diff --git a/crates/chatd/src/adapters/tiny.rs b/crates/chatd/src/adapters/tiny.rs
index 71f6092a..b64faa5a 100644
--- a/crates/chatd/src/adapters/tiny.rs
+++ b/crates/chatd/src/adapters/tiny.rs
@@ -48,6 +48,11 @@ pub fn prove_legacy_saved_speakers() -> Result<(), String> {
     messages::prove_legacy_saved_speakers()
 }
 
+/// The lab's keycap-reaction proof. See `messages::prove_keycap_reaction`.
+pub fn prove_keycap_reaction() -> Result<(), String> {
+    messages::prove_keycap_reaction()
+}
+
 /// The lab's cache-open-without-device proof. See `messages::prove_cache_open_without_device`.
 pub fn prove_cache_open_without_device() -> Result<(), String> {
     messages::prove_cache_open_without_device()
diff --git a/crates/chatd/src/adapters/tiny/messages.rs b/crates/chatd/src/adapters/tiny/messages.rs
index 83ae665f..48333909 100644
--- a/crates/chatd/src/adapters/tiny/messages.rs
+++ b/crates/chatd/src/adapters/tiny/messages.rs
@@ -881,6 +881,21 @@ pub(super) fn prove_sealed_preview() -> Result<(), String> {
     }
 }
 
+/// The lab's keycap-reaction proof. A keycap is taken as a reaction, and a bare digit still is not.
+pub(super) fn prove_keycap_reaction() -> Result<(), String> {
+    for keycap in ["1\u{FE0F}\u{20E3}", "0\u{20E3}", "#\u{FE0F}\u{20E3}"] {
+        if !unicode_emoji(keycap) {
+            return Err(format!("the keycap {keycap:?} was refused as a reaction"));
+        }
+    }
+    for not_one in ["1", "10\u{FE0F}\u{20E3}", "1\u{FE0F}", ":one:"] {
+        if unicode_emoji(not_one) {
+            return Err(format!("{not_one:?} was taken as a reaction"));
+        }
+    }
+    Ok(())
+}
+
 /// The lab's sealed-preview-attachments proof. A wiped sqlite row still restores a captionless
 /// image and a captioned attachment from the authenticated sealed cache. The lab sets
 /// `TINYCLIENT_PROVE=sealed-preview-attachments`.
@@ -1887,14 +1902,17 @@ fn media_kind(kind: &str, mime: &str) -> MediaKind {
     }
 }
 
-/// Unicode emoji only. A shortcode, a word, or a path is not one.
+/// Unicode emoji only. A shortcode, a word, or a path is not one. A keycap is one, though it
+/// starts with an ASCII digit (1️⃣ is `1`, U+FE0F, U+20E3), and tinyserver takes it.
 fn unicode_emoji(emoji: &str) -> bool {
     let mut textual = false;
     if emoji.is_empty() {
         return false;
     }
-    for ch in emoji.chars() {
-        if ch.is_ascii_alphanumeric() || ch == ':' || ch == '/' {
+    for (at, ch) in emoji.char_indices() {
+        let after = &emoji[at + ch.len_utf8()..];
+        let keycap = ch.is_ascii_digit() && (after.starts_with('\u{20E3}') || after.starts_with("\u{FE0F}\u{20E3}"));
+        if !keycap && (ch.is_ascii_alphanumeric() || ch == ':' || ch == '/') {
             return false;
         }
         if !ch.is_ascii() {
@@ -2881,6 +2899,20 @@ mod tests {
         assert!(!unicode_emoji(""));
     }
 
+    /// A keycap reaction (1️⃣) was refused when sent and dropped when another app had put one on
+    /// a message, because it starts with an ASCII digit.
