feat(chat): reasoning-effort selector — sticky per-conversation thinking-depth knob

Consume the backend's reasoning-effort handoff ([email protected] ReasoningEffort +
[email protected] GET/PUT /conversations/:id/reasoning-effort,
ChatRequest.reasoningEffort): a 5-level selector in the sidebar Model view,
under the provider + model dropdowns. null renders as 'high (default)' per
the server-owned resolution chain; PUT on change (effective next turn);
error + revert on 400; per-conversation re-mount incl. drafts (the draft id
survives promotion, so an effort set on a draft applies from turn 1).
Re-mirrored .dispatch references; GLOSSARY 'reasoning effort'; handoff
updated. 616 tests green; live curl probe passed.

3 files changed, 197 insertions, 3 deletions

diff --git a/src/app/App.svelte b/src/app/App.svelte
index 4c5a82b..dffa937 100644
--- a/src/app/App.svelte
+++ b/src/app/App.svelte
@@ -1,4 +1,5 @@
 <script lang="ts">
+	import type { ReasoningEffort } from "@dispatch/transport-contract";
 	import type { InvokeMessage } from "@dispatch/ui-contract";
 	import { tick } from "svelte";
 	import Table from "../components/Table.svelte";
@@ -12,6 +13,8 @@
 		Composer,
 		manifest as chatManifest,
 		ModelSelector,
+		ReasoningEffortSelector,
+		type ReasoningEffortSaveResult,
 	} from "../features/chat";
 	import { manifest as conversationCacheManifest } from "../features/conversation-cache";
 	import { manifest as markdownManifest } from "../features/markdown";
@@ -154,6 +157,17 @@
 			: { ok: false, error: result.error };
 	}
 
+	// Adapt the store's reasoning-effort result to the chat feature's port.
+	async function saveReasoningEffort(
+		level: ReasoningEffort,
+	): Promise<ReasoningEffortSaveResult | null> {
+		const result = await store.setReasoningEffort(level);
+		if (result === null) return null;
+		return result.ok
+			? { ok: true, reasoningEffort: result.reasoningEffort }
+			: { ok: false, error: result.error };
+	}
+
 	// Adapt the store's cwd/LSP results to the workspace feature's ports.
 	async function saveCwd(cwd: string): Promise<CwdSaveResult | null> {
 		const result = await store.setCwd(cwd);
@@ -295,10 +309,11 @@
 	{#if kind === "model"}
 		<div class="flex flex-col gap-3">
 			<ModelSelector models={store.models} selected={store.activeModel} onSelect={handleSelectModel} />
-			<!-- Keyed on the workspace conversation (active tab OR draft) so the input
-			     re-mounts per conversation — incl. switching between drafts — and can't
-			     bleed across tabs. Editable for a draft too (cwd applies from turn 1). -->
+			<!-- Keyed on the workspace conversation (active tab OR draft) so the inputs
+			     re-mount per conversation — incl. switching between drafts — and can't
+			     bleed across tabs. Editable for a draft too (cwd + effort apply from turn 1). -->
 			{#key store.currentConversationId}
+				<ReasoningEffortSelector persisted={store.reasoningEffort} save={saveReasoningEffort} />
 				<CwdField cwd={store.cwd} canEdit={true} save={saveCwd} />
 			{/key}
 		</div>
diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index 999f2be..05577a6 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -6,7 +6,10 @@ import type {
 	CwdResponse,
 	LspStatusResponse,
 	ModelsResponse,
+	ReasoningEffort,
+	ReasoningEffortResponse,
 	SetCwdRequest,
+	SetReasoningEffortRequest,
 	WarmRequest,
 	WarmResponse,
 } from "@dispatch/transport-contract";
@@ -52,6 +55,11 @@ export type LspResult =
 	| { readonly ok: true; readonly response: LspStatusResponse }
 	| { readonly ok: false; readonly error: string };
 
