diff --git a/CHANGELOG.md b/CHANGELOG.md
index e77f0621..e2caf2e3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * [BC Break] Remove the `providerClass` argument of `#[CompletionProvider]`. Use `provider:`, which takes the same class-string and is now the first positional argument.
 * Add `HttpTransport::getSessionId()` to read the server-minted `Mcp-Session-Id`: a request-scoped caller can persist it and pass it back through the constructor's `$headers` on a later transport. Always `null` on `2026-07-28`, which removed protocol-level sessions.
 * Fix OIDC discovery rejecting issuers with a trailing slash (e.g. Authentik, Auth0).
+* [BC Break] Validate a tool result's `structuredContent` against the tool's `outputSchema`, which the specification requires the server to honour. A mismatch is answered with a `CallToolResult` carrying `isError: true` instead of the non-conforming value, matching the TypeScript, Python and Java SDKs. Skipped when the tool declares no `outputSchema`, when the result carries no `structuredContent`, and when the result is already an error.
 
 0.8.0
 -----
diff --git a/docs/servers/tools.md b/docs/servers/tools.md
index a919b401..258039f3 100644
--- a/docs/servers/tools.md
+++ b/docs/servers/tools.md
@@ -200,6 +200,32 @@ Either way the data reaches the client: a return value with no structured repres
 A tool that wants to branch on the revision itself can read it from the injected `RequestContext`, see
 [Talking back to the client](../handlers/client-communication.md#clientgateway).
 
+#### Output validation
+
+The specification states that a server **must** produce structured results that conform to the declared schema. The SDK
+holds you to it: when a tool declares an `outputSchema` and the result carries `structuredContent`, the SDK validates
+that value against the schema before sending it. A mismatch becomes a tool error result, so the model reads the reason
+and can retry:
+
+```json
+{
+  "content": [
+    {
+      "type": "text",
+      "text": "Invalid structured output for tool 'get_weather': Missing required properties: `temperature`."
+    }
+  ],
+  "isError": true
+}
+```
+
+The check applies to a `CallToolResult` you build yourself as well as to one the SDK wraps for you. It is skipped in
+three cases:
+
+- The tool declares no `outputSchema`.
+- The result carries no `structuredContent`, which is the warning case described above.
+- The result is already marked `isError: true`, because its content is a failure message and not the declared output.
+
 [sep-2106]: https://modelcontextprotocol.io/specification/2026-07-28/server/tools#structured-content
 
 ### Error Handling
diff --git a/src/Server/Handler/Request/CallToolHandler.php b/src/Server/Handler/Request/CallToolHandler.php
index 72f3c21f..eb4ccaaf 100644
--- a/src/Server/Handler/Request/CallToolHandler.php
+++ b/src/Server/Handler/Request/CallToolHandler.php
@@ -24,6 +24,7 @@
 use Mcp\Schema\Request\CallToolRequest;
 use Mcp\Schema\Result\CallToolResult;
 use Mcp\Schema\Result\InputRequiredResult;
+use Mcp\Schema\Tool;
 use Mcp\Server\RequestContext;
 use Mcp\Server\Session\SessionInterface;
 use Psr\Log\LoggerInterface;
@@ -81,18 +82,7 @@ public function handle(Request $request, SessionInterface $session): Response|Er
         $inputSchema = $reference->tool->inputSchema;
         $validationErrors = $this->schemaValidator->validateAgainstJsonSchema($arguments, $inputSchema);
         if (!empty($validationErrors)) {
-            $errorMessages = [];
-
-            foreach ($validationErrors as $errorDetail) {
-                $pointer = $errorDetail['pointer'] ?? '';
-                $message = $errorDetail['message'] ?? 'Unknown validation error';
-                $errorMessages[] = ('/' !== $pointer && '' !== $pointer ? "Property '{$pointer}': " : '').$message;
-            }
-
-            $summaryMessage = "Invalid parameters for tool '{$toolName}': ".implode('; ', \array_slice($errorMessages, 0, 3));
-            if (\count($errorMessages) > 3) {
-                $summaryMessage .= '; ...and more errors.';
-            }
+            $summaryMessage = "Invalid parameters for tool '{$toolName}': ".self::summarizeValidationErrors($validationErrors);
 
             return Error::forInvalidParams($summaryMessage, $request->getId(), ['validation_errors' => $validationErrors]);
         }
@@ -146,7 +136,7 @@ public function handle(Request $request, SessionInterface $session): Response|Er
                 'structured_content' => $structuredContent,
             ]);
 