+    #[test]
+    fn a_keycap_is_a_reaction_and_a_bare_digit_is_not() {
+        assert!(unicode_emoji("1\u{FE0F}\u{20E3}"));
+        assert!(unicode_emoji("7\u{20E3}"), "a keycap without its variation selector");
+        assert!(unicode_emoji("*\u{FE0F}\u{20E3}"));
+        assert!(!unicode_emoji("1"));
+        assert!(!unicode_emoji("1\u{FE0F}"), "a digit and a variation selector are not a keycap");
+        assert!(!unicode_emoji("10\u{FE0F}\u{20E3}"), "only the digit right before the keycap mark is part of it");
+        assert!(!unicode_emoji("a\u{20E3}"));
+        assert!(super::prove_keycap_reaction().is_ok());
+    }
+
     #[test]
     fn a_new_direct_event_records_the_peer_on_either_side_before_the_first_message() {
         for (low, high) in [(ME, PEER), (PEER, ME)] {
diff --git a/crates/chatd/src/main.rs b/crates/chatd/src/main.rs
index 7bc3e40c..68d52c6e 100644
--- a/crates/chatd/src/main.rs
+++ b/crates/chatd/src/main.rs
@@ -236,6 +236,15 @@ fn main() -> io::Result<()> {
         eprintln!("prove-pending-message-identity:0");
         return Ok(());
     }
+    // The lab's keycap-reaction driver. A build without this returns to the server below and stays up.
+    if std::env::var("TINYCLIENT_PROVE").ok().as_deref() == Some("keycap-reaction") {
+        if let Err(err) = chatd::adapters::tiny::prove_keycap_reaction() {
+            eprintln!("keycap-reaction: {err}");
+            std::process::exit(1);
+        }
+        eprintln!("prove-exit:0");
+        return Ok(());
+    }
     let _log = protocol::log_policy::install("chatd", protocol::log_policy::LogPolicy::from_env());
     protocol::crash::install_panic_hook("chatd");
     let dir = protocol::support_dir();
diff --git a/e2e/scenarios/keycap-reaction/drive.linux.sh b/e2e/scenarios/keycap-reaction/drive.linux.sh
new file mode 100644
index 00000000..60e0c91d
--- /dev/null
+++ b/e2e/scenarios/keycap-reaction/drive.linux.sh
@@ -0,0 +1,10 @@
+#!/bin/bash
+# keycap-reaction on Linux. The lab runs this with bash while ffmpeg films the display.
+set -u
+HERE=$(cd "$(dirname "$0")" && pwd)
+if command -v python3 >/dev/null 2>&1; then
+    exec python3 "$HERE/drive.py"
+fi
+echo "python3 is not on PATH" >&2
+exit 1
+
diff --git a/e2e/scenarios/keycap-reaction/drive.macos.sh b/e2e/scenarios/keycap-reaction/drive.macos.sh
new file mode 100644
index 00000000..1982bc1e
--- /dev/null
+++ b/e2e/scenarios/keycap-reaction/drive.macos.sh
@@ -0,0 +1,10 @@
+#!/bin/bash
+# keycap-reaction on macOS. The lab runs this with bash while the helper films the app window.
+set -u
+HERE=$(cd "$(dirname "$0")" && pwd)
+if command -v python3 >/dev/null 2>&1; then
+    exec python3 "$HERE/drive.py"
+fi
+echo "python3 is not on PATH" >&2
+exit 1
+
diff --git a/e2e/scenarios/keycap-reaction/drive.py b/e2e/scenarios/keycap-reaction/drive.py
new file mode 100644
index 00000000..7e216716
--- /dev/null
+++ b/e2e/scenarios/keycap-reaction/drive.py
@@ -0,0 +1,86 @@
+#!/usr/bin/env python3
+"""A keycap emoji such as 1️⃣ is taken as a Tiny reaction.
+
+Starts this build's chatd with TINYCLIENT_PROVE=keycap-reaction. That build checks the reaction
+filter the Tiny adapter applies to sent and received reactions, and exits 0 only when keycaps pass
+and a bare digit does not. A build from before the fix ignores the variable and keeps serving, so
+this driver fails there.
+
+Env from the lab: E2E_OUT, E2E_T0, E2E_APP_DIR.
