refactor(core): share session model requests - #42680

What

Route durable Session steps and transient session.generate calls through one SessionModelRequest.prepare boundary for provider-visible request shaping.

Transient generation now receives the same context-hook tool reconciliation, unsupported-media handling, image bounds, Session headers, prompt-cache identity, and HTTP request/response hooks as normal Session execution.

Before / After

Before: the runner used SessionModelRequest.prepare, while transient generation independently rebuilt a similar LLMRequest. The generation path omitted media compatibility, image bounding, HTTP hooks, identity-aware renamed-tool reconciliation, and shared transport policy.

After: callers select their own transcript, then pass { scope, transcript, toolChoice, webSocket } into the shared preparation stage. Both paths produce requests through the same provider-visible transformations. Durable runner calls explicitly opt into the stateful per-Session WebSocket channel; transient generation remains HTTP-only and cannot mutate or contend for runner transport checkpoints.

How

• packages/core/src/session/model-request.ts now accepts a selected Session/model/tool scope plus a caller-built transcript and owns all shared request transformations.
• packages/core/src/session/runner/llm.ts owns step-limit prompts, tool-call rejection, continuation policy, and prompt-cache sequence diagnostics before/after preparation.
• packages/core/src/session/generate-node.ts owns preview-history selection and the private user prompt, then calls llm.generate with the shared prepared request and options.
• Merged with current v2: the shared preparation stage now also carries the model.request plugin hook (fix(core): prefer websocket with request hooks #43234) and fork-aware prompt cache keys (both previously duplicated per path), so transient generation inherits those too.
• HTTP-hook coverage verifies transient generation forwards middleware through LLMClient.generate.
• A patch changeset records the @opencode-ai/core behavior change.

Scope

• V2 only; packages/opencode is unchanged.
• No Protocol, Server HttpApi, or generated-client changes.
• No changes to durable Session history or transient generation persistence semantics.
• Stateful Session WebSocket transport remains exclusive to durable runner calls.

Testing

• cd packages/core && bun typecheck
• cd packages/core && bun run build
• cd packages/core && bun run test test/session-model-request.test.ts test/session-generate.test.ts test/session-runner.test.ts (157 passed, 534 assertions)
• Full Core suite review run: 1,794 passed, 16 skipped
• Push hook: bun turbo typecheck --concurrency=3 (33 packages passed)

Flow

sequenceDiagram
    participant Caller as Runner / Generate
    participant Prepare as SessionModelRequest.prepare
    participant Hooks as Plugin hooks
    participant LLM as LLMClient

    Caller->>Caller: Select transcript and execution policy
    Caller->>Prepare: scope + transcript + toolChoice + WebSocket policy
    Prepare->>Hooks: session.context
    Hooks-->>Prepare: transformed system, messages, tools
    Prepare->>Prepare: reconcile tools and normalize media
    Prepare-->>Caller: canonical request + HTTP/transport options
    Caller->>LLM: stream or generate