diff --git a/packages/coding-agent/src/sdk.ts b/packages/coding-agent/src/sdk.ts
index bca73c40e..3321e78ac 100644
--- a/packages/coding-agent/src/sdk.ts
+++ b/packages/coding-agent/src/sdk.ts
@@ -1525,10 +1525,19 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		// entries capture it at fetch time and are dropped at injection if a newer
 		// mutation (any tool) bumped it in the meantime.
 		const fileMutationVersions = new Map<string, number>();
+		const activeToolNames = new Set<string>();
+		const setActiveToolNames = (names: Iterable<string>): void => {
+			activeToolNames.clear();
+			for (const name of names) {
+				activeToolNames.add(name);
+			}
+		};
 		const toolSession: ToolSession = {
 			get cwd() {
 				return sessionManager.getCwd();
 			},
+			isToolActive: name => activeToolNames.has(name),
+			setActiveToolNames,
 			hasUI: options.hasUI ?? false,
 			enableLsp,
 			get hasEditTool() {
@@ -2540,6 +2549,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		});
 		hasRegistered = true;
 
+		setActiveToolNames(initialToolNames);
 		const { systemPrompt } = await logger.time(
 			"buildSystemPrompt",
 			rebuildSystemPrompt,
@@ -2834,6 +2844,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			rebuildSystemPrompt,
 			reloadSshTool,
 			requestedToolNames: requestedToolNameSet,
+			setActiveToolNames,
 			getMcpServerInstructions: mcpManager
 				? () => {
 						const raw = mcpManager.getServerInstructions();
diff --git a/packages/coding-agent/src/session/agent-session.ts b/packages/coding-agent/src/session/agent-session.ts
index 5b36e37a6..8be2067d4 100644
--- a/packages/coding-agent/src/session/agent-session.ts
+++ b/packages/coding-agent/src/session/agent-session.ts
@@ -686,6 +686,8 @@ export interface AgentSessionConfig {
 	toolRegistry?: Map<string, AgentTool>;
 	/** Tool names whose current registry entry is still the built-in implementation. */
 	builtInToolNames?: Iterable<string>;
+	/** Update tool-session predicates that render guidance from the live active tool set. */
+	setActiveToolNames?: (names: Iterable<string>) => void;
 	/** Current session pre-LLM message transform pipeline */
 	transformContext?: (messages: AgentMessage[], signal?: AbortSignal) => AgentMessage[] | Promise<AgentMessage[]>;
 	/**
@@ -1717,6 +1719,7 @@ export class AgentSession {
 		| undefined;
 	#getMcpServerInstructions: (() => Map<string, string> | undefined) | undefined;
 	#reloadSshTool: (() => Promise<AgentTool | null>) | undefined;
+	#setActiveToolNames: ((names: Iterable<string>) => void) | undefined;
 	#disconnectOwnedMcpManager: (() => Promise<void>) | undefined;
 	#requestedToolNames: ReadonlySet<string> | undefined;
 	#baseSystemPrompt: string[];
@@ -2155,6 +2158,7 @@ export class AgentSession {
 		this.#rebuildSystemPrompt = config.rebuildSystemPrompt;
 		this.#getMcpServerInstructions = config.getMcpServerInstructions;
 		this.#reloadSshTool = config.reloadSshTool;
+		this.#setActiveToolNames = config.setActiveToolNames;
 		this.#disconnectOwnedMcpManager = config.disconnectOwnedMcpManager;
 		this.#baseSystemPrompt = this.agent.state.systemPrompt;
 		this.#promptModelKey = this.#currentPromptModelKey();
@@ -6290,6 +6294,7 @@ export class AgentSession {
 				),
 			);
 		}
+		this.#setActiveToolNames?.(validToolNames);
 		const activeNameSet = new Set(validToolNames);
 		for (const name of Array.from(this.#selectedDiscoveredToolNames)) {
 			if (!activeNameSet.has(name) || isMCPToolName(name) || !this.#toolRegistry.has(name)) {
@@ -6395,6 +6400,7 @@ export class AgentSession {
 	async refreshBaseSystemPrompt(): Promise<void> {
 		if (!this.#rebuildSystemPrompt) return;
 		const activeToolNames = this.getActiveToolNames();
+		this.#setActiveToolNames?.(activeToolNames);
