diff --git a/.changeset/frontend-mobile-layout.md b/.changeset/frontend-mobile-layout.md
new file mode 100644
index 0000000..d2034f7
--- /dev/null
+++ b/.changeset/frontend-mobile-layout.md
@@ -0,0 +1,7 @@
+---
+"frontend": patch
+---
+
+The wizard now fits a phone screen. It previously had no width breakpoints at all, so the fixed 210px step rail left roughly 165px of usable content on a 375px device. Below 700px the rail becomes a horizontal top bar — the same nodes and connector rotated a quarter turn, each label under its node — so every step stays visible without hiding navigation behind a menu. The remaining layouts follow: the landing cards stack and scroll from the top instead of centering and clipping on a short screen, the JSON preview drawer takes the full width now that there is no side rail to sit beside, the Variables description/type pair and the Authors paired fields drop to one column, picker and action rows wrap, and the JSON viewer's per-level indent tightens so deep nesting no longer runs off the screen.
+
+Form controls are also held at 16px on small screens: iOS Safari auto-zooms whenever a focused control is smaller than that, and every field here was around 14px, so tapping one zoomed the page in and left it scrolled sideways.
diff --git a/packages/frontend/src/components/AppShell.module.css b/packages/frontend/src/components/AppShell.module.css
index 4f8c2db..fe84c27 100644
--- a/packages/frontend/src/components/AppShell.module.css
+++ b/packages/frontend/src/components/AppShell.module.css
@@ -11,6 +11,23 @@
   background-color: var(--c-bg);
 }
 
+/* Phone: the sidebar becomes a top bar (see Sidebar.module.css), so the shell
+   stacks and the content takes the full width. `min-height: 0` lets the content
+   scroll inside the shell rather than pushing it taller than the viewport —
+   without it a flex column child refuses to shrink below its content height. */
+@media (max-width: 700px) {
+  .shell {
+    flex-direction: column;
+  }
+
+  .content {
+    min-height: 0;
+    /* Bottom padding clears the fixed Preview pill so it can't cover the last
+       control on the page (usually Continue). */
+    padding: 1.5rem 1rem 4.5rem;
+  }
+}
+
 .previewPill {
   position: fixed;
   bottom: 1.5rem;
@@ -30,6 +47,15 @@
   transition: border-color 0.15s, color 0.15s, background-color 0.15s;
 }
 
+/* Phone: tuck the pill closer into the corner. This has to sit *after* the base
+   rule above — same specificity, so source order is what decides. */
+@media (max-width: 700px) {
+  .previewPill {
+    bottom: 1rem;
+    right: 1rem;
+  }
+}
+
 .previewPill:hover {
   color: var(--c-ink);
   border-color: var(--c-ink-3);
diff --git a/packages/frontend/src/components/JsonViewer.module.css b/packages/frontend/src/components/JsonViewer.module.css
index 23cc0ce..505cd6c 100644
--- a/packages/frontend/src/components/JsonViewer.module.css
+++ b/packages/frontend/src/components/JsonViewer.module.css
@@ -19,6 +19,19 @@
   padding-left: 1.5rem;
 }
 
+/* Phone: 1.5rem per nesting level eats a 375px screen within a few levels, so
+   tighten the indent (and the gutters) rather than force sideways scrolling. */
+@media (max-width: 700px) {
+  .viewer {
+    padding: 0.85rem 0.75rem;
+    font-size: 0.78rem;
+  }
+
+  .children {
+    padding-left: 0.9rem;
+  }
+}
+
 /* Syntax token colors — light mode (jsPsych logo palette, darkened for contrast on white) */
 .key   { color: #006838; }  /* logo dark green — 7:1 on white */
 .str   { color: #b06210; }  /* orange-amber (hue ~31°) — 4.5:1 on white */
diff --git a/packages/frontend/src/components/PreviewDrawer.module.css b/packages/frontend/src/components/PreviewDrawer.module.css
index 980638e..42375bb 100644
--- a/packages/frontend/src/components/PreviewDrawer.module.css
+++ b/packages/frontend/src/components/PreviewDrawer.module.css
@@ -23,6 +23,16 @@
   background: rgba(0, 0, 0, 0.35);
 }
 
+/* Phone: there's no side rail to sit beside, so the drawer takes the screen.
