diff --git a/app/Http/Controllers/MicropubController.php b/app/Http/Controllers/MicropubController.php
index ded95242..1d26894f 100644
--- a/app/Http/Controllers/MicropubController.php
+++ b/app/Http/Controllers/MicropubController.php
@@ -12,9 +12,12 @@ use App\Models\Place;
 use App\Models\SyndicationTarget;
 use App\Services\Micropub\Data\MicropubData;
 use App\Services\Micropub\MicropubHandlerRegistry;
+use App\Services\Micropub\NoteResolver;
+use App\Services\Micropub\NoteSource;
 use Illuminate\Database\Eloquent\ModelNotFoundException;
 use Illuminate\Http\JsonResponse;
 use Illuminate\Http\Request;
+use Illuminate\Support\Arr;
 
 class MicropubController extends Controller
 {
@@ -46,8 +49,15 @@ class MicropubController extends Controller
         try {
             $handler = $this->handlerRegistry->getHandler($type);
             $dataClass = $handler->dataClass();
-            /** @var MicropubData $data */
-            $data = $dataClass::fromRequest($request);
+
+            try {
+                /** @var MicropubData $data */
+                $data = $dataClass::fromRequest($request);
+            } catch (\TypeError) {
+                // the data objects are typed, so this means a property has the wrong shape
+                return $this->invalidRequest('A property in the request has an invalid value');
+            }
+
             $result = $handler->handle($data);
 
             if ($result['response'] === 'updated') {
@@ -89,40 +99,36 @@ class MicropubController extends Controller
      * Respond to a GET request to the micropub endpoint.
      *
      * Token validation is handled by the VerifyMicropubToken middleware.
-     * Supports q=syndicate-to, q=config, and q=geo:<lat>,<lng> queries.
+     * Supports q=syndicate-to, q=config, q=source, and q=geo:<lat>,<lng> queries.
      * The default response returns the token metadata.
      */
     public function get(Request $request): JsonResponse
     {
-        if ($request->input('q') === 'syndicate-to') {
+        $q = $request->input('q');
+
+        if ($q === 'syndicate-to') {
             return response()->json([
                 'syndicate-to' => SyndicationTarget::all(),
             ]);
         }
 
-        if ($request->input('q') === 'config') {
+        if ($q === 'config') {
             return response()->json([
                 'syndicate-to' => SyndicationTarget::all(),
                 'media-endpoint' => route('media-endpoint'),
             ]);
         }
 
-        if ($request->has('q') && str_starts_with($request->input('q'), 'geo:')) {
-            preg_match_all(
-                '/([0-9.\-]+)/',
-                $request->input('q'),
-                $matches
-            );
-            $distance = (count($matches[0]) === 3) ? 100 * $matches[0][2] : 1000;
-            $places = Place::near(
-                (object) ['latitude' => $matches[0][0], 'longitude' => $matches[0][1]],
-                $distance
-            )->get();
+        if ($q === 'source') {
+            return $this->sourceResponse($request);
+        }
 
-            return response()->json([
-                'response' => 'places',
-                'places' => $places,
-            ]);
+        if (is_string($q) && str_starts_with($q, 'geo:')) {
+            return $this->placesResponse($q);
+        }
+
+        if ($request->has('q')) {
+            return $this->invalidRequest('This server does not know how to handle this q parameter');
         }
 
