diff --git a/CHANGELOG.md b/CHANGELOG.md
index 115768cdc..40ce826db 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,13 @@
 
 ## Unreleased
 
+- Add `version` parameter to `openai-chat` provider config to support different API versions:
+  - **Version 1**: Uses `max_tokens` parameter (for Mistral AI and older OpenAI-compatible endpoints)
+  - **Version 2** (default): Uses `max_completion_tokens` parameter (for OpenAI and modern endpoints)
+  - Fixes Mistral AI 422 errors by allowing configuration of appropriate token parameter
+  - Supports both provider-level and model-level version configuration with provider taking precedence
+  - Maintains full backward compatibility by defaulting to version 2
+
 ## 0.86.0
 
 - Improve agent behavior prompt to mention usage of editor_diagnostics tool. #230
diff --git a/docs/configuration.md b/docs/configuration.md
index 786e31979..9d8ef7ec1 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -582,6 +582,7 @@ To configure, add your OTLP collector config via `:otlp` map following [otlp aut
             completionUrlRelativePath?: string;
             thinkTagStart?: string;
             thinkTagEnd?: string;
+            version?: 1 | 2; // API version for openai-chat providers (1 for Mistral/compatible, 2 for OpenAI/modern, default: 2)
             models: {[key: string]: {
               modelName?: string;
               extraPayload?: {[key: string]: any}
diff --git a/docs/models.md b/docs/models.md
index 609d6a22c..cc308a673 100644
--- a/docs/models.md
+++ b/docs/models.md
@@ -146,6 +146,56 @@ Defaults by API type:
 
 Only set this when your provider uses a different path or expects query parameters at the endpoint (e.g., Azure API versioning).
 
+#### API Version (for openai-chat providers)
+
+The `openai-chat` API supports different versions to accommodate various provider requirements:
+
+- **`version: 1`**: Uses `max_tokens` parameter (for Mistral AI and older OpenAI-compatible endpoints)
+- **`version: 2`** (default): Uses `max_completion_tokens` parameter (for OpenAI and modern endpoints)
+
+Set the version in your provider config to ensure compatibility:
+
+```javascript title="~/.config/eca/config.json"
+{
+  "providers": {
+    "mistral": {
+      "api": "openai-chat",
+      "version": 1,
+      "url": "https://api.mistral.ai",
+      "key": "${env:MISTRAL_API_KEY}",
+      "models": {
+        "labs-devstral-small-2512": {
+          "extraPayload": {
+            "max_tokens": 2048
+          }
+        }
+      }
+    }
+  }
+}
+```
+
+For OpenAI and most modern providers, you can omit the version (defaults to 2):
+
+```javascript title="~/.config/eca/config.json"
+{
+  "providers": {
+    "openai": {
+      "api": "openai-chat",
+      "url": "https://api.openai.com",
+      "key": "${env:OPENAI_API_KEY}",
+      "models": {
+        "gpt-4o": {
+          "extraPayload": {
+            "max_completion_tokens": 2048
+          }
+        }
+      }
+    }
+  }
+}
+```
+
 ### Credential File Authentication
 