+   (The --sidebar-w allowance above is meaningless once the nav is a top bar.) */
+@media (max-width: 700px) {
+  .drawer {
+    width: 100%;
+    max-width: 100%;
+    border-left: none;
+  }
+}
+
 @keyframes slideIn {
   from { transform: translateX(100%); }
   to   { transform: translateX(0); }
diff --git a/packages/frontend/src/components/Sidebar.module.css b/packages/frontend/src/components/Sidebar.module.css
index c50a3fd..3272788 100644
--- a/packages/frontend/src/components/Sidebar.module.css
+++ b/packages/frontend/src/components/Sidebar.module.css
@@ -216,6 +216,79 @@
 :global([data-theme="dark"]) .psychdsLink { color: var(--c-ink-2); }
 :global([data-theme="dark"]) .psychdsLink:hover { color: var(--c-forest-bright); }
 
+/* ─── Phone: the step nav becomes a top bar ──────────────────────────────────
+   A 210px side rail leaves ~165px of content on a 375px screen, so below 700px
+   the nav turns horizontal and sits above the content: the same nodes and
+   connector, rotated a quarter turn, with each label under its node.
+   Progress stays visible at all times — no hamburger, nothing hidden. */
+
+@media (max-width: 700px) {
+  .sidebar {
+    width: 100%;
+    min-width: 0;
+    padding: 0.75rem 0 0.5rem;
+    border-right: none;
+    border-bottom: 1px solid var(--c-border-sub);
+  }
+
+  .header {
+    padding: 0 1rem 0.75rem;
+  }
+
+  /* Steps run left-to-right, each an equal-width column (node above label). */
+  .stepList {
+    flex-direction: row;
+    flex: 0 0 auto;
+    gap: 0;
+    padding: 0 0.5rem;
+  }
+
+  .stepList li {
+    flex: 1 1 0;
+    min-width: 0;
+  }
+
+  .step {
+    flex-direction: column;
+    gap: 0.3rem;
+    padding: 0.4rem 0.15rem;
+    font-size: 0.68rem;
+    line-height: 1.25;
+    text-align: center;
+  }
+
+  /* The connector rotates with the flow: it now runs from this node's center
+     across to the next node's. Equal-width columns make width:100% land exactly
+     on the next node, the same trick the vertical rule uses with its height. */
+  .step::before {
+    left: 50%;
+    top: calc(0.4rem + 11px); /* step padding-top + node radius = node center */
+    width: 100%;
+    height: 2px;
+    transform: none;
+  }
+
+  /* Footer flattens into a single row under the steps so it costs one line of
+     height instead of two stacked blocks. */
+  .footer {
+    flex-direction: row;
+    align-items: center;
+    justify-content: space-between;
+    gap: 0.75rem;
+    padding: 0.6rem 1rem 0;
+    margin-top: 0.4rem;
+  }
+
+  .psychdsLink {
+    padding: 0;
+  }
+
+  .startOver {
+    width: auto;
+    flex-shrink: 0;
+  }
+}
+
 /* Start over confirmation modal — native <dialog> for free focus trap + Escape */
 
 .dialog {
@@ -223,7 +296,9 @@
   border: 1px solid var(--c-border);
   border-radius: 12px;
   padding: 1.75rem;
-  width: 320px;
+  /* Never wider than the viewport it's centred in (320px overflows a 375px
+     phone once the UA's dialog margins are counted). */
+  width: min(320px, calc(100vw - 2rem));
   display: flex;
   flex-direction: column;
   gap: 0.75rem;
diff --git a/packages/frontend/src/index.css b/packages/frontend/src/index.css
index cc60063..352efc4 100644
--- a/packages/frontend/src/index.css
+++ b/packages/frontend/src/index.css
@@ -236,6 +236,23 @@ a:hover {
   outline-offset: 2px;
 }
 
+/* Phone: iOS Safari auto-zooms the page whenever a focused form control has a
+   font-size under 16px — every field here is ~14px, so tapping one would zoom
+   in and leave the layout scrolled sideways with no way back short of a pinch.
+   Holding the controls at 16px on small screens is the standard prevention.
