diff --git a/src/Builders/PromptBuilder.php b/src/Builders/PromptBuilder.php
index 538392db..b4b189c3 100644
--- a/src/Builders/PromptBuilder.php
+++ b/src/Builders/PromptBuilder.php
@@ -613,7 +613,12 @@ public function isSupported(?CapabilityEnum $capability = null): bool
 
             // If still no capability, infer from output modalities
             if ($capability === null) {
-                $capability = $this->inferCapabilityFromOutputModalities();
+                try {
+                    $capability = $this->inferCapabilityFromOutputModalities();
+                } catch (RuntimeException $e) {
+                    // The output modality maps to no capability, so no model can support it.
+                    return false;
+                }
             }
         }
 
diff --git a/tests/unit/Builders/PromptBuilderTest.php b/tests/unit/Builders/PromptBuilderTest.php
index 23a58959..c3b5b899 100644
--- a/tests/unit/Builders/PromptBuilderTest.php
+++ b/tests/unit/Builders/PromptBuilderTest.php
@@ -3864,6 +3864,19 @@ public function testIsSupportedWithInferredCapability(): void
         $this->assertTrue($builder->isSupported());
     }
 
+    /**
+     * Tests isSupported returns false for an output modality without a matching capability.
+     *
+     * @return void
+     */
+    public function testIsSupportedReturnsFalseForUnsupportedOutputModality(): void
+    {
+        $builder = new PromptBuilder($this->registry, 'Test prompt');
+        $builder->asOutputModalities(ModalityEnum::document());
+
+        $this->assertFalse($builder->isSupported());
+    }
+
     /**
      * Tests isSupported method with inferred capability from model interfaces.
      *