diff --git a/docs/rpc.md b/docs/rpc.md
index f1f27db30..eb324074c 100644
--- a/docs/rpc.md
+++ b/docs/rpc.md
@@ -124,6 +124,7 @@ Important edge behavior from runtime:
 ### State
 
 - `{ id?, type: "get_state" }`
+- `{ id?, type: "set_fast_mode", enabled: boolean }`
 - `{ id?, type: "get_available_commands" }`
 - `{ id?, type: "set_todos", phases: TodoPhase[] }`
 - `{ id?, type: "set_host_tools", tools: RpcHostToolDefinition[] }`
@@ -231,6 +232,10 @@ Local-only slash commands may emit `command_output` frames before completing via
 
 ### `get_state` payload
 
+`tokensPerSecond` is a number when output throughput is available and `null`
+otherwise. `fastModeEnabled` reports the session setting, while
+`fastModeActive` reports whether the current model realizes fast mode.
+
 ```json
 {
   "model": { "provider": "...", "id": "..." },
@@ -243,6 +248,9 @@ Local-only slash commands may emit `command_output` frames before completing via
   "sessionFile": "...",
   "sessionId": "...",
   "sessionName": "...",
+  "fastModeEnabled": false,
+  "tokensPerSecond": null,
+  "fastModeActive": false,
   "autoCompactionEnabled": true,
   "messageCount": 0,
   "queuedMessageCount": 0,
@@ -275,6 +283,56 @@ Local-only slash commands may emit `command_output` frames before completing via
 }
 ```
 
+### `set_fast_mode` payload
+
+`set_fast_mode` changes whether fast mode is enabled for the session. The
+request is:
+
+```json
+{ "id": "req_fast_on", "type": "set_fast_mode", "enabled": true }
+```
+
+On success, `data` always contains both `enabled` and `active`. `enabled`
+reports the session setting; `active` reports whether fast mode is realized by
+the current model:
+
+```json
+{
+  "id": "req_fast_on",
+  "type": "response",
+  "command": "set_fast_mode",
+  "success": true,
+  "data": { "enabled": true, "active": true }
+}
+```
+
+Enabling fast mode on a model without a service-tier family fails with the
+exact error below:
+
+```json
+{
+  "id": "req_fast_on",
+  "type": "response",
+  "command": "set_fast_mode",
+  "success": false,
+  "error": "Fast mode is unavailable for the current model."
+}
+```
+
+Disabling fast mode is idempotent, including on an unsupported model. It
+succeeds as an off/no-op result and leaves `get_state.fastModeEnabled` and
+`get_state.fastModeActive` false:
+
+```json
+{
+  "id": "req_fast_off",
+  "type": "response",
+  "command": "set_fast_mode",
+  "success": true,
+  "data": { "enabled": false, "active": false }
+}
+```
+
 ### `set_todos` payload
 
 Replaces the in-memory todo state for the current session and returns the normalized phase list:
diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 1953032df..630029a34 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -5,6 +5,8 @@
 ### Added
 
 - Added tokens-per-second throughput to RPC `get_state` responses for non-TUI clients.
+- Added the RPC `set_fast_mode` command for live fast-mode control.
+- Added `fastModeEnabled` and `fastModeActive` to RPC `get_state` responses.
 
 ## [17.1.8] - 2026-07-28
 
diff --git a/packages/coding-agent/src/modes/rpc/rpc-mode.ts b/packages/coding-agent/src/modes/rpc/rpc-mode.ts
index 66d567a53..952079365 100644
--- a/packages/coding-agent/src/modes/rpc/rpc-mode.ts
+++ b/packages/coding-agent/src/modes/rpc/rpc-mode.ts
@@ -1104,7 +1104,8 @@ export async function runRpcMode(
 			}
 
 			case "set_fast_mode": {
-				if (!session.setFastMode(command.enabled)) {
+				const supported = session.setFastMode(command.enabled);
+				if (command.enabled && !supported) {
 					return error(id, "set_fast_mode", "Fast mode is unavailable for the current model.");
 				}
 				return success(id, "set_fast_mode", {
diff --git a/packages/coding-agent/test/rpc.test.ts b/packages/coding-agent/test/rpc.test.ts
index 817c6de75..8cc867afb 100644
--- a/packages/coding-agent/test/rpc.test.ts
+++ b/packages/coding-agent/test/rpc.test.ts
@@ -315,3 +315,44 @@ describe.skipIf(!e2eApiKey("ANTHROPIC_API_KEY"))("RPC mode", () => {
 		expect(text).toContain("test123");
 	}, 90000);
 });
+
+describe("RPC fast mode with unsupported model", () => {
+	let client: RpcClient;
+	let sessionDir: string;
+
+	beforeEach(() => {
+		sessionDir = path.join(os.tmpdir(), `omp-rpc-fast-mode-test-${Snowflake.next()}`);
+		client = new RpcClient({
+			cliPath: path.join(import.meta.dir, "..", "src", "cli.ts"),
+			cwd: path.join(import.meta.dir, ".."),
+			env: {
+				PI_CODING_AGENT_DIR: sessionDir,
+				FIREWORKS_API_KEY: "test-fireworks-key",
+			},
+			provider: "fireworks",
+			model: "deepseek-v4-flash",
+		});
+	});
+
+	afterEach(async () => {
+		await client.stop();
+		if (sessionDir && fs.existsSync(sessionDir)) {
+			removeSyncWithRetries(sessionDir);
+		}
+	});
+
+	test("rejects enable but idempotently accepts disable", async () => {
+		await client.start();
+
+		await expect(client.setFastMode(true)).rejects.toMatchObject({
+			message: "Fast mode is unavailable for the current model.",
+		});
+
+		const disabled = await client.setFastMode(false);
+		expect(disabled).toEqual({ enabled: false, active: false });
+
+		const state = await client.getState();
+		expect(state.fastModeEnabled).toBe(false);
+		expect(state.fastModeActive).toBe(false);
+	}, 30000);
+});