+/** Outcome of `PUT /conversations/:id/reasoning-effort`. */
+export type ReasoningEffortResult =
+	| { readonly ok: true; readonly reasoningEffort: ReasoningEffort }
+	| { readonly ok: false; readonly error: string };
+
 export interface AppStore {
 	readonly tabs: readonly Tab[];
 	readonly activeConversationId: string | null;
@@ -85,6 +93,18 @@ export interface AppStore {
 	 */
 	setCwd(cwd: string): Promise<CwdResult | null>;
 	/**
+	 * The workspace conversation's persisted reasoning effort, or null when never
+	 * set (the server then resolves turns at the default, `"high"`).
+	 */
+	readonly reasoningEffort: ReasoningEffort | null;
+	/**
+	 * Persist the workspace conversation's reasoning effort
+	 * (`PUT /conversations/:id/reasoning-effort`). Works for a draft too (its id
+	 * survives promotion), so the first turn already runs at the chosen level.
+	 * Takes effect from the NEXT turn; resolution stays server-owned.
+	 */
+	setReasoningEffort(level: ReasoningEffort): Promise<ReasoningEffortResult | null>;
+	/**
 	 * Fetch the workspace conversation's language-server status (`GET /conversations/:id/lsp`).
 	 * The backend lazily spawns servers, so this may take a moment on the first call for a cwd.
 	 */
@@ -234,6 +254,29 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 		}
 	}
 
+	// The workspace conversation's persisted reasoning effort. Seeded from the
+	// backend on focus change; null = never set (the server default applies).
+	let reasoningEffort = $state<ReasoningEffort | null>(null);
+
+	/** Refetch the workspace conversation's reasoning effort (works for a draft too). */
+	async function refreshReasoningEffort(): Promise<void> {
+		const id = workspaceConversationId();
+		// Clear immediately so a switch never shows the PREVIOUS conversation's level
+		// while the fetch is in flight (null renders as the server default).
+		reasoningEffort = null;
+		try {
+			const res = await fetchImpl(
+				`${httpBase}/conversations/${encodeURIComponent(id)}/reasoning-effort`,
+			);
+			if (!res.ok) return;
+			const data = (await res.json()) as ReasoningEffortResponse;
+			// Guard a slow response losing a race with a conversation switch.
+			if (workspaceConversationId() === id) reasoningEffort = data.reasoningEffort ?? null;
+		} catch {
+			// Non-fatal: an effort fetch failure just leaves the default rendering.
+		}
+	}
+
 	function getActiveChat(): ChatStore {
 		const activeId = tabsStore.activeConversationId;
 		if (activeId === null) {
@@ -434,6 +477,7 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 
 	refreshActiveChat();
 	void refreshCwd();
+	void refreshReasoningEffort();
 
 	return {
 		get tabs(): readonly Tab[] {
@@ -468,6 +512,9 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 		get cwd(): string | null {
 			return cwd;
 		},
+		get reasoningEffort(): ReasoningEffort | null {
+			return reasoningEffort;
+		},
 		get currentConversationId(): string {
 			return workspaceConversationId();
 		},
@@ -499,6 +546,7 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 				// surfaces (e.g. cache-warming) to its id.
 				syncSubscriptions();
 				void refreshCwd();
+				void refreshReasoningEffort();
 				// Now send on the promoted store
 				chatStores.get(conversationId)?.send(text);
 			} else {
@@ -525,6 +573,7 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 			refreshActiveChat();
 			syncSubscriptions();
 			void refreshCwd();
+			void refreshReasoningEffort();
 		},
 
 		selectTab(conversationId: string): void {
@@ -536,6 +585,7 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 			refreshActiveChat();
 			syncSubscriptions();
 			void refreshCwd();
+			void refreshReasoningEffort();
 		},
 
 		closeTab(conversationId: string): void {
@@ -554,6 +604,7 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 			refreshActiveChat();
 			syncSubscriptions();
 			void refreshCwd();
+			void refreshReasoningEffort();
 		},
 
 		invoke(surfaceId: string, actionId: string, payload?: unknown): void {
@@ -612,6 +663,37 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 			}
 		},
 
+		async setReasoningEffort(level: ReasoningEffort): Promise<ReasoningEffortResult | null> {
+			const id = workspaceConversationId();
+			const body: SetReasoningEffortRequest = { reasoningEffort: level };
+			try {
+				const res = await fetchImpl(
+					`${httpBase}/conversations/${encodeURIComponent(id)}/reasoning-effort`,
+					{
+						method: "PUT",
+						headers: { "content-type": "application/json" },
+						body: JSON.stringify(body),
+					},
+				);
+				if (!res.ok) {
+					const errBody = (await res.json().catch(() => null)) as { error?: string } | null;
+					return {
+						ok: false,
+						error: errBody?.error ?? `Set reasoning effort failed (HTTP ${res.status})`,
+					};
+				}
+				const data = (await res.json()) as ReasoningEffortResponse;
+				const next = data.reasoningEffort ?? level;
+				if (workspaceConversationId() === id) reasoningEffort = next;
+				return { ok: true, reasoningEffort: next };
+			} catch (err) {
+				return {
+					ok: false,
+					error: err instanceof Error ? err.message : "Set reasoning effort request failed",
+				};
+			}
+		},
+
 		async lspStatus(): Promise<LspResult | null> {
 			const id = workspaceConversationId();
 			try {
diff --git a/src/app/store.test.ts b/src/app/store.test.ts
index f4b5a0f..db6fdaa 100644
--- a/src/app/store.test.ts
+++ b/src/app/store.test.ts
@@ -708,6 +708,103 @@ describe("createAppStore", () => {
 		store.dispose();
 	});
 
