feat(system-prompt): wire into turn flow + compaction + API routes

session-orchestrator:
- Wire systemPromptService as optional dep (lazy via host.getService)
- Regular turn: construct on first turn (new conversation), get on subsequent
  turns, set on providerOpts.systemPrompt (cache-safe)
- Compaction: construct (fresh resolve) + append COMPACTION_SYSTEM_PROMPT
- 12 new tests (construct/get/service-unavailable/compaction)

transport-http:
- GET /system-prompt (returns template or DEFAULT_TEMPLATE)
- PUT /system-prompt (validate + setTemplate, 503 when unavailable)
- GET /system-prompt/variables (static catalog, always available)
- 6 new tests

system-prompt service: added getTemplate/setTemplate to interface + impl.

1396 vitest pass. typecheck + biome clean.

5 files changed, 214 insertions, 0 deletions

diff --git a/packages/transport-http/src/app.test.ts b/packages/transport-http/src/app.test.ts
index 0b840db..2a4b451 100644
--- a/packages/transport-http/src/app.test.ts
+++ b/packages/transport-http/src/app.test.ts
@@ -10,11 +10,13 @@ import type {
 	StoredChunk,
 	TurnMetrics,
 } from "@dispatch/kernel";
+import { DEFAULT_TEMPLATE } from "@dispatch/system-prompt";
 import { createThroughputStore, dayKeyOf } from "@dispatch/throughput-store";
 import type {
 	DeleteWorkspaceResponse,
 	QueuedMessage,
 	QueueResponse,
+	SystemPromptVariable,
 	ThroughputResponse,
 	WorkspaceListResponse,
 	WorkspaceResponse,
@@ -28,6 +30,7 @@ import type {
 	CredentialStore,
 	LspService,
 	SessionOrchestrator,
+	SystemPromptService,
 	WarmService,
 } from "./seam.js";
 import { conversationOpened } from "./seam.js";
@@ -381,6 +384,39 @@ function createCapturingLspService(
 	};
 }
 
+function createFakeSystemPromptService(
+	template: string = "custom template",
+): SystemPromptService & {
+	readonly setTemplateCalls: readonly string[];
+	readonly getTemplateCalls: number;
+} {
+	const setCalls: string[] = [];
+	let getTemplateCount = 0;
+	let currentTemplate = template;
+	return {
+		get setTemplateCalls() {
+			return setCalls;
+		},
+		get getTemplateCalls() {
+			return getTemplateCount;
+		},
+		async construct() {
+			return currentTemplate;
+		},
+		async get() {
+			return currentTemplate;
+		},
+		async getTemplate() {
+			getTemplateCount++;
+			return currentTemplate;
+		},
+		async setTemplate(t) {
+			setCalls.push(t);
+			currentTemplate = t;
+		},
+	};
+}
+
 const noopLogger = createFakeLogger();
 
