Server Tools fail when using model fallback via models

[judemont]

When using the @openrouter/agent SDK with a single model, OpenRouter Server Tools work correctly.

However, when using the models parameter to configure model fallback, the request fails with:

{ code: 400, message: 'Server tool request failed' }

Working Example :

import { OpenRouter, serverTool, tool } from "@openrouter/agent";

const openrouter = new OpenRouter({ apiKey: process.env.OPENROUTER_API_KEY!, });

const result = openrouter.callModel({
  model: "mistralai/mistral-small-2603",
  input: "What day of the week is it today?",
  tools: [
    serverTool({
      type: "openrouter:datetime",
    }),
  ],
});

const text = await result.getText();
console.log(text);

Output :

Today is **Friday**.

Failing example :

import { OpenRouter, serverTool, tool } from "@openrouter/agent";

const openrouter = new OpenRouter({ apiKey: process.env.OPENROUTER_API_KEY!, });

const result = openrouter.callModel({
  models: ["mistralai/mistral-small-2603", "mistralai/mistral-large-2512"],
  input: "What day of the week is it today?",
  tools: [
    serverTool({
      type: "openrouter:datetime",
    }),
  ],
});

const text = await result.getText();
console.log(text);

Output :

file:///home/jdm/autre/openrouter-issue-poc/node_modules/@openrouter/sdk/esm/models/errors/badrequestresponseerror.js:41
    return new BadRequestResponseError(remapped, {
           ^

BadRequestResponseError: Server tool request failed
    at Object.transform (file:///home/jdm/autre/openrouter-issue-poc/node_modules/@openrouter/sdk/esm/models/errors/badrequestresponseerror.js:41:12)
    at inst._zod.parse (file:///home/jdm/autre/openrouter-issue-poc/node_modules/zod/v4/classic/schemas.js:1084:28)
    at handlePipeResult (file:///home/jdm/autre/openrouter-issue-poc/node_modules/zod/v4/core/schemas.js:1975:22)
    at inst._zod.parse (file:///home/jdm/autre/openrouter-issue-poc/node_modules/zod/v4/core/schemas.js:1966:16)
    at Module.<anonymous> (file:///home/jdm/autre/openrouter-issue-poc/node_modules/zod/v4/core/parse.js:6:32)
    at inst.parse (file:///home/jdm/autre/openrouter-issue-poc/node_modules/zod/v4/classic/schemas.js:73:42)
    at safeParseResponse.request.request (file:///home/jdm/autre/openrouter-issue-poc/node_modules/@openrouter/sdk/esm/lib/matchers.js:167:74)
    at safeParseResponse (file:///home/jdm/autre/openrouter-issue-poc/node_modules/@openrouter/sdk/esm/lib/matchers.js:192:19)
    at matchFunc (file:///home/jdm/autre/openrouter-issue-poc/node_modules/@openrouter/sdk/esm/lib/matchers.js:167:28)
    at process.processTicksAndRejections (node:internal/process/task_queues:104:5) {
  statusCode: 400,
  body: '{"error":{"message":"Server tool request failed","code":400,"metadata":{"provider_name":null,"previous_errors":[{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"}]}},"user_id":"user_374Pd0xxWDijSbXDj65djGvgzqv"}',
  headers: Headers {},
  contentType: 'application/json',
  rawResponse: Response {},
  'data$': {
    error: {
      code: 400,
      message: 'Server tool request failed',
      metadata: {
        provider_name: null,
        previous_errors: [
          { code: 400, message: 'Server tool request failed' },
          { code: 400, message: 'Server tool request failed' },
          { code: 400, message: 'Server tool request failed' },
          { code: 400, message: 'Server tool request failed' },
          { code: 400, message: 'Server tool request failed' },
          { code: 400, message: 'Server tool request failed' },
          { code: 400, message: 'Server tool request failed' },
          { code: 400, message: 'Server tool request failed' },
          { code: 400, message: 'Server tool request failed' },
          { code: 400, message: 'Server tool request failed' }
        ]
      }
    },
    userId: 'user_374Pd0xxWDijSbXDj65djGvgzqv',
    'request$': Request {},
    'response$': Response {},
    'body$': '{"error":{"message":"Server tool request failed","code":400,"metadata":{"provider_name":null,"previous_errors":[{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"},{"code":400,"message":"Server tool request failed"}]}},"user_id":"user_374Pd0xxWDijSbXDj65djGvgzqv"}'
  },
  error: {
    code: 400,
    message: 'Server tool request failed',
    metadata: {
      provider_name: null,
      previous_errors: [
        { code: 400, message: 'Server tool request failed' },
        { code: 400, message: 'Server tool request failed' },
        { code: 400, message: 'Server tool request failed' },
        { code: 400, message: 'Server tool request failed' },
        { code: 400, message: 'Server tool request failed' },
        { code: 400, message: 'Server tool request failed' },
        { code: 400, message: 'Server tool request failed' },
        { code: 400, message: 'Server tool request failed' },
        { code: 400, message: 'Server tool request failed' },
        { code: 400, message: 'Server tool request failed' }
      ]
    }
  },
  userId: 'user_374Pd0xxWDijSbXDj65djGvgzqv'
}

The same result occurs with any model and any server tool.