diff --git a/KNOWLEDGE_BASE.md b/KNOWLEDGE_BASE.md
index 9a0f5fd9f..385288ced 100644
--- a/KNOWLEDGE_BASE.md
+++ b/KNOWLEDGE_BASE.md
@@ -437,6 +437,7 @@ A signed request sees what the web shows you, such as your own pending commits.
 
 - read your account, logs, replies, notifications, and saved commits;
 - commit, reply, and quote, with images, and delete your commits;
+- preview commit drafts without publishing them;
 - follow or unfollow, mute or unmute, and block or unblock, and list who you muted and blocked;
 - star or unstar, and save or unsave;
 - create polls on commits that are neither replies nor quotes, and vote on polls;
diff --git a/app/Http/Controllers/Api/CommitPreviewController.php b/app/Http/Controllers/Api/CommitPreviewController.php
new file mode 100644
index 000000000..bc74dddb4
--- /dev/null
+++ b/app/Http/Controllers/Api/CommitPreviewController.php
@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Controllers\Api;
+
+use App\Http\Requests\CreateApiCommitPreviewRequest;
+use App\Services\CommitBodies;
+use App\Services\EmojiShortcodes;
+use App\Services\Mentions;
+use Illuminate\Http\JsonResponse;
+
+final readonly class CommitPreviewController
+{
+    /**
+     * Shows a draft as the commit it would become, as the web's composer previews it: the body as it would be kept, the
+     * segments the app draws it from, the excerpt a list would fold it to, and where it mentions someone. Nothing is
+     * kept, so previews don't count towards the commits a user may make a minute.
+     */
+    public function store(CreateApiCommitPreviewRequest $request, EmojiShortcodes $emojiShortcodes, CommitBodies $commitBodies, Mentions $mentions): JsonResponse
+    {
+        $body = $emojiShortcodes->replace($request->string('body')->value());
+
+        return response()->json(['data' => [
+            'body' => $body,
+            ...$commitBodies->segments($body),
+            'mentions' => $mentions->positions($body),
+        ]]);
+    }
+}
diff --git a/app/Http/Requests/CreateApiCommitPreviewRequest.php b/app/Http/Requests/CreateApiCommitPreviewRequest.php
new file mode 100644
index 000000000..849c11962
--- /dev/null
+++ b/app/Http/Requests/CreateApiCommitPreviewRequest.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Requests;
+
+use Illuminate\Contracts\Validation\ValidationRule;
+use Illuminate\Foundation\Http\FormRequest;
+
+final class CreateApiCommitPreviewRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * Blank lines are collapsed first, as they are when the draft is committed.
+     */
+    public function prepareForValidation(): void
+    {
+        CreateCommitRequest::normalizeBody($this);
+    }
+
+    /**
+     * @return array<string, array<int, string|ValidationRule>>
+     */
+    public function rules(): array
+    {
+        return [
+            'body' => CreateCommitRequest::bodyRules(),
+        ];
+    }
+}
diff --git a/routes/api.php b/routes/api.php
index 48652d8c2..78e9d0f13 100644
--- a/routes/api.php
+++ b/routes/api.php
@@ -6,6 +6,7 @@
 use App\Http\Controllers\Api\CommitController as ApiCommitController;
 use App\Http\Controllers\Api\CommitPastelController;
 use App\Http\Controllers\Api\CommitPollVoteController;
+use App\Http\Controllers\Api\CommitPreviewController;
 use App\Http\Controllers\Api\CommitReplyController;
 use App\Http\Controllers\Api\CommitReportController;
 use App\Http\Controllers\Api\CommitStarController;
@@ -121,6 +122,9 @@
             ->name('api-commit.store');
         Route::delete('commits/{commit}', [ApiCommitController::class, 'destroy'])->name('api-commit.destroy');
 
+        // Commit Preview: a draft as the commit it would become, which keeps nothing...
+        Route::post('commits/preview', [CommitPreviewController::class, 'store'])->name('api-commit-preview.store');
+
         // Commit Reply...
