diff --git a/src/Mcp/Tools/Routers/EntriesRouter.php b/src/Mcp/Tools/Routers/EntriesRouter.php
index d92d251..710c293 100644
--- a/src/Mcp/Tools/Routers/EntriesRouter.php
+++ b/src/Mcp/Tools/Routers/EntriesRouter.php
@@ -455,6 +455,36 @@ private function updateEntry(array $arguments): array
                 unset($data['published']);
             }
 
+            // Handle slug separately *before* blueprint validation so the
+            // FieldsValidator never sees the slug column. Letting the merged
+            // payload include the slug forces UniqueEntryValue to compare the
+            // current slug against the entry being updated and reject it as
+            // "already taken" — even when the caller never asked to change
+            // the slug. Mirrors the createEntry() flow but passes the
+            // entry's id as the $except argument so the rule excludes the
+            // current entry from the uniqueness check.
+            if (array_key_exists('slug', $data)) {
+                $newSlug = is_string($data['slug']) ? $data['slug'] : '';
+                $slugValidator = Validator::make(['slug' => $newSlug], [
+                    'slug' => [
+                        'required',
+                        'string',
+                        new UniqueEntryValue($entry->collectionHandle(), $entry->id(), $site),
+                    ],
+                ]);
+
+                if ($slugValidator->fails()) {
+                    $errors = $slugValidator->errors()->get('slug');
+                    /** @var array<string> $flatErrors */
+                    $flatErrors = array_map(fn ($error) => is_string($error) ? $error : implode(', ', (array) $error), $errors);
+
+                    return $this->createErrorResponse('Slug validation failed: ' . implode(', ', $flatErrors))->toArray();
+                }
+
+                $entry->slug($newSlug);
+                unset($data['slug']);
+            }
+
             // For dated collections, parse the date and set it on the entry.
             // Keep a normalized copy in data so the FieldsValidator sees the required field.
             if (array_key_exists('date', $data) && $entry->collection()->dated()) {
@@ -487,9 +517,14 @@ private function updateEntry(array $arguments): array
                 // TypeError (common with third-party fieldtypes like SEO Pro
                 // whose preProcessValidatable can't handle stored data formats),
                 // we fall back to validating only the incoming fields.
+                // NOTE: do NOT inject slug into mergedData. Any slug change
+                // was already applied to the entry object above; the
+                // FieldsValidator does not need the slug column and including
+                // it forces UniqueEntryValue to reject the current entry's
+                // own slug. See the slug-handling block earlier in this
+                // method (issue #27).
                 /** @var array<string, mixed> $mergedData */
                 $mergedData = array_merge($entry->data()->all(), $data);
-                $mergedData['slug'] = $entry->slug();
                 $mergedData = $this->sanitizeStoredFieldDataForValidation($blueprint, $mergedData);
 
                 $validationContext = [
diff --git a/tests/Feature/Routers/EntriesRouterTest.php b/tests/Feature/Routers/EntriesRouterTest.php
index 64de092..8884333 100644
--- a/tests/Feature/Routers/EntriesRouterTest.php
+++ b/tests/Feature/Routers/EntriesRouterTest.php
@@ -184,6 +184,143 @@ public function test_create_entry_without_slug_generates_from_title(): void
         $this->assertEquals('auto-slug-generation-test', $result['data']['entry']['slug']);
     }
 