 ECA also supports standard plain-text .netrc file format for reading credentials.
diff --git a/integration-test/integration/chat/custom_provider_test.clj b/integration-test/integration/chat/custom_provider_test.clj
index 8fae82a31..06bc9efa8 100644
--- a/integration-test/integration/chat/custom_provider_test.clj
+++ b/integration-test/integration/chat/custom_provider_test.clj
@@ -125,7 +125,53 @@
                       {:role "user" :content [{:type "input_text" :text "Who's there?"}]}
                       {:role "assistant" :content [{:type "output_text" :text "Foo"}]}
                       {:role "user" :content [{:type "input_text" :text "What foo?"}]}]}
-             (llm.mocks/get-req-body :simple-text-2)))))))
+             (llm.mocks/get-req-body :simple-text-2))))))
+
+(deftest openai-chat-version-parameter
+  (eca/start-process!)
+
+  (eca/request! (fixture/initialize-request
+                 {:initializationOptions
+                  (merge fixture/default-init-options
+                         {:defaultModel "mistral/labs-devstral-small-2512"
+                          :providers
+                          {"mistral"
+                           {:api "openai-chat"
+                            :version 1  ;; Test version 1 for Mistral
+                            :url (str "http://localhost:" llm-mock.server/port "/openai-chat")
+                            :key "mistral-key"
+                            :models {"labs-devstral-small-2512" {}}}}})
+                  :capabilities {:codeAssistant {:chat {}}}}))
+
+  (eca/notify! (fixture/initialized-notification))
+  (testing "Mistral provider with version 1 uses max_tokens parameter"
+    (is (match?
+         {:chat {:models (m/embeds ["mistral/labs-devstral-small-2512"])
+                 :selectModel "mistral/labs-devstral-small-2512"}}
+         (eca/client-awaits-server-notification :config/updated)))
+
+    (let [chat-id* (atom nil)]
+      (testing "Request body contains max_tokens instead of max_completion_tokens"
+        (llm.mocks/set-case! :simple-text-0)
+        (let [resp (eca/request! (fixture/chat-prompt-request
+                                  {:model "mistral/labs-devstral-small-2512"
+                                   :message "Test Mistral compatibility"}))
+              chat-id (reset! chat-id* (:chatId resp))]
+
+          (is (match?
+               {:chatId (m/pred string?)
+                :model "mistral/labs-devstral-small-2512"
+                :status "prompting"}
+               resp))
+
+          ;; Verify that the request body contains max_tokens (version 1) instead of max_completion_tokens
+          (let [req-body (llm.mocks/get-req-body :simple-text-0)]
+            (is (contains? req-body :max_tokens) "Should contain max_tokens for version 1")
+            (is (not (contains? req-body :max_completion_tokens)) "Should not contain max_completion_tokens for version 1")
+            (is (= 32000 (:max_tokens req-body)) "Should default to 32000 max_tokens"))
+
+          (match-content chat-id "user" {:type "text" :text "Test Mistral compatibility\n"})
+          (match-content chat-id "system" {:type "progress" :state "finished"})))))))
 
 (deftest openai-chat-simple-text
   (eca/start-process!)
diff --git a/integration-test/llm_mock/openai_chat.clj b/integration-test/llm_mock/openai_chat.clj
index d3ab65dea..17b006833 100644
--- a/integration-test/llm_mock/openai_chat.clj
+++ b/integration-test/llm_mock/openai_chat.clj
@@ -100,7 +100,9 @@
   (let [body (some-> (slurp (:body req)) (json/parse-string true))
         messages (:messages body)
         normalized (messages->normalized-input messages)
-        normalized-body (merge normalized (select-keys body [:tools]))]
+        ;; Capture token parameters for version testing
+        token-params (select-keys body [:max_tokens :max_completion_tokens])
+        normalized-body (merge normalized (select-keys body [:tools]) token-params)]
     (hk/as-channel
      req
      {:on-open (fn [ch]
diff --git a/src/eca/llm_api.clj b/src/eca/llm_api.clj
index 064c644c4..2a6d0abaf 100644
--- a/src/eca/llm_api.clj
+++ b/src/eca/llm_api.clj
@@ -140,6 +140,7 @@
                                 extra-payload)
           :api-url api-url
           :api-key api-key
+          :provider-config provider-config
           :extra-headers {"openai-intent" "conversation-panel"
                           "x-request-id" (str (random-uuid))
                           "vscode-sessionid" ""
@@ -165,7 +166,8 @@
                                   {:extra_body {:google {:thinking_config {:include_thoughts true}}}})
                                 extra-payload)
           :api-url api-url
-          :api-key api-key}
+          :api-key api-key
+          :provider-config provider-config}
          callbacks)
 
         (= "ollama" provider)
@@ -207,6 +209,7 @@
             :think-tag-start think-tag-start
             :think-tag-end think-tag-end
             :http-client http-client
+            :provider-config provider-config
             :api-url api-url
             :api-key api-key}
            callbacks))
diff --git a/src/eca/llm_providers/openai_chat.clj b/src/eca/llm_providers/openai_chat.clj
index f9d1e4b5f..da3f444d6 100644
--- a/src/eca/llm_providers/openai_chat.clj
+++ b/src/eca/llm_providers/openai_chat.clj
@@ -287,6 +287,41 @@
                     (emit-text! (.substring buf 0 emit-len))
                     (reset! content-buffer* (.substring buf emit-len))))))))))))
 