         // the default response is just to return the token data
@@ -138,4 +144,59 @@ class MicropubController extends Controller
             ],
         ]);
     }
+
+    /**
+     * Respond to a q=source query with the microformats2 JSON of a post.
+     */
+    private function sourceResponse(Request $request): JsonResponse
+    {
+        $url = $request->input('url');
+
+        if (! is_string($url)) {
+            return $this->invalidRequest('The url parameter is required');
+        }
+
+        try {
+            $note = resolve(NoteResolver::class)->fromUrl($url);
+        } catch (MicropubUnsupportedModelException $e) {
+            return $this->invalidRequest($e->getMessage());
+        } catch (ModelNotFoundException) {
+            return $this->invalidRequest('No known note with given ID', 404);
+        }
+
+        $requested = array_values(array_filter(Arr::wrap($request->input('properties')), 'is_string'));
+
+        return response()->json(resolve(NoteSource::class)->toArray($note, $requested));
+    }
+
+    /**
+     * Respond to a q=geo:<lat>,<lng> query with the nearby places.
+     */
+    private function placesResponse(string $q): JsonResponse
+    {
+        preg_match_all('/([0-9.\-]+)/', $q, $matches);
+
+        if (count($matches[0]) < 2 || count(array_filter($matches[0], 'is_numeric')) !== count($matches[0])) {
+            return $this->invalidRequest('The geo query must contain a latitude and longitude');
+        }
+
+        $distance = (count($matches[0]) === 3) ? 100 * $matches[0][2] : 1000;
+        $places = Place::near(
+            (object) ['latitude' => $matches[0][0], 'longitude' => $matches[0][1]],
+            $distance
+        )->get();
+
+        return response()->json([
+            'response' => 'places',
+            'places' => $places,
+        ]);
+    }
+
+    private function invalidRequest(string $description, int $status = 400): JsonResponse
+    {
+        return response()->json([
+            'error' => 'invalid_request',
+            'error_description' => $description,
+        ], $status);
+    }
 }
diff --git a/app/Services/Micropub/Handlers/UpdateHandler.php b/app/Services/Micropub/Handlers/UpdateHandler.php
index 136a0840..0840e5ee 100644
--- a/app/Services/Micropub/Handlers/UpdateHandler.php
+++ b/app/Services/Micropub/Handlers/UpdateHandler.php
@@ -11,6 +11,7 @@ use App\Models\Media;
 use App\Models\Note;
 use App\Services\Micropub\Data\MicropubData;
 use App\Services\Micropub\Data\UpdateData;
+use App\Services\Micropub\NoteResolver;
 use Illuminate\Support\Str;
 
 class UpdateHandler implements MicropubHandlerInterface
@@ -36,18 +37,18 @@ class UpdateHandler implements MicropubHandlerInterface
             throw new InvalidTokenScopeException;
         }
 
-        $urlPath = parse_url($data->updateUrl, PHP_URL_PATH);
-
-        if (mb_substr($urlPath, 1, 5) !== 'notes') {
-            throw new MicropubUnsupportedModelException('This implementation currently only supports the updating of notes');
+        if ($data->updateUrl === null) {
+            throw new MicropubHandlerException('The url parameter is required');
         }
 
-        $note = Note::nb60(basename($urlPath))->firstOrFail();
+        $note = resolve(NoteResolver::class)->fromUrl($data->updateUrl);
 
         if ($data->updateReplace === null && $data->updateAdd === null && $data->updateDelete === null) {
             throw new MicropubHandlerException('Unsupported update operation');
         }
 
+        $this->validateOperations($data);
+
         if ($data->updateReplace !== null) {
             foreach ($data->updateReplace as $property => $value) {
                 match ($property) {
@@ -89,6 +90,66 @@ class UpdateHandler implements MicropubHandlerInterface
         ];
     }
 
