diff --git a/packages/coding-agent/src/session/agent-session.ts b/packages/coding-agent/src/session/agent-session.ts
index 1a0f9c51a..93416b98d 100644
--- a/packages/coding-agent/src/session/agent-session.ts
+++ b/packages/coding-agent/src/session/agent-session.ts
@@ -2296,11 +2296,15 @@ export class AgentSession {
 	 *
 	 * The signature covers:
 	 *   1. Active tool names in order (the prompt renders them in this order).
-	 *   2. Active tool labels and descriptions (both are echoed in the prompt body;
-	 *      see `system-prompt.md` `{{label}}: \`{{name}}\`` rendering).
-	 *   3. When MCP discovery is on, every registry tool's name+label+description,
-	 *      since `rebuildSystemPrompt` summarizes discoverable MCP tools that are
-	 *      not in the active set.
+	 *   2. Active tool labels, descriptions, and wire-visible names — all are
+	 *      rendered into the prompt body (see `system-prompt.md` `{{label}}: \`{{name}}\``
+	 *      and `toolPromptNames` in `buildSystemPrompt`). The wire name comes from
+	 *      `tool.customWireName` and overrides the internal name on the model wire
+	 *      (e.g. `edit` exposes itself as `apply_patch` to GPT-5 in apply_patch mode);
+	 *      a stale wire name would desync prompt guidance from actual tool routing.
+	 *   3. When MCP discovery is on, every registry tool's name+label+description+
+	 *      customWireName, since `rebuildSystemPrompt` summarizes discoverable MCP
+	 *      tools that are not in the active set.
 	 *   4. MCP server instructions text (per server), since `rebuildSystemPrompt`
 	 *      embeds these in the appended prompt under "## MCP Server Instructions".
 	 *      A server upgrade can change instructions while keeping tools identical.
@@ -2314,7 +2318,8 @@ export class AgentSession {
 		// Order-preserving join: any reorder must produce a different signature so
 		// the rebuild fires and the new tool list reaches the API.
 		const nameSegment = toolNames.join("\u0001");
-		const describeTool = (tool: AgentTool): string => `${tool.name}=${tool.label ?? ""}|${tool.description ?? ""}`;
+		const describeTool = (tool: AgentTool): string =>
+			`${tool.name}=${tool.label ?? ""}|${tool.description ?? ""}|${tool.customWireName ?? ""}`;
 		const descriptionSegment = tools.map(describeTool).join("\u0002");
 		let registrySegment = "";
 		if (this.#mcpDiscoveryEnabled) {
diff --git a/packages/coding-agent/test/agent-session-tool-rebuild-skip.test.ts b/packages/coding-agent/test/agent-session-tool-rebuild-skip.test.ts
index c19e149f5..afa4f5aaf 100644
--- a/packages/coding-agent/test/agent-session-tool-rebuild-skip.test.ts
+++ b/packages/coding-agent/test/agent-session-tool-rebuild-skip.test.ts
@@ -300,4 +300,38 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 		await session.refreshMCPTools([active, discoverableV2]);
 		expect(rebuildCount).toBe(baseline + 1);
 	});
+	it("rebuilds when an MCP tool's customWireName changes", async () => {
+		// `customWireName` overrides the model-facing tool name (e.g. `edit` exposes
+		// itself as `apply_patch` to GPT-5). The wire name is rendered into the prompt
+		// body via `toolPromptNames`, so a wire-name flip with the rest of the metadata
+		// constant would otherwise leave a stale system prompt that advertises the wrong
+		// callable name to the model. The signature must catch this.
+		let rebuildCount = 0;
+		const { session } = newSession(async toolNames => {
+			rebuildCount++;
+			return `tools:${toolNames.join(",")}`;
+		});
+
+		// Attach a custom wire name to the MCP tool. `applyToolProxy` forwards arbitrary
+		// properties from the underlying CustomTool to the wrapper, so the AgentTool the
+		// signature inspects exposes `customWireName` as if it were declared on the type.
+		const v1 = createMcpCustomTool("mcp__nucleus_search", "nucleus", "search", "Search");
+		const v1WithWire = { ...v1, customWireName: "wire_v1" } as typeof v1 & { customWireName: string };
+		await session.refreshMCPTools([v1WithWire]);
+		expect(rebuildCount).toBe(1);
+
+		// Same wire name: skip.
+		await session.refreshMCPTools([v1WithWire]);
+		expect(rebuildCount).toBe(1);
+
+		// Wire name changes while name/label/description stay constant: must rebuild.
+		const v2WithWire = { ...v1, customWireName: "wire_v2" } as typeof v1 & { customWireName: string };
+		await session.refreshMCPTools([v2WithWire]);
+		expect(rebuildCount).toBe(2);
+
+		// Drop wire name entirely: must rebuild (signature must differ from `wire_v2`).
+		await session.refreshMCPTools([v1]);
+		expect(rebuildCount).toBe(3);
+	});
+
 });