fix(client): skip output schema validation on error results

[codefromthecrypt]

Motivation and Context

The MCP Inspector client failed tool calls that returned isError: true alongside structuredContent, because the SDK still validated the output schema even on error results.

How Has This Been Tested?

new tests and manual

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Verified with MCP inspector against IntelliJ

Found the port of IntelliJ with MCP server enabled (Note: HTTP Streamable isn't default yet, so this is SSE)

$ grep -i mcp $HOME/Library/Application\ Support/JetBrains/*/options/mcpServer.xml
/Users/codefromthecrypt/Library/Application Support/JetBrains/IntelliJIdea2025.3/options/mcpServer.xml:  <component name="McpServerSettings">
/Users/codefromthecrypt/Library/Application Support/JetBrains/IntelliJIdea2025.3/options/mcpServer.xml:    <option name="enableMcpServer" value="true" />
/Users/codefromthecrypt/Library/Application Support/JetBrains/IntelliJIdea2025.3/options/mcpServer.xml:    <option name="mcpServerPort" value="64344" />

MCP inspector against IntelliJ before this fix

Tried the CLI

$ npx @modelcontextprotocol/inspector \
    http://127.0.0.1:64344/sse \
    --cli \
    --method tools/call \
    --tool-name get_repositories
Failed to call tool get_repositories: MCP error -32602: Structured content does not match the tool's output schema: data must have required property 'roots'

Failed with exit code: 1

Tried the UI

$ MCP_SERVER_URL=http://127.0.0.1:64344/sse npm start
Starting MCP inspector...
⚙️ Proxy server listening on localhost:6277
🔑 Session token: c4aa2d10d863a6672d4bc85eae9f5f372041ac8b1d268e016af9c16a04391933
   Use this token to authenticate requests or set DANGEROUSLY_OMIT_AUTH=true to disable auth

🚀 MCP Inspector is up and running at:
   http://localhost:6274/?MCP_PROXY_AUTH_TOKEN=c4aa2d10d863a6672d4bc85eae9f5f372041ac8b1d268e016af9c16a04391933

🌐 Opening browser...

MCP inspector against IntelliJ after this fix

Installation

Patching all inspector package.json like so:

   "dependencies": {
-    "@modelcontextprotocol/sdk": "^1.25.2",
+    "@modelcontextprotocol/sdk": "file:../../typescript-sdk",

Applied like this:

npm run clean
npm run build

Tried the CLI and it works as you can see the user error missing the projectPath, which is friendly due to showing valid choices:

$ node ~/oss/inspector/cli/build/cli.js \
    http://127.0.0.1:64344/sse \
    --cli \
    --method tools/call \
    --tool-name get_repositories
{
  "content": [
    {
      "type": "text",
      "text": "No exact project is specified while multiple projects are opened.\n You may specify the project path via `projectPath` parameter when calling a tool. \n If you're aware of the current working directory you may pass it as `projectPath`. \n In the case when it's unobvious which project to use you have to ASK the USER about a project providing him a numbered list of the projects.\n Currently open projects: {\"projects\":[{\"path\":\"/Users/codefromthecrypt/oss/intellij-community/plugins/mcp-server\"},{\"path\":\"/Users/codefromthecrypt/oss/koog\"}]}"
    }
  ],
  "structuredContent": {
    "projects": [
      {
        "path": "/Users/codefromthecrypt/oss/intellij-community/plugins/mcp-server"
      },
      {
        "path": "/Users/codefromthecrypt/oss/koog"
      }
    ]
  },
  "isError": true
}

Tried the UI and it shows valid error choices (relies on an inspector bug to fully correct the UI)

$ MCP_SERVER_URL=http://127.0.0.1:64344/sse npm start

> @modelcontextprotocol/inspector@0.19.0 start
> node client/bin/start.js

Starting MCP inspector...
⚙️ Proxy server listening on localhost:6277
🔑 Session token: f06b424e9e240977d39e44a7444a25099d90b1fd7f432b9428ab86df58e03087
   Use this token to authenticate requests or set DANGEROUSLY_OMIT_AUTH=true to disable auth

🚀 MCP Inspector is up and running at:
   http://localhost:6274/?MCP_PROXY_AUTH_TOKEN=f06b424e9e240977d39e44a7444a25099d90b1fd7f432b9428ab86df58e03087

🌐 Opening browser...

MCP inspector follow-up

Inspector has a bug here which will be fixed once this PR is merged and released as a part of modelcontextprotocol/inspector#1045

--- a/client/src/components/ToolResults.tsx
+++ b/client/src/components/ToolResults.tsx
@@ -115,7 +115,7 @@ const ToolResults = ({
           error:
             "Tool has an output schema but did not return structured content",
         };
-      } else if (structuredResult.structuredContent) {
+      } else if (structuredResult.structuredContent && !isError) {
         validationResult = validateToolOutput(
           selectedTool.name,
           structuredResult.structuredContent,

[changeset-bot]

⚠️ No Changeset found

Latest commit: a9ff608

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[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1428

commit: a9ff608

[codefromthecrypt]

@cliffhall if you have a chance to take a look. this is subtle but breaks ability to see tool errors in MCP inspector. For example, the jetbrains MCP server will fail with a nice reason why, but we can't see it. note I targeted this to 1.x as that is what inspector uses, but can raise another for main.

[cliffhall]

@cliffhall if you have a chance to take a look. this is subtle but breaks ability to see tool errors in MCP inspector. For example, the jetbrains MCP server will fail with a nice reason why, but we can't see it. note I targeted this to 1.x as that is what inspector uses, but can raise another for main.

@codefromthecrypt Code looks good, but could you throw some before & after screenshots into the description?

[codefromthecrypt]

@cliffhall fully inventoried for your approval!

[cliffhall]

Ok, looking at your error screenshot, I have to ask why you have an isError condition and still have populated the structuredContent field. The behavior is questionable. If the agent sees isError and sees a content block with an error message, what could the purpose of the structuredContent possibly be?

The situation we have here with outputSchema validation happening when structuredContent is present is the correct behavior. Not only is it a signal that the result from your tool is probably going to be confusing to an agent, it is actually following the specification:

There is no caveat that clients can skip validation if the result is an error. That's probably because it is a condition that doesn't make sense.

[codefromthecrypt]

@cliffhall to be clear I was unable to use MCP inspector to debug a problem because the schema was being applied to what I understand is an incorrect interpretation of the spec. Given I don't own the server and MCP inspector is a tool to use to help troubleshoot servers, what do you recommend? basically at the moment, I will see no info at all. Is there another way out or should folks use a different proxy when there are potential mismatches to diagnose?

[codefromthecrypt]

nevermind I will follow-up here as it is the exact concern, and cite this as reason to move the SEP forward modelcontextprotocol/modelcontextprotocol#2145