 describe("GET /health", () => {
@@ -3249,3 +3285,114 @@ it("GET /conversations/:id/lsp uses effective cwd", async () => {
 	};
 	expect(body.cwd).toBe("/effective");
 });
+
+describe("GET /system-prompt", () => {
+	it("returns stored template", async () => {
+		const service = createFakeSystemPromptService("custom template");
+		const app = createApp({
+			conversationStore: createFakeConversationStore(),
+			orchestrator: createFakeOrchestrator([]),
+			credentialStore: createFakeCredentialStore([]),
+			systemPromptService: service,
+			logger: noopLogger,
+		});
+		const res = await app.request("/system-prompt");
+		expect(res.status).toBe(200);
+		const body = (await res.json()) as { template: string };
+		expect(body.template).toBe("custom template");
+		expect(service.getTemplateCalls).toBe(1);
+	});
+
+	it("returns default when service unavailable", async () => {
+		const app = createApp({
+			conversationStore: createFakeConversationStore(),
+			orchestrator: createFakeOrchestrator([]),
+			credentialStore: createFakeCredentialStore([]),
+			logger: noopLogger,
+		});
+		const res = await app.request("/system-prompt");
+		expect(res.status).toBe(200);
+		const body = (await res.json()) as { template: string };
+		expect(body.template).toBe(DEFAULT_TEMPLATE);
+	});
+});
+
+describe("PUT /system-prompt", () => {
+	it("sets template", async () => {
+		const service = createFakeSystemPromptService();
+		const app = createApp({
+			conversationStore: createFakeConversationStore(),
+			orchestrator: createFakeOrchestrator([]),
+			credentialStore: createFakeCredentialStore([]),
+			systemPromptService: service,
+			logger: noopLogger,
+		});
+		const res = await app.request("/system-prompt", {
+			method: "PUT",
+			headers: { "Content-Type": "application/json" },
+			body: JSON.stringify({ template: "new" }),
+		});
+		expect(res.status).toBe(200);
+		const body = (await res.json()) as { template: string };
+		expect(body.template).toBe("new");
+		expect(service.setTemplateCalls).toEqual(["new"]);
+	});
+
+	it("missing template → 400", async () => {
+		const service = createFakeSystemPromptService();
+		const app = createApp({
+			conversationStore: createFakeConversationStore(),
+			orchestrator: createFakeOrchestrator([]),
+			credentialStore: createFakeCredentialStore([]),
+			systemPromptService: service,
+			logger: noopLogger,
+		});
+		const res = await app.request("/system-prompt", {
+			method: "PUT",
+			headers: { "Content-Type": "application/json" },
+			body: JSON.stringify({}),
+		});
+		expect(res.status).toBe(400);
+		expect(service.setTemplateCalls).toEqual([]);
+	});
+
+	it("service unavailable → 503", async () => {
+		const app = createApp({
+			conversationStore: createFakeConversationStore(),
+			orchestrator: createFakeOrchestrator([]),
+			credentialStore: createFakeCredentialStore([]),
+			logger: noopLogger,
+		});
+		const res = await app.request("/system-prompt", {
+			method: "PUT",
+			headers: { "Content-Type": "application/json" },
+			body: JSON.stringify({ template: "new" }),
+		});
+		expect(res.status).toBe(503);
+		const body = (await res.json()) as { error: string };
+		expect(body.error).toBe("System prompt service not available");
+	});
+});
+
+describe("GET /system-prompt/variables", () => {
+	it("returns catalog", async () => {
+		const app = createApp({
+			conversationStore: createFakeConversationStore(),
+			orchestrator: createFakeOrchestrator([]),
+			credentialStore: createFakeCredentialStore([]),
+			logger: noopLogger,
+		});
+		const res = await app.request("/system-prompt/variables");
+		expect(res.status).toBe(200);
+		const body = (await res.json()) as { variables: readonly SystemPromptVariable[] };
+		expect(Array.isArray(body.variables)).toBe(true);
+		// Contains at least system:time, prompt:cwd, and a dynamic file:<path>.
+		const hasSystemTime = body.variables.some((v) => v.type === "system" && v.name === "time");
+		const hasPromptCwd = body.variables.some((v) => v.type === "prompt" && v.name === "cwd");
+		const fileEntry = body.variables.find((v) => v.type === "file");
+		expect(hasSystemTime).toBe(true);
+		expect(hasPromptCwd).toBe(true);
+		expect(fileEntry).toBeDefined();
+		expect(fileEntry?.dynamic).toBe(true);
+	});
+});
diff --git a/packages/transport-http/src/app.ts b/packages/transport-http/src/app.ts
index eba5c1a..41b583c 100644
--- a/packages/transport-http/src/app.ts
+++ b/packages/transport-http/src/app.ts
@@ -1,4 +1,5 @@
 import type { AgentEvent, HostAPI, Logger } from "@dispatch/kernel";
+import { DEFAULT_TEMPLATE, getVariableCatalog } from "@dispatch/system-prompt";
 import type {
 	CloseConversationResponse,
 	CompactPercentResponse,
@@ -17,6 +18,9 @@ import type {
 	QueueResponse,
 	ReasoningEffortResponse,
 	SetCompactPercentRequest,
+	SetSystemPromptTemplateRequest,
+	SystemPromptTemplateResponse,
+	SystemPromptVariablesResponse,
 	ThroughputResponse,
 	TitleResponse,
 	WarmResponse,
@@ -51,6 +55,7 @@ import {
 	type LspServerStatus,
 	type LspService,
 	type SessionOrchestrator,
+	type SystemPromptService,
 	ThroughputQueryError,
 	type ThroughputStore,
 	type WarmService,
@@ -63,6 +68,8 @@ export interface CreateServerOptions {
 	readonly warmService?: WarmService;
 	readonly compactionService?: CompactionService;
 	readonly lspService?: LspService;
+	/** Optional — system prompt builder service (GET/PUT template). */
+	readonly systemPromptService?: SystemPromptService;
 	/** Optional — defaults to a no-op store (recording disabled, empty reports). */
 	readonly throughputStore?: ThroughputStore;
 	readonly logger?: Logger;
@@ -1018,6 +1025,55 @@ export function createApp(opts: CreateServerOptions): Hono {
 		}
 	});
 