+    /**
+     * Check the whole request before applying any of it, so that a malformed
+     * value can't leave a note partially updated. The spec requires the values
+     * of each property to be arrays.
+     *
+     * @throws MicropubHandlerException
+     */
+    private function validateOperations(UpdateData $data): void
+    {
+        foreach ([$data->updateReplace ?? [], $data->updateAdd ?? []] as $operation) {
+            foreach ($operation as $property => $values) {
+                if (! is_string($property) || ! is_array($values) || $values === [] || ! array_is_list($values)) {
+                    throw new MicropubHandlerException('The values of each property must be a non-empty array');
+                }
+
+                $this->validateValues($property, $values);
+            }
+        }
+
+        $delete = $data->updateDelete ?? [];
+
+        if (array_is_list($delete)) {
+            foreach ($delete as $property) {
+                if (! is_string($property)) {
+                    throw new MicropubHandlerException('The properties to delete must be strings');
+                }
+            }
+
+            return;
+        }
+
+        foreach ($delete as $property => $values) {
+            if (! is_array($values) || ! array_is_list($values)) {
+                throw new MicropubHandlerException('The values of each property must be an array');
+            }
+
+            $this->validateValues($property, $values);
+        }
+    }
+
+    /**
+     * @throws MicropubHandlerException
+     */
+    private function validateValues(string $property, array $values): void
+    {
+        if (! in_array($property, ['content', 'syndication', 'photo'], true)) {
+            return;
+        }
+
+        foreach ($values as $value) {
+            if ($property === 'content' && is_array($value)) {
+                $value = $value['html'] ?? $value['value'] ?? null;
+            }
+
+            if (! is_string($value)) {
+                throw new MicropubHandlerException("The {$property} property has an invalid value");
+            }
+        }
+    }
+
     private function applyDelete(Note $note, array $delete): void
     {
         if (array_is_list($delete)) {
diff --git a/app/Services/Micropub/NoteResolver.php b/app/Services/Micropub/NoteResolver.php
new file mode 100644
index 00000000..26d8d39a
--- /dev/null
+++ b/app/Services/Micropub/NoteResolver.php
@@ -0,0 +1,43 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Services\Micropub;
+
+use App\Exceptions\MicropubUnsupportedModelException;
+use App\Models\Note;
+use Illuminate\Database\Eloquent\ModelNotFoundException;
+use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
+
+/**
+ * Finds the note a Micropub request refers to by its URL.
+ */
+class NoteResolver
+{
+    /**
+     * @throws MicropubUnsupportedModelException if the URL is not a note on this site
+     * @throws ModelNotFoundException if no such note exists
+     */
+    public function fromUrl(string $url): Note
+    {
+        $host = parse_url($url, PHP_URL_HOST);
+        $ourHost = parse_url((string) config('app.url'), PHP_URL_HOST);
+
+        if (! is_string($host) || ! is_string($ourHost) || strcasecmp($host, $ourHost) !== 0) {
+            throw new MicropubUnsupportedModelException('The URL is not a post on this site');
+        }
+
+        $path = (string) parse_url($url, PHP_URL_PATH);
+
+        if (preg_match('#^/notes/([0-9A-Za-z_]+)/?$#', $path, $matches) !== 1) {
+            throw new MicropubUnsupportedModelException('This implementation currently only supports notes');
+        }
+
+        try {
+            return Note::nb60($matches[1])->firstOrFail();
+        } catch (NotFoundHttpException) {
+            // Note::scopeNb60() aborts when the ID is too big for the database
+            throw (new ModelNotFoundException)->setModel(Note::class);
+        }
+    }
+}
diff --git a/app/Services/Micropub/NoteSource.php b/app/Services/Micropub/NoteSource.php
new file mode 100644
index 00000000..457953f4
--- /dev/null
+++ b/app/Services/Micropub/NoteSource.php
@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Services\Micropub;
+
+use App\Models\Note;
+
+/**
+ * Builds the microformats2 JSON for a q=source query.
+ */
+class NoteSource
+{
+    /**
+     * Per the spec, the type is only included when all properties are returned.
+     *
+     * @param  string[]  $requested  The properties to return, or empty for all of them
+     */
+    public function toArray(Note $note, array $requested = []): array
+    {
+        $properties = $this->properties($note);
+
+        if ($requested !== []) {
+            return ['properties' => (object) array_intersect_key($properties, array_flip($requested))];
+        }
+
+        return ['type' => ['h-entry'], 'properties' => (object) $properties];
+    }
+
+    private function properties(Note $note): array
+    {
+        $media = $note->media;
+
+        return array_filter([
+            // the raw source, as the note accessor returns the rendered HTML
+            'content' => array_filter([$note->getRawOriginal('note')], fn ($value) => $value !== null),
+            'published' => [$note->created_at->toIso8601String()],
+            'in-reply-to' => array_filter([$note->in_reply_to]),
+            'category' => $note->tags->pluck('tag')->all(),
+            'photo' => $media->where('type', 'image')->map(fn ($item) => $item->pivot->alt_text
+                ? ['value' => $item->url, 'alt' => $item->pivot->alt_text]
+                : $item->url)->values()->all(),
+            'video' => $media->where('type', 'video')->pluck('url')->all(),
+            'audio' => $media->where('type', 'audio')->pluck('url')->all(),
+            'syndication' => array_values(array_filter([
+                $note->tweet_id ? 'https://twitter.com/i/web/status/'.$note->tweet_id : null,
+                $note->facebook_url,
+                $note->swarm_url,
+                $note->instagram_url,
+                $note->mastodon_url,
+                $note->bluesky_url,
+            ])),
+            'location' => $this->location($note),
+        ], fn (array $values) => $values !== []);
+    }
+
+    private function location(Note $note): array
+    {
+        if ($note->place !== null) {
+            return [$note->place->uri];
+        }
+
+        if ($note->getRawOriginal('location') !== null) {
+            return ['geo:'.$note->latitude.','.$note->longitude];
+        }
+
+        return [];
+    }
+}
diff --git a/app/Services/NoteService.php b/app/Services/NoteService.php
index 2fbe45a2..87c4007f 100644
--- a/app/Services/NoteService.php
+++ b/app/Services/NoteService.php
@@ -30,6 +30,9 @@ class NoteService
             $content = null;
         }
 
