microsoft/TypeScript · error · Error

getClassifier is not available using the server interface.

Error message

getClassifier is not available using the server interface.

What it means

Thrown by the server-backed language service adapter (harnessLanguageService.ts:807) when a test invokes getClassifier() on a TestSession/SessionClientHost. The classifier is a compiler-internal API that the tsserver protocol does not expose, so the server adapter deliberately throws rather than pretend to support it.

Source

Thrown at src/harness/harnessLanguageService.ts:807

        if (options) {
            client.setCompilerOptionsForInferredProjects(ts.optionMapToObject(ts.serializeCompilerOptions(options)) as ts.server.protocol.CompilerOptions);
        }

        // Set the properties
        this.client = client;
        this.host = clientHost;
    }
    getLogger(): LoggerWithInMemoryLogs {
        return this.logger;
    }
    getHost(): SessionClientHost {
        return this.host;
    }
    getLanguageService(): ts.LanguageService {
        return this.client;
    }
    getClassifier(): ts.Classifier {
        throw new Error("getClassifier is not available using the server interface.");
    }
    getPreProcessedFileInfo(): ts.PreProcessedFileInfo {
        throw new Error("getPreProcessedFileInfo is not available using the server interface.");
    }
    assertTextConsistent(fileName: string): void {
        const serverText = this.server.getText(fileName);
        const clientText = this.host.readFile(fileName);
        ts.Debug.assert(
            serverText === clientText,
            [
                "Server and client text are inconsistent.",
                "",
                "\x1b[1mServer\x1b[0m\x1b[31m:",
                serverText,
                "",
                "\x1b[1mClient\x1b[0m\x1b[31m:",
                clientText,
                "",

View on GitHub (pinned to b465fdbfe1)

Solutions

1. Gate classifier tests to run only against the in-process adapter (skip when the adapter is the server/session variant).
2. Move classifier assertions into a separate test that does not exercise the server code path.
3. If you must assert server-side classification, use the EncodedSyntacticClassifications protocol command instead of getClassifier.

Example fix

// before — runs against every adapter, throws on server variant
test.each(adapters)("classifies", adapter => {
  const cls = adapter.getClassifier().getClassificationsForLine(...);
  assert.ok(cls);
});

// after — skip server variants for classifier-only tests
test.each(adapters.filter(a => !isServerAdapter(a)))("classifies", adapter => {
  const cls = adapter.getClassifier().getClassificationsForLine(...);
  assert.ok(cls);
});

// Detect server-backed adapters and skip classifier tests for them.
function supportsClassifier(adapter: unknown): boolean {
  // Server-backed adapters throw on getClassifier; detect by capability rather than type.
  return typeof (adapter as any).getClassifier === 'function' && !isServerAdapter(adapter);
}

function isServerAdapter(adapter: unknown): boolean {
  // SessionClientHost advertises getHost/getLanguageService but lacks a real classifier.
  return typeof (adapter as any)?.getHost === 'function' && !(adapter as any)._classifierAvailable;
}

When it happens

Trigger: A language service test is parameterised to run both against the in-process LanguageService and against a server Session, and the test body calls `adapter.getClassifier()`. The in-process variant returns a real Classifier; the server variant throws this message.

Common situations: Writing a classifier/syntax-classification test and running it through the server harness; sharing one test function across both adapter kinds without gating classifier-only calls.

Related errors

• getPreProcessedFileInfo is not available using the server in
• No script with name '" + fileName + "'
• verifyCurrentLineContent\n
• verifyFileContent in file '${fileName}' failed:\n${showTextD
• Expected at least one js file to be emitted or at least one

AI-assisted analysis of microsoft/TypeScript@b465fdbfe1 (2026-08-12). Data as JSON: /api/errors/4e63ec7176b63cb9. Report an issue: GitHub.