>BPU EMJ IJL</code></td>
+          <td><b>Platform 105</b> — ES / EM / BP, AA55-era meter bus</td>
+          <td><b>SUPPORTED</b></td></tr>
+      <tr><td><code>SPB SPN</code></td><td>SBP <b>G2</b> (GW…-SBP-20) — platform 745</td><td><b>NO</b></td></tr>
+      <tr><td><code>ESN ESC</code></td><td>ES <b>G2</b> (GW…-ES-20) — platform 745</td><td><b>NO</b></td></tr>
+      <tr><td><code>ETU EHU BTU …</code></td><td>ET / EH / BT hybrids — platform 205/745/753</td><td><b>NO</b></td></tr>
+      <tr><td><code>SDT DST MSU NSU …</code></td><td>PV-only inverters — <b>no battery to control</b></td><td><b>NO</b></td></tr>
+    </table>
+    <p style="margin:0"><small>Reference unit: <code>95000BPS225W0290</code> → <code>BPS</code> → supported.
+    A “-20” or “G2” suffix in the model name means a different platform with a different meter
+    protocol. <b>If the tag is not on the supported list, stop — do not quote the job.</b></small></p>
+  </div>
+
   <div class="cols">
     <div class="box">
       <h3>Confirm with the client</h3>