+        // an invalid date throws, so parse it before anything is saved
+        $published = $this->getPublished($data);
+
         $note = Note::create(
             [
                 'note' => $content,
@@ -38,7 +41,7 @@ class NoteService
             ]
         );
 
-        if ($published = $this->getPublished($data)) {
+        if ($published !== null) {
             $note->created_at = $note->updated_at = $published;
         }
 
@@ -205,11 +208,11 @@ class NoteService
                 if (is_string($photo)) {
                     $photoUrl = $photo;
                 } elseif (is_array($photo)) {
-                    $photoUrl = $photo['value'];
-                    $photoAlt = $photo['alt'];
+                    $photoUrl = $photo['value'] ?? null;
+                    $photoAlt = is_string($photo['alt'] ?? null) ? $photo['alt'] : null;
                 }
 
-                if (empty($photoUrl)) {
+                if (! is_string($photoUrl) || $photoUrl === '') {
                     continue;
                 }
 
diff --git a/tests/Feature/MicropubControllerTest.php b/tests/Feature/MicropubControllerTest.php
index 7a350dc1..60c5ec96 100644
--- a/tests/Feature/MicropubControllerTest.php
+++ b/tests/Feature/MicropubControllerTest.php
@@ -1116,4 +1116,188 @@ class MicropubControllerTest extends TestCase
         $response->assertStatus(201);
         Queue::assertPushed(SyndicateNoteToMastodon::class);
     }
+
+    #[Test]
+    public function source_query_returns_the_raw_note_as_microformats2(): void
+    {
+        $note = Note::factory()->create([
+            'note' => 'Some *markdown* #indieweb',
+            'in_reply_to' => 'https://example.org/post',
+            'mastodon_url' => 'https://mastodon.example/@me/1',
+        ]);
+        $media = Media::factory()->create(['path' => 'media/alt.jpg', 'type' => 'image']);
+        $note->media()->attach($media->id, ['alt_text' => 'A description', 'order' => 0]);
+
+        $response = $this->getJson(
+            '/api/post?q=source&url='.urlencode($note->uri),
+            ['HTTP_Authorization' => 'Bearer '.$this->getToken()]
+        );
+
+        $response->assertOk()->assertExactJson([
+            'type' => ['h-entry'],
+            'properties' => [
+                'content' => ['Some *markdown* #indieweb'],
+                'published' => [$note->created_at->toIso8601String()],
+                'in-reply-to' => ['https://example.org/post'],
+                'category' => ['indieweb'],
+                'photo' => [['value' => $media->url, 'alt' => 'A description']],
+                'syndication' => ['https://mastodon.example/@me/1'],
+            ],
+        ]);
+    }
+
+    #[Test]
+    public function source_query_returns_the_location_of_a_note(): void
+    {
+        $headers = ['HTTP_Authorization' => 'Bearer '.$this->getToken()];
+        $geoNote = Note::factory()->create(['location' => '1.23, 4.56']);
+        $place = Place::factory()->create();
+        $placeNote = Note::factory()->create();
+        $placeNote->place()->associate($place)->save();
+
+        $this->getJson('/api/post?q=source&properties[]=location&url='.urlencode($geoNote->uri), $headers)
+            ->assertExactJson(['properties' => ['location' => ['geo:1.23,4.56']]]);
