diff --git a/apps/server/src/mcp/McpHttpServer.test.ts b/apps/server/src/mcp/McpHttpServer.test.ts
index 9d28aeb4dfc1..90c4af4d6e0d 100644
--- a/apps/server/src/mcp/McpHttpServer.test.ts
+++ b/apps/server/src/mcp/McpHttpServer.test.ts
@@ -34,7 +34,10 @@ import * as McpToolAccess from "./McpToolAccess.ts";
 import * as McpToolAccessTestkit from "./McpToolAccess.testkit.ts";
 import * as McpInvocationContext from "./McpInvocationContext.ts";
 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 { ProjectToolkit } from "./toolkits/project/tools.ts";
 
 const environmentId = EnvironmentId.make("environment-mcp-test");
 const threadId = ThreadId.make("thread-mcp-test");
@@ -125,17 +128,55 @@ const serveSnapshots = (clientId: string, result: unknown) =>
     return inputs;
   });
 
-const callSnapshot = (args: Record<string, unknown>) =>
+const callTool = (name: string, args: Record<string, unknown>) =>
   Effect.gen(function* () {
     const server = yield* McpServer.McpServer;
     return yield* server
-      .callTool({ name: "preview_snapshot", arguments: args })
+      .callTool({ name, arguments: args })
       .pipe(
         Effect.provideService(McpInvocationContext.McpInvocationContext, invocation),
         Effect.provideService(McpSchema.McpServerClient, client),
       );
   });
 
+const callSnapshot = (args: Record<string, unknown>) => callTool("preview_snapshot", args);
+
+// Real tool definitions with handlers that record what they decoded.
+const layerRecordingTools = (calls: Array<unknown>) => {
+  const RecordingToolkit = Toolkit.make(
+    OrchestratorToolkit.tools.delegate_task,
+    OrchestratorToolkit.tools.t3_thread_read,
+    OrchestratorToolkit.tools.t3_thread_wait,
+    ProjectToolkit.tools.t3_project_update,
+  );
+  const record = McpToolAccess.reads((params: unknown) =>
+    Effect.sync(() => calls.push(params)).pipe(
+      Effect.andThen(
+        Effect.fail(new OrchestratorMcpFailure({ code: "thread_not_found", message: "Recorded." })),
+      ),
+    ),
+  );
+  return McpHttpServer.toolkitRegistration(
+    RecordingToolkit,
+    McpToolAccess.toLayer(RecordingToolkit, {
+      delegate_task: record,
+      t3_thread_read: record,
+      t3_thread_wait: record,
+      t3_project_update: record,
+    }),
+  ).pipe(
+    Layer.provideMerge(McpServer.McpServer.layer),
+    Layer.provide(
+      Layer.mergeAll(
+        McpToolAccessTestkit.liveThreadsLayer,
+        Layer.mock(OrchestratorMcpService.OrchestratorMcpService)({}),
+        Layer.mock(ProjectService.ProjectService)({}),
+        NodeServices.layer,
+      ),
+    ),
+  );
+};
+
 it("normalizes empty successful notification responses to accepted", () => {
   const notificationResponse = McpHttpServer.normalizeMcpHttpResponse(
     HttpServerResponse.text("", { status: 200, contentType: "application/json" }),
@@ -390,7 +431,7 @@ it.effect("refuses preview tools to a client outside a thread before they run",
 it.effect("rejects non-boolean snapshot image options before selecting a browser host", () =>
   Effect.gen(function* () {
     const server = yield* McpServer.McpServer;
-    for (const includeImage of ["false", 0, null]) {
+    for (const includeImage of ["false", 0]) {
       const result = yield* server
         .callTool({
           name: "preview_snapshot",
@@ -447,6 +488,58 @@ it.effect("saves the snapshot PNG on request and reports its path", () =>
   ).pipe(Effect.provide(layerTest)),
 );
 
