diff --git a/apps/server/src/mcp/advertisedJsonSchema.test.ts b/apps/server/src/mcp/advertisedJsonSchema.test.ts
index 09a90556a9dd0..4fb0b58895202 100644
--- a/apps/server/src/mcp/advertisedJsonSchema.test.ts
+++ b/apps/server/src/mcp/advertisedJsonSchema.test.ts
@@ -33,7 +33,11 @@ it("drops JSON noise, merges tagged unions, and keeps null where it is required"
       target: {
         type: "object",
         properties: {
-          kind: { type: "string", enum: ["url", "number"] },
+          kind: {
+            type: "string",
+            enum: ["url", "number"],
+            description: "`url` requires `url`. `number` requires `number`.",
+          },
           url: { type: "string" },
           number: { type: "integer" },
         },
diff --git a/apps/server/src/mcp/advertisedJsonSchema.ts b/apps/server/src/mcp/advertisedJsonSchema.ts
index 10001307f2d6c..2ba71da70a844 100644
--- a/apps/server/src/mcp/advertisedJsonSchema.ts
+++ b/apps/server/src/mcp/advertisedJsonSchema.ts
@@ -64,6 +64,18 @@ const mergeTaggedObjects = (branches: ReadonlyArray<Json>): JsonObject | undefin
   const requiredByAll = first.filter(
     (name) => name !== tag && others.every((required) => required.includes(name)),
   );
+  // Merging drops each tag's own required fields, so the tag says which they are.
+  const perTag = objects.flatMap((object, index) => {
+    const own = (index === 0 ? first : others[index - 1]!).filter(
+      (name) => name !== tag && !requiredByAll.includes(name),
+    );
+    return own.length === 0
+      ? []
+      : [
+          `\`${String(tagOf(object, tag))}\` requires ${own.map((name) => `\`${name}\``).join(", ")}.`,
+        ];
+  });
+  if (perTag.length > 0) (properties[tag] as JsonObject).description = perTag.join(" ");
   return { type: "object", properties, required: [tag, ...requiredByAll] };
 };
 
diff --git a/apps/server/src/provider/agentContextBudget.json b/apps/server/src/provider/agentContextBudget.json
index 4439c50940722..8143a8e1e4f9c 100644
--- a/apps/server/src/provider/agentContextBudget.json
+++ b/apps/server/src/provider/agentContextBudget.json
@@ -1,5 +1,5 @@
 {
   "claudeSystemAppendBytes": 5597,
   "codexAdditionalContextBytes": 7327,
-  "mcpToolListJsonBytes": 48217
+  "mcpToolListJsonBytes": 48682
 }