         Route::get('commits/{commit}/replies', [CommitReplyController::class, 'index'])
             ->withTrashed()
diff --git a/tests/Feature/Controllers/Api/CommitPreviewControllerTest.php b/tests/Feature/Controllers/Api/CommitPreviewControllerTest.php
new file mode 100644
index 000000000..79d38ec4e
--- /dev/null
+++ b/tests/Feature/Controllers/Api/CommitPreviewControllerTest.php
@@ -0,0 +1,88 @@
+<?php
+
+declare(strict_types=1);
+
+use App\Jobs\UnfurlLink;
+use App\Models\Commit;
+use App\Models\User;
+use App\Rules\CommitBodyLength;
+use App\Services\Urls;
+use Illuminate\Support\Facades\Queue;
+
+beforeEach(function (): void {
+    $this->user = User::factory()->create();
+    $this->keyPair = apiKeyPairFor($this->user);
+});
+
+it('previews a draft the way the commit it becomes is shown', function (string $body): void {
+    $preview = signedApiRequest($this->keyPair, 'POST', route('api-commit-preview.store'), ['body' => $body]);
+
+    $commit = signedApiRequest($this->keyPair, 'POST', route('api-commit.store'), ['body' => $body])->json('data');
+
+    $preview->assertOk()
+        ->assertExactJson(['data' => [
+            'body' => $commit['body'],
+            'body_segments' => $commit['body_segments'],
+            'preview_segments' => $commit['preview_segments'],
+            'mentions' => $commit['mentions'],
+        ]]);
+})->with([
+    'code, mentions and emoji' => "  shipped :rocket: with @nunomaduro\n```php\n\$name = 'devnull';\n```\nrun `composer test` :fire:  ",
+    'runs of blank lines' => "Shipped it.\n\n\n\nFinally.",
+    'too long to show whole in a list' => implode("\n", array_fill(0, 12, 'One more line of the changelog.')),
+]);
+
+it('previews publishable text longer than 4000 raw characters when it contains links', function (): void {
+    Queue::fake([UnfurlLink::class]);
+    $body = implode(' ', array_fill(0, 3, mb_str_pad('https://example.com/', 1400, 'a')));
+
+    signedApiRequest($this->keyPair, 'POST', route('api-commit-preview.store'), ['body' => $body])
+        ->assertOk()
+        ->assertJsonPath('data.body', $body);
+
+    expect(Commit::query()->exists())->toBeFalse();
+    Queue::assertNothingPushed();
+
+    signedApiRequest($this->keyPair, 'POST', route('api-commit.store'), ['body' => $body])
+        ->assertCreated()
+        ->assertJsonPath('data.body', $body);
+});
+
+it('folds a long draft as a list would, and leaves a short one whole', function (): void {
+    signedApiRequest($this->keyPair, 'POST', route('api-commit-preview.store'), ['body' => str_repeat('word ', 200)])
+        ->assertOk()
+        ->assertJsonPath('data.preview_segments.0.type', 'text');
+
+    signedApiRequest($this->keyPair, 'POST', route('api-commit-preview.store'), ['body' => 'It was DNS.'])
+        ->assertOk()
+        ->assertJsonPath('data.body_segments', [['type' => 'text', 'text' => 'It was DNS.']])
+        ->assertJsonPath('data.preview_segments', null)
+        ->assertJsonPath('data.mentions', []);
+});
+
+it('previews drafts without committing them or spending the commit budget', function (): void {
+    foreach (range(1, 11) as $draft) {
+        signedApiRequest($this->keyPair, 'POST', route('api-commit-preview.store'), ['body' => "Draft {$draft}"])->assertOk();
+    }
+
+    expect(Commit::query()->exists())->toBeFalse();
+
+    signedApiRequest($this->keyPair, 'POST', route('api-commit.store'), ['body' => 'Shipped it.'])->assertCreated();
+});
+
+it('requires a body the composer could commit', function (array $data, string $message): void {
+    signedApiRequest($this->keyPair, 'POST', route('api-commit-preview.store'), $data)
+        ->assertUnprocessable()
+        ->assertJsonValidationErrors(['body' => $message]);
+})->with([
+    'missing' => [[], 'The body field is required.'],
+    'blank' => [['body' => "  \n  "], 'The body field is required.'],
+    'too long' => [['body' => str_repeat('a', Commit::MAX_BODY_LENGTH + 1)], 'The body field must not be greater than '.Commit::MAX_BODY_LENGTH.' characters.'],
+    'oversized link' => [['body' => mb_str_pad('https://example.com/', Urls::MAX_LENGTH + 1, 'a')], 'Each link must not be greater than '.Urls::MAX_LENGTH.' characters.'],
+    'raw body too long' => [['body' => implode(' ', array_fill(0, 20, mb_str_pad('https://example.com/', Urls::MAX_LENGTH, 'a')))], 'The body field must not be greater than '.CommitBodyLength::MAX_RAW_LENGTH.' characters.'],
+]);
+
+it('requires a signed request', function (): void {
+    $this->postJson(route('api-commit-preview.store'), ['body' => 'It was DNS.'])
+        ->assertUnauthorized();
+});