fix: bound per-model context windows on both AI config write paths

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/windmill-ai/src/ai_types.rs

@@ -210,7 +210,9 @@ pub fn validate_model_pricing_json(ai_config: &serde_json::Value) -> Result<(),
             return Err(format!("Price override for {} is not an object", key));
         };
         for field in ["input", "output", "cache_read", "cache_write"] {
-            let Some(rate) = price.get(field) else { continue };
+            let Some(rate) = price.get(field) else {
+                continue;
+            };
             let rate = rate
                 .as_f64()
                 .filter(|r| r.is_finite() && *r >= 0.0 && *r <= MAX_MODEL_RATE);
@@ -223,9 +225,66 @@ pub fn validate_model_pricing_json(ai_config: &serde_json::Value) -> Result<(),
         }
         for required in ["input", "output"] {
             if !price.contains_key(required) {
-                return Err(format!("Price override for {} is missing {}", key, required));
+                return Err(format!(
+                    "Price override for {} is missing {}",
+                    key, required
+                ));
             }
         }
     }
     Ok(())
 }
+
+// ============================================================================
+// Context windows
+// ============================================================================
+
+/// The chat compacts its history against `context_window_per_model`, so a window near
+/// or below zero would compact on every turn or drop the whole history.
+pub const MIN_CONTEXT_WINDOW: i64 = 1000;
+pub const MAX_CONTEXT_WINDOW: i64 = 10_000_000;
+
+pub fn validate_context_window(key: &str, tokens: i64) -> Result<(), String> {
+    if !(MIN_CONTEXT_WINDOW..=MAX_CONTEXT_WINDOW).contains(&tokens) {
+        return Err(format!(
+            "Context window for {key} must be between {MIN_CONTEXT_WINDOW} and {MAX_CONTEXT_WINDOW} tokens"
+        ));
+    }
+    Ok(())
+}
+
+/// The untyped counterpart for the instance config, for the reason given on
+/// `validate_model_pricing_json`.
+pub fn validate_context_windows_json(ai_config: &serde_json::Value) -> Result<(), String> {
+    let windows = match ai_config.get("context_window_per_model") {
+        None | Some(serde_json::Value::Null) => return Ok(()),
+        Some(v) => v
+            .as_object()
+            .ok_or_else(|| "context_window_per_model must be an object".to_string())?,
+    };
+    for (key, tokens) in windows {
+        let tokens = tokens
+            .as_i64()
+            .ok_or_else(|| format!("Context window for {key} must be an integer"))?;
+        validate_context_window(key, tokens)?;
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod context_window_tests {
+    use super::validate_context_windows_json;
+    use serde_json::json;
+
+    #[test]
+    fn instance_context_windows_are_bounded() {
+        let check = |w| validate_context_windows_json(&json!({ "context_window_per_model": w }));
+        assert!(validate_context_windows_json(&json!({})).is_ok());
+        assert!(check(json!({ "customai:m": 32000 })).is_ok());
+        assert!(check(json!({ "customai:m": 0 })).is_err());
+        assert!(check(json!({ "customai:m": -5 })).is_err());
+        assert!(check(json!({ "customai:m": 5_000_000_000i64 })).is_err());
+        assert!(check(json!({ "customai:m": "32000" })).is_err());
+        assert!(check(json!([32000])).is_err());
+    }
+}

backend/windmill-api-settings/src/lib.rs

@@ -945,6 +945,8 @@ async fn run_setting_pre_write_hook(
         AI_CONFIG_SETTING => {
             windmill_ai::ai_types::validate_model_pricing_json(value)
                 .map_err(error::Error::BadRequest)?;
+            windmill_ai::ai_types::validate_context_windows_json(value)
+                .map_err(error::Error::BadRequest)?;
         }
         AUTOMATE_USERNAME_CREATION_SETTING => {
             if value.as_bool().unwrap_or(false) {

backend/windmill-api/src/ai.rs

@@ -512,6 +512,14 @@ impl AIConfig {
         Ok(())
     }
 
+    pub fn validate_context_windows(&self) -> Result<()> {
+        for (key, tokens) in self.context_window_per_model.iter().flatten() {
+            windmill_ai::ai_types::validate_context_window(key, i64::from(*tokens))
+                .map_err(Error::BadRequest)?;
+        }
+        Ok(())
+    }
+
     pub fn validate_sessions_retention(&self) -> Result<()> {
         match self.sessions_retention_days {
             Some(days) if !(1..=MAX_SESSIONS_RETENTION_DAYS).contains(&days) => {

backend/windmill-api/src/workspaces.rs

@@ -108,6 +108,7 @@ async fn edit_copilot_config(
     }
 
     ai_config.validate_model_pricing()?;
+    ai_config.validate_context_windows()?;
     ai_config.validate_sessions_retention()?;
 
     let mut tx = db.begin().await?;