fix: remap max_completion_tokens to max_tokens for OpenAI-compatible providers

When using OpenAI-compatible providers like Mistral via the declarative
provider system, the OpenAI-specific parameter max_completion_tokens
(used for O-series models) is not recognized and causes 422 errors.

This adds a sanitize_request_for_compat method to OpenAiProvider that
remaps max_completion_tokens to max_tokens for any non-native OpenAI
provider, ensuring compatibility with Mistral and other OpenAI-compatible
APIs.

Closes #7762

Signed-off-by: fre <anonwurcod@proton.me>

Author: fresh3nough

crates/goose/src/providers/openai.rs

@@ -251,6 +251,20 @@ impl OpenAiProvider {
         Self::is_responses_model(model_name)
     }
 
+    fn sanitize_request_for_compat(&self, mut payload: serde_json::Value) -> serde_json::Value {
+        if self.name == OPEN_AI_PROVIDER_NAME {
+            return payload;
+        }
+
+        if let Some(obj) = payload.as_object_mut() {
+            if let Some(value) = obj.remove("max_completion_tokens") {
+                obj.entry("max_tokens").or_insert(value);
+            }
+        }
+
+        payload
+    }
+
     fn map_base_path(base_path: &str, target: &str, fallback: &str) -> String {
         let normalized = Self::normalize_base_path(base_path);
         if normalized.ends_with(target) || normalized.contains(&format!("/{target}")) {
@@ -457,6 +471,7 @@ impl Provider for OpenAiProvider {
                 &ImageFormat::OpenAi,
                 self.supports_streaming,
             )?;
+            let payload = self.sanitize_request_for_compat(payload);
             let mut log = RequestLog::start(model_config, &payload)?;
 
             let response = self
@@ -568,7 +583,82 @@ impl EmbeddingCapable for OpenAiProvider {
 
 #[cfg(test)]
 mod tests {
-    use super::OpenAiProvider;
+    use super::*;
+    use serde_json::json;
+
+    fn make_provider(name: &str) -> OpenAiProvider {
+        OpenAiProvider {
+            api_client: ApiClient::new("http://localhost".to_string(), AuthMethod::NoAuth).unwrap(),
+            base_path: "v1/chat/completions".to_string(),
+            organization: None,
+            project: None,
+            model: ModelConfig::new_or_fail("test-model"),
+            custom_headers: None,
+            supports_streaming: true,
+            name: name.to_string(),
+        }
+    }
+
+    #[test]
+    fn sanitize_remaps_max_completion_tokens_for_compat_provider() {
+        let provider = make_provider("mistral");
+        let payload = json!({
+            "model": "mistral-medium-latest",
+            "messages": [],
+            "max_completion_tokens": 16384
+        });
+
+        let result = provider.sanitize_request_for_compat(payload);
+        let obj = result.as_object().unwrap();
+
+        assert!(!obj.contains_key("max_completion_tokens"));
+        assert_eq!(obj.get("max_tokens").unwrap(), &json!(16384));
+    }
+
+    #[test]
+    fn sanitize_preserves_existing_max_tokens_for_compat_provider() {
+        let provider = make_provider("mistral");
+        let payload = json!({
+            "model": "mistral-medium-latest",
+            "messages": [],
+            "max_tokens": 4096,
+            "max_completion_tokens": 16384
+        });
+
+        let result = provider.sanitize_request_for_compat(payload);
+        let obj = result.as_object().unwrap();
+
+        assert!(!obj.contains_key("max_completion_tokens"));
+        assert_eq!(obj.get("max_tokens").unwrap(), &json!(4096));
+    }
+
+    #[test]
+    fn sanitize_noop_for_native_openai_provider() {
+        let provider = make_provider("openai");
+        let payload = json!({
+            "model": "o3",
+            "messages": [],
+            "max_completion_tokens": 16384
+        });
+
+        let result = provider.sanitize_request_for_compat(payload);
+        let obj = result.as_object().unwrap();
+
+        assert!(obj.contains_key("max_completion_tokens"));
+        assert!(!obj.contains_key("max_tokens"));
+    }
+
+    #[test]
+    fn sanitize_no_token_params() {
+        let provider = make_provider("groq");
+        let payload = json!({
+            "model": "llama-3.3-70b-versatile",
+            "messages": []
+        });
+
+        let result = provider.sanitize_request_for_compat(payload.clone());
+        assert_eq!(result, payload);
+    }
 
     #[test]
     fn gpt_5_2_codex_uses_responses_when_base_path_is_default() {