+it.effect("treats null snapshot options like omitted ones", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const inputs = yield* serveSnapshots("mcp-null-options-client", snapshotResult);
+
+      const omitted = yield* callSnapshot({});
+      const nulls = yield* callSnapshot({ tabId: null, includeImage: null, save: null });
+
+      expect(nulls).toEqual(omitted);
+      expect(inputs).toEqual([{}, {}]);
+    }),
+  ).pipe(Effect.provide(layerTest)),
+);
+
+it.effect("decodes null optional arguments the way it decodes omitted ones", () => {
+  const calls: Array<unknown> = [];
+  return Effect.gen(function* () {
+    const thread = { threadId: "thread-null-arguments" };
+    yield* callTool("t3_thread_read", thread);
+    yield* callTool("t3_thread_read", { ...thread, afterPosition: null, view: null });
+    yield* callTool("delegate_task", { task: "Inspect", target: {} });
+    yield* callTool("delegate_task", { task: "Inspect", title: null, target: { model: null } });
+    // A null the schema declares keeps its meaning: here it clears the project icon.
+    yield* callTool("t3_project_update", { projectId: "project-1", projectIcon: null });
+
+    expect(calls).toHaveLength(5);
+    expect(calls[1]).toEqual(calls[0]);
+    expect(calls[3]).toEqual(calls[2]);
+    expect(calls[4]).toStrictEqual({ projectId: "project-1", projectIcon: null });
+  }).pipe(Effect.provide(layerRecordingTools(calls)));
+});
+
+it.effect("accepts finite wait budgets and refuses non-finite ones", () => {
+  const calls: Array<unknown> = [];
+  return Effect.gen(function* () {
+    yield* callTool("delegate_task", { task: "Inspect", timeoutMs: 1500.5 });
+    yield* callTool("t3_thread_wait", { threadId: "thread-wait", timeoutMs: 5000 });
+    expect(calls).toMatchObject([{ timeoutMs: 1500.5 }, { timeoutMs: 5000 }]);
+
+    for (const [name, args] of [
+      ["delegate_task", { task: "Inspect", timeoutMs: "Infinity" }],
+      ["t3_thread_wait", { threadId: "thread-wait", timeoutMs: "NaN" }],
+      ["t3_thread_wait", { threadId: "thread-wait", timeoutMs: "-Infinity" }],
+      ["t3_thread_read", { threadId: "thread-wait", afterPosition: "2" }],
+    ] as const) {
+      const refused = yield* callTool(name, args).pipe(Effect.flip);
+      expect(refused._tag, `${name} ${JSON.stringify(args)}`).toBe("InvalidParams");
+    }
+    expect(calls).toHaveLength(2);
+  }).pipe(Effect.provide(layerRecordingTools(calls)));
+});
+
 it.effect("reports a tagged error when the screenshot cannot be saved", () =>
   Effect.scoped(
     Effect.gen(function* () {
diff --git a/apps/server/src/mcp/McpHttpServer.ts b/apps/server/src/mcp/McpHttpServer.ts
index cd2ccf141638..80d43043df6a 100644
--- a/apps/server/src/mcp/McpHttpServer.ts
+++ b/apps/server/src/mcp/McpHttpServer.ts
@@ -11,7 +11,7 @@ import * as Schema from "effect/Schema";
 import * as Sink from "effect/Sink";
 import * as Stream from "effect/Stream";
 import type * as Types from "effect/Types";
-import { AiError, McpProtocol, McpSchema, McpServer, Tool, type Toolkit } from "effect/ai";
+import { AiError, McpProtocol, McpSchema, McpServer, Tool, Toolkit } from "effect/ai";
 import { HttpRouter, HttpServerRequest, HttpServerResponse } from "effect/http";
 import { OrchestratorMcpFailure, PreviewAutomationError } from "@t3tools/contracts";
 
@@ -386,6 +386,22 @@ const saveScreenshot = Effect.fn("McpHttpServer.saveScreenshot")(function* (
   return screenshotPath;
 });
 