+        $this->getJson('/api/post?q=source&properties[]=location&url='.urlencode($placeNote->uri), $headers)
+            ->assertExactJson(['properties' => ['location' => [$place->uri]]]);
+    }
+
+    #[Test]
+    public function source_query_only_returns_the_requested_properties(): void
+    {
+        $note = Note::factory()->create(['note' => 'Hello']);
+
+        $response = $this->getJson(
+            '/api/post?q=source&properties[]=content&url='.urlencode($note->uri),
+            ['HTTP_Authorization' => 'Bearer '.$this->getToken()]
+        );
+
+        $response->assertOk()->assertExactJson(['properties' => ['content' => ['Hello']]]);
+    }
+
+    #[Test]
+    public function source_query_with_no_matching_properties_returns_an_empty_object(): void
+    {
+        $note = Note::factory()->create();
+
+        $response = $this->getJson(
+            '/api/post?q=source&properties[]=nonsense&url='.urlencode($note->uri),
+            ['HTTP_Authorization' => 'Bearer '.$this->getToken()]
+        );
+
+        $this->assertSame('{"properties":{}}', $response->assertOk()->getContent());
+    }
+
+    #[Test]
+    public function source_query_reports_invalid_urls(): void
+    {
+        $headers = ['HTTP_Authorization' => 'Bearer '.$this->getToken()];
+        $note = Note::factory()->create();
+
+        $this->getJson('/api/post?q=source', $headers)
+            ->assertStatus(400)->assertJson(['error' => 'invalid_request']);
+        $this->getJson('/api/post?q=source&url='.urlencode('https://other.example/notes/'.$note->nb60id), $headers)
+            ->assertStatus(400)->assertJson(['error' => 'invalid_request']);
+        $this->getJson('/api/post?q=source&url='.urlencode(config('app.url').'/blog/A'), $headers)
+            ->assertStatus(400)->assertJson(['error' => 'invalid_request']);
+        $this->getJson('/api/post?q=source&url='.urlencode(config('app.url').'/notes/ZZZZ'), $headers)
+            ->assertStatus(404)->assertJson(['error' => 'invalid_request']);
+        $this->getJson('/api/post?q=source&url='.urlencode(config('app.url').'/notes/zzzzzzzzzz'), $headers)
+            ->assertStatus(404)->assertJson(['error' => 'invalid_request']);
+    }
+
+    #[Test]
+    public function unknown_and_malformed_queries_return_an_error(): void
+    {
+        $headers = ['HTTP_Authorization' => 'Bearer '.$this->getToken()];
+
+        foreach (['q=nonsense', 'q[]=config', 'q=geo:junk', 'q=geo:1.2.3,4'] as $query) {
+            $this->getJson('/api/post?'.$query, $headers)
+                ->assertStatus(400)->assertJson(['error' => 'invalid_request']);
+        }
+    }
+
+    #[Test]
+    public function update_with_a_malformed_value_is_rejected_without_changing_the_note(): void
+    {
+        $headers = ['HTTP_Authorization' => 'Bearer '.$this->getToken()];
+        $note = Note::factory()->create(['note' => 'original']);
+
+        $operations = [
+            ['replace' => ['content' => 'hello']],
+            ['replace' => ['content' => []]],
+            ['replace' => ['content' => [123]]],
+            ['replace' => ['content' => ['replaced'], 'syndication' => 'https://www.swarmapp.com/1']],
+            ['replace' => ['content']],
+            ['add' => ['syndication' => [['nested']]]],
+            ['delete' => [['content']]],