+"""
+
+import os
+import subprocess
+import sys
+import time
+
+OUT = os.environ.get("E2E_OUT", "")
+T0 = float(os.environ.get("E2E_T0") or time.time())
+APP = os.environ.get("E2E_APP_DIR", "")
+
+FAILED = False
+STEPS = ""
+
+
+def now():
+    return time.time() - T0
+
+
+def step(words, ok):
+    global FAILED
+    if not ok:
+        FAILED = True
+    line = '{"t": %s, "step": "%s", "ok": %s}' % (round(now(), 1), words.replace('"', "'"), "true" if ok else "false")
+    print(("[ok] " if ok else "[FAILED] ") + words, flush=True)
+    if STEPS:
+        with open(STEPS, "a", encoding="utf-8") as handle:
+            handle.write(line + "\n")
+
+
+def chatd_bin(app_dir):
+    if sys.platform == "win32":
+        return os.path.join(app_dir, "chatd.exe")
+    if sys.platform == "darwin" and app_dir.endswith(".app"):
+        return os.path.join(app_dir, "Contents", "MacOS", "chatd")
+    return os.path.join(app_dir, "chatd")
+
+
+def main():
+    global STEPS
+    if not OUT or not APP:
+        print("E2E_OUT and E2E_APP_DIR are required", file=sys.stderr)
+        return 1
+    os.makedirs(OUT, exist_ok=True)
+    STEPS = os.path.join(OUT, "steps.jsonl")
+    open(STEPS, "w", encoding="utf-8").close()
+    binary = chatd_bin(APP)
+    if not os.path.isfile(binary):
+        step("chatd is in E2E_APP_DIR", False)
+        return 1
+
+    data = os.path.join(OUT, "data")
+    os.makedirs(data, exist_ok=True)
+    env = os.environ.copy()
+    env["TINYCLIENT_DIR"] = data
+    env["TINYCLIENT_PROVE"] = "keycap-reaction"
+    env["TINYCLIENT_LOG_LEVEL"] = "info"
+    kwargs = {"cwd": data, "env": env, "stdin": subprocess.DEVNULL}
+    if sys.platform == "win32":
+        kwargs["creationflags"] = 0x08000000
+    with open(os.path.join(OUT, "chatd.out.log"), "w", encoding="utf-8") as out, open(os.path.join(OUT, "chatd.err.log"), "w", encoding="utf-8") as err:
+        kwargs["stdout"] = out
+        kwargs["stderr"] = err
+        proc = subprocess.Popen([binary], **kwargs)
+        try:
+            code = proc.wait(timeout=45)
+        except subprocess.TimeoutExpired:
+            proc.kill()
+            proc.wait(timeout=5)
+            code = None
+    step("a keycap emoji is taken as a reaction", code == 0)
+    return 1 if FAILED else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
+
diff --git a/e2e/scenarios/keycap-reaction/drive.windows.ps1 b/e2e/scenarios/keycap-reaction/drive.windows.ps1
new file mode 100644
index 00000000..188364c9
--- /dev/null
+++ b/e2e/scenarios/keycap-reaction/drive.windows.ps1
@@ -0,0 +1,18 @@
+# keycap-reaction on Windows. The lab runs this in the console session it is already filming.
+$ErrorActionPreference = 'Stop'
+$drive = Join-Path $PSScriptRoot 'drive.py'
+if (Get-Command py -ErrorAction SilentlyContinue) {
+    & py -3 $drive
+    exit $LASTEXITCODE
+}
+if (Get-Command python -ErrorAction SilentlyContinue) {
+    & python $drive
+    exit $LASTEXITCODE
+}
+if (Get-Command python3 -ErrorAction SilentlyContinue) {
+    & python3 $drive
+    exit $LASTEXITCODE
+}
+Write-Host 'python is not on PATH (tried py -3, python, python3)'
+exit 1
+
diff --git a/e2e/scenarios/keycap-reaction/scenario.json b/e2e/scenarios/keycap-reaction/scenario.json
new file mode 100644
index 00000000..23689879
--- /dev/null
+++ b/e2e/scenarios/keycap-reaction/scenario.json
@@ -0,0 +1,12 @@
+{
+  "id": "keycap-reaction",
+  "title": "A keycap emoji such as 1️⃣ is taken as a Tiny reaction",
+  "platforms": ["windows", "linux", "macos"],
+  "not_applicable": {
+    "android": "The phone proof build does not run desktop chatd. The same reaction filter is what this desktop run checks.",
+    "ios": "The phone proof build does not run desktop chatd. The same reaction filter is what this desktop run checks."
+  },
+  "needs": [],
+  "release": false,
+  "max_still_seconds": 60
+}