OpenCode server-API: fix agent→model binding, native fork & per-session diff

[shrimpwtf]

Summary

Closes gaps in Paseo's OpenCode HTTP server-API integration. OpenCode already runs on the server API (not ACP) — this PR fixes the agent→model binding bug and lights up unused endpoints, all provider-scoped.

Headline fix: agent→model binding (atlas→kimi)

OpenCode's GET /agent returns each agent's bound model (e.g. oh-my-openagent binds atlas→kimi). Paseo was discarding it, so switching agents never switched the model. Now:

• AgentMode carries an optional model (providerID/modelID).
• Selecting an agent/mode applies its bound model; an explicit model pick overrides until the next mode switch; the override does not persist across switches.
• AgentManager.setAgentMode() surfaces the re-bound model into runtimeInfo.model so the composer picker updates.

Also included

• Permission "Allow always" — documented/verified it persists server-side via OpenCode's reply:"always" (session-scoped); no nonexistent remember flag. Added a CI-gated e2e.
• Native fork — "Fork chat" now routes into session.fork() for OpenCode (real provider child, history preserved), gated by an agentNativeFork capability. Other providers keep the generic flow.
• Per-session diff — session.diff() mapped to a provider-agnostic changed-files shape, gated by an opencodeSessionDiff capability with a client update-host affordance (no fallback path).

Investigated / deferred (see mission backlog)

• Workspace search (find*), agent-context shell, and user-facing diff/search UI panels were evaluated and deferred (duplicates or need a larger UI subsystem).
• In-app provider auth was a spike only — feasible for code/api-key flows but blocked on a SECURITY.md policy decision ("Paseo never stores or transmits provider API keys").

Compatibility

• All protocol schema additions are .optional(); new capabilities gated in server_info.features.* with dated COMPAT() tags (v0.2.0). No shared ACP files (copilot/cursor/kiro/trae/generic) were behavior-changed.

Testing

• Per-feature unit tests via the fake OpenCode harness; real behaviors covered by CI-gated daemon-e2e/opencode-*.real.e2e.test.ts. npm run typecheck + targeted lint green. Full suite intentionally not run locally (per repo rules) — relies on CI.

[greptile-apps]

Greptile Summary

This PR closes three gaps in Paseo's OpenCode HTTP server-API integration: it fixes the agent\u2192model binding bug (switching agents now switches the effective model), adds a native fork path that preserves provider-side history, and maps session.diff() to a provider-agnostic changed-files shape. All new capabilities are guarded by server_info.features.* flags and all protocol additions are .optional().

• Agent\u2192model binding: mapOpenCodeAgentToMode now carries the agent's bound model, setMode applies it via applyAgentBoundModelForModeSelection, and AgentManager.setAgentMode surfaces the result in runtimeInfo so the UI model picker updates.
• Native fork: forkSession on OpenCodeAgentClient calls OpenCode's fork endpoint preserving provider-side history; session.ts adds handleAgentForkRequest mirroring the import flow; view.tsx gates OpenCode forks through it while leaving all other providers unchanged.
• Per-session diff: diffSession() on OpenCodeAgentSession calls session.diff() and maps FileDiff entries to SessionChangedFile, with a client-side capability guard that shows an "update host" affordance on older daemons.

Confidence Score: 4/5

Safe to merge for the fork and diff features; the agent-model binding path has a fragility where a mode switch can throw if the OpenCode server is unavailable.

The three headline features are well-structured and correctly gated by capability flags. The one concern is that setMode in OpenCodeAgentSession now calls getAvailableModes() (an HTTP round-trip to the OpenCode daemon) to resolve the bound model. If that call throws, the entire mode switch is rejected rather than succeeding without the bound-model side-effect. Before this PR, setMode had no live-server dependency.

packages/server/src/server/agent/providers/opencode-agent.ts — specifically resolveAgentBoundModel / applyAgentBoundModelForModeSelection inside setMode.