+	it("seeds reasoningEffort from GET /conversations/:id/reasoning-effort (null = never set)", async () => {
+		const base = fakeFetchImpl();
+		const fetchImpl: typeof fetch = async (input, init) => {
+			const url = typeof input === "string" ? input : input instanceof URL ? input.href : input.url;
+			if (url.endsWith("/reasoning-effort")) {
+				return new Response(JSON.stringify({ conversationId: "x", reasoningEffort: "xhigh" }), {
+					status: 200,
+				});
+			}
+			return base(input, init);
+		};
+		const ws = fakeSocket();
+		const store = createAppStore({
+			socketFactory: () => ws,
+			fetchImpl,
+			localStorage: createFakeStorage(),
+		});
+		ws.resolveOpen();
+
+		await vi.waitFor(() => {
+			expect(store.reasoningEffort).toBe("xhigh");
+		});
+
+		store.dispose();
+	});
+
+	it("setReasoningEffort PUTs the level and updates local state from the echo", async () => {
+		const calls: { url: string; method: string; body: string | undefined }[] = [];
+		const base = fakeFetchImpl();
+		const fetchImpl: typeof fetch = async (input, init) => {
+			const url = typeof input === "string" ? input : input instanceof URL ? input.href : input.url;
+			calls.push({ url, method: init?.method ?? "GET", body: init?.body as string | undefined });
+			if (url.endsWith("/reasoning-effort") && init?.method === "PUT") {
+				const sent = JSON.parse(init.body as string) as { reasoningEffort: string };
+				return new Response(
+					JSON.stringify({ conversationId: "x", reasoningEffort: sent.reasoningEffort }),
+					{ status: 200 },
+				);
+			}
+			if (url.endsWith("/reasoning-effort")) {
+				return new Response(JSON.stringify({ conversationId: "x", reasoningEffort: null }), {
+					status: 200,
+				});
+			}
+			return base(input, init);
+		};
+		const ws = fakeSocket();
+		const store = createAppStore({
+			socketFactory: () => ws,
+			fetchImpl,
+			localStorage: createFakeStorage(),
+		});
+		ws.resolveOpen();
+
+		const result = await store.setReasoningEffort("max");
+		expect(result).toEqual({ ok: true, reasoningEffort: "max" });
+		expect(store.reasoningEffort).toBe("max");
+
+		const put = calls.find((c) => c.method === "PUT" && c.url.endsWith("/reasoning-effort"));
+		expect(put).toBeDefined();
+		// The PUT targets the workspace conversation (draft id works too) and
+		// carries exactly the SetReasoningEffortRequest body.
+		expect(put?.url).toContain(`/conversations/${store.currentConversationId}/`);
+		expect(JSON.parse(put?.body ?? "{}")).toEqual({ reasoningEffort: "max" });
+
+		store.dispose();
+	});
+
+	it("setReasoningEffort surfaces a 400 error and leaves state unchanged", async () => {
+		const base = fakeFetchImpl();
+		const fetchImpl: typeof fetch = async (input, init) => {
+			const url = typeof input === "string" ? input : input instanceof URL ? input.href : input.url;
+			if (url.endsWith("/reasoning-effort") && init?.method === "PUT") {
+				return new Response(JSON.stringify({ error: "bad level" }), { status: 400 });
+			}
+			if (url.endsWith("/reasoning-effort")) {
+				return new Response(JSON.stringify({ conversationId: "x", reasoningEffort: null }), {
+					status: 200,
+				});
+			}
+			return base(input, init);
+		};
+		const ws = fakeSocket();
+		const store = createAppStore({
+			socketFactory: () => ws,
+			fetchImpl,
+			localStorage: createFakeStorage(),
+		});
+		ws.resolveOpen();
+
+		const result = await store.setReasoningEffort("max");
+		expect(result).toEqual({ ok: false, error: "bad level" });
+		expect(store.reasoningEffort).toBeNull();
+
+		store.dispose();
+	});
+
 	it("does NOT re-scope a scope:'global' surface on conversation switch (no churn)", () => {
 		const ws = fakeSocket();
 		const store = createAppStore({