Fall back to title/name for missing MCP tool description

Per the MCP spec, tool `description` is optional; some servers omit it
or send an empty string. Anthropic's API rejects this with a 400
(`tools.<n>.custom.description: Input should be a valid string`),
breaking chats whenever such a server is enabled.

Coerce at the MCP boundary in `tool->internal` to `:description ->
:title -> "MCP tool: <name>"` so every provider downstream receives a
non-null string.

🤖 Generated with [eca](https://eca.dev)

Co-Authored-By: eca-agent <git@eca.dev>

Author: ericdallo

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- Bugfix: MCP tools without a `description` (which the MCP spec marks optional) no longer break Anthropic chat requests with `tools.<n>.custom.description: Input should be a valid string`. Missing/empty descriptions now fall back to the tool's `title`, then to a synthesized `MCP tool: <name>` string at the MCP boundary so all providers receive a non-null string.
+
 ## 0.131.0
 
 - Add `${plugin:root}` dynamic interpolation for plugin-provided config, hooks, commands, and rules.

src/eca/features/tools/mcp.clj

@@ -226,10 +226,16 @@
       :else nil)))
 
 (defn ^:private tool->internal
-  "Adapt plumcp tool map to ECA's internal tool shape."
+  "Adapt plumcp tool map to ECA's internal tool shape.
+
+  Per the MCP spec, tool `description` is optional; some servers omit it. We
+  normalize a missing/empty description here so providers that require a
+  non-null string (e.g. Anthropic) don't reject the request."
   [tool]
   {:name (:name tool)
-   :description (:description tool)
+   :description (or (not-empty (:description tool))
+                    (not-empty (:title tool))
+                    (str "MCP tool: " (:name tool)))
    :parameters (:inputSchema tool)})
 
 (defn ^:private format-jsonrpc-error

test/eca/features/tools/mcp_test.clj

@@ -428,3 +428,40 @@
       (is (= 2 @refresh-count*) "should have tried token refresh on attempts 1 and 2")
       (is (= :failed (get-in @db* [:mcp-clients "fail-server" :status]))
           "server should be in failed state after exhausting retries"))))
+
+(deftest tool->internal-description-fallback-test
+  (testing "uses :description when present"
+    (is (= "Generate an image"
+           (:description (#'mcp/tool->internal
+                          {:name "create-image"
+                           :description "Generate an image"
+                           :inputSchema {}})))))
+
+  (testing "falls back to :title when :description is missing"
+    (is (= "Create Image"
+           (:description (#'mcp/tool->internal
+                          {:name "create-image"
+                           :title "Create Image"
+                           :inputSchema {}})))))
+
+  (testing "falls back to :title when :description is empty string"
+    (is (= "Create Image"
+           (:description (#'mcp/tool->internal
+                          {:name "create-image"
+                           :description ""
+                           :title "Create Image"
+                           :inputSchema {}})))))
+
+  (testing "synthesizes from :name when both :description and :title are missing"
+    (is (= "MCP tool: create-image"
+           (:description (#'mcp/tool->internal
+                          {:name "create-image"
+                           :inputSchema {}})))))
+
+  (testing "synthesizes from :name when both :description and :title are empty"
+    (is (= "MCP tool: edit-image"
+           (:description (#'mcp/tool->internal
+                          {:name "edit-image"
+                           :description ""
+                           :title ""
+                           :inputSchema {}}))))))