diff --git a/src/agents/system-prompt.test.ts b/src/agents/system-prompt.test.ts
index ae28c0b43dfe..a3aecace113f 100644
--- a/src/agents/system-prompt.test.ts
+++ b/src/agents/system-prompt.test.ts
@@ -825,7 +825,9 @@ describe("buildAgentSystemPrompt", () => {
     });
 
     expect(prompt).toContain("## Model Aliases");
-    expect(prompt).toContain("Model override: prefer alias");
+    expect(prompt).toContain(
+      "Model override: aliases are shortcuts for unqualified model requests. Use explicit provider/model references verbatim; do not substitute an alias or another provider.",
+    );
     expect(prompt).toContain("- Opus: anthropic/claude-opus-4-5");
   });
 
diff --git a/src/agents/system-prompt.ts b/src/agents/system-prompt.ts
index e6510fea835d..ef8b09c451f0 100644
--- a/src/agents/system-prompt.ts
+++ b/src/agents/system-prompt.ts
@@ -1288,7 +1288,7 @@ export function buildAgentSystemPrompt(params: {
         ? "## Model Aliases"
         : "",
       params.modelAliasLines && params.modelAliasLines.length > 0 && !isMinimal
-        ? "Model override: prefer alias; provider/model also accepted."
+        ? "Model override: aliases are shortcuts for unqualified model requests. Use explicit provider/model references verbatim; do not substitute an alias or another provider."
         : "",
       params.modelAliasLines && params.modelAliasLines.length > 0 && !isMinimal
         ? params.modelAliasLines.join("\n")