Important Files Changed

Filename	Overview
packages/server/src/server/agent/providers/opencode-agent.ts	Adds agent→model binding, native fork, and per-session diff for OpenCode. Core logic is sound, but setMode now makes a live network call to getAvailableModes() to resolve the bound model — if the OpenCode server is unavailable the mode switch fails entirely rather than degrading gracefully.
packages/server/src/server/agent/agent-manager.ts	Extracts registerImportedProviderSession shared between import and fork, and surfaces the re-bound model after a mode switch via getRuntimeInfo(). Refactor is clean; model surfacing is correct for OpenCode and a no-op for other providers.
packages/server/src/server/session.ts	Adds handleAgentForkRequest for the native-fork RPC path, mirroring the existing import pattern and correctly threading the parent agent label.
packages/server/src/server/agent/import-sessions.ts	Adds forkProviderSession as a thin orchestration wrapper mirroring importProviderSession. Consistent with existing pattern; no issues.
packages/server/src/server/agent/agent-sdk-types.ts	Adds fork/diff types and parentSessionId to ImportedProviderSession. parentSessionId is populated by OpenCode's fork response but never consumed in production code.
packages/protocol/src/messages.ts	Adds AgentForkRequest/Response message pair and opencodeSessionDiff/agentNativeFork capability flags. All additions are .optional() and backward-compatible.
packages/app/src/agent-stream/view.tsx	Routes OpenCode fork through the new native-fork RPC when agentNativeFork capability is present; all other providers fall through unchanged.
packages/client/src/daemon-client.ts	Adds forkAgent client method following the same request/response pattern as other RPC methods. No issues.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant UI as view.tsx
    participant DC as DaemonClient
    participant Sess as session.ts
    participant AM as AgentManager
    participant OC as OpenCodeAgentClient

    Note over UI,OC: Native Fork flow
    UI->>DC: forkAgent(agentId, opts)
    DC->>Sess: agent.fork.request
    Sess->>AM: forkProviderSession(...)
    AM->>OC: forkSession(providerHandleId, cwd)
    OC->>OC: client.session.fork(sessionID, directory)
    OC-->>AM: ImportedProviderSession
    AM->>AM: registerImportedProviderSession(forked)
    AM-->>Sess: ManagedAgent
    Sess-->>DC: agent.fork.response
    DC-->>UI: AgentSnapshotPayload
    UI->>UI: navigateToAgent(forkedAgent)

    Note over UI,OC: Agent-Model binding flow
    UI->>DC: setAgentMode(agentId, modeId)
    DC->>Sess: set_agent_mode_request
    Sess->>AM: setAgentMode(agentId, modeId)
    AM->>OC: session.setMode(modeId)
    OC->>OC: getAvailableModes() [network call]
    OC->>OC: applyAgentBoundModelForModeSelection
    AM->>OC: session.getRuntimeInfo()
    OC-->>AM: model bound to agent
    AM->>AM: "agent.runtimeInfo.model = effectiveModel"
    AM-->>Sess: emitState(agent)
    Sess-->>DC: agent_state (model updated)

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sequenceDiagram
    participant UI as view.tsx
    participant DC as DaemonClient
    participant Sess as session.ts
    participant AM as AgentManager
    participant OC as OpenCodeAgentClient

    Note over UI,OC: Native Fork flow
    UI->>DC: forkAgent(agentId, opts)
    DC->>Sess: agent.fork.request
    Sess->>AM: forkProviderSession(...)
    AM->>OC: forkSession(providerHandleId, cwd)
    OC->>OC: client.session.fork(sessionID, directory)
    OC-->>AM: ImportedProviderSession
    AM->>AM: registerImportedProviderSession(forked)
    AM-->>Sess: ManagedAgent
    Sess-->>DC: agent.fork.response
    DC-->>UI: AgentSnapshotPayload
    UI->>UI: navigateToAgent(forkedAgent)

    Note over UI,OC: Agent-Model binding flow
    UI->>DC: setAgentMode(agentId, modeId)
    DC->>Sess: set_agent_mode_request
    Sess->>AM: setAgentMode(agentId, modeId)
    AM->>OC: session.setMode(modeId)
    OC->>OC: getAvailableModes() [network call]
    OC->>OC: applyAgentBoundModelForModeSelection
    AM->>OC: session.getRuntimeInfo()
    OC-->>AM: model bound to agent
    AM->>AM: "agent.runtimeInfo.model = effectiveModel"
    AM-->>Sess: emitState(agent)
    Sess-->>DC: agent_state (model updated)

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_{Reviews (3): Last reviewed commit: "fix(opencode): address greptile review o..." | Re-trigger Greptile}

[shrimpwtf]

Closing in favor of #2256, which splits out just the agent→model-on-mode-switch fix as a self-contained, provider-clean change: no if(provider==opencode) branches, and no opencode details in the protocol (all OpenCode logic stays inside opencode-agent.ts). The native-fork and per-session-diff pieces can follow as their own focused PRs if wanted. Thanks for the review feedback.