diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index a5e2586df..22e0d8bdc 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -18,6 +18,7 @@
 - Fixed the `/mcp` sign-in URL not being clickable when it wraps across lines, by emitting it as a terminal hyperlink with a `Cmd/Ctrl+click to open` line like `/login` ([#10186](https://github.com/earendil-works/pi/issues/10186)).
 - Fixed codemode `image()` accepting malformed base64 data or unsupported image types, which persisted an invalid image block that made every later provider request fail with HTTP 400 ([#10215](https://github.com/earendil-works/pi/issues/10215)).
 - Fixed codemode failing to start its script worker from the standalone Windows executable ([#10204](https://github.com/earendil-works/pi/issues/10204)).
+- Fixed prompt submission slowing down with session length, because resolving the session's model selection looked up the model catalog once per assistant message ([#10198](https://github.com/earendil-works/pi/issues/10198)).
 
 ## [0.99.1] - 2026-09-29
 
diff --git a/packages/coding-agent/src/core/virtual-models.ts b/packages/coding-agent/src/core/virtual-models.ts
index c6b5e6998..269ef6858 100644
--- a/packages/coding-agent/src/core/virtual-models.ts
+++ b/packages/coding-agent/src/core/virtual-models.ts
@@ -122,26 +122,37 @@ export function findLatestResponse(messages: readonly AgentMessage[]): Assistant
  * `model_change`, because responses name the physical models it routed to. Otherwise the latest
  * physical response wins, as in sessions without virtual models. A virtual model that is no longer
  * registered does not hold, so the selection falls back to the physical model that answered last.
+ *
+ * Only the last `model_change` can hold, so this looks up at most one model in the catalog.
  */
 export function getBranchSelection(
 	branch: readonly SessionEntry[],
 	getModel: (provider: string, modelId: string) => Model<Api> | undefined,
 ): { provider: string; modelId: string } | undefined {
-	const isVirtual = (provider: string, modelId: string) => {
-		const model = getModel(provider, modelId);
-		return model !== undefined && isVirtualModel(model);
-	};
-	let selection: { provider: string; modelId: string } | undefined;
-	for (const entry of branch) {
+	for (let i = branch.length - 1; i >= 0; i--) {
+		const entry = branch[i];
 		if (entry.type === "model_change") {
-			selection = { provider: entry.provider, modelId: entry.modelId };
-		} else if (entry.type === "message" && entry.message.role === "assistant" && !isVirtualModel(entry.message)) {
-			if (!selection || !isVirtual(selection.provider, selection.modelId)) {
-				selection = { provider: entry.message.provider, modelId: entry.message.model };
-			}
+			return { provider: entry.provider, modelId: entry.modelId };
+		}
+		if (entry.type === "message" && entry.message.role === "assistant" && !isVirtualModel(entry.message)) {
+			const response = { provider: entry.message.provider, modelId: entry.message.model };
+			const change = findLastModelChange(branch, i);
+			const model = change && getModel(change.provider, change.modelId);
+			return change && model && isVirtualModel(model) ? change : response;
 		}
 	}
-	return selection;
+	return undefined;
+}
+
+function findLastModelChange(
+	branch: readonly SessionEntry[],
+	before: number,
+): { provider: string; modelId: string } | undefined {
+	for (let i = before - 1; i >= 0; i--) {
+		const entry = branch[i];
+		if (entry.type === "model_change") return { provider: entry.provider, modelId: entry.modelId };
+	}
+	return undefined;
 }
 
