fix(server): align ProtocolError re-throw with spec error classification

.changeset/protocol-error-rethrow.md

@@ -0,0 +1,9 @@
+---
+'@modelcontextprotocol/server': minor
+---
+
+Re-throw all `ProtocolError` instances from `tools/call` handler as JSON-RPC errors instead of wrapping them in `isError: true` results.
+
+**Breaking change:** Output validation failures (missing or schema-mismatched `structuredContent`) now surface as JSON-RPC `InternalError` rejections instead of `{ isError: true }` results. Input validation failures continue to return `{ isError: true }` per the MCP spec's tool-execution-error classification.
+
+This also means tool handlers that deliberately `throw new ProtocolError(...)` will now propagate that as a JSON-RPC error, matching the python-sdk behavior.

docs/migration-SKILL.md

@@ -163,6 +163,20 @@ if (error instanceof SdkError && error.code === SdkErrorCode.ClientHttpFailedToO
 }
 ```
 
+### Tool error classification (McpServer tools/call)
+
+`McpServer` now re-throws any `ProtocolError` from the `tools/call` handler as a JSON-RPC error. Previously only `UrlElicitationRequired` was re-thrown; other protocol errors were wrapped as `{ isError: true }` results.
+
+Behavior changes in `callTool` results:
+
+- Input validation failure: `{ isError: true }` → `{ isError: true }` (unchanged)
+- Output validation failure: `{ isError: true }` → throws `ProtocolError` (`InternalError`)
+- Task-required without task: `{ isError: true }` → throws `ProtocolError` (`InvalidParams`)
+- Handler throws `ProtocolError`: `{ isError: true }` → re-thrown as JSON-RPC error
+- Handler throws plain `Error`: `{ isError: true }` → `{ isError: true }` (unchanged)
+
+Migration: if code checks `result.isError` to detect output-schema violations or deliberate `ProtocolError` throws, add a `try/catch` around `callTool`. If a handler throws `ProtocolError` expecting tool-level wrapping, change it to throw a plain `Error`.
+
 ### OAuth error consolidation
 
 Individual OAuth error classes replaced with single `OAuthError` class and `OAuthErrorCode` enum:

docs/migration.md

@@ -607,6 +607,44 @@ try {
 }
 ```
 
+#### Tool error classification
+
+The `tools/call` handler in `McpServer` now re-throws any `ProtocolError` as a JSON-RPC error instead of wrapping it in an `isError: true` result. Previously, only `UrlElicitationRequired` was re-thrown.
+
+This aligns error surfaces with the MCP spec's classification:
+
+- **Input validation failure** — unchanged, still returns `{ isError: true }` (spec classifies this as a tool-execution error)
+- **Output validation failure** — now throws `ProtocolError` with `InternalError` code (was `{ isError: true }`)
+- **Task-required tool called without task** — now throws `ProtocolError` with `InvalidParams` code (was `{ isError: true }`)
+- **Handler throws `ProtocolError`** — now re-thrown as a JSON-RPC error (was `{ isError: true }`)
+- **Handler throws plain `Error`** — unchanged, still returns `{ isError: true }`
+
+**Before (v1):**
+
+```typescript
+const result = await client.callTool({ name: 'test', arguments: {} });
+if (result.isError) {
+    // caught output-schema mismatches, task misconfig, handler ProtocolErrors
+}
+```
+
+**After (v2):**
+
+```typescript
+try {
+    const result = await client.callTool({ name: 'test', arguments: {} });
+    if (result.isError) {
+        // only input validation and ordinary handler exceptions land here
+    }
+} catch (error) {
+    if (error instanceof ProtocolError) {
+        // output validation failure, task misconfig, or handler-thrown ProtocolError
+    }
+}
+```
+
+If your tool handler was throwing `ProtocolError` expecting it to be wrapped as `isError: true`, throw a plain `Error` instead.
+
 #### New `SdkErrorCode` enum
 
 The new `SdkErrorCode` enum contains string-valued codes for local SDK errors:

packages/server/src/server/mcp.ts

@@ -183,7 +183,7 @@ export class McpServer {
                 // Handle taskSupport 'required' without task augmentation
                 if (taskSupport === 'required' && !isTaskRequest) {
                     throw new ProtocolError(
-                        ProtocolErrorCode.MethodNotFound,
+                        ProtocolErrorCode.InvalidParams,
                         `Tool ${request.params.name} requires task augmentation (taskSupport: 'required')`
                     );
                 }
@@ -206,8 +206,8 @@ export class McpServer {
                 await this.validateToolOutput(tool, result, request.params.name);
                 return result;
             } catch (error) {
-                if (error instanceof ProtocolError && error.code === ProtocolErrorCode.UrlElicitationRequired) {
-                    throw error; // Return the error to the caller without wrapping in CallToolResult
+                if (error instanceof ProtocolError) {
+                    throw error;
                 }
                 return this.createToolError(error instanceof Error ? error.message : String(error));
             }
@@ -251,10 +251,9 @@ export class McpServer {
 
