diff --git a/tools/deno/api-diff.ts b/tools/deno/api-diff.ts
new file mode 100755
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--- /dev/null
+++ b/tools/deno/api-diff.ts
@@ -0,0 +1,122 @@
+#! /usr/bin/env -S deno run --allow-run --allow-net --allow-read --allow-write=/tmp --allow-env
+
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * Copyright Oxide Computer Company
+ */
+import * as flags from 'https://deno.land/std@0.208.0/flags/mod.ts'
+import { exists } from 'https://deno.land/std@0.208.0/fs/mod.ts'
+import { $, CommandBuilder } from 'https://deno.land/x/dax@0.35.0/mod.ts'
+
+const HELP = `
+Display changes to API client caused by a given Omicron PR. Works by downloading
+the OpenAPI spec before and after, generating clients in temp dirs, and diffing.
+
+Requirements:
+  - Deno (which you have if you're seeing this message)
+  - GitHub CLI (gh)
+
+Usage:
+  ./tools/deno/api-diff.ts [-f] [PR number]
+  ./tools/deno/api-diff.ts -h
+
+Flags:
+  -f, --force          Download spec and regen client even if dir already exists
+  -h, --help           Show this help message
+
+Parameters:
+  PR number <int>: If left out, interactive picker is shown
+`.trim()
+
+// inspired by: https://github.com/dsherret/dax/issues/137#issuecomment-1603848769
+declare module 'https://deno.land/x/dax@0.35.0/mod.ts' {
+  interface CommandBuilder {
+    pipe(next: CommandBuilder): CommandBuilder
+  }
+}
+
+CommandBuilder.prototype.pipe = function (next: CommandBuilder): CommandBuilder {
+  const p = this.stdout('piped').spawn()
+  return next.stdin(p.stdout())
+}
+
+// have to do this this way because I couldn't figure out how to get
+// my stupid bash function to show up here. I'm sure it's possible
+async function pickPr() {
+  const listPRs = () =>
+    $`gh pr list -R oxidecomputer/omicron --limit 100 
+        --json number,title,updatedAt,author 
+        --template '{{range .}}{{tablerow .number .title .author.name (timeago .updatedAt)}}{{end}}'`
+  const picker = () => $`fzf --height 25% --reverse`
+  const cut = () => $`cut -f1 -d ' '`
+
+  const prNum = await listPRs().pipe(picker()).pipe(cut()).text()
+  if (!/^\d+$/.test(prNum)) {
+    console.error(`Error picking PR. Expected number, got '${prNum}'`)
+    Deno.exit()
+  }
+  return parseInt(prNum, 10)
+}
+
+async function getPrRange(prNum: number) {
+  const query = `{
+      repository(owner: "oxidecomputer", name: "omicron") {
+        pullRequest(number: ${prNum}) {
+          baseRefOid
+          headRefOid
+        }
+      }
+    }`
+  const pr = await $`gh api graphql -f query=${query}`.json()
+  const { baseRefOid: base, headRefOid: head } = pr.data.repository.pullRequest
+  return { base, head } as { base: string; head: string }
+}
+
+async function genForCommit(commit: string, force: boolean) {
+  const tmpDir = `/tmp/api-diff/${commit}`
+  const alreadyExists = await exists(tmpDir)
+
+  // if the directory already exists, skip it
+  if (force || !alreadyExists) {
+    await $`rm -rf ${tmpDir}`
+    await $`mkdir -p ${tmpDir}`
+    await $`npm run --silent --prefix ../oxide.ts gen-from ${commit} ${tmpDir}`
+    await $`npx prettier --write --log-level error ${tmpDir}`
+  }
+
+  return tmpDir
+}
+
+//////////////////////////////
+// ACTUAL SCRIPT FOLLOWS
+//////////////////////////////
+
+if (!$.commandExistsSync('gh')) throw Error('Need gh (GitHub CLI)')
+
+const args = flags.parse(Deno.args, {
+  alias: { force: ['f'], h: 'help' },
+  boolean: ['force', 'help'],
+})
+
+if (args.help) {
+  console.log(HELP)
+  Deno.exit()
+}
+
+const prNum = args._[0] ? args._[0] : await pickPr()
+
+if (typeof prNum !== 'number') {
+  console.error(`PR number must be a number. Got '${prNum}' instead.`)
+  Deno.exit()
+}
+
+const { base, head } = await getPrRange(prNum)
+
+const tmpDirBase = await genForCommit(base, args.force)
+const tmpDirHead = await genForCommit(head, args.force)
+
+// git difftool is a trick to diff with whatever you have git set to use
+await $`git --no-pager difftool ${tmpDirBase}/Api.ts ${tmpDirHead}/Api.ts || true`