From 6afba7e2e12ec50378b0a25a2cee46de96062b15 Mon Sep 17 00:00:00 2001
From: Theo Browne <me@theo.li>
Date: Mon, 26 Oct 2026 23:00:48 -0700
Subject: [PATCH] fix(server): MCP toolkit tools advertise the compact input
 schema

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 apps/server/src/mcp/McpHttpServer.test.ts     | 14 ++++++++
 apps/server/src/mcp/McpHttpServer.ts          | 33 ++++++++++++++++++-
 .../src/provider/agentContextBudget.json      |  2 +-
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/apps/server/src/mcp/McpHttpServer.test.ts b/apps/server/src/mcp/McpHttpServer.test.ts
index 6c502c1ef1305..8d7fb85a9e9b8 100644
--- a/apps/server/src/mcp/McpHttpServer.test.ts
+++ b/apps/server/src/mcp/McpHttpServer.test.ts
@@ -21,6 +21,7 @@ import { HttpBody, HttpClient, HttpClientRequest, HttpRouter } from "effect/http
 import * as ProjectService from "../project/ProjectService.ts";
 import * as PullRequestReviewGuideService from "../pullRequest/PullRequestReviewGuideService.ts";
 import * as ServerConfig from "../config.ts";
+import { advertisedJsonSchema } from "./advertisedJsonSchema.ts";
 import * as McpHttpServer from "./McpHttpServer.ts";
 import * as McpToolAccess from "./McpToolAccess.ts";
 import * as McpToolAccessTestkit from "./McpToolAccess.testkit.ts";
@@ -29,6 +30,7 @@ import * as McpSessionRegistry from "./McpSessionRegistry.ts";
 import * as OrchestratorMcpService from "./OrchestratorMcpService.ts";
 import * as PreviewAutomationBroker from "./PreviewAutomationBroker.ts";
 import { OrchestratorToolkit } from "./toolkits/orchestrator/tools.ts";
+import { PreviewToolkit } from "./toolkits/preview/tools.ts";
 import { ProjectToolkit } from "./toolkits/project/tools.ts";
 
 const environmentId = EnvironmentId.make("environment-mcp-test");
@@ -356,6 +358,18 @@ it.effect("refuses preview tools to a client outside a thread before they run",
   }).pipe(Effect.provide(layerTest)),
 );
 
+it.effect("advertises the compact input schema for every registered tool", () =>
+  Effect.gen(function* () {
+    const server = yield* McpServer.McpServer;
+    const tools: Record<string, Tool.Any> = PreviewToolkit.tools;
+
+    expect(server.tools.length).toBe(Object.keys(tools).length);
+    for (const { tool } of server.tools) {
+      expect(tool.inputSchema, tool.name).toEqual(advertisedJsonSchema(tools[tool.name]!));
+    }
+  }).pipe(Effect.provide(layerTest)),
+);
+
 it.effect("checks each preview_act action against that action's own input", () =>
   Effect.scoped(
     Effect.gen(function* () {
diff --git a/apps/server/src/mcp/McpHttpServer.ts b/apps/server/src/mcp/McpHttpServer.ts
index 044a43baeab1e..0d8837f0ab875 100644
--- a/apps/server/src/mcp/McpHttpServer.ts
+++ b/apps/server/src/mcp/McpHttpServer.ts
@@ -948,6 +948,35 @@ const withToolAnalytics =
     return yield* registration.pipe(Effect.provideService(McpServer.McpServer, recordingServer));
   });
 
+/** `McpServer.registerToolkit` advertises the full codec schema; agents get the compact one. */
+const withAdvertisedSchemas = Effect.fn("McpHttpServer.withAdvertisedSchemas")(function* <
+  Tools extends Record<string, Tool.Any>,
+  A,
+  E,
+  R,
+>(toolkit: Toolkit.Toolkit<Tools>, registration: Effect.Effect<A, E, R>) {
+  const server = yield* McpServer.McpServer;
+  const tools: Record<string, Tool.Any> = toolkit.tools;
+  const advertisingServer = McpServer.McpServer.of({
+    ...server,
+    addTool: Effect.fn("McpServer.addTool")(function (options) {
+      const tool = tools[options.tool.name];
+      return server.addTool(
+        tool === undefined
+          ? options
+          : {
+              ...options,
+              tool: new McpSchema.Tool({
+                ...options.tool,
+                inputSchema: advertisedJsonSchema(tool),
+              }),
+            },
+      );
+    }),
+  });
+  return yield* registration.pipe(Effect.provideService(McpServer.McpServer, advertisingServer));
+});
+
 /**
  * `McpServer.toolkit` asks for every service its tools declare when it
  * registers them, but the auth middleware provides `McpInvocationContext` to
@@ -959,7 +988,9 @@ export const toolkitRegistration = <Tools extends Record<string, Tool.Any>, EX,
   handlers: McpToolAccess.HandlersLayer<Tools, EX, RX>,
 ) => {
   const registration = Layer.effectDiscard(
-    withToolAnalytics(McpServer.registerToolkit(decodeAsAdvertised(toolkit))),
+    withToolAnalytics(
+      withAdvertisedSchemas(toolkit, McpServer.registerToolkit(decodeAsAdvertised(toolkit))),
+    ),
   ).pipe(Layer.provide(McpServer.McpServer.layer));
   // @effect-diagnostics-next-line unsafeEffectTypeAssertion:off - the auth middleware provides it per request.
   const registered = registration as Layer.Layer<
diff --git a/apps/server/src/provider/agentContextBudget.json b/apps/server/src/provider/agentContextBudget.json
index e3272bfdf8021..a54335979eb4c 100644
--- a/apps/server/src/provider/agentContextBudget.json
+++ b/apps/server/src/provider/agentContextBudget.json
@@ -1,5 +1,5 @@
 {
   "claudeSystemAppendBytes": 4857,
   "codexAdditionalContextBytes": 5409,
-  "mcpToolListJsonBytes": 54964
+  "mcpToolListJsonBytes": 42328
 }