Fix promtps that send assistant messages not working for anthropic.

Author: ericdallo

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Support http-streamable http servers (non auth support for now)
+- Fix promtps that send assistant messages not working for anthropic.
 
 ## 0.51.3
 

docs/configuration.md

@@ -68,7 +68,7 @@ For MCP servers configuration, use the `mcpServers` config, examples:
           "command": "npx",
           "args": ["-y", "@modelcontextprotocol/server-memory"],
           // optional
-          "env": {"FOO" "bar"}
+          "env": {"FOO" "bar"} 
         }
       }
     }

src/eca/llm_providers/anthropic.clj

@@ -17,6 +17,44 @@
 
 (def ^:private messages-path "/v1/messages")
 
+(defn ^:private any-assistant-message-without-thinking-previously?
+  "If there is a assistant message, which has no previous any role message with thinking content, returns true."
+  [messages]
+  (loop [msgs messages
+         seen-thinking? false]
+    (if-let [msg (first msgs)]
+      (let [is-assistant? (= "assistant" (:role msg))
+            has-thinking? (and (vector? (:content msg))
+                               (some #(= "thinking" (:type %)) (:content msg)))]
+        (cond
+          ;; If this is an assistant message and we haven't seen thinking before, return true
+          (and is-assistant? (not seen-thinking?) (not has-thinking?))
+          true
+
+          ;; If this message has thinking content, mark it as seen
+          has-thinking?
+          (recur (rest msgs) true)
+
+          ;; Otherwise continue
+          :else
+          (recur (rest msgs) seen-thinking?)))
+      ;; No assistant message found without previous thinking
+      false)))
+
+(defn ^:private fix-non-thinking-assistant-messages [messages]
+  (if (any-assistant-message-without-thinking-previously? messages)
+    ;; Anthropic doesn't like assistant messages without thinking blocks,
+    ;; we force to be a user one when this happens
+    ;; (MCP prompts that return assistant messages as initial step like clojureMCP)
+    ;; https://clojurians.slack.com/archives/C093426FPUG/p1757622242502969
+    (mapv (fn [{:keys [role content] :as msg}]
+            (if (= "assistant" role)
+              {:role "user"
+               :content content}
+              msg))
+          messages)
+    messages))
+
 (defn ^:private ->tools [tools web-search]
   (cond->
    (mapv (fn [tool]
@@ -105,7 +143,7 @@
            tools web-search extra-payload]}
    {:keys [on-message-received on-error on-reason on-prepare-tool-call on-tools-called on-usage-updated]}]
   (let [messages (concat (normalize-messages past-messages)
-                         (normalize-messages user-messages))
+                         (normalize-messages (fix-non-thinking-assistant-messages user-messages)))
         body (merge (assoc-some
                      {:model model
                       :messages (add-cache-to-last-message messages)