 /** Latest router state a session branch stores for a virtual model. */
diff --git a/packages/coding-agent/test/virtual-models.test.ts b/packages/coding-agent/test/virtual-models.test.ts
index 7d1d0b651..3650c2ccd 100644
--- a/packages/coding-agent/test/virtual-models.test.ts
+++ b/packages/coding-agent/test/virtual-models.test.ts
@@ -14,7 +14,7 @@ import { AuthStorage } from "../src/core/auth-storage.ts";
 import { ModelRuntime } from "../src/core/model-runtime.ts";
 import { createAgentSession } from "../src/core/sdk.ts";
 import { SessionManager } from "../src/core/session-manager.ts";
-import type { ModelRouteRequest, VirtualModelDefinition } from "../src/core/virtual-models.ts";
+import { getBranchSelection, type ModelRouteRequest, type VirtualModelDefinition } from "../src/core/virtual-models.ts";
 import { createTestResourceLoader } from "./utilities.ts";
 
 async function createRuntime(requests: ModelRouteRequest[] = []) {
@@ -51,6 +51,68 @@ function assistantFrom(model: Model<string>, text: string): AssistantMessage {
 	return { ...fauxAssistantMessage(text), api: model.api, provider: model.provider, model: model.id };
 }
 
+describe("getBranchSelection", () => {
+	function select(build: (sessionManager: SessionManager) => void, runtime: ModelRuntime) {
+		const sessionManager = SessionManager.inMemory();
+		build(sessionManager);
+		const lookups: string[] = [];
+		const selection = getBranchSelection(sessionManager.getBranch(), (provider, modelId) => {
+			lookups.push(`${provider}/${modelId}`);
+			return runtime.getModel(provider, modelId);
+		});
+		return { selection, lookups };
+	}
+
+	// #10198: the selection must not cost one catalog lookup per assistant message.
+	it("looks up only the last model_change", async () => {
+		const { runtime, virtual } = await createRuntime();
+		const small = runtime.getModel("faux", "small")!;
+		const large = runtime.getModel("faux", "large")!;
+
+		const physical = select((sessionManager) => {
+			sessionManager.appendModelChange(small.provider, small.id);
+			for (let i = 0; i < 100; i++) sessionManager.appendMessage(assistantFrom(large, "ok"));
+		}, runtime);
+		expect(physical.selection).toEqual({ provider: "faux", modelId: "large" });
+		expect(physical.lookups).toEqual(["faux/small"]);
+
+		const routed = select((sessionManager) => {
+			sessionManager.appendModelChange(small.provider, small.id);
+			sessionManager.appendMessage(assistantFrom(small, "ok"));
+			sessionManager.appendModelChange(virtual.provider, virtual.id);
+			for (let i = 0; i < 100; i++) sessionManager.appendMessage(assistantFrom(large, "ok"));
+		}, runtime);
+		expect(routed.selection).toEqual({ provider: "router", modelId: "auto" });
+		expect(routed.lookups).toEqual(["router/auto"]);
+	});
+
+	it("uses the last model_change without responses after it", async () => {
+		const { runtime, virtual } = await createRuntime();
+		const small = runtime.getModel("faux", "small")!;
+		const { selection, lookups } = select((sessionManager) => {
+			sessionManager.appendModelChange(virtual.provider, virtual.id);
+			sessionManager.appendMessage(assistantFrom(small, "ok"));
+			sessionManager.appendModelChange(small.provider, small.id);
+		}, runtime);
+		expect(selection).toEqual({ provider: "faux", modelId: "small" });
+		expect(lookups).toEqual([]);
+	});
+
+	it("uses the latest physical response without a model_change", async () => {
+		const { runtime, virtual } = await createRuntime();
+		const small = runtime.getModel("faux", "small")!;
+		const large = runtime.getModel("faux", "large")!;
+		const { selection, lookups } = select((sessionManager) => {
+			sessionManager.appendMessage(assistantFrom(small, "ok"));
+			sessionManager.appendMessage(assistantFrom(large, "ok"));
+			// Failed routing leaves the virtual model on its message.
+			sessionManager.appendMessage({ ...assistantFrom(virtual, ""), stopReason: "error" });
+		}, runtime);
+		expect(selection).toEqual({ provider: "faux", modelId: "large" });
+		expect(lookups).toEqual([]);
+	});
+});
+
 describe("ModelRuntime virtual models", () => {
 	it("lists a virtual model and routes it to a physical model with a clamped thinking level", async () => {
 		const requests: ModelRouteRequest[] = [];