fix: normalize nullable schemas for Vertex Gemini compatibility (#8930)

Signed-off-by: Daniel Nicoletti <dantti12@gmail.com>
Signed-off-by: Douwe Osinga <douwe@squareup.com>
Co-authored-by: Douwe Osinga <douwe@squareup.com>

crates/goose/src/providers/formats/openai.rs

@@ -710,6 +710,7 @@ fn ensure_valid_json_schema(schema: &mut Value) {
             if let Some(properties) = params_obj.get_mut("properties") {
                 if let Some(properties_obj) = properties.as_object_mut() {
                     for (_key, prop) in properties_obj.iter_mut() {
+                        normalize_nullable(prop);
                         if prop.is_object()
                             && prop.get("type").and_then(|t| t.as_str()) == Some("object")
                         {
@@ -722,6 +723,61 @@ fn ensure_valid_json_schema(schema: &mut Value) {
     }
 }
 
+/// Normalizes nullable type representations that some providers (e.g. Vertex Gemini via Bifrost)
+/// don't support:
+/// - `"type": ["integer", "null"]` → `"type": "integer"` (drops the null variant)
+/// - `"anyOf": [T, {"type": "null"}]` → T (unwraps to the non-null schema)
+///
+/// Optional-ness is already conveyed by the field being absent from `required`.
+fn normalize_nullable(schema: &mut Value) {
+    let Some(obj) = schema.as_object_mut() else {
+        return;
+    };
+
+    // Handle type: ["T", "null"] array form (schemars 1.x style for nullable primitives)
+    if let Some(type_val) = obj.get("type").cloned() {
+        if let Some(types) = type_val.as_array() {
+            let non_null: Vec<&Value> = types
+                .iter()
+                .filter(|t| t.as_str() != Some("null"))
+                .collect();
+            if non_null.len() == 1 {
+                let scalar = non_null[0].clone();
+                obj.insert("type".to_string(), scalar);
+                return;
+            }
+        }
+    }
+
+    // Handle anyOf: [T, {type: "null"}] form — merge the non-null variant's fields
+    // into the current object (preserving sibling keys like "description" or "default")
+    // rather than replacing the whole schema.
+    if let Some(any_of) = obj.remove("anyOf") {
+        if let Some(variants) = any_of.as_array() {
+            if variants.len() == 2 {
+                let is_null = |v: &Value| v.get("type").and_then(|t| t.as_str()) == Some("null");
+                let non_null = if is_null(&variants[0]) {
+                    Some(&variants[1])
+                } else if is_null(&variants[1]) {
+                    Some(&variants[0])
+                } else {
+                    None
+                };
+                if let Some(replacement) = non_null {
+                    if let Some(replacement_obj) = replacement.as_object() {
+                        for (k, v) in replacement_obj {
+                            obj.entry(k.clone()).or_insert(v.clone());
+                        }
+                        return;
+                    }
+                }
+            }
+        }
+        // Put it back if we couldn't simplify
+        obj.insert("anyOf".to_string(), any_of);
+    }
+}
+
 fn strip_data_prefix(line: &str) -> Option<&str> {
     // SSE spec allows both "data: value" and "data:value" (space after colon is optional)
     line.strip_prefix("data: ")
@@ -1139,6 +1195,85 @@ mod tests {
         let mut tools = vec![original_schema.clone()];
         validate_tool_schemas(&mut tools);
         assert_eq!(tools[0], original_schema);
+
+        // Test case 4: anyOf nullable is unwrapped, preserving sibling metadata
+        let mut tools = vec![json!({
+            "type": "function",
+            "function": {
+                "name": "shell",
+                "description": "run shell",
+                "parameters": {
+                    "type": "object",
+                    "properties": {
+                        "command": { "type": "string" },
+                        "timeout_secs": {
+                            "description": "timeout in seconds",
+                            "anyOf": [
+                                { "type": "integer", "format": "uint64", "minimum": 0 },
+                                { "type": "null" }
+                            ]
+                        }
+                    },
+                    "required": ["command"]
+                }
+            }
+        })];
+        validate_tool_schemas(&mut tools);
+        let timeout_schema = &tools[0]["function"]["parameters"]["properties"]["timeout_secs"];
+        assert_eq!(timeout_schema["type"], "integer");
+        assert_eq!(timeout_schema["format"], "uint64");
+        assert_eq!(timeout_schema["description"], "timeout in seconds");
+        assert!(timeout_schema.get("anyOf").is_none());
+
+        // Test case 4b: type array form (schemars 1.x style for nullable primitives)
+        let mut tools = vec![json!({
+            "type": "function",
+            "function": {
+                "name": "shell",
+                "description": "run shell",
+                "parameters": {
+                    "type": "object",
+                    "properties": {
+                        "command": { "type": "string" },
+                        "timeout_secs": {
+                            "type": ["integer", "null"],
+                            "format": "uint64",
+                            "minimum": 0
+                        }
+                    },
+                    "required": ["command"]
+                }
+            }
+        })];
+        validate_tool_schemas(&mut tools);
+        let timeout_schema = &tools[0]["function"]["parameters"]["properties"]["timeout_secs"];
+        assert_eq!(timeout_schema["type"], "integer");
+        assert!(!timeout_schema["type"].is_array());
+
+        // Test case 5: Verify the actual ShellParams schema is compatible (no anyOf for timeout_secs)
+        use crate::agents::platform_extensions::developer::shell::ShellParams;
+        use schemars::schema_for;
+        let schema_value = serde_json::to_value(schema_for!(ShellParams)).unwrap();
+        let schema_obj = schema_value.as_object().unwrap().clone();
+        let tool = rmcp::model::Tool::new("shell", "run shell", schema_obj);
+        let mut tools = vec![json!({
+            "type": "function",
+            "function": {
+                "name": tool.name,
+                "description": tool.description,
+                "parameters": tool.input_schema,
+            }
+        })];
+        validate_tool_schemas(&mut tools);
+        let timeout = &tools[0]["function"]["parameters"]["properties"]["timeout_secs"];
+        assert!(
+            timeout.get("anyOf").is_none(),
+            "timeout_secs should not have anyOf after validation, got: {timeout}"
+        );
+        assert_eq!(
+            timeout["type"], "integer",
+            "timeout_secs should have type=integer"
+        );
     }
 
     const OPENAI_TOOL_USE_RESPONSE: &str = r#"{