Diagnose "Model is not assignable to type LanguageModelV1"

TL;DR

Run npm ls ai @ai-sdk/openai (or your provider). Delete node_modules and reinstall, then re-run tsc.

Steps

1. Symptom: tsc fails with Model is not assignable to type "LanguageModelV1" (or similar spec-type errors) on code that looks correct.
2. Cause: version drift. The ai core expects LanguageModel spec v2/v3 shapes; the installed @ai-sdk/* provider package implements an older spec. The types no longer line up.
3. Confirmation:
4. Run npm ls ai @ai-sdk/openai (or your provider). If the majors do not belong to the same era (e.g. ai@7 with @ai-sdk/openai@2), that is the bug.
5. Check for nested duplicates: npm ls @ai-sdk/provider showing multiple versions means two spec generations are loaded at once.
6. Fix:
7. npm install ai@latest @ai-sdk/react@latest @ai-sdk/openai@latest (all satellites, one command, one commit).
8. Delete node_modules and reinstall, then re-run tsc. Stale nested copies survive naive upgrades.
9. If a community provider is involved, check whether it has a release for your ai major. If not, pin ai back or replace the provider.
10. Verification: tsc --noEmit passes. Then run the app and confirm the model call works at runtime, not just at type level.
11. The runtime sibling of this error is UnsupportedModelVersionError. Same root cause, same fix.

When to use

You are seeing this: Symptom: tsc fails with Model is not assignable to type "LanguageModelV1" (or similar spec-type errors) on code that looks correct. Use this skill when you run into "Diagnose "Model is not assignable to type LanguageModelV1"".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

From the AI SDK troubleshooting index (Model is not assignable to type LanguageModelV1 entry), verified 2026-09-26. The errors reference documents UnsupportedModelVersionError for provider/spec mismatches. npm registry confirms the v7 era line-up (ai 7.0.x, @ai-sdk/provider 4.0.x).