diff --git a/understand-anything-plugin/hooks/auto-update-prompt.md b/understand-anything-plugin/hooks/auto-update-prompt.md
index 13d5584..5fe88cc 100644
--- a/understand-anything-plugin/hooks/auto-update-prompt.md
+++ b/understand-anything-plugin/hooks/auto-update-prompt.md
@@ -37,6 +37,58 @@ Incrementally update the knowledge graph using deterministic structural fingerpr
    mkdir -p $PROJECT_ROOT/.understand-anything/intermediate
    ```
 
+9. **Apply `.understandignore` exclusions** (same semantics as `/understand` Step 2.5 in `agents/project-scanner.md`).
+
+   Without this step, files in user-excluded paths (migrations, vendored code, tests) are counted as structural changes and can spuriously escalate the action to `FULL_UPDATE` even when the real change set is tiny.
+
+   1. If neither `$PROJECT_ROOT/.understand-anything/.understandignore` nor `$PROJECT_ROOT/.understandignore` exists, the step 7 extension filter is sufficient — skip to Phase 1.
+
+   2. Write the step 7 file list to `$PROJECT_ROOT/.understand-anything/intermediate/changed-files-pre.json` as a JSON array of relative paths.
+
+   3. Resolve `$PLUGIN_ROOT`:
+      - Use `$CLAUDE_PLUGIN_ROOT` if set (Claude Code's hook context sets this).
+      - Otherwise try `$HOME/.understand-anything-plugin`.
+      - Validate the chosen candidate by checking `$candidate/packages/core/dist/ignore-filter.js` exists.
+      - If neither resolves: report "Cannot locate plugin install at `$CLAUDE_PLUGIN_ROOT` or `$HOME/.understand-anything-plugin`; auto-update aborted. Run `/understand` to re-baseline." and **STOP**. Do **not** silently skip — silent skip reproduces issue #153.
+
+   4. Write `$PROJECT_ROOT/.understand-anything/intermediate/ignore-filter.mjs`:
+      ```javascript
+      import { readFileSync, writeFileSync } from 'node:fs';
+      import { pathToFileURL } from 'node:url';
+      import path from 'node:path';
+
+      const PROJECT_ROOT = process.cwd();
+      const PLUGIN_ROOT = process.argv[2];
+      const inputPath = process.argv[3];
+
+      const modUrl = pathToFileURL(
+        path.join(PLUGIN_ROOT, 'packages/core/dist/ignore-filter.js'),
+      ).href;
+      const { createIgnoreFilter } = await import(modUrl);
+      const filter = createIgnoreFilter(PROJECT_ROOT);
+
+      const input = JSON.parse(readFileSync(inputPath, 'utf-8'));
+      const kept = input.filter((p) => !filter.isIgnored(p));
+      const removed = input.length - kept.length;
+
+      writeFileSync(
+        path.join(PROJECT_ROOT, '.understand-anything/intermediate/changed-files.json'),
+        JSON.stringify({ kept, removed, total: input.length }, null, 2),
+      );
+      console.log(`.understandignore: kept ${kept.length}/${input.length} (removed ${removed})`);
+      ```
+
+   5. Run it:
+      ```bash
+      node $PROJECT_ROOT/.understand-anything/intermediate/ignore-filter.mjs \
+        "$PLUGIN_ROOT" \
+        $PROJECT_ROOT/.understand-anything/intermediate/changed-files-pre.json
+      ```
+
+   6. Read `$PROJECT_ROOT/.understand-anything/intermediate/changed-files.json`. Pass the `kept` array as the input file list for Phase 1's fingerprint-check script.
+
+   7. If `kept.length === 0`: update `meta.json` with the new commit hash, report "All changed source files are in ignored paths. Metadata updated." and **STOP**.
+
 ---
 
 ## Phase 1 — Structural Fingerprint Check (Zero LLM Tokens)