+    /**
+     * Regression for https://github.com/cboxdk/statamic-mcp/issues/27.
+     *
+     * The previous updateEntry() flow injected the entry's current
+     * slug into the FieldsValidator payload, which made
+     * UniqueEntryValue compare the slug against the entry being
+     * updated and reject it as "already taken" — even when the caller
+     * never passed a slug at all.
+     */
+    public function test_update_entry_without_slug_in_data_succeeds(): void
+    {
+        $entry = Entry::make()
+            ->collection($this->collectionHandle)
+            ->slug("update-no-slug-{$this->testId}")
+            ->data(['title' => 'Original Title']);
+        $entry->save();
+
+        $result = $this->router->execute([
+            'action' => 'update',
+            'collection' => $this->collectionHandle,
+            'id' => $entry->id(),
+            'data' => [
+                'title' => 'Updated Title',
+            ],
+        ]);
+
+        $this->assertTrue(
+            $result['success'],
+            'updating an entry without changing the slug must succeed; got: '
+            . json_encode($result['errors'] ?? []),
+        );
+
+        $reloaded = Entry::find($entry->id());
+        $this->assertSame('Updated Title', $reloaded->get('title'));
+        $this->assertSame("update-no-slug-{$this->testId}", $reloaded->slug());
+    }
+
+    /**
+     * Resending the entry's *current* slug as part of `data` is
+     * idempotent and must not trigger UniqueEntryValue against the
+     * entry itself.
+     */
+    public function test_update_entry_with_unchanged_slug_succeeds(): void
+    {
+        $slug = "update-same-slug-{$this->testId}";
+        $entry = Entry::make()
+            ->collection($this->collectionHandle)
+            ->slug($slug)
+            ->data(['title' => 'Original']);
+        $entry->save();
+
+        $result = $this->router->execute([
+            'action' => 'update',
+            'collection' => $this->collectionHandle,
+            'id' => $entry->id(),
+            'data' => [
+                'title' => 'Updated',
+                'slug' => $slug,
+            ],
+        ]);
+
+        $this->assertTrue(
+            $result['success'],
+            'resending the existing slug must be idempotent; got: '
+            . json_encode($result['errors'] ?? []),
+        );
+
+        $reloaded = Entry::find($entry->id());
+        $this->assertSame($slug, $reloaded->slug());
+        $this->assertSame('Updated', $reloaded->get('title'));
+    }
+
+    /**
+     * Renaming an entry to a slug that does not collide with any
+     * other entry in the collection succeeds.
+     */
+    public function test_update_entry_with_new_unique_slug_succeeds(): void
+    {
+        $entry = Entry::make()
+            ->collection($this->collectionHandle)
+            ->slug("update-rename-from-{$this->testId}")
+            ->data(['title' => 'Will Be Renamed']);
+        $entry->save();
+
+        $newSlug = "update-rename-to-{$this->testId}";
+        $result = $this->router->execute([
+            'action' => 'update',
+            'collection' => $this->collectionHandle,
+            'id' => $entry->id(),
+            'data' => [
+                'slug' => $newSlug,
+            ],
+        ]);
+
+        $this->assertTrue(
+            $result['success'],
+            'updating to a unique new slug must succeed; got: '
+            . json_encode($result['errors'] ?? []),
+        );
+
+        $reloaded = Entry::find($entry->id());
+        $this->assertSame($newSlug, $reloaded->slug());
+    }
+
+    /**
+     * Renaming an entry to a slug that another entry already owns
+     * must surface the validation error.
+     */
+    public function test_update_entry_to_existing_slug_returns_error(): void
+    {
+        $other = Entry::make()
+            ->collection($this->collectionHandle)
+            ->slug("update-collision-{$this->testId}")
+            ->data(['title' => 'Other']);
+        $other->save();
+
+        $entry = Entry::make()
+            ->collection($this->collectionHandle)
+            ->slug("update-target-{$this->testId}")
+            ->data(['title' => 'Target']);
+        $entry->save();
+
+        $result = $this->router->execute([
+            'action' => 'update',
+            'collection' => $this->collectionHandle,
+            'id' => $entry->id(),
+            'data' => [
+                'slug' => $other->slug(),
+            ],
+        ]);
+
+        $this->assertFalse(
+            $result['success'],
+            'updating to a slug already owned by another entry must fail',
+        );
+    }
+
     public function test_update_nonexistent_entry_returns_error(): void
     {
         $result = $this->router->execute([