+(defn- build-request-body-with-version
+  "Build request body with version-specific max tokens parameter.
+  
+  version 1: uses max_tokens (for Mistral, older OpenAI-compatible endpoints)
+  version 2 (default): uses max_completion_tokens (for OpenAI, modern endpoints)
+  
+  Checks for version in both provider-config and extra-payload, with provider-config taking precedence."
+  [extra-payload provider-config model messages stream? temperature tools]
+  (let [version (or (get provider-config :version)
+                     (get extra-payload :version)
+                     2)
+        max-tokens-param (if (= version 1) :max_tokens :max_completion_tokens)
+        base-request (-> {:model model
+                         :messages messages
+                         :stream stream?}
+                        (assoc max-tokens-param 32000)
+                        (assoc-some
+                         :temperature temperature
+                         :tools (when (seq tools) (->tools tools))))
+        ;; Remove version from extra-payload to avoid sending it to the API
+        extra-payload (dissoc extra-payload :version)
+        ;; Handle potential conflicts between extra-payload and version-based tokens
+        extra-payload (if (= version 1)
+                        ;; For version 1, remove max_completion_tokens if present and ensure max_tokens
+                        (-> extra-payload
+                            (dissoc :max_completion_tokens)
+                            (cond-> (not (contains? extra-payload :max_tokens))
+                              (assoc :max_tokens 32000)))
+                        ;; For version 2, remove max_tokens if present and ensure max_completion_tokens
+                        (-> extra-payload
+                            (dissoc :max_tokens)
+                            (cond-> (not (contains? extra-payload :max_completion_tokens))
+                              (assoc :max_completion_tokens 32000))))]
+    (deep-merge base-request extra-payload)))
+
 (defn chat-completion!
   "Primary entry point for OpenAI chat completions with streaming support.
 
@@ -295,7 +330,7 @@
    Compatible with OpenRouter and other OpenAI-compatible providers."
   [{:keys [model user-messages instructions temperature api-key api-url url-relative-path
            past-messages tools extra-payload extra-headers supports-image?
-           think-tag-start think-tag-end http-client]}
+           think-tag-start think-tag-end http-client provider-config]}
    {:keys [on-message-received on-error on-prepare-tool-call on-tools-called on-reason on-usage-updated] :as callbacks}]
   (let [think-tag-start (or think-tag-start "<think>")
         think-tag-end (or think-tag-end "</think>")
@@ -305,15 +340,7 @@
                        (normalize-messages past-messages supports-image? think-tag-start think-tag-end)
                        (normalize-messages user-messages supports-image? think-tag-start think-tag-end)))
 
-        body (deep-merge
-              (assoc-some
-               {:model model
-                :messages messages
-                :stream stream?
-                :max_completion_tokens 32000}
-               :temperature temperature
-               :tools (when (seq tools) (->tools tools)))
-              extra-payload)
+        body (build-request-body-with-version extra-payload provider-config model messages stream? temperature tools)
 