 		const built = await this.#rebuildSystemPrompt(activeToolNames, this.#toolRegistry);
 		this.#baseSystemPrompt = built.systemPrompt;
 		this.#baseSystemPromptBeforeMemoryPromotion = undefined;
diff --git a/packages/coding-agent/src/tools/bash.ts b/packages/coding-agent/src/tools/bash.ts
index 9feef6a76..ea9f4a031 100644
--- a/packages/coding-agent/src/tools/bash.ts
+++ b/packages/coding-agent/src/tools/bash.ts
@@ -376,7 +376,21 @@ export class BashTool implements AgentTool<typeof bashSchemaBase | typeof bashSc
 	};
 	readonly label = "Bash";
 	readonly loadMode = "essential";
-	readonly description: string;
+	get description(): string {
+		const evalBackends = resolveEvalBackends(this.session);
+		const isToolActive = (name: string, fallback: boolean): boolean => this.session.isToolActive?.(name) ?? fallback;
+		return prompt.render(bashDescription, {
+			asyncEnabled: this.#asyncEnabled,
+			autoBackgroundEnabled: this.#autoBackgroundEnabled,
+			autoBackgroundThresholdSeconds: Math.max(0, Math.floor(this.#autoBackgroundThresholdMs / 1000)),
+			hasAstGrep: isToolActive("ast_grep", this.session.settings.get("astGrep.enabled")),
+			hasAstEdit: isToolActive("ast_edit", this.session.settings.get("astEdit.enabled")),
+			hasGrep: isToolActive("grep", this.session.settings.get("grep.enabled")),
+			hasGlob: isToolActive("glob", this.session.settings.get("glob.enabled")),
+			hasRead: isToolActive("read", true),
+			hasEval: isToolActive("eval", evalBackends.python || evalBackends.js || evalBackends.ruby || evalBackends.julia),
+		});
+	}
 	readonly parameters: BashToolSchema;
 	// Non-pty calls run alongside each other (the executor isolates overlapping
 	// runs on the same shell session); pty takes over the terminal UI and must
@@ -398,19 +412,6 @@ export class BashTool implements AgentTool<typeof bashSchemaBase | typeof bashSc
 			),
 		);
 		this.parameters = this.#asyncEnabled ? bashSchemaWithAsync : bashSchemaBase;
-		const evalBackends = resolveEvalBackends(this.session);
-		const isToolActive = (name: string, fallback: boolean): boolean => this.session.isToolActive?.(name) ?? fallback;
-		this.description = prompt.render(bashDescription, {
-			asyncEnabled: this.#asyncEnabled,
-			autoBackgroundEnabled: this.#autoBackgroundEnabled,
-			autoBackgroundThresholdSeconds: Math.max(0, Math.floor(this.#autoBackgroundThresholdMs / 1000)),
-			hasAstGrep: isToolActive("ast_grep", this.session.settings.get("astGrep.enabled")),
-			hasAstEdit: isToolActive("ast_edit", this.session.settings.get("astEdit.enabled")),
-			hasGrep: isToolActive("grep", this.session.settings.get("grep.enabled")),
-			hasGlob: isToolActive("glob", this.session.settings.get("glob.enabled")),
-			hasRead: isToolActive("read", true),
-			hasEval: isToolActive("eval", evalBackends.python || evalBackends.js || evalBackends.ruby || evalBackends.julia),
-		});
 	}
 
 	#formatResultOutput(result: BashResult | BashInteractiveResult): string {
diff --git a/packages/coding-agent/src/tools/index.ts b/packages/coding-agent/src/tools/index.ts
index 83e39a517..8ea30c85d 100644
--- a/packages/coding-agent/src/tools/index.ts
+++ b/packages/coding-agent/src/tools/index.ts
@@ -226,6 +226,8 @@ export interface ToolSession {
 	getToolByName?: (name: string) => AgentTool | undefined;
 	/** Return whether a built-in tool is active in this turn's tool set. */
 	isToolActive?: (name: string) => boolean;
+	/** Update the active built-in tool predicate when a session changes tools mid-run. */
+	setActiveToolNames?: (names: Iterable<string>) => void;
 	/** Agent registry for IRC routing across live sessions. */
 	agentRegistry?: AgentRegistry;
 	/** Get artifacts directory for artifact:// URLs */
@@ -650,7 +652,11 @@ export async function createTools(session: ToolSession, toolNames?: string[]): P
 				];
 