-            return new Response($request->getId(), $result);
+            return new Response($request->getId(), $this->validateStructuredContent($reference->tool, $result) ?? $result);
         } catch (MissingRequiredClientCapabilityException $e) {
             // Not a tool failure — the request was unservable, and the client
             // needs to retry declaring the capability. Rendered as -32021.
@@ -171,6 +161,59 @@ public function handle(Request $request, SessionInterface $session): Response|Er
         }
     }
 
+    /**
+     * A tool declaring an `outputSchema` promises every `structuredContent` it sends
+     * conforms to it, in every revision. A mismatch is the server's own bug, but it
+     * is reported as a tool execution error rather than a protocol error so that the
+     * model sees it and can fall back to `content`.
+     *
+     * @return CallToolResult|null the error result to send instead, or null when there is nothing to report
+     */
+    private function validateStructuredContent(Tool $tool, CallToolResult $result): ?CallToolResult
+    {
+        // An error result carries a failure message, not the tool's declared output.
+        // A null `structuredContent` is absent from the wire, and the caller has
+        // already warned about it.
+        if (null === $tool->outputSchema || $result->isError || null === $result->structuredContent) {
+            return null;
+        }
+
+        $validationErrors = $this->schemaValidator->validateAgainstJsonSchema($result->structuredContent, $tool->outputSchema);
+        if ([] === $validationErrors) {
+            return null;
+        }
+
+        $summaryMessage = "Invalid structured output for tool '{$tool->name}': ".self::summarizeValidationErrors($validationErrors);
+
+        $this->logger->error($summaryMessage, [
+            'name' => $tool->name,
+            'validation_errors' => $validationErrors,
+        ]);
+
+        return CallToolResult::error([new TextContent($summaryMessage)]);
+    }
+
+    /**
+     * @param list<array{pointer: string, keyword: string, message: string}> $validationErrors
+     */
+    private static function summarizeValidationErrors(array $validationErrors): string
+    {
+        $errorMessages = [];
+
+        foreach ($validationErrors as $errorDetail) {
+            $pointer = $errorDetail['pointer'] ?? '';
+            $message = $errorDetail['message'] ?? 'Unknown validation error';
+            $errorMessages[] = ('/' !== $pointer && '' !== $pointer ? "Property '{$pointer}': " : '').$message;
+        }
+
+        $summary = implode('; ', \array_slice($errorMessages, 0, 3));
+        if (\count($errorMessages) > 3) {
+            $summary .= '; ...and more errors.';
+        }
+
+        return $summary;
+    }
+
     /**
      * Whether a `structuredContent` value encodes as a JSON object — the only shape
      * revisions predating SEP-2106 accept.
diff --git a/tests/Unit/Server/Handler/Request/CallToolHandlerTest.php b/tests/Unit/Server/Handler/Request/CallToolHandlerTest.php
index 308c9809..d1a457d5 100644
--- a/tests/Unit/Server/Handler/Request/CallToolHandlerTest.php
+++ b/tests/Unit/Server/Handler/Request/CallToolHandlerTest.php
@@ -31,6 +31,15 @@
 
 class CallToolHandlerTest extends TestCase
 {
+    private const WEATHER_OUTPUT_SCHEMA = [
+        'type' => 'object',
+        'properties' => [
+            'temperature' => ['type' => 'number'],
+            'conditions' => ['type' => 'string'],
+        ],
+        'required' => ['temperature', 'conditions'],
+    ];
+
     private CallToolHandler $handler;
     private RegistryInterface&MockObject $registry;
     private ReferenceHandlerInterface&MockObject $referenceHandler;
@@ -699,6 +708,116 @@ public function testValidationError(): void
         $this->assertEquals(Error::INVALID_PARAMS, $response->code);
     }
 