+            ['delete' => ['syndication' => 'https://www.swarmapp.com/1']],
+        ];
+
+        foreach ($operations as $operation) {
+            $this->postJson('/api/post', ['action' => 'update', 'url' => $note->uri, ...$operation], $headers)
+                ->assertStatus(400)->assertJson(['error' => 'invalid_request']);
+        }
+
+        $this->assertSame('original', $note->refresh()->getRawOriginal('note'));
+    }
+
+    #[Test]
+    public function update_with_an_unusable_url_is_rejected(): void
+    {
+        $headers = ['HTTP_Authorization' => 'Bearer '.$this->getToken()];
+        $note = Note::factory()->create(['note' => 'original']);
+        $replace = ['replace' => ['content' => ['changed']]];
+
+        $this->postJson('/api/post', ['action' => 'update', ...$replace], $headers)->assertStatus(400);
+        $this->postJson('/api/post', ['action' => 'update', 'url' => ['a'], ...$replace], $headers)->assertStatus(400);
+        $this->postJson('/api/post', ['action' => 'update', 'url' => 'https://other.example/notes/'.$note->nb60id, ...$replace], $headers)->assertStatus(400);
+        $this->postJson('/api/post', ['action' => 'update', 'url' => config('app.url').'/notes/zzzzzzzzzz', ...$replace], $headers)->assertStatus(404);
+
+        $this->assertSame('original', $note->refresh()->getRawOriginal('note'));
+    }
+
+    #[Test]
+    public function creating_with_a_wrongly_shaped_property_returns_an_error(): void
+    {
+        $headers = ['HTTP_Authorization' => 'Bearer '.$this->getToken()];
+        $noteCount = Note::count();
+
+        $this->postJson('/api/post', ['type' => ['h-entry'], 'properties' => ['content' => ['x'], 'mp-syndicate-to' => 'https://a.example']], $headers)
+            ->assertStatus(400)->assertJson(['error' => 'invalid_request']);
+        $this->postJson('/api/post', ['type' => ['h-entry'], 'properties' => ['content' => [123]]], $headers)
+            ->assertStatus(400)->assertJson(['error' => 'invalid_request']);
+        $this->post('/api/post', ['h' => 'entry', 'content' => ['a', 'b']], $headers)
+            ->assertStatus(400)->assertJson(['error' => 'invalid_request']);
+
+        $this->assertSame($noteCount, Note::count());
+    }
+
+    #[Test]
+    public function creating_with_an_invalid_published_date_does_not_leave_a_note_behind(): void
+    {
+        Queue::fake();
+        $noteCount = Note::count();
+
+        $this->postJson(
+            '/api/post',
+            ['type' => ['h-entry'], 'properties' => ['content' => ['x'], 'published' => ['not a date']]],
+            ['HTTP_Authorization' => 'Bearer '.$this->getToken()]
+        )->assertStatus(400)->assertJson(['error' => 'invalid_request']);
+
+        $this->assertSame($noteCount, Note::count());
+    }
+
+    #[Test]
+    public function creating_with_a_photo_object_without_alt_text_succeeds(): void
+    {
+        Queue::fake();
+
+        $this->postJson(
+            '/api/post',
+            ['type' => ['h-entry'], 'properties' => ['content' => ['x'], 'photo' => [['value' => 'https://example.org/no-alt.jpg']]]],
+            ['HTTP_Authorization' => 'Bearer '.$this->getToken()]
+        )->assertStatus(201);
+
+        $this->assertDatabaseHas('media_endpoint', ['path' => 'https://example.org/no-alt.jpg']);
+    }
 }