+   Only the controls you actually type into: a checkbox or radio can't be zoomed
+   into, and forcing a font-size on one only risks disturbing a styled control
+   that sizes itself from it. !important because each page's CSS module sets its
+   own font-size on these controls by class, which outranks a bare element
+   selector. */
+@media (max-width: 700px) {
+  input:not([type="checkbox"]):not([type="radio"]),
+  select,
+  textarea {
+    font-size: 16px !important;
+  }
+}
+
 /* Respect users who ask for less motion: collapse transitions and the preview
    drawer's slide-in to a near-instant, non-animated state change. */
 @media (prefers-reduced-motion: reduce) {
diff --git a/packages/frontend/src/pages/Authors.module.css b/packages/frontend/src/pages/Authors.module.css
index 9692cd1..21c5569 100644
--- a/packages/frontend/src/pages/Authors.module.css
+++ b/packages/frontend/src/pages/Authors.module.css
@@ -410,3 +410,26 @@
 .continueBtn:hover {
   background-color: var(--c-cta-hover);
 }
+
+/* ─── Phone ─────────────────────────────────────────────────────────────────── */
+
+@media (max-width: 700px) {
+  /* Paired fields (name/email, etc.) stack — two columns on a phone leaves each
+     input too narrow to read what's typed in it. */
+  .fieldRow {
+    grid-template-columns: 1fr;
+    gap: 0.7rem;
+  }
+
+  .card {
+    padding: 0.85rem 0.85rem 0.75rem;
+  }
+
+  .bulkActions {
+    flex-wrap: wrap;
+  }
+
+  .continueBtn {
+    width: 100%;
+  }
+}
diff --git a/packages/frontend/src/pages/DataUpload.module.css b/packages/frontend/src/pages/DataUpload.module.css
index c7daedb..de2312e 100644
--- a/packages/frontend/src/pages/DataUpload.module.css
+++ b/packages/frontend/src/pages/DataUpload.module.css
@@ -485,3 +485,42 @@
 .replaceConfirmYes:hover {
   filter: brightness(0.94);
 }
+
+/* ─── Phone ─────────────────────────────────────────────────────────────────── */
+
+@media (max-width: 700px) {
+  /* Button + "or" + folder name is too much for one phone line; let it wrap,
+     and let the (ellipsised) folder name shrink rather than push the row wide. */
+  .pickerRow {
+    flex-wrap: wrap;
+    gap: 0.6rem 0.85rem;
+  }
+
+  .folderName {
+    min-width: 0;
+    flex: 1 1 100%;
+  }
+
+  /* Action rows keep their spacing but drop to a second line when needed. */
+  .joinKeyActions,
+  .replaceConfirmBtns {
+    flex-wrap: wrap;
+    gap: 0.75rem 1rem;
+  }
+
+  /* Size the lists to the viewport rather than to a fixed 220/260px. In portrait
+     that lands near the desktop cap; in landscape, where 40% of the height is
+     well under it, the list gives the rest of the page room instead of filling
+     the screen. dvh so the retractable URL bar doesn't inflate it; the vh line
+     is the fallback for browsers without dvh. */
+  .fileList,
+  .statusList {
+    max-height: 40vh;
+    max-height: 40dvh;
+  }
+
+  .continueBtn,
+  .processBtn {
+    width: 100%;
+  }
+}
diff --git a/packages/frontend/src/pages/Landing.module.css b/packages/frontend/src/pages/Landing.module.css
index b3d2a28..7c66625 100644
--- a/packages/frontend/src/pages/Landing.module.css
+++ b/packages/frontend/src/pages/Landing.module.css
@@ -180,3 +180,40 @@
   padding: 0.1em 0.3em;
   border-radius: 3px;
 }
+
+/* ─── Phone ───────────────────────────────────────────────────────────────────
+   Two 260px cards side by side need 544px, so they stack. Once stacked the
+   screen can no longer centre everything vertically without clipping the top on
+   a short phone, so the column scrolls from the top instead. */
+
+@media (max-width: 700px) {
+  .landing {
+    justify-content: flex-start;
+    overflow-y: auto;
