diff --git a/actions/setup/js/assign_agent_helpers.cjs b/actions/setup/js/assign_agent_helpers.cjs
index a199efa7462..b431ec1cc66 100644
--- a/actions/setup/js/assign_agent_helpers.cjs
+++ b/actions/setup/js/assign_agent_helpers.cjs
@@ -359,10 +359,13 @@ async function assignAgentToIssue(assignableId, agentId, currentAssignees, agent
     if (customInstructions) debugMsg += `, customInstructions=${customInstructions.substring(0, 50)}...`;
     core.debug(debugMsg);
 
+    // Build GraphQL-Features header - include coding_agent_model_selection when model is provided
+    const graphqlFeatures = model ? "issues_copilot_assignment_api_support,coding_agent_model_selection" : "issues_copilot_assignment_api_support";
+
     const response = await github.graphql(mutation, {
       ...variables,
       headers: {
-        "GraphQL-Features": "issues_copilot_assignment_api_support",
+        "GraphQL-Features": graphqlFeatures,
       },
     });
 
diff --git a/actions/setup/js/assign_agent_helpers.test.cjs b/actions/setup/js/assign_agent_helpers.test.cjs
index 03328968147..f2088e79271 100644
--- a/actions/setup/js/assign_agent_helpers.test.cjs
+++ b/actions/setup/js/assign_agent_helpers.test.cjs
@@ -258,6 +258,11 @@ describe("assign_agent_helpers.cjs", () => {
           actorIds: ["AGENT_456", "USER_1"],
         })
       );
+
+      // Should only include issues_copilot_assignment_api_support when model is not provided
+      const calledArgs = mockGithub.graphql.mock.calls[0];
+      const variables = calledArgs[1];
+      expect(variables.headers["GraphQL-Features"]).toBe("issues_copilot_assignment_api_support");
     });
 
     it("should preserve existing assignees when adding agent", async () => {
@@ -324,6 +329,8 @@ describe("assign_agent_helpers.cjs", () => {
       expect(mutation).toContain("agentAssignment");
       expect(mutation).toContain("model: $model");
       expect(variables.model).toBe("claude-opus-4.6");
+      // Should include coding_agent_model_selection feature flag when model is provided
+      expect(variables.headers["GraphQL-Features"]).toBe("issues_copilot_assignment_api_support,coding_agent_model_selection");
     });
 
     it("should include customAgent in agentAssignment when provided", async () => {