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Empty-Prompt Local-to-Cloud Handoff — Stage 1 Sub-Tech-Spec (warp-4)

Sub-tech-spec for what Stage 1 of REMOTE-1499 delivers on the warp-4 side. The full end-to-end architecture lives in TECH.md; the full product behavior lives in PRODUCT.md. This document is scoped to the contents of harry/empty-prompt-handoff-wire-contract. Branch: harry/empty-prompt-handoff-wire-contract (warp-4 half of the paired-branch cross-repo PR shared with warp-server-4). Sibling: ../../../warp-server-4/specs/REMOTE-1499/STAGE-1.md (server-side relaxations).

Scope

Stage 1 widens the wire shape of SpawnAgentRequest.prompt from String to Option<String> so a Stage 2 client can omit the field when the user submits an empty handoff. By itself Stage 1 introduces no interactive behavior changes: every interactive call site continues to send Some(...) of the same string, and the only non-test sites that emit prompt: None at runtime are the oz agent run CLI skill-only and conversation-only invocations. The skill-only path is end-to-end functional pre-Stage-2 because the warp-server-4 prompt+skill gate accepts the omitted field as the Go zero value. The conversation-only path requires the server-side validator relaxations that ship in the sibling warp-server-4 Stage 1 PR. The server-side relaxations that accept additional shapes (empty prompt paired with ConversationID, InitialSnapshotToken, or rehydration metadata) land in the sibling warp-server-4 PR on the same branch name. Cross-repo coordination is purely through the JSON wire shape — there are no shared edit surfaces.

Wire shape

SpawnAgentRequest in app/src/server/server_api/ai.rs:206-208:

/// None for skill-only or conversation-only invocations; omitted on the wire.
#[serde(skip_serializing_if = "Option::is_none")]
pub prompt: Option<String>,

Option<T> serializes transparently under serde: Some("hello") emits "prompt": "hello", and skip_serializing_if = "Option::is_none" causes None to omit the field entirely. The struct derives only Serialize, not Deserialize, so wire compatibility only has to hold client→server. Warp-server-4 deserializers that treat prompt as a string see the omitted field as the Go zero value "", which the existing skill-only validator already accepts — so the wire shape is compatible with both pre- and post-Stage-1 servers.

Construction sites

All twelve SpawnAgentRequest { … } construction sites wrap their prompt value in Some(...):

  • app/src/terminal/view/ambient_agent/model.rs:632 (build_handoff_spawn_request) and :1120 (spawn_agent): both wrap the result of extract_user_query_mode(prompt).
  • app/src/pane_group/pane/terminal_pane.rs:2137: orchestration-spawned child runs.
  • app/src/ai/agent_sdk/ambient.rs:481-482: oz agent run CLI (see CLI path below).
  • Test fixtures: spawn_tests.rs:702/770/838/901/1047, model_tests.rs:54, view_tests.rs:1323, mcp_config_tests.rs:272, ai_tests.rs:39.

CLI path

app/src/ai/agent_sdk/ambient.rs:267-313 resolves the prompt as Option<String> directly:

  • Some(Prompt::PlainText(text)) → Some(text)
  • Some(Prompt::SavedPrompt(id)) → resolves to Some(prompt_text.to_string()) on hit; fatal-errors on miss
  • None → None (skill-only or conversation-only invocations: --skill alone, --conversation alone, or --skill + --conversation with no prompt) ambient.rs:474-480 then computes the (prompt, mode) pair via a match that runs extract_user_query_mode only on the Some branch and defaults mode to UserQueryMode::Normal when the prompt is None. UserQueryMode is imported at the top of the file (ambient.rs:6). The resulting Option<String> flows directly into the constructed SpawnAgentRequest at ambient.rs:481-482. These are the only non-test sites that emit prompt: None at runtime. The warp-server-4 prompt+skill gate at agent_webhooks.go:343-347 accepts the skill-only case independently of the Stage 1 server relaxations, so the CLI's --skill foo flow continues to pass server validation even against an unupdated server. The --conversation-only flow depends on the Stage 1 server-side relaxations in the sibling warp-server-4 PR.

