Wizard: warn in-UI when default dest table already exists.

Previously the existing-dest check fired on submit and surfaced as a raw
JSON 400. Now step 3 introspects the default dest up front and renders a
yellow banner listing existing columns; submit-time mismatches render
wizard_error.html (409) with missing vs. existing side-by-side and a back
link that re-plumbs the form qvals.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pipekit/web/app.py

@@ -13,6 +13,7 @@ wizard, editors, and SSE-driven live run watch come next.
 from __future__ import annotations
 
 from pathlib import Path
+from urllib.parse import urlencode
 
 from fastapi import APIRouter, FastAPI, HTTPException, Query, Request
 from fastapi.responses import HTMLResponse, RedirectResponse
@@ -340,6 +341,23 @@ def wizard_step3(request: Request,
     default_dest_conn_id = conn.get("default_dest_connection_id")
     default_dest_schema = conn.get("default_dest_schema") or ""
 
+    # Proactive warning — if the default dest table already exists, surface
+    # its columns now so the user can align picks before submit.
+    dest_warn: dict | None = None
+    if not fetch_error and default_dest_conn_id and default_dest_schema:
+        dest_conn_row = repo.get_connection(default_dest_conn_id)
+        if dest_conn_row is not None:
+            try:
+                existing = _existing_dest_columns(
+                    dest_conn_row, default_dest_schema, default_module_name)
+            except jrunner.JrunnerError:
+                existing = None
+            if existing is not None:
+                dest_warn = {
+                    "qualified": f"{default_dest_schema}.{default_module_name}",
+                    "columns": sorted(existing),
+                }
+
     return _templates.TemplateResponse(
         request,
         "wizard_step3.html",
@@ -349,7 +367,8 @@ def wizard_step3(request: Request,
              table_description=table_description,
              fetch_error=fetch_error, default_module_name=default_module_name,
              default_dest_conn_id=default_dest_conn_id,
-             default_dest_schema=default_dest_schema),
+             default_dest_schema=default_dest_schema,
+             dest_warn=dest_warn),
     )
 
 
@@ -448,12 +467,23 @@ async def wizard_create(request: Request):
         missing = [c["dest_name"] for c in chosen
                    if c["dest_name"].lower() not in existing_cols]
         if missing:
-            raise HTTPException(
+            back_qs = urlencode(
-                400,
+                [("source_connection_id", source_connection_id),
-                f"dest table {qualified_dest} already exists but is missing "
+                 ("table", table),
-                f"columns: {', '.join(missing)}. Drop the table, choose a "
+                 ("table_schema", qvals.get("schema") or qvals.get("library") or ""),
-                f"different dest_table, or align your column picks to match "
+                 *qvals.items()])
-                f"the existing schema.")
+            return _templates.TemplateResponse(
+                request,
+                "wizard_error.html",
+                _ctx(
+                    title="Dest table column mismatch",
+                    qualified_dest=qualified_dest,
+                    missing=missing,
+                    existing=sorted(existing_cols),
+                    back_qs=back_qs,
+                ),
+                status_code=409,
+            )
 
     try:
         jrunner.run_dest_sql(

pipekit/web/static/style.css

@@ -277,3 +277,4 @@ table.picker tbody tr:hover td { background: #1c2128; }
 }
 .flash.ok { border-color: #2f6b35; background: #16261a; color: #b6dcb8; }
 .flash.err { border-color: #6b2f2f; background: #261616; color: #dcb6b6; }
+.flash.warn { border-color: #6b5a2f; background: #261f16; color: #e1c98a; }

pipekit/web/templates/wizard_error.html

@@ -0,0 +1,57 @@
+{% extends "base.html" %}
+{% set section = "modules" %}
+{% block title %}New module — error{% endblock %}
+
+{% block content %}
+<div class="panel">
+    <header>
+        Wizard error
+        <span class="subtitle">{{ title }}</span>
+        <span style="margin-left:auto">
+            <a href="/wizard/columns?{{ back_qs }}">&larr; back to step 3</a>
+        </span>
+    </header>
+    <div class="body">
+        <div class="flash err">
+            Dest table <code>{{ qualified_dest }}</code> already exists, but your
+            picks don't match its schema. Drop the table, choose a different
+            dest table, or align your column picks (dest names) with the
+            existing columns below.
+        </div>
+
+        <div class="two-col">
+            <div class="panel">
+                <header>
+                    Missing from existing table
+                    <span class="subtitle">{{ missing|length }}</span>
+                </header>
+                <div class="body tight">
+                    <table class="grid">
+                        <tbody>
+                        {% for m in missing %}
+                            <tr><td class="mono">{{ m }}</td></tr>
+                        {% endfor %}
+                        </tbody>
+                    </table>
+                </div>
+            </div>
+
+            <div class="panel">
+                <header>
+                    Columns in existing dest
+                    <span class="subtitle">{{ existing|length }}</span>
+                </header>
+                <div class="body tight">
+                    <table class="grid">
+                        <tbody>
+                        {% for c in existing %}
+                            <tr><td class="mono">{{ c }}</td></tr>
+                        {% endfor %}
+                        </tbody>
+                    </table>
+                </div>
+            </div>
+        </div>
+    </div>
+</div>
+{% endblock %}

pipekit/web/templates/wizard_step3.html

@@ -18,6 +18,16 @@
     </div>
 </div>
 
+{% if dest_warn %}
+<div class="flash warn">
+    Dest table <code>{{ dest_warn.qualified }}</code> already exists on the
+    default destination. If you proceed, pipekit will <strong>not</strong> drop
+    or recreate it — your picks below (dest names) must match the existing
+    columns, or the create will fail. Existing columns:
+    <span class="mono">{{ dest_warn.columns|join(', ') }}</span>
+</div>
+{% endif %}
+
 {% if not fetch_error %}
 <form method="post" action="/wizard/create">
     <input type="hidden" name="source_connection_id" value="{{ connection.id }}">