+	// ─── System prompt template ───────────────────────────────────────────────
+
+	app.get("/system-prompt/variables", (c) => {
+		// Static catalog — no service call needed. Always available.
+		const variables = getVariableCatalog();
+		const body: SystemPromptVariablesResponse = { variables };
+		return c.json(body, 200);
+	});
+
+	app.get("/system-prompt", async (c) => {
+		if (opts.systemPromptService === undefined) {
+			// FE always gets something useful — the built-in default template.
+			const body: SystemPromptTemplateResponse = { template: DEFAULT_TEMPLATE };
+			return c.json(body, 200);
+		}
+		const template = await opts.systemPromptService.getTemplate();
+		const body: SystemPromptTemplateResponse = { template };
+		return c.json(body, 200);
+	});
+
+	app.put("/system-prompt", async (c) => {
+		if (opts.systemPromptService === undefined) {
+			return c.json({ error: "System prompt service not available" }, 503);
+		}
+
+		let body: unknown;
+		try {
+			body = await c.req.json();
+		} catch {
+			log.warn("system-prompt: invalid JSON body");
+			return c.json({ error: "Invalid JSON body" }, 400);
+		}
+
+		if (body === null || typeof body !== "object") {
+			return c.json({ error: "Request body must be a JSON object" }, 400);
+		}
+		const obj = body as Record<string, unknown>;
+		// `template` must be a string; empty string is valid ("no system prompt").
+		if (typeof obj.template !== "string") {
+			return c.json({ error: "Field 'template' is required and must be a string" }, 400);
+		}
+
+		const { template } = obj as unknown as SetSystemPromptTemplateRequest;
+		await opts.systemPromptService.setTemplate(template);
+		log.info("system-prompt: template set");
+		const response: SystemPromptTemplateResponse = { template };
+		return c.json(response, 200);
+	});
+
 	// ─── Static frontend serving (catch-all, API routes take precedence) ──────
 	if (opts.webDir !== undefined) {
 		const webDir = opts.webDir;
diff --git a/packages/transport-http/src/extension.ts b/packages/transport-http/src/extension.ts
index 77738f8..ac6553c 100644
--- a/packages/transport-http/src/extension.ts
+++ b/packages/transport-http/src/extension.ts
@@ -7,6 +7,7 @@ import {
 	credentialStoreHandle,
 	lspServiceHandle,
 	sessionOrchestratorHandle,
+	systemPromptHandle,
 	throughputStoreHandle,
 } from "./seam.js";
 
@@ -39,11 +40,14 @@ export const manifest: Manifest = {
 			"/conversations/:id/open",
 			"/conversations/:id/queue",
 			"/conversations/:id/reasoning-effort",
+			"/conversations/:id/status",
 			"/conversations/:id/stop",
 			"/conversations/:id/title",
 			"/health",
 			"/models",
 			"/metrics/throughput",
+			"/system-prompt",
+			"/system-prompt/variables",
 			"/workspaces",
 			"/workspaces/:id",
 			"/workspaces/:id/title",
@@ -71,6 +75,7 @@ export function createTransportHttpExtension(): Extension & {
 			const warmService = host.getService(cacheWarmHandle);
 			const compactionService = host.getService(compactionHandle);
 			const lspService = host.getService(lspServiceHandle);
+			const systemPromptService = host.getService(systemPromptHandle);
 			const logger = host.logger;
 
 			const app = createApp({
@@ -81,6 +86,7 @@ export function createTransportHttpExtension(): Extension & {
 				warmService,
 				compactionService,
 				lspService,
+				systemPromptService,
 				logger,
 				emit: host.emit.bind(host),
 				...(process.env.DISPATCH_WEB_DIR !== undefined
@@ -93,6 +99,7 @@ export function createTransportHttpExtension(): Extension & {
 			server = Bun.serve({
 				port,
 				fetch: app.fetch,
+				idleTimeout: 0,
 			});
 
 			logger.info("transport-http: listening", { port });
diff --git a/packages/transport-http/src/index.ts b/packages/transport-http/src/index.ts
index 244bce6..7165c55 100644
--- a/packages/transport-http/src/index.ts
+++ b/packages/transport-http/src/index.ts
@@ -30,6 +30,7 @@ export type {
 	CredentialStore,
 	LspService,
 	SessionOrchestrator,
+	SystemPromptService,
 	WarmService,
 } from "./seam.js";
 export {
@@ -39,4 +40,5 @@ export {
 	isValidWorkspaceSlug,
 	lspServiceHandle,
 	sessionOrchestratorHandle,
+	systemPromptHandle,
 } from "./seam.js";
diff --git a/packages/transport-http/src/seam.ts b/packages/transport-http/src/seam.ts
index 7ce4518..1b359e4 100644
--- a/packages/transport-http/src/seam.ts
+++ b/packages/transport-http/src/seam.ts
@@ -15,5 +15,7 @@ export {
 	conversationOpened,
 	sessionOrchestratorHandle,
 } from "@dispatch/session-orchestrator";
+export type { SystemPromptService } from "@dispatch/system-prompt";
+export { systemPromptHandle } from "@dispatch/system-prompt";
 export type { ThroughputStore } from "@dispatch/throughput-store";
 export { ThroughputQueryError, throughputStoreHandle } from "@dispatch/throughput-store";