diff --git a/README.md b/README.md
index 7480b79..3e8a0ab 100644
--- a/README.md
+++ b/README.md
@@ -80,7 +80,7 @@ The process exits `1` when any finding reaches the `--fail-on` threshold (defaul
 
 ## Rule catalog
 
-18 built-in rules, each with a stable id, severity, and remediation. Toggle any of
+19 built-in rules, each with a stable id, severity, and remediation. Toggle any of
 them via config.
 
 | Rule | Severity | Category | Description |
@@ -88,6 +88,7 @@ them via config.
 | `MCP001` | high | permissions | Destructive tool lacks scoping or confirmation |
 | `MCP002` | critical | permissions | Tool exposes arbitrary command execution |
 | `MCP003` | medium | permissions | Mutating tool has an unscoped input surface |
+| `MCP004` | high | permissions | Mutating tool advertises readOnlyHint |
 | `MCP010` | medium | schema | Tool is missing an input schema |
 | `MCP011` | low | schema | Schema allows unbounded additional properties |
 | `MCP012` | low | schema | Unconstrained string argument |
diff --git a/src/rules/permissions.ts b/src/rules/permissions.ts
index 3197c32..71fbf13 100644
--- a/src/rules/permissions.ts
+++ b/src/rules/permissions.ts
@@ -94,6 +94,40 @@ export const execTool: Rule = {
  * A mutating tool that accepts a free-form object with no properties can write
  * anything anywhere.
  */
+/**
+ * MCP004 - Mutating/destructive tool advertises readOnlyHint.
+ * Clients use readOnlyHint for auto-approval; marking a write tool read-only
+ * is a confused-deputy / approval-bypass primitive.
+ */
+export const readOnlyHintMismatch: Rule = {
+  id: "MCP004",
+  title: "Mutating tool advertises readOnlyHint",
+  description:
+    "A tool that mutates or deletes state must not advertise readOnlyHint, which clients use to auto-approve safe reads.",
+  severity: "high",
+  category: "permissions",
+  evaluate(target, ctx): Finding[] {
+    const mutatingVerbs = [...WRITE_VERBS, ...DESTRUCTIVE_VERBS];
+    const findings: Finding[] = [];
+    for (const tool of target.tools) {
+      if (tool.annotations?.readOnlyHint !== true) continue;
+      const haystack = `${tool.name} ${tool.description ?? ""}`;
+      const hit = containsAny(haystack, mutatingVerbs);
+      if (!hit) continue;
+      findings.push(
+        ctx.report({
+          title: "Write tool incorrectly marked read-only",
+          message: `Tool "${tool.name}" advertises readOnlyHint but implies a mutating action ("${hit}").`,
+          remediation:
+            "Set readOnlyHint to false (or omit it) for tools that delete, update, or otherwise change state.",
+          location: tool.name,
+        }),
+      );
+    }
+    return findings;
+  },
+};
+
 export const unscopedWriteTool: Rule = {
   id: "MCP003",
   title: "Mutating tool has an unscoped input surface",
@@ -128,5 +162,6 @@ export const unscopedWriteTool: Rule = {
 export const permissionRules: Rule[] = [
   destructiveNoScoping,
   execTool,
+  readOnlyHintMismatch,
   unscopedWriteTool,
 ];
diff --git a/src/types.ts b/src/types.ts
index 835d3a1..c1ca8aa 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -34,11 +34,21 @@ export interface JsonSchema {
   [key: string]: unknown;
 }
 
+/** MCP tool annotations (e.g. hints for clients and auto-approval). */
+export interface ToolAnnotations {
+  readOnlyHint?: boolean;
+  destructiveHint?: boolean;
+  idempotentHint?: boolean;
+  openWorldHint?: boolean;
+  [key: string]: unknown;
+}
+
 /** A tool as enumerated from an MCP server (or a static manifest). */
 export interface ToolSpec {
   name: string;
   description?: string;
   inputSchema?: JsonSchema;
+  annotations?: ToolAnnotations;
 }
 
 /** A resource exposed by an MCP server. */
diff --git a/test/rules.test.ts b/test/rules.test.ts
index 4a5f9bb..9eb9f29 100644
--- a/test/rules.test.ts
+++ b/test/rules.test.ts
@@ -58,6 +58,46 @@ describe("rules against the insecure surface", () => {
   }
 });
 
+describe("MCP004 readOnlyHint mismatch", () => {
+  function findingsFor(tools: AuditTarget["tools"]) {
+    return audit(makeTarget({ tools })).findings.filter((f) => f.ruleId === "MCP004");
+  }
+
+  it("fires when a write-verb tool advertises readOnlyHint", () => {
+    const findings = findingsFor([
+      {
+        name: "cleanup_records",
+        description: "deletes stale records",
+        annotations: { readOnlyHint: true },
+      },
+    ]);
+    expect(findings).toHaveLength(1);
+    expect(findings[0].location).toBe("cleanup_records");
+  });
+
+  it("does not fire when readOnlyHint is false", () => {
+    const findings = findingsFor([
+      {
+        name: "cleanup_records",
+        description: "deletes stale records",
+        annotations: { readOnlyHint: false },
+      },
+    ]);
+    expect(findings).toHaveLength(0);
+  });
+
+  it("does not fire for a read-only tool with readOnlyHint", () => {
+    const findings = findingsFor([
+      {
+        name: "get_weather",
+        description: "returns weather for a city",
+        annotations: { readOnlyHint: true },
+      },
+    ]);
+    expect(findings).toHaveLength(0);
+  });
+});
+
 describe("MCP002 exec detection", () => {
   it("flags a shell tool as critical", () => {
     const target = makeTarget({