fix(opencode): wire OpenAI previous_response_id session caching

[wildoranges]

Issue for this PR

Closes #20847

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR fixes OpenAI Responses cache continuity in two parts:

1. Persist and reuse previous_response_id across session turns.
2. Ensure promptCacheKey is set not only for providerID === "openai", but also for OpenAI SDK-based providers (@ai-sdk/openai) and GitHub Copilot SDK (@ai-sdk/github-copilot).

Without (2), custom OpenAI providers can still miss cache routing even when previous_response_id is present.

I also added provider transform tests to lock this behavior.

How did you verify your code works?

• bun test test/provider/transform.test.ts
• bun test test/session/llm.test.ts
• bun typecheck (including pre-push turbo typecheck)

Screenshots / recordings

N/A (non-UI change)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[Copilot]

Pull request overview

This PR wires OpenAI Responses API previous_response_id reuse through OpenCode session state by persisting the latest OpenAI response_id on the session and forwarding it into @ai-sdk/openai request options for subsequent turns.

Changes:

• Add last_response_id to the session schema + migration, expose it on Session.Info, and persist updates via a new Session.setResponseId.
• Capture openai.responseId from provider metadata at the end of a step and save it to the session; pass it into LLM/provider option transforms as previousResponseId.
• Remove OpenAI input item ID stripping in the provider fetch path; add transform tests asserting previousResponseId behavior.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/opencode/test/provider/transform.test.ts	Adds unit tests ensuring previousResponseId is only set for @ai-sdk/openai models.
packages/opencode/src/session/session.sql.ts	Adds last_response_id column to the session table definition.
packages/opencode/src/session/prompt.ts	Reads session per loop iteration and passes session.lastResponseId into LLM processing.
packages/opencode/src/session/processor.ts	Extracts OpenAI responseId from provider metadata on finish-step and persists it to the session.
packages/opencode/src/session/llm.ts	Introduces lastResponseId on stream input and forwards it as previousResponseId into provider option transforms.
packages/opencode/src/session/index.ts	Adds lastResponseId to session row mapping/schema and introduces setResponseId API.
packages/opencode/src/provider/transform.ts	Adds previousResponseId option wiring for @ai-sdk/openai.
packages/opencode/src/provider/provider.ts	Removes logic that stripped OpenAI input item IDs from POST bodies.
packages/opencode/migration/20260403085443_add_last_response_id/migration.sql	Adds SQLite migration to append last_response_id to session.
packages/opencode/migration/20260403085443_add_last_response_id/snapshot.json	Updates migration snapshot to include last_response_id.

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[Copilot]

Session.setResponseId publishes a Session.Event.Updated patch with lastResponseId, but session updates are persisted via the projector’s toPartialRow(...) mapping. That mapper currently doesn’t include lastResponseId -> last_response_id, so this value will never be written to SQLite (and sessions.get(...) will keep returning undefined). Add the missing field mapping in packages/opencode/src/session/projectors.ts so updates to lastResponseId are persisted.

[wildoranges]

Thanks for catching this — fixed in 30dc759. I added last_response_id: grab(info, "lastResponseId") in packages/opencode/src/session/projectors.ts, so Session.Event.Updated patches now persist lastResponseId to SQLite.

[wildoranges]

Follow-up update: I pushed one additional cache-hit fix in f1247c5.

Besides persisting/reusing previous_response_id, the branch now also applies promptCacheKey for SDK-based OpenAI providers (@ai-sdk/openai and @ai-sdk/github-copilot), which was the missing piece for my custom OpenAI provider setup.

[wildoranges]

Follow-up commit pushed: a064a2f66.

This addresses a Copilot compatibility regression: we now stop sending previousResponseId on @ai-sdk/github-copilot requests because that backend returns previous_response_id is not supported.

previousResponseId remains enabled for @ai-sdk/openai, and existing promptCacheKey logic is unchanged.

[wildoranges]

Follow-up commit pushed: a064a2f66.

This addresses a Copilot compatibility regression: we now stop sending previousResponseId on @ai-sdk/github-copilot requests because that backend returns previous_response_id is not supported.

previousResponseId remains enabled for @ai-sdk/openai, and existing promptCacheKey logic is unchanged.

[rekram1-node]

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