diff --git a/packages/coding-agent/src/session/agent-session.ts b/packages/coding-agent/src/session/agent-session.ts
index e5658acac..da3a30ff4 100644
--- a/packages/coding-agent/src/session/agent-session.ts
+++ b/packages/coding-agent/src/session/agent-session.ts
@@ -4039,10 +4039,6 @@ export class AgentSession {
 		return this.#tools.applyActiveToolsByName(toolNames);
 	}
 
-	#takePendingXdevMountNotice(): CustomMessage | undefined {
-		return this.#tools.takePendingXdevMountNotice();
-	}
-
 	/** Rediscovers reloadable skills and refreshes prompt metadata. */
 	refreshSkills(): Promise<void> {
 		return this.#tools.refreshSkills();
@@ -5018,11 +5014,10 @@ export class AgentSession {
 			}
 
 			// A pending xd:// delta accompanies the next user-authored prompt,
-			// never an agent-initiated continuation.
-			const xdevMountNotice = isUserQueuedMessage(message) ? this.#takePendingXdevMountNotice() : undefined;
-			if (xdevMountNotice) {
-				messages.push(xdevMountNotice);
-			}
+			// never an agent-initiated continuation. Reserve its pre-user position,
+			// but consume it only after before_agent_start determines whether the
+			// final provider prompt still carries the base xd:// catalog.
+			const xdevMountNoticeIndex = messages.length;
 			messages.push(message);
 			// Inject any pending "nextTurn" messages as context alongside the user message
 			for (const msg of this.#pendingNextTurnMessages) {
@@ -5053,6 +5048,7 @@ export class AgentSession {
 			if ((this.#isDisposed && !disposingBeforeTransition) || this.#promptGeneration !== generation) return;
 			const beforeAgentStartSystemPrompt = await this.#buildSystemPromptForAgentStart(expandedText);
 
+			let baseXdevCatalogDelivered = true;
 			// Emit before_agent_start extension event
 			if (this.#extensionRunner) {
 				const result = await this.#extensionRunner.emitBeforeAgentStart(
@@ -5087,6 +5083,7 @@ export class AgentSession {
 				}
 
 				if (result?.systemPrompt !== undefined) {
+					baseXdevCatalogDelivered = false;
 					this.agent.setSystemPrompt(result.systemPrompt);
 				} else {
 					this.agent.setSystemPrompt(beforeAgentStartSystemPrompt);
@@ -5110,6 +5107,12 @@ export class AgentSession {
 					return;
 				}
 			}
+			const xdevMountNotice = isUserQueuedMessage(message)
+				? this.#tools.takePendingXdevMountNotice(baseXdevCatalogDelivered)
+				: undefined;
+			if (xdevMountNotice) {
+				messages.splice(xdevMountNoticeIndex, 0, xdevMountNotice);
+			}
 
 			await this.#maintenance.runPrePromptCompactionIfNeeded(messages);
 			if (this.#promptGeneration !== generation) {
diff --git a/packages/coding-agent/src/session/session-tools.ts b/packages/coding-agent/src/session/session-tools.ts
index cf7b1642f..56ea27b92 100644
--- a/packages/coding-agent/src/session/session-tools.ts
+++ b/packages/coding-agent/src/session/session-tools.ts
@@ -779,20 +779,23 @@ export class SessionTools {
 	}
 
 	/** Consumes the hidden notice for unannounced `xd://` mount changes. */
-	takePendingXdevMountNotice(): CustomMessage<XdevMountNoticeDetails> | undefined {
+	takePendingXdevMountNotice(baseCatalogDelivered: boolean): CustomMessage<XdevMountNoticeDetails> | undefined {
 		const pending = this.#pendingXdevMountDelta;
 		if (!pending) return undefined;
 		this.#pendingXdevMountDelta = undefined;
 		this.#ensureAnnouncedMountsSeeded();
 		// A pending add for a device the outgoing base prompt already lists in its
-		// catalog needs no notice line — but the delivery makes the model aware of
-		// it, so record it announced. Doing this here (at delivery) rather than when
-		// the prompt was rebuilt keeps add/remove coalescing intact: a device mounted
-		// then unmounted before any prompt is sent cancels out in
-		// {@link #notifyXdevMountDelta} and never emits a spurious "No longer mounted"
-		// notice for a device the model never saw (issue #7139 review).
-		for (const name of pending.added) {
-			if (this.#basePromptXdevNames.has(name)) this.#announcedMounts.add(name);
+		// catalog needs no notice line — but only when the final provider prompt
+		// still carries that base catalog. A `before_agent_start` replacement drops
+		// it, so its additions must remain in the notice. Record prompt-carried
+		// devices announced here, after the final prompt is known and immediately
+		// before delivery. The pending delta remains untouched by rebuilds, letting
+		// {@link #notifyXdevMountDelta} cancel a mount followed by an unmount before
+		// any request is sent (issue #7139 reviews).
+		if (baseCatalogDelivered) {
+			for (const name of pending.added) {
+				if (this.#basePromptXdevNames.has(name)) this.#announcedMounts.add(name);
+			}
 		}
 		// Only announce a net change relative to what the model already knows (from
 		// this session and persisted history): a re-mount of an already-announced
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 dc3471b6b..0cebefd9c 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
@@ -5,6 +5,7 @@ import { createMockModel, type MockResponseSource } from "@oh-my-pi/pi-ai/provid
 import { buildModel } from "@oh-my-pi/pi-catalog/build";
 import { Settings } from "@oh-my-pi/pi-coding-agent/config/settings";
 import type { CustomTool } from "@oh-my-pi/pi-coding-agent/extensibility/custom-tools/types";
+import type { ExtensionRunner } from "@oh-my-pi/pi-coding-agent/extensibility/extensions";
 import { AgentSession } from "@oh-my-pi/pi-coding-agent/session/agent-session";
 import { type CustomMessage, convertToLlm } from "@oh-my-pi/pi-coding-agent/session/messages";
 import { SessionManager } from "@oh-my-pi/pi-coding-agent/session/session-manager";
@@ -110,6 +111,8 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 		 * path. Existing tests leave this off, so their rebuild carries no catalog.
 		 */
 		exposeXdevCatalog?: boolean;
+		/** Optional per-turn system prompt replacement returned by before_agent_start. */
+		beforeAgentStartSystemPrompt?: string[];
 	}
 
