diff --git a/src/agent/executor.ts b/src/agent/executor.ts
index 3d1ead9..f98e25c 100644
--- a/src/agent/executor.ts
+++ b/src/agent/executor.ts
@@ -20,6 +20,28 @@ export class Executor {
     sessionId: string,
     toolCall: ToolCall
   ): Promise<ToolResult> {
+    const validationError = this.registry.validateInput(
+      toolCall.name,
+      toolCall.input
+    );
+    if (validationError) {
+      await this.eventStore.append(sessionId, "tool_call", {
+        id: toolCall.id,
+        tool: toolCall.name,
+        input: toolCall.input,
+      });
+
+      await this.eventStore.append(sessionId, "tool_result", {
+        toolCallId: toolCall.id,
+        tool: toolCall.name,
+        content: validationError.content.slice(0, 2000),
+        isError: true,
+        metadata: validationError.metadata,
+      });
+
+      return validationError;
+    }
+
     const decision = this.policyEngine.evaluate(toolCall.name, toolCall.input);
 
     await this.eventStore.append(sessionId, "policy_decision", {
diff --git a/src/tools/registry.ts b/src/tools/registry.ts
index 5746ab7..72724d4 100644
--- a/src/tools/registry.ts
+++ b/src/tools/registry.ts
@@ -1,5 +1,14 @@
 import type { ToolDefinition, ToolResult, ToolSchema } from "../types/tools.js";
 
+const SUPPORTED_PROPERTY_TYPES = new Set([
+  "string",
+  "number",
+  "integer",
+  "boolean",
+  "object",
+  "array",
+]);
+
 export class ToolRegistry {
   private tools = new Map<string, ToolDefinition>();
 
@@ -29,6 +38,118 @@ export class ToolRegistry {
     if (!tool) {
       return { content: `Unknown tool: ${name}`, isError: true };
     }
+
+    const validationError = this.validateInput(name, input);
+    if (validationError) return validationError;
+
     return tool.execute(input);
   }
+
+  validateInput(
+    name: string,
+    input: Record<string, unknown>
+  ): ToolResult | null {
+    const tool = this.tools.get(name);
+    if (!tool) return null;
+
+    const errors = validateAgainstSchema(tool.inputSchema, input);
+    if (errors.length === 0) return null;
+
+    return {
+      content: `Invalid input for tool "${name}": ${errors.join(" ")}`,
+      isError: true,
+      metadata: {
+        validationErrors: errors,
+      },
+    };
+  }
+}
+
+function validateAgainstSchema(
+  schema: Record<string, unknown>,
+  input: Record<string, unknown>
+): string[] {
+  const errors: string[] = [];
+  const required = readRequiredFields(schema.required);
+
+  for (const field of required) {
+    if (
+      !Object.prototype.hasOwnProperty.call(input, field) ||
+      input[field] === undefined
+    ) {
+      errors.push(`missing required field "${field}".`);
+    }
+  }
+
+  const properties = readProperties(schema.properties);
+  for (const [field, propertySchema] of Object.entries(properties)) {
+    if (
+      !Object.prototype.hasOwnProperty.call(input, field) ||
+      input[field] === undefined
+    ) {
+      continue;
+    }
+
+    const expectedType = readExpectedType(propertySchema);
+    if (!expectedType) continue;
+
+    const value = input[field];
+    if (!matchesType(value, expectedType)) {
+      errors.push(`field "${field}" must be ${formatExpectedType(expectedType)}.`);
+    }
+  }
+
+  return errors;
+}
+
+function readRequiredFields(required: unknown): string[] {
+  if (!Array.isArray(required)) return [];
+  return required.filter((field): field is string => typeof field === "string");
+}
+
+function readProperties(
+  properties: unknown
+): Record<string, Record<string, unknown>> {
+  if (!isPlainObject(properties)) return {};
+
+  const result: