fix: keep the provider user agent when called through the AI SDK - #603

Description

Bug. Requests made through the AI SDK lose the provider's ai-sdk/openrouter/<version> user agent identifier. The AI SDK passes its own user-agent (ai/<version>) in the call-level headers, and every model merged headers with combineHeaders(this.config.headers(), options.headers), which puts call headers last. So the SDK's value replaced ours. A direct doGenerate call has no call-level UA, so it kept the identifier, which is why the existing tests didn't catch this.

Repro.

const openrouter = createOpenRouter({ apiKey, fetch: logUserAgent });
await generateText({ model: openrouter('openai/gpt-4o-mini'), prompt: 'hi' });
// user-agent contains ai/7.0.103 but not ai-sdk/openrouter/<version>

What other providers do. @ai-sdk/openai (latest) builds its config headers the same way, with withUserAgentSuffix({...}, 'ai-sdk/openai/<v>') and then combineHeaders(this.config.headers?.(), options.headers). combineHeaders in @ai-sdk/provider-utils (latest) is still a plain spread, so first-party providers lose their identifier through ai too. Running withUserAgentSuffix after combining won't work here either: our version (#362, fixes #300) uses any existing UA verbatim, so it would just keep ai/<v>.

Fix. New combineHeadersWithUserAgent(configHeaders, callHeaders) in src/utils/with-user-agent-suffix.ts. It combines the headers like before. When the config UA is the default provider identifier and the call-level UA comes from the AI SDK (it contains an ai/<version> token) without the identifier, it sends <call UA> ai-sdk/openrouter/<v>. For example, ai/7.0.103 ai-sdk/openrouter/3.1.0 ai-sdk/provider-utils/... runtime/....

• A user-specified provider-level User-Agent (Openrouter AI SDK provider overrides/silently rewrites user-specified headers #300) is still never modified.
• A custom call-level UA without the ai/<v> token, e.g. doGenerate({ headers: { 'user-agent': 'my-app/1.0' } }), is still sent verbatim (raised in review). Through ai, a user's call-level UA becomes my-app/1.0 ai/<v>, and that one does get the identifier appended.
• The identifier is never added twice.

It replaces combineHeaders everywhere headers are combined: chat (doGenerate, doStream), completion (doGenerate, doStream), embedding, image, video, reranking and evaluation.

Not changed. Through ai, a user-specified provider-level User-Agent is still replaced by the SDK's call UA. That is existing behavior and a separate question from this bug.

Tests.

• src/tests/user-agent.test.ts drives the real ai functions with a mocked fetch and asserts the final UA contains both ai/<v> and ai-sdk/openrouter/<v>: generateText (chat and completion), embed, generateImage, experimental_generateVideo, experimental_evaluate. streamText and rerank in the pinned ai 7.0.103 send no ai/<v> UA, so for those the test asserts that the provider identifier is present. It also checks that a user-specified provider UA, and a custom call-level UA on a direct doGenerate, don't get the identifier appended.
• src/utils/with-user-agent-suffix.test.ts adds unit tests for the helper: append, case-insensitive match, no call UA, no duplication, custom call UA without the ai/ token, custom provider UA.

I checked that the 6 ai-driven assertions (chat, completion, embed, image, video, evaluate) fail on origin/main and pass with this fix.

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

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