         ;; Atom to accumulate tool call data from streaming chunks.
         ;; OpenAI streams tool call arguments across multiple chunks, so we need to
diff --git a/test/eca/llm_providers/openai_chat_test.clj b/test/eca/llm_providers/openai_chat_test.clj
index 819b0f306..6a1d6f0a3 100644
--- a/test/eca/llm_providers/openai_chat_test.clj
+++ b/test/eca/llm_providers/openai_chat_test.clj
@@ -255,4 +255,97 @@
            [:text "re"]]}
          (process-text-think-aware
           ["<" "thi" "nk>H" "um.." ".</" "thi" "nk>H"
-           "ello " "the" "re " "mat" "e!"])))))
+           "ello " "the" "re " "mat" "e!"]))))
+
+;; Test helper to capture the request body from chat-completion!
+(defn capture-request-body
+  "Helper to capture the request body that would be sent to the API"
+  [config]
+  (let [request-body* (atom nil)
+        callbacks {:on-message-received (fn [_])
+                   :on-error (fn [_])
+                   :on-prepare-tool-call (fn [_])
+                   :on-tools-called (fn [_])
+                   :on-reason (fn [_])
+                   :on-usage-updated (fn [_])}
+        ;; Mock the base-chat-request! function to capture the body
+        original-base-chat-request! @(resolve 'eca.llm-providers.openai-chat/base-chat-request!)
+        _ (with-redefs [eca.llm-providers.openai-chat/base-chat-request!
+                        (fn [{:keys [body] :as _all-args}]
+                          (reset! request-body* body)
+                          ;; Return a mock successful response
+                          {:usage {:prompt_tokens 10 :completion_tokens 5 :total_tokens 15}
+                           :choices [{:message {:content "test response" :reasoning "test reasoning"}}]})]
+          (eca.llm-providers.openai-chat/chat-completion! config callbacks))
+        result @request-body*]
+    result))
+
+(deftest version-parameter-test
+  (testing "Version 1 uses max_tokens parameter"
+    (let [body (capture-request-body
+                {:model "test-model"
+                 :user-messages [{:role "user" :content "test"}]
+                 :extra-payload {:version 1}})]
+      (is (contains? body :max_tokens) "Should contain max_tokens for version 1")
+      (is (not (contains? body :max_completion_tokens)) "Should not contain max_completion_tokens for version 1")
+      (is (= 32000 (:max_tokens body)) "Should default to 32000 max_tokens for version 1")))
+
+  (testing "Version 2 uses max_completion_tokens parameter"
+    (let [body (capture-request-body
+                {:model "test-model"
+                 :user-messages [{:role "user" :content "test"}]
+                 :extra-payload {:version 2}})]
+      (is (contains? body :max_completion_tokens) "Should contain max_completion_tokens for version 2")
+      (is (not (contains? body :max_tokens)) "Should not contain max_tokens for version 2")
+      (is (= 32000 (:max_completion_tokens body)) "Should default to 32000 max_completion_tokens for version 2")))
+
+  (testing "Default version (missing) uses max_completion_tokens parameter"
+    (let [body (capture-request-body
+                {:model "test-model"
+                 :user-messages [{:role "user" :content "test"}]
+                 :extra-payload {}})]
+      (is (contains? body :max_completion_tokens) "Should contain max_completion_tokens by default")
+      (is (not (contains? body :max_tokens)) "Should not contain max_tokens by default")
+      (is (= 32000 (:max_completion_tokens body)) "Should default to 32000 max_completion_tokens")))
+
+  (testing "Version 1 with user-provided max_tokens in extraPayload"
+    (let [body (capture-request-body
+                {:model "test-model"
+                 :user-messages [{:role "user" :content "test"}]
+                 :extra-payload {:version 1 :max_tokens 2048}})]
+      (is (contains? body :max_tokens) "Should contain max_tokens for version 1")
+      (is (= 2048 (:max_tokens body)) "Should use user-provided max_tokens for version 1")
+      (is (not (contains? body :max_completion_tokens)) "Should not contain max_completion_tokens for version 1")))
+
+  (testing "Version 2 with user-provided max_completion_tokens in extraPayload"
+    (let [body (capture-request-body
+                {:model "test-model"
+                 :user-messages [{:role "user" :content "test"}]
+                 :extra-payload {:version 2 :max_completion_tokens 1024}})]
+      (is (contains? body :max_completion_tokens) "Should contain max_completion_tokens for version 2")
+      (is (= 1024 (:max_completion_tokens body)) "Should use user-provided max_completion_tokens for version 2")
+      (is (not (contains? body :max_tokens)) "Should not contain max_tokens for version 2")))
+
+  (testing "Version 1 removes conflicting max_completion_tokens from extraPayload"
+    (let [body (capture-request-body
+                {:model "test-model"
+                 :user-messages [{:role "user" :content "test"}]
+                 :extra-payload {:version 1 :max_completion_tokens 5000}})]
+      (is (contains? body :max_tokens) "Should contain max_tokens for version 1")
+      (is (not (contains? body :max_completion_tokens)) "Should remove conflicting max_completion_tokens for version 1")))
+
+  (testing "Version 2 removes conflicting max_tokens from extraPayload"
+    (let [body (capture-request-body
+                {:model "test-model"
+                 :user-messages [{:role "user" :content "test"}]
+                 :extra-payload {:version 2 :max_tokens 5000}})]
+      (is (contains? body :max_completion_tokens) "Should contain max_completion_tokens for version 2")
+      (is (not (contains? body :max_tokens)) "Should remove conflicting max_tokens for version 2")))
+
+  (testing "Version parameter is not included in final request body"
+    (let [body (capture-request-body
+                {:model "test-model"
+                 :user-messages [{:role "user" :content "test"}]
+                 :extra-payload {:version 1 :temperature 0.7}})]
+      (is (not (contains? body :version)) "Should not contain version in final request body")
+      (is (= 0.7 (:temperature body)) "Should preserve other extraPayload parameters")))))