         const parseResult = await validateStandardSchema(tool.inputSchema, args ?? {});
         if (!parseResult.success) {
-            throw new ProtocolError(
-                ProtocolErrorCode.InvalidParams,
-                `Input validation error: Invalid arguments for tool ${toolName}: ${parseResult.error}`
-            );
+            // Per spec, input validation failures are tool-execution errors (isError: true),
+            // not protocol errors — throw plain Error so the catch wraps it as a tool result.
+            throw new Error(`Input validation error: Invalid arguments for tool ${toolName}: ${parseResult.error}`);
         }
 
         return parseResult.data as unknown as Args;
@@ -279,7 +278,7 @@ export class McpServer {
 
         if (!result.structuredContent) {
             throw new ProtocolError(
-                ProtocolErrorCode.InvalidParams,
+                ProtocolErrorCode.InternalError,
                 `Output validation error: Tool ${toolName} has an output schema but no structured content was provided`
             );
         }
@@ -288,7 +287,7 @@ export class McpServer {
         const parseResult = await validateStandardSchema(tool.outputSchema, result.structuredContent);
         if (!parseResult.success) {
             throw new ProtocolError(
-                ProtocolErrorCode.InvalidParams,
+                ProtocolErrorCode.InternalError,
                 `Output validation error: Invalid structured content for tool ${toolName}: ${parseResult.error}`
             );
         }
@@ -333,7 +332,11 @@ export class McpServer {
         }
 
         // Return the final result
-        return (await ctx.task.store.getTaskResult(taskId)) as CallToolResult;
+        const result = (await ctx.task.store.getTaskResult(taskId)) as CallToolResult;
+        if (task.status === 'completed') {
+            await this.validateToolOutput(tool, result, request.params.name);
+        }
+        return result;
     }
 
     private _completionHandlerInitialized = false;

test/integration/test/server/mcp.test.ts

@@ -1418,25 +1418,15 @@ describe('Zod v4', () => {
 
             await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
 
-            // Call the tool and expect it to throw an error
-            const result = await client.callTool({
-                name: 'test',
-                arguments: {
-                    input: 'hello'
-                }
-            });
-
-            expect(result.isError).toBe(true);
-            expect(result.content).toEqual(
-                expect.arrayContaining([
-                    {
-                        type: 'text',
-                        text: expect.stringContaining(
-                            'Output validation error: Tool test has an output schema but no structured content was provided'
-                        )
+            // Output validation failure is a server-side bug → JSON-RPC InternalError
+            await expect(
+                client.callTool({
+                    name: 'test',
+                    arguments: {
+                        input: 'hello'
                     }
-                ])
-            );
+                })
+            ).rejects.toThrow('Output validation error: Tool test has an output schema but no structured content was provided');
         });
         /***
          * Test: Tool with Output Schema Must Provide Structured Content
@@ -1550,23 +1540,15 @@ describe('Zod v4', () => {
 
             await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
 
-            // Call the tool and expect it to throw a server-side validation error
-            const result = await client.callTool({
-                name: 'test',
-                arguments: {
-                    input: 'hello'
-                }
-            });
-
-            expect(result.isError).toBe(true);
-            expect(result.content).toEqual(
-                expect.arrayContaining([
-                    {
-                        type: 'text',
-                        text: expect.stringContaining('Output validation error: Invalid structured content for tool test')
+            // Output validation failure is a server-side bug → JSON-RPC InternalError
+            await expect(
+                client.callTool({
+                    name: 'test',
+                    arguments: {
+                        input: 'hello'
                     }
-                ])
-            );
+                })
+            ).rejects.toThrow(/Output validation error: Invalid structured content for tool test/);
         });
 
         /***
@@ -6441,16 +6423,13 @@ describe('Zod v4', () => {
 
             await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
 
-            // Call the tool WITHOUT task augmentation - should return error
-            const result = await client.callTool({
-                name: 'long-running-task',
-                arguments: { input: 'test data' }
-            });
-
-            // Should receive error result
-            expect(result.isError).toBe(true);
-            const content = result.content as TextContent[];
-            expect(content[0]!.text).toContain('requires task augmentation');
+            // Call the tool WITHOUT task augmentation - should throw JSON-RPC error
+            await expect(
+                client.callTool({
+                    name: 'long-running-task',
+                    arguments: { input: 'test data' }
+                })
+            ).rejects.toThrow(/requires task augmentation/);
 
             taskStore.cleanup();
         });