fix(coding-agent): rebuild system prompt on model switch

packages/coding-agent/src/core/agent-session.ts

@@ -2231,6 +2231,16 @@ export class AgentSession {
 		});
 	}
 
+	/**
+	 * Rebuild the cached base system prompt so the # Environment block reflects the
+	 * current model. Model switches update agent.state.model but the prompt is
+	 * otherwise only rebuilt at init, so without this the model reports a stale id.
+	 */
+	private _refreshSystemPromptForModel(): void {
+		this._baseSystemPrompt = this._rebuildSystemPrompt(this.getActiveToolNames());
+		this.agent.state.systemPrompt = this._baseSystemPrompt;
+	}
+
 	/**
 	 * Set model directly.
 	 * Validates that auth is configured, saves to session and settings.
@@ -2250,6 +2260,8 @@ export class AgentSession {
 		// Re-clamp thinking level for new model's capabilities
 		this.setThinkingLevel(thinkingLevel);
 
+		this._refreshSystemPromptForModel();
+
 		await this._emitModelSelect(model, previousModel, "set");
 	}
 
@@ -2290,6 +2302,8 @@ export class AgentSession {
 		// setThinkingLevel clamps to model capabilities.
 		this.setThinkingLevel(thinkingLevel);
 
+		this._refreshSystemPromptForModel();
+
 		await this._emitModelSelect(next.model, currentModel, "cycle");
 
 		return { model: next.model, thinkingLevel: this.thinkingLevel, isScoped: true };
@@ -2315,6 +2329,8 @@ export class AgentSession {
 		// Re-clamp thinking level for new model's capabilities
 		this.setThinkingLevel(thinkingLevel);
 
+		this._refreshSystemPromptForModel();
+
 		await this._emitModelSelect(nextModel, currentModel, "cycle");
 
 		return { model: nextModel, thinkingLevel: this.thinkingLevel, isScoped: false };

packages/coding-agent/test/suite/agent-session-model-extension.test.ts

@@ -44,6 +44,34 @@ describe("AgentSession model and extension characterization", () => {
 		).toEqual([`${nextModel.provider}/${nextModel.id}`]);
 	});
 
+	it("setModel refreshes the system prompt so the provider sees the new model", async () => {
+		const harness = await createHarness({
+			models: [
+				{ id: "faux-1", name: "One", reasoning: true },
+				{ id: "faux-2", name: "Two", reasoning: true },
+			],
+		});
+		harnesses.push(harness);
+
+		const seenSystemPrompts: string[] = [];
+		const capture = (context: { systemPrompt?: string }) => {
+			seenSystemPrompts.push(context.systemPrompt ?? "");
+			return fauxAssistantMessage("ok");
+		};
+
+		harness.setResponses([capture]);
+		await harness.session.prompt("first");
+
+		await harness.session.setModel(harness.getModel("faux-2")!);
+
+		harness.appendResponses([capture]);
+		await harness.session.prompt("second");
+
+		expect(seenSystemPrompts[0]).toContain("Active model: faux-1");
+		expect(seenSystemPrompts[1]).toContain("Active model: faux-2");
+		expect(seenSystemPrompts[1]).not.toContain("Active model: faux-1");
+	});
+
 	it("cycles through scoped models and preserves the scoped thinking preference", async () => {
 		const harness = await createHarness({
 			models: [