 	function newSession(
@@ -119,6 +122,8 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 		session: AgentSession;
 		/** Provider-call message snapshots (LLM-converted), one per model request. */
 		contexts: Message[][];
+		/** Provider-call system prompt snapshots, one per model request. */
+		systemPrompts: string[][];
 		/** Mutable registry shared with the session, for lifecycle-only mount fixtures. */
 		toolRegistry: Map<string, AgentTool>;
 	} {
@@ -131,6 +136,7 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 		if (options.xdev && !options.lazyWrite) toolRegistry.set(writeTool.name, writeTool);
 		const mock = options.responses ? createMockModel({ responses: options.responses }) : undefined;
 		const contexts: Message[][] = [];
+		const systemPrompts: string[][] = [];
 		const agent = new Agent({
 			getApiKey: () => "test-key",
 			initialState: {
@@ -147,6 +153,7 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 			streamFn: mock
 				? (model, context, streamOptions) => {
 						contexts.push([...context.messages]);
+						systemPrompts.push([...(context.systemPrompt ?? [])]);
 						return mock.stream(model, context, streamOptions);
 					}
 				: undefined,
@@ -163,6 +170,12 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 				if (!toolRegistry.has("write")) toolRegistry.set("write", writeTool);
 				return true;
 			},
+			extensionRunner: options.beforeAgentStartSystemPrompt
+				? ({
+						emitBeforeAgentStart: async () => ({ systemPrompt: options.beforeAgentStartSystemPrompt }),
+						emit: async () => undefined,
+					} as unknown as ExtensionRunner)
+				: undefined,
 			rebuildSystemPrompt: async (toolNames, _tools) => {
 				const base = await rebuildSystemPrompt(toolNames);
 				if (!options.exposeXdevCatalog) return { systemPrompt: [base] };
@@ -174,7 +187,7 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 			xdev: options.xdev,
 		});
 		sessions.push(session);
-		return { session, contexts, toolRegistry };
+		return { session, contexts, systemPrompts, toolRegistry };
 	}
 
 	it("skips rebuild when an MCP refresh produces an identical tool set", async () => {
@@ -1035,6 +1048,28 @@ These tools became available:
 		).toHaveLength(0);
 	});
 
+	it("keeps the mount notice when before_agent_start replaces the catalog prompt (#7139)", async () => {
+		const replacementPrompt = ["extension replacement"];
+		const { session, contexts, systemPrompts } = newSession(async toolNames => `tools:${toolNames.join(",")}`, {
+			xdev: createTestXdevState(),
+			responses: [{ content: ["ok"] }],
+			exposeXdevCatalog: true,
+			beforeAgentStartSystemPrompt: replacementPrompt,
+		});
+		const search = createMcpCustomTool("mcp__nucleus_search", "nucleus", "search", "Search nucleus");
+
+		// The base prompt rebuild exposes the device, but the per-turn extension
+		// replaces that prompt before the provider call. The mount notice is now
+		// the only channel making the newly mounted device visible on this turn.
+		await session.refreshMCPTools([search]);
+		await session.prompt("hi");
+
+		expect(systemPrompts[0]).toEqual(replacementPrompt);
+		const notices = mountNoticesIn(contexts[0]);
+		expect(notices).toHaveLength(1);
+		expect(notices[0]).toContain("xd://mcp__nucleus_search");
+	});
+
 	it("does not emit an unmount notice for a catalog device unmounted before delivery (#7139)", async () => {
 		const { session } = newSession(async toolNames => `tools:${toolNames.join(",")}`, {
 			xdev: createTestXdevState(),