diff --git a/README.md b/README.md
index b8133ff..2389a60 100644
--- a/README.md
+++ b/README.md
@@ -96,8 +96,11 @@ What it was asked matters, because "can this be run" has two halves here.
 Twenty-two of the twenty-nine have no `main` at all. Six of the remaining
 seven have one and want the filesystem, which a page does not have, so `needs`
 records the capabilities they asked for. That leaves exactly one example this
-page can start, and Run is off for the other twenty-eight with the reason
-beside it, rather than letting somebody press it and be refused.
+page can start through `main`. In the picker that example says
+`Run program`; a library or a program needing filesystem capabilities
+says `Run tests` instead and names why its `main` cannot
+start in the tab. Every shown example therefore has a real primary action
+rather than a disabled Run button.
 
 The picker shows fourteen of the twenty-nine, and that list is in
 `tools/artifact.mjs` because both tools need it. The corpus is not a menu:
diff --git a/assets/play.js b/assets/play.js
index c465c60..78ae509 100644
--- a/assets/play.js
+++ b/assets/play.js
@@ -29,6 +29,7 @@ const GUTTER = document.getElementById("gutter");
 const CONTROLS = document.querySelector(".controls");
 const CONSOLE = document.querySelector(".console");
 const VERBS = Array.from(document.querySelectorAll("[data-verb]"));
+const EXAMPLE_ACTION = document.getElementById("example-action");
 
 const encoder = new TextEncoder();
 const decoder = new TextDecoder();
@@ -123,11 +124,10 @@ function running(yes) {
   }
 }
 
-// Most of the corpus cannot be started here, for two different reasons, and
-// pressing Run to find that out is a bad way to be told. Twenty-one are
-// libraries with no `main` to enter through. Six more have one and want the
-// filesystem, which this page does not have, so they would answer with a list
-// of capabilities rather than with anything about the program.
+// Most of the corpus cannot start a `main` here, for two different reasons.
+// Some are libraries with no entry point; others have one and want filesystem
+// capabilities this page deliberately does not hold. They do have tests, so
+// the primary action runs those instead of becoming a disabled dead end.
 //
 // `examples/index.json` records both answers from the pinned artifact, so the
 // button is off before it is reached for, and the note says which of the two
@@ -143,6 +143,36 @@ function thisIsRunnable(yes, why = "") {
   if (ready) running(false);
 }
 
+function configureExampleAction(entry) {
+  if (!entry) {
+    EXAMPLE_ACTION.dataset.verb = "deed_run";
+    EXAMPLE_ACTION.textContent = "Run program";
+    thisIsRunnable(true);
+    return;
+  }
+
+  if (entry.runs && entry.needs.length === 0) {
+    EXAMPLE_ACTION.dataset.verb = "deed_run";
+    EXAMPLE_ACTION.textContent = "Run program";
+    thisIsRunnable(true, "Program example. Its `main` runs in this tab.");
+    return;
+  }
+
+  if (entry.tests > 0) {
+    EXAMPLE_ACTION.dataset.verb = "deed_test";
+    EXAMPLE_ACTION.textContent = "Run tests";
+    const reason = entry.runs
+      ? `Its \`main\` needs ${entry.needs.join(", ")}, which this page cannot provide.`
+      : "Library example: there is no `main` to start.";
+    thisIsRunnable(true, `${reason} The button runs its tests in this tab.`);
+    return;
+  }
+
+  EXAMPLE_ACTION.dataset.verb = "deed_run";
+  EXAMPLE_ACTION.textContent = "Cannot run here";
+  thisIsRunnable(false, whyNot(entry));
+}
+
 function lines(text) {
   return text.split("\n").filter((line) => line.trim() !== "");
 }
@@ -492,7 +522,7 @@ const check = () => run("deed_check").then(paint);
 SOURCE.addEventListener("input", () => {
   // Whatever was known about the example is now known about a different
   // program.
-  if (!runnable) thisIsRunnable(true);
+  configureExampleAction(null);
   drawGutter(SOURCE.value);
   schedule(paint, 150);
   schedule(check, 500);
@@ -651,7 +681,7 @@ async function loadFromLink() {
 // tests, and the summary under the picker is the comment at the top of the
 // file rather than a description written here.
 //
-// The picker shows twelve of the twenty-eight. The rest are the corpus doing
+// The picker shows fourteen of the twenty-nine. The rest are the corpus doing
 // its other job, one language feature at a time so the compiler's tests have
 // something to read, and a menu of those is a menu of somebody else's test
 // suite. `shown` in the index carries the choice and its order; every file is
@@ -678,7 +708,10 @@ async function loadExamples() {
     .sort((a, b) => a.shown - b.shown)) {
     const option = document.createElement("option");
     option.value = entry.file;
-    option.textContent = entry.file.replace(/\.deed$/, "");
+    const name = entry.file.replace(/\.deed$/, "");
+    option.textContent = entry.runs && entry.needs.length === 0
+      ? `${name} · runs here`
+      : `${name} · tests`;
     EXAMPLE.append(option);
   }
   EXAMPLE.disabled = false;
@@ -686,7 +719,7 @@ async function loadExamples() {
   EXAMPLE.addEventListener("change", async () => {
     const file = EXAMPLE.value;
     if (!file) {
-      thisIsRunnable(true);
+      configureExampleAction(null);
       return;
     }
     const entry = summaries.get(file);
@@ -696,8 +729,8 @@ async function loadExamples() {
     const response = await fetch(`../examples/${encodeURIComponent(file)}`);
     SOURCE.value = await response.text();
     marked = new Map();
-    thisIsRunnable(entry ? entry.runs && entry.needs.length === 0 : true, whyNot(entry));
-    paint();
+    configureExampleAction(entry);
+    schedule(paint, 150);
   });
 }
 