Reader sites

Two reader sites use .as_deref() so the Some case dereferences to &str and the None case short-circuits cleanly:

  • app/src/terminal/view/ambient_agent/block/entry.rs:160 — entry-block title fallback chain: 
    request.prompt.as_deref().and_then(Self::meaningful_title)
    A None prompt skips the fallback and the chain proceeds to the default title.
  • app/src/terminal/view/ambient_agent/view_impl.rs:159-164 — Cloud Mode Setup V2 queued-prompt insertion: 
    request.prompt.as_deref()
    .map(|prompt| display_user_query_with_mode(request.mode, prompt))
    The existing if !prompt.is_empty() guard at view_impl.rs:166 suppresses the queued-prompt block insertion when the prompt is None. Stage 2 reuses this short-circuit for the substituted-prompt UI variants. No other code in warp-4 pattern-matches or destructures SpawnAgentRequest.prompt.

Testing

Unit tests

  • app/src/server/server_api/ai_tests.rs:66-89 — spawn_agent_request_omits_prompt_when_none constructs a SpawnAgentRequest { prompt: None, ... }, serializes to serde_json::Value, and asserts value.get("prompt").is_none(). This is the only test that exercises the None branch directly and pins the skip_serializing_if contract.
  • ai_tests.rs:37-64 (spawn_agent_request_serializes_agent_uid_as_agent_identity_uid) uses Some("hello") and round-trips the full struct through serde_json::to_value. Its assertions on the agent_identity_uid field name implicitly verify that Some(String) serializes transparently — a stray {"Some": ...} wrapping would break the round-trip.
  • app/src/ai/agent_sdk/mcp_config_tests.rs:272 (serializes_mcp_servers_as_object_not_string) uses Some("hello") and round-trips the struct to verify nested MCP config serialization; provides the same implicit guarantee for the prompt shape.
  • app/src/terminal/view/ambient_agent/model_tests.rs:143, 276, 339 and app/src/terminal/view_tests.rs:920, 965 assert handoff auto-submit and cloud-mode dispatch payloads via assert_eq!(request.prompt.as_deref(), Some("...")), pinning the exact prompt string under the Option<String> shape.
  • spawn_tests.rs fixtures at :702/770/838/901/1047 exercise the struct shape in spawn-task polling tests.

Validation

  • cargo fmt --all --check.
  • cargo check -p warp --tests. Nextest and full clippy are intentionally not part of the per-PR validation for this change — the diff is a mechanical type widening plus targeted reader updates, and the listed checks plus the per-stage unit tests cover the relevant surfaces.

Risks and mitigations

  • Pre-Stage-1 servers receiving Stage-1+ client payloads. Mitigated by Option<T>'s transparent serialization plus skip_serializing_if = "Option::is_none": the common Some case emits the same JSON shape that pre-Stage-1 servers always accepted. Of the two None-emitting CLI paths, skill-only is already accepted by the agent_webhooks.go prompt+skill gate as the Go zero-value prompt: ""; conversation-only depends on the Stage 1 server-side relaxations and was already rejected by pre-Stage-1 servers regardless of whether the client sent "" or omitted the field, so Stage 1 does not regress that path.
  • Post-Stage-1 servers receiving pre-Stage-1 client payloads. Not a concern: the field is non-optional on the wire from a pre-Stage-1 client; the server deserializer tolerates presence or absence of the field equivalently.
  • Borrow-site regressions. The two &request.prompt borrows in the repo go through .as_deref() chains; the Some case dereferences to &str identically to the pre-Stage-1 shape and the None case short-circuits cleanly. There are no match / if let destructures of request.prompt to migrate.
  • Stage-coupling risk. Stage 1 alone never produces a None runtime value from any interactive flow — only the CLI skill-only and conversation-only paths can — so the additive server-side relaxations in the sibling warp-server-4 PR are not load-bearing for the skill-only CLI path. They are load-bearing for the conversation-only CLI path, but that path was already broken against pre-Stage-1 servers (which reject prompt: "" + no skill), so Stage 1 does not regress it. Reverting the server-side PR independently is safe modulo the conversation-only CLI flow.

Follow-ups

Stage 1 is scaffolding for Stage 2. The behaviors that justify the wire-contract change — empty-prompt handoff via chip / & / /handoff, continue in the cloud substitution against an in-progress source, the queued-prompt indicator label variants, the worker-derived skip-initial-turn wiring, and the CloudModeSetupPhaseEnded setup-phase teardown — are specced under STAGE-2.md on harry/empty-prompt-handoff-local. Stage 1 introduces no FeatureFlag::EmptyPromptHandoff itself; that flag lands on the Stage 2 branch.

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