+    padding: 2rem 1rem;
+  }
+
+  .logo {
+    height: 44px;
+  }
+
+  .title {
+    font-size: 1.75rem;
+  }
+
+  .description {
+    font-size: 0.95rem;
+  }
+
+  .cards {
+    flex-direction: column;
+    gap: 1rem;
+    width: 100%;
+    max-width: 400px;
+  }
+
+  .card {
+    width: 100%;
+    padding: 1.25rem;
+  }
+}
diff --git a/packages/frontend/src/pages/ProjectInfo.module.css b/packages/frontend/src/pages/ProjectInfo.module.css
index a1b4201..ab24306 100644
--- a/packages/frontend/src/pages/ProjectInfo.module.css
+++ b/packages/frontend/src/pages/ProjectInfo.module.css
@@ -401,3 +401,37 @@
 .continueBtn:hover {
   background-color: var(--c-cta-hover);
 }
+
+/* ─── Phone ─────────────────────────────────────────────────────────────────── */
+
+@media (max-width: 700px) {
+  .form {
+    gap: 1.25rem;
+  }
+
+  /* "Upload JSON" + its hint stop sharing one line. */
+  .uploadRow {
+    flex-wrap: wrap;
+    gap: 0.5rem 0.75rem;
+  }
+
+  /* The 4.5rem label gutter leaves too little for the value, so the conflict
+     rows read as label-above-value instead. */
+  .conflictDetailRow {
+    flex-direction: column;
+    gap: 0.15rem;
+  }
+
+  .conflictDetailLabel {
+    width: auto;
+  }
+
+  .conflictBtns {
+    flex-wrap: wrap;
+  }
+
+  .continueBtn {
+    align-self: stretch;
+    width: 100%;
+  }
+}
diff --git a/packages/frontend/src/pages/Review.module.css b/packages/frontend/src/pages/Review.module.css
index b47562b..ea01727 100644
--- a/packages/frontend/src/pages/Review.module.css
+++ b/packages/frontend/src/pages/Review.module.css
@@ -246,3 +246,20 @@
   white-space: pre-wrap;
   word-break: break-word;
 }
+
+/* ─── Phone ─────────────────────────────────────────────────────────────────── */
+
+@media (max-width: 700px) {
+  /* Full-width so the primary action is an easy thumb target. */
+  .downloadBtn,
+  .saveJsonBtn,
+  .validateBtn {
+    align-self: stretch;
+    width: 100%;
+    text-align: center;
+  }
+
+  .validatorNote {
+    padding: 0.85rem 0.9rem;
+  }
+}
diff --git a/packages/frontend/src/pages/Variables.module.css b/packages/frontend/src/pages/Variables.module.css
index 562cffb..c83b990 100644
--- a/packages/frontend/src/pages/Variables.module.css
+++ b/packages/frontend/src/pages/Variables.module.css
@@ -374,3 +374,42 @@
 .continueBtn:hover {
   background-color: var(--c-cta-hover);
 }
+
+/* ─── Phone ─────────────────────────────────────────────────────────────────── */
+
+@media (max-width: 700px) {
+  /* Subtext and the "show system variables" switch stop competing for one line. */
+  .introRow {
+    flex-direction: column;
+    align-items: flex-start;
+    gap: 0.75rem;
+    margin-bottom: 1.25rem;
+  }
+
+  /* A long column name plus its badges no longer forces the row wider than the
+     screen — the name wraps and the badges keep their own line if they must. */
+  .rowHeader {
+    flex-wrap: wrap;
+    gap: 0.4rem 0.6rem;
+    padding: 0.6rem 0.75rem;
+  }
+
+  .rowName {
+    min-width: 0;
+    overflow-wrap: anywhere;
+  }
+
+  /* Description over type, rather than a 140px column squeezing both. */
+  .bodyMain {
+    grid-template-columns: 1fr;
+    gap: 0.85rem;
+  }
+
+  .rowBody {
+    padding: 0.6rem 0.75rem 0.85rem;
+  }
+
+  .continueBtn {
+    width: 100%;
+  }
+}
diff --git a/website/src/pages/contact.module.css b/website/src/pages/contact.module.css
index 6ae7590..a017e73 100644
--- a/website/src/pages/contact.module.css
+++ b/website/src/pages/contact.module.css
@@ -3,9 +3,18 @@
   align-items: center;
   justify-content: center;
   min-height: calc(100vh - var(--ifm-navbar-height) - 6rem);