diff --git a/assets/style.css b/assets/style.css
index e5e978b..ddcf08d 100644
--- a/assets/style.css
+++ b/assets/style.css
@@ -999,6 +999,65 @@ button {
   min-width: calc(19ch + 1.6rem + 2px);
 }
 
+#example-action {
+  min-width: calc(16ch + 1.6rem + 2px);
+}
+
+@media (max-width: 42rem) {
+  body.fits header.site h1 {
+    font-size: 1.55rem;
+  }
+
+  body.fits header.site h2,
+  body.fits main > h1,
+  body.fits main > .lede {
+    display: none;
+  }
+
+  body.fits nav.site {
+    margin-bottom: 0.5rem;
+  }
+
+  body.fits .status {
+    margin-bottom: 0.5rem;
+  }
+
+  body.fits .controls {
+    display: grid;
+    gap: 0.5rem;
+    width: 100%;
+    min-width: 0;
+  }
+
+  body.fits .picker {
+    display: grid;
+    grid-template-columns: auto minmax(0, 1fr);
+    width: 100%;
+    min-width: 0;
+    justify-content: stretch;
+  }
+
+  body.fits .picker select {
+    width: 100%;
+    min-width: 0;
+    max-width: 100%;
+  }
+
+  body.fits .verbs {
+    display: grid;
+    grid-template-columns: repeat(2, minmax(0, 1fr));
+    width: 100%;
+    min-width: 0;
+  }
+
+  body.fits .verbs button,
+  body.fits #share,
+  body.fits #example-action {
+    width: 100%;
+    min-width: 0;
+  }
+}
+
 button:hover:not(:disabled),
 select:hover:not(:disabled) {
   border-color: var(--black);
diff --git a/play/index.html b/play/index.html
index 03cb41a..1596f02 100644
--- a/play/index.html
+++ b/play/index.html
@@ -73,7 +73,7 @@ <h1>Playground</h1>
 
           <div class="verbs">
             <button data-verb="deed_check" disabled>Check</button>
-            <button data-verb="deed_run" disabled>Run</button>
+            <button id="example-action" data-verb="deed_run" disabled>Run program</button>
             <button data-verb="deed_test" disabled>Test</button>
             <button data-verb="deed_fmt" disabled>Format</button>
             <button id="stop" type="button" disabled>Stop</button>
diff --git a/tools/check.mjs b/tools/check.mjs
index c48cf16..cac3d64 100644
--- a/tools/check.mjs
+++ b/tools/check.mjs
@@ -301,7 +301,7 @@ if (deed) {
   }
 }
 
-// The picker's dozen. `shown` is the position in `SHOWN` and -1 for the rest,
+// The picker's fourteen. `shown` is the position in `SHOWN` and -1 for the rest,
 // so a file leaving the corpus takes its place in the picker with it and says
 // so here rather than shortening the menu quietly.
 for (const file of SHOWN) {
@@ -318,6 +318,24 @@ for (const entry of index.examples) {
   }
 }
 
+// A selected example always gets one honest primary action. A program with no
+// external capability runs its main; everything else shown in the picker must
+// have tests for the dynamic `Run tests` action. Without this, a newly shown
+// library with no tests silently brings back the disabled Run dead end.
+const shownEntries = index.examples.filter((entry) => entry.shown >= 0);
+if (!shownEntries.some((entry) => entry.runs && entry.needs.length === 0)) {
+  complain("examples/index.json", "the picker has no program whose main runs in the browser");
+}
+for (const entry of shownEntries) {
+  const mainRunsHere = entry.runs && entry.needs.length === 0;
+  if (!mainRunsHere && entry.tests < 1) {
+    complain(
+      "examples/index.json",
+      `${entry.file} is shown but has neither a browser-runnable main nor tests`,
+    );
+  }
+}
+
 // This site names one release, and the pin is which one. Everything else that
 // says a version is prose: the filenames on the install page, the `deed 0.2.2`
 // under each `--version`, and the version a share link carries.