diff --git a/src/lib/api-handler.ts b/src/lib/api-handler.ts
index 2a8386d6e..18bd0d40c 100644
--- a/src/lib/api-handler.ts
+++ b/src/lib/api-handler.ts
@@ -16,6 +16,11 @@ import { SCOPE_HEALTH_READ, SCOPE_HEALTH_WRITE } from "./mcp/oauth/config";
 /**
  * HTTP methods a read-only credential may use on the REST surface. A request
  * with any other method (POST / PUT / PATCH / DELETE) is a write.
+ *
+ * The MCP-audience guard below assumes these methods are side-effect-free:
+ * an MCP-bound token is admitted on them. A future side-effecting GET (or HEAD)
+ * would silently widen that token's reach over REST — do NOT add one without
+ * revisiting the MCP audience binding here.
  */
 const READ_HTTP_METHODS = new Set(["GET", "HEAD", "OPTIONS"]);
 
diff --git a/src/lib/mcp/__tests__/write-tools.test.ts b/src/lib/mcp/__tests__/write-tools.test.ts
index ac6fa49b4..d677f3ba6 100644
--- a/src/lib/mcp/__tests__/write-tools.test.ts
+++ b/src/lib/mcp/__tests__/write-tools.test.ts
@@ -3,10 +3,14 @@ import { describe, it, expect, vi, beforeEach } from "vitest";
 const logMcpMeasurement = vi.fn();
 const logMcpMood = vi.fn();
 const logMcpBloodPressure = vi.fn();
+const checkMcpMeasurement = vi.fn();
+const checkMcpBloodPressure = vi.fn();
 vi.mock("../writes", () => ({
   logMcpMeasurement: (...a: unknown[]) => logMcpMeasurement(...a),
   logMcpMood: (...a: unknown[]) => logMcpMood(...a),
   logMcpBloodPressure: (...a: unknown[]) => logMcpBloodPressure(...a),
+  checkMcpMeasurement: (...a: unknown[]) => checkMcpMeasurement(...a),
+  checkMcpBloodPressure: (...a: unknown[]) => checkMcpBloodPressure(...a),
 }));
 
 const checkMcpWriteRateLimit = vi.fn();
@@ -41,6 +45,10 @@ beforeEach(() => {
     remaining: 59,
     resetAt: Date.now() + 1000,
   });
+  // Default: the pre-write validation passes, so a preview echoes a clean
+  // would-be record. Individual tests override to assert wouldFail.
+  checkMcpMeasurement.mockReturnValue({ ok: true });
+  checkMcpBloodPressure.mockReturnValue({ ok: true });
 });
 
 describe("write-tool surface", () => {
@@ -167,6 +175,27 @@ describe("confirm gate — log_measurement", () => {
     expect(result.written).toBe(false);
     expect(logMcpMeasurement).not.toHaveBeenCalled();
   });
+
+  it("a preview that would be refused on commit carries wouldFail + reason", async () => {
+    checkMcpMeasurement.mockReturnValue({
+      ok: false,
+      error: "out_of_range",
+      reason: "Timestamp must not be in the future",
+    });
+    const result = (await tool("log_measurement").run(CTX, {
+      type: "WEIGHT",
+      value: 80,
+      measuredAt: "9999-01-01T00:00:00Z",
+      idempotencyKey: "k-1",
+    })) as Record<string, unknown>;
+    // Still a preview — nothing written — but the commit verdict is surfaced.
+    expect(result.requiresConfirmation).toBe(true);
+    expect(result.written).toBe(false);
+    expect(result.wouldFail).toBe(true);
+    expect(result.error).toBe("out_of_range");
+    expect(result.reason).toBe("Timestamp must not be in the future");
+    expect(logMcpMeasurement).not.toHaveBeenCalled();
+  });
 });
 
 describe("confirm gate — log_mood", () => {
diff --git a/src/lib/mcp/write-tools.ts b/src/lib/mcp/write-tools.ts
index feba0f78c..508e027b1 100644
--- a/src/lib/mcp/write-tools.ts
+++ b/src/lib/mcp/write-tools.ts
@@ -35,7 +35,13 @@ import type { MeasurementType } from "@/generated/prisma/client";
 import { annotate } from "@/lib/logging/context";
 import { checkMcpWriteRateLimit } from "@/lib/rate-limit";
 import type { McpToolDefinition } from "./tools";
-import { logMcpMeasurement, logMcpMood, logMcpBloodPressure } from "./writes";
+import {
+  logMcpMeasurement,
+  logMcpMood,
+  logMcpBloodPressure,
+  checkMcpMeasurement,
+  checkMcpBloodPressure,
+} from "./writes";
 
 /**
  * Write-tool annotations (MCP 2025-11-25). NOT read-only and NOT destructive
@@ -66,6 +72,10 @@ const writeOutput: z.ZodRawShape = {
   instruction: z.string().optional(),
   preview: z.unknown().optional(),
   record: z.unknown().optional(),
+  // Set on a preview that would be REFUSED on commit (type / range / instant),
+  // with `error` + `reason` carrying the same verdict the commit would return —
+  // so a preview and its commit never disagree.
+  wouldFail: z.boolean().optional(),
 };
 
 /** Shared confirm-gate inputs every write tool carries. */
@@ -156,6 +166,9 @@ export const MCP_WRITE_TOOLS: McpToolDefinition[] = [
       }
 
       if (!confirm) {
+        // Run the SAME validation the commit core runs so the preview cannot
+        // show a value that would then be refused on commit.
+        const check = checkMcpMeasurement(type, value, measuredAt);
         annotate({
           action: { name: "mcp.tool.write" },
           meta: { tool: "log_measurement", status: "preview" },
@@ -171,6 +184,9 @@ export const MCP_WRITE_TOOLS: McpToolDefinition[] = [
             source: "MCP",
           },
           instruction: CONFIRM_INSTRUCTION,
+          ...(check.ok
+            ? {}
+            : { wouldFail: true, error: check.error, reason: check.reason }),
         };
       }
 
@@ -310,6 +326,8 @@ export const MCP_WRITE_TOOLS: McpToolDefinition[] = [
       }
 
       if (!confirm) {
+        // Same validation the commit core runs — preview == commit verdict.
+        const check = checkMcpBloodPressure(systolic, diastolic, measuredAt);
         annotate({
           action: { name: "mcp.tool.write" },
           meta: { tool: "log_blood_pressure", status: "preview" },
@@ -325,6 +343,9 @@ export const MCP_WRITE_TOOLS: McpToolDefinition[] = [
             source: "MCP",
           },
           instruction: CONFIRM_INSTRUCTION,
+          ...(check.ok
+            ? {}
+            : { wouldFail: true, error: check.error, reason: check.reason }),
         };
       }