+/**
+ * Tool input schemas are advertised through each schema's JSON codec, which
+ * offers `null` for every `Schema.optional` field, but Toolkit decodes with the
+ * schema itself and rejects that `null`. Decoding through the same codec makes
+ * a tool accept what it advertises.
+ */
+const decodeAsAdvertised = <Tools extends Record<string, Tool.Any>>(
+  toolkit: Toolkit.Toolkit<Tools>,
+): Toolkit.Toolkit<Tools> =>
+  // The clones keep each tool's id, name and decoded parameter type, so handlers still match.
+  Toolkit.make(
+    ...Object.values(toolkit.tools).map((tool) =>
+      tool.setParameters(Schema.toCodecJson(tool.parametersSchema)),
+    ),
+  ) as unknown as Toolkit.Toolkit<Tools>;
+
 const isPreviewAutomationError = Schema.is(PreviewAutomationError);
 
 const previewSnapshotFailure = <E>(cause: Cause.Cause<E>) => {
@@ -431,7 +447,7 @@ const registerPreviewSnapshot = Effect.fn("McpHttpServer.registerPreviewSnapshot
   const saveServices = yield* Effect.context<
     ServerConfig.ServerConfig | FileSystem.FileSystem | Path.Path | Crypto.Crypto
   >();
-  const built = yield* PreviewSnapshotToolkit;
+  const built = yield* decodeAsAdvertised(PreviewSnapshotToolkit);
   const tool = PreviewSnapshotTool;
   yield* server.addTool({
     tool: new McpSchema.Tool({
@@ -687,7 +703,7 @@ const registerImageTool = <T extends Tool.Any, E, R>(
 const registerDeviceScreenshot = Effect.fn("McpHttpServer.registerDeviceScreenshot")(function* () {
   const devices = yield* DeviceService.DeviceService;
   const threads = yield* ThreadManagementService.ThreadManagementService;
-  const built = yield* DeviceScreenshotToolkit;
+  const built = yield* decodeAsAdvertised(DeviceScreenshotToolkit);
   yield* registerImageTool(
     DeviceScreenshotTool,
     (payload) =>
@@ -709,7 +725,7 @@ const isOrchestratorMcpFailure = Schema.is(OrchestratorMcpFailure);
 const registerHtmlPreview = Effect.fn("McpHttpServer.registerHtmlPreview")(function* () {
   const htmlRender = yield* HtmlRender.HtmlRender;
   const threads = yield* ThreadManagementService.ThreadManagementService;
-  const built = yield* HtmlPreviewToolkit;
+  const built = yield* decodeAsAdvertised(HtmlPreviewToolkit);
   yield* registerImageTool(
     HtmlPreviewTool,
     (payload) =>
@@ -740,7 +756,7 @@ export const toolkitRegistration = <Tools extends Record<string, Tool.Any>, EX,
   toolkit: Toolkit.Toolkit<Tools>,
   handlers: McpToolAccess.HandlersLayer<Tools, EX, RX>,
 ) => {
-  const registration = McpServer.toolkit(toolkit);
+  const registration = McpServer.toolkit(decodeAsAdvertised(toolkit));
   // @effect-diagnostics-next-line unsafeEffectTypeAssertion:off - the auth middleware provides it per request.
   const registered = registration as Layer.Layer<
     never,
diff --git a/packages/contracts/src/orchestratorMcp.ts b/packages/contracts/src/orchestratorMcp.ts
index 382cea55bb15..5e178c21551a 100644
--- a/packages/contracts/src/orchestratorMcp.ts
+++ b/packages/contracts/src/orchestratorMcp.ts
@@ -173,7 +173,7 @@ export const OrchestratorMcpDelegateTaskInput = Schema.Struct({
         "Defaults to async. Use wait only when this turn needs the child's result before you can continue.",
     }),
   ),
-  timeoutMs: Schema.optional(Schema.Number).annotate({
+  timeoutMs: Schema.optional(Schema.Finite).annotate({
     description:
       "Wait budget for mode=wait only. Default 10 minutes. Elapsing it returns waitTimedOut=true on that call and does not cancel the child.",
   }),
@@ -443,7 +443,7 @@ export type OrchestratorMcpThreadSendResult = typeof OrchestratorMcpThreadSendRe
 export const OrchestratorMcpThreadWaitInput = Schema.Struct({
   threadId: ThreadId,
   runId: Schema.optional(RunId),
-  timeoutMs: Schema.optional(Schema.Number),
+  timeoutMs: Schema.optional(Schema.Finite),
 });
 export type OrchestratorMcpThreadWaitInput = typeof OrchestratorMcpThreadWaitInput.Type;