+  /* dvh so the retractable URL bar on mobile doesn't push the page taller than
+     the screen; the vh line above is the fallback. */
+  min-height: calc(100dvh - var(--ifm-navbar-height) - 6rem);
   padding: 3rem 1.5rem;
 }
 
+@media (max-width: 700px) {
+  .wrap {
+    padding: 2rem 1.15rem;
+  }
+}
+
 .inner {
   max-width: 720px;
   text-align: left;
diff --git a/website/src/pages/index.module.css b/website/src/pages/index.module.css
index 6e3becb..f01b8d2 100644
--- a/website/src/pages/index.module.css
+++ b/website/src/pages/index.module.css
@@ -232,3 +232,55 @@
   font-size: 0.95rem;
   margin: 0;
 }
+
+/* --- Phone -------------------------------------------------------------- */
+
+/* The 900px step above already stacks the hero; this one is about the phone
+   itself — trimming the outer gutters and the display type, which still
+   overhangs a 375px screen at 2.4rem. */
+@media (max-width: 600px) {
+  .hero {
+    padding: 2.25rem 1.15rem 1.5rem;
+  }
+
+  .heroLogo {
+    height: 60px;
+    margin-bottom: 1.15rem;
+  }
+
+  .heroTitle {
+    font-size: 1.9rem;
+    letter-spacing: -0.02em;
+  }
+
+  .heroLede {
+    font-size: 1.05rem;
+  }
+
+  .pathCard {
+    padding: 1.15rem;
+  }
+
+  /* Side-by-side button + link would leave the button too narrow to read, so
+     they stack full-width — the link centres to stay aligned with the button
+     above it rather than hanging off its left edge. */
+  .pathCardActions {
+    align-items: stretch;
+    flex-direction: column;
+    gap: 0.85rem;
+  }
+
+  .pathLink {
+    text-align: center;
+  }
+
+  .features,
+  .showcase {
+    padding-left: 1.15rem;
+    padding-right: 1.15rem;
+  }
+
+  .showcaseCard {
+    padding: 1.25rem 1.15rem;
+  }
+}
diff --git a/website/src/pages/wizard.module.css b/website/src/pages/wizard.module.css
index 2f9e791..85b59de 100644
--- a/website/src/pages/wizard.module.css
+++ b/website/src/pages/wizard.module.css
@@ -6,5 +6,20 @@
      4.5rem is the footer's single-row height; it wraps taller on narrow screens,
      where a small scroll is fine. */
   height: calc(100vh - var(--ifm-navbar-height) - 4.5rem);
+  /* Mobile browsers count their retractable URL bar in 100vh, so the frame
+     would hang below the fold until the bar hides. dvh tracks the bar as it
+     moves; the vh line above stays as the fallback for browsers without it. */
+  height: calc(100dvh - var(--ifm-navbar-height) - 4.5rem);
   border: 0;
 }
+
+/* Phone: the footer wraps to three rows here, so the 4.5rem single-row
+   allowance is far too small — subtracting it would still leave the frame
+   overhanging. Give the wizard the whole viewport below the navbar and let the
+   footer live below the fold; the wizard scrolls internally either way. */
+@media (max-width: 700px) {
+  .wizardFrame {
+    height: calc(100vh - var(--ifm-navbar-height));
+    height: calc(100dvh - var(--ifm-navbar-height));
+  }
+}
diff --git a/website/src/theme/Footer/styles.module.css b/website/src/theme/Footer/styles.module.css
index 17da2c3..30686ce 100644
--- a/website/src/theme/Footer/styles.module.css
+++ b/website/src/theme/Footer/styles.module.css
@@ -64,3 +64,23 @@
   color: var(--ifm-color-primary);
   text-decoration: none;
 }
+
+/* Phone: the row already wraps, but a 1.75rem link gap plus space-between
+   scatters the wrapped items across the width. Centre everything into a tidy
+   stack and close the gaps up. */
+@media (max-width: 700px) {
+  .footer {
+    padding: 1.15rem 1.15rem;
+  }
+
+  .inner {
+    justify-content: center;
+    text-align: center;
+    gap: 0.9rem 1.25rem;
+  }
+
+  .links {
+    justify-content: center;
+    gap: 0.75rem 1.25rem;
+  }
+}