fix(client): drop sampling params for the Claude 5 generation and Haiku 5.5

crates/tui/src/client/anthropic.rs

@@ -229,9 +229,11 @@ impl CodewhaleClient {
             _ => {}
         }
 
-        // Sampling parameters: Claude 4.7+ rejects temperature/top_p
-        // entirely; earlier models reject the two together. Send at most one
-        // (temperature wins), or neither for models that forbid them.
+        // Sampling parameters: every model from Claude 4.7 on (including the
+        // Claude 5 generation and Haiku 5.5) rejects non-default
+        // temperature/top_p; earlier models reject the two together. Send at
+        // most one (temperature wins), or neither for models that forbid
+        // them.
         if !anthropic_model_rejects_sampling(&request.model) {
             if let Some(temperature) = request.temperature {
                 body["temperature"] = json!(temperature);
@@ -650,11 +652,25 @@ fn assistant_tool_use_ids(message: &Value) -> Vec<String> {
         .unwrap_or_default()
 }
 
-/// Models that reject `temperature` / `top_p` outright (Claude 4.7+).
+/// Models that reject non-default `temperature` / `top_p` outright (every
+/// model released after Opus 4.6). The Messages API reference states those
+/// models "do not support" setting `temperature` / `top_p` beyond their
+/// backwards-compatible defaults (`temperature: 1.0`, `top_p >= 0.99`), and
+/// the migration guides restate it per family: "Setting `temperature`,
+/// `top_p`, or `top_k` to any non-default value on Claude Opus 4.7 and
+/// later models, including Claude Opus 5.5, returns a 400 error". That
+/// covers the Claude 5 generation (`claude-opus-5(-5)`, `claude-sonnet-5(-5)`)
+/// and Haiku 5.5 (which admits only its defaults — `temperature: 1`, a lone
+/// `top_p: 0.99`; `top_p: 1`, any `top_k`, or both params together 400),
+/// besides the Fable/Mythos lines. Dropping the params entirely is always
+/// safe; sending a forbidden one bricks the request. Re-checked 2026-10-08.
 fn anthropic_model_rejects_sampling(model: &str) -> bool {
     let lower = model.to_ascii_lowercase();
     lower.contains("opus-4-7")
         || lower.contains("opus-4-8")
+        || ["opus-5", "sonnet-5", "haiku-5"]
+            .iter()
+            .any(|family| lower.contains(family))
         || lower.contains("fable")
         || lower.contains("mythos")
 }
@@ -1801,15 +1817,39 @@ mod tests {
         assert!(body.get("temperature").is_none(), "{body}");
         assert!(body.get("top_p").is_none(), "{body}");
 
+        // The Claude 5 generation and Haiku 5.5 reject sampling too. The
+        // datestamped spelling pins the substring match against snapshot
+        // suffixes.
+        for model in [
+            "claude-opus-5",
+            "claude-opus-5-5",
+            "claude-opus-5-20260101",
+            "claude-sonnet-5",
+            "claude-sonnet-5-5",
+            "claude-haiku-5-5",
+            "claude-fable-5",
+            "claude-mythos-5",
+        ] {
+            let body =
+                client.build_anthropic_body(&request_with(model, None, Some(0.7), Some(0.9)), true);
+            assert!(
+                body.get("temperature").is_none() && body.get("top_p").is_none(),
+                "{model} must not receive sampling params: {body}"
+            );
+        }
+
         // Older models accept ONE of temperature / top_p (temperature wins).
-        let body = client.build_anthropic_body(
-            &request_with("claude-sonnet-4-6", None, Some(0.7), Some(0.9)),
-            true,
-        );
-        assert_eq!(
-            body.get("temperature").and_then(Value::as_f64),
-            Some(f64::from(0.7f32))
-        );
+        // The pre-4.7 generations stay ungated, so a future over-broad arm
+        // like `contains("opus-4-")` cannot slip through.
+        for model in ["claude-sonnet-4-6", "claude-opus-4-6", "claude-haiku-4-5"] {
+            let body =
+                client.build_anthropic_body(&request_with(model, None, Some(0.7), Some(0.9)), true);
+            assert_eq!(
+                body.get("temperature").and_then(Value::as_f64),
+                Some(f64::from(0.7f32)),
+                "{model} keeps temperature"
+            );
+        }
         assert!(body.get("top_p").is_none(), "never send both: {body}");
     }