+    public function testStructuredContentMissingARequiredPropertyIsReportedAsAToolError(): void
+    {
+        $request = $this->createCallToolRequest('get_weather', []);
+        $toolReference = $this->createToolReference('get_weather', static fn () => ['conditions' => 'sunny'], self::WEATHER_OUTPUT_SCHEMA);
+
+        $this->registry->method('getTool')->willReturn($toolReference);
+        $this->referenceHandler->method('handle')->willReturn(['conditions' => 'sunny']);
+        $toolReference->method('formatResult')->willReturn([new TextContent('{"conditions":"sunny"}')]);
+
+        $response = $this->handler->handle($request, $this->session);
+
+        $this->assertInstanceOf(Response::class, $response);
+        $this->assertTrue($response->result->isError);
+        $this->assertNull($response->result->structuredContent);
+        $this->assertStringContainsString("Invalid structured output for tool 'get_weather'", $this->firstText($response->result));
+        $this->assertStringContainsString('temperature', $this->firstText($response->result));
+    }
+
+    public function testStructuredContentOfTheWrongTypeIsReportedAsAToolError(): void
+    {
+        $structuredContent = ['temperature' => 'warm', 'conditions' => 'sunny'];
+        $request = $this->createCallToolRequest('get_weather', []);
+        $toolReference = $this->createToolReference('get_weather', static fn () => $structuredContent, self::WEATHER_OUTPUT_SCHEMA);
+
+        $this->registry->method('getTool')->willReturn($toolReference);
+        $this->referenceHandler->method('handle')->willReturn($structuredContent);
+        $toolReference->method('formatResult')->willReturn([new TextContent('{"temperature":"warm","conditions":"sunny"}')]);
+
+        $response = $this->handler->handle($request, $this->session);
+
+        $this->assertInstanceOf(Response::class, $response);
+        $this->assertTrue($response->result->isError);
+        $this->assertStringContainsString("Invalid structured output for tool 'get_weather'", $this->firstText($response->result));
+    }
+
+    public function testConformingStructuredContentIsSentUnchanged(): void
+    {
+        $structuredContent = ['temperature' => 22.5, 'conditions' => 'sunny'];
+        $request = $this->createCallToolRequest('get_weather', []);
+        $toolReference = $this->createToolReference('get_weather', static fn () => $structuredContent, self::WEATHER_OUTPUT_SCHEMA);
+
+        $this->registry->method('getTool')->willReturn($toolReference);
+        $this->referenceHandler->method('handle')->willReturn($structuredContent);
+        $toolReference->method('formatResult')->willReturn([new TextContent('{"temperature":22.5,"conditions":"sunny"}')]);
+
+        $response = $this->handler->handle($request, $this->session);
+
+        $this->assertInstanceOf(Response::class, $response);
+        $this->assertFalse($response->result->isError);
+        $this->assertSame($structuredContent, $response->result->structuredContent);
+    }
+
+    public function testStructuredContentIsNotValidatedWithoutAnOutputSchema(): void
+    {
+        $structuredContent = ['conditions' => 'sunny'];
+        $request = $this->createCallToolRequest('get_weather', []);
+        $toolReference = $this->createToolReference('get_weather', static fn () => $structuredContent);
+
+        $this->registry->method('getTool')->willReturn($toolReference);
+        $this->referenceHandler->method('handle')->willReturn($structuredContent);
+        $toolReference->method('formatResult')->willReturn([new TextContent('{"conditions":"sunny"}')]);
+
+        $response = $this->handler->handle($request, $this->session);
+
+        $this->assertInstanceOf(Response::class, $response);
+        $this->assertFalse($response->result->isError);
+        $this->assertSame($structuredContent, $response->result->structuredContent);
+    }
+
+    public function testSelfBuiltResultIsValidatedAgainstTheOutputSchema(): void
+    {
+        $request = $this->createCallToolRequest('get_weather', []);
+        $toolReference = $this->createToolReference('get_weather', static fn () => null, self::WEATHER_OUTPUT_SCHEMA);
+        $callToolResult = new CallToolResult([new TextContent('Built by hand')], false, ['conditions' => 'sunny']);
+
+        $this->registry->method('getTool')->willReturn($toolReference);
+        $this->referenceHandler->method('handle')->willReturn($callToolResult);
+
+        $response = $this->handler->handle($request, $this->session);
+
+        $this->assertInstanceOf(Response::class, $response);
+        $this->assertNotSame($callToolResult, $response->result);
+        $this->assertTrue($response->result->isError);
+        $this->assertStringContainsString("Invalid structured output for tool 'get_weather'", $this->firstText($response->result));
+    }
+
+    public function testErrorResultIsNotValidatedAgainstTheOutputSchema(): void
+    {
+        $request = $this->createCallToolRequest('get_weather', []);
+        $toolReference = $this->createToolReference('get_weather', static fn () => null, self::WEATHER_OUTPUT_SCHEMA);
+        $callToolResult = new CallToolResult([new TextContent('The weather service is down.')], true, ['reason' => 'timeout']);
+
+        $this->registry->method('getTool')->willReturn($toolReference);
+        $this->referenceHandler->method('handle')->willReturn($callToolResult);
+
+        $response = $this->handler->handle($request, $this->session);
+
+        $this->assertInstanceOf(Response::class, $response);
+        $this->assertSame($callToolResult, $response->result);
+        $this->assertSame('The weather service is down.', $this->firstText($response->result));
+    }
+
+    private function firstText(CallToolResult $result): string
+    {
+        $content = $result->content[0];
+        $this->assertInstanceOf(TextContent::class, $content);
+
+        return $content->text;
+    }
+
     /**
      * @param array<string, mixed> $arguments
      */