 	const activeToolNames = new Set(baseEntries.map(([name]) => name));
-	session.isToolActive = name => activeToolNames.has(name);
+	if (session.setActiveToolNames) {
+		session.setActiveToolNames(activeToolNames);
+	} else {
+		session.isToolActive = name => activeToolNames.has(name);
+	}
 
 	const baseResults = await Promise.all(
 		baseEntries.map(async ([name, factory]) => {
diff --git a/packages/coding-agent/test/agent-session-tool-rebuild-skip.test.ts b/packages/coding-agent/test/agent-session-tool-rebuild-skip.test.ts
index 6cefd9c5b..221720e06 100644
--- a/packages/coding-agent/test/agent-session-tool-rebuild-skip.test.ts
+++ b/packages/coding-agent/test/agent-session-tool-rebuild-skip.test.ts
@@ -176,6 +176,52 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 		expect(rebuildCount).toBe(baseline + 2);
 	});
 
+	it("updates live active-tool predicates before rebuilding the prompt", async () => {
+		const activeToolNames = new Set(["read", "bash", "grep"]);
+		const readTool = createBasicTool("read", "Read");
+		const bashTool = createBasicTool("bash", "Bash");
+		const grepTool = createBasicTool("grep", "Grep");
+		Object.defineProperty(bashTool, "description", {
+			get: () => (activeToolNames.has("grep") ? "bash sees grep" : "bash hides grep"),
+			enumerable: true,
+			configurable: true,
+		});
+		const toolRegistry = new Map<string, AgentTool>([
+			[readTool.name, readTool],
+			[bashTool.name, bashTool],
+			[grepTool.name, grepTool],
+		]);
+		const agent = new Agent({
+			initialState: {
+				model: createModel(),
+				systemPrompt: ["initial"],
+				tools: [readTool, bashTool, grepTool],
+				messages: [],
+			},
+		});
+		const session = new AgentSession({
+			agent,
+			sessionManager: SessionManager.inMemory(),
+			settings: Settings.isolated({ "compaction.enabled": false }),
+			modelRegistry: {} as never,
+			toolRegistry,
+			setActiveToolNames: names => {
+				activeToolNames.clear();
+				for (const name of names) {
+					activeToolNames.add(name);
+				}
+			},
+			rebuildSystemPrompt: async (_toolNames, tools) => ({
+				systemPrompt: [tools.get("bash")?.description ?? "missing bash"],
+			}),
+		});
+		sessions.push(session);
+
+		await session.setActiveToolsByName(["read", "bash"]);
+
+		expect(agent.state.systemPrompt).toEqual(["bash hides grep"]);
+	});
+
 	it("does not skip when refreshBaseSystemPrompt is called explicitly", async () => {
 		let rebuildCount = 0;
 		const { session } = newSession(async toolNames => {
diff --git a/packages/coding-agent/test/tools/index.test.ts b/packages/coding-agent/test/tools/index.test.ts
index 8978dbfb3..845a285b1 100644
--- a/packages/coding-agent/test/tools/index.test.ts
+++ b/packages/coding-agent/test/tools/index.test.ts
@@ -273,6 +273,28 @@ describe("createTools", () => {
 		expect(session.isToolActive?.("read")).toBe(false);
 	});
 
+	it("renders bash guidance from the live active tool predicate", async () => {
+		const activeToolNames = new Set<string>();
+		const session = createTestSession({
+			isToolActive: name => activeToolNames.has(name),
+			setActiveToolNames: names => {
+				activeToolNames.clear();
+				for (const name of names) {
+					activeToolNames.add(name);
+				}
+			},
+		});
+
+		const tools = await createTools(session, ["bash", "grep", "read", "glob"]);
+		const bash = tools.find(tool => tool.name === "bash");
+
+		expect(bash?.description).toContain("`grep` tool");
+		session.setActiveToolNames?.(["bash"]);
+		expect(bash?.description).not.toContain("`grep` tool");
+		expect(bash?.description).not.toContain("`ls` → `read`");
+		expect(bash?.description).not.toContain("`find` → the `glob` tool");
+	});
+
 	it("includes search_tool_bm25 when MCP tool discovery is enabled and executable", async () => {
 		const session = createTestSession({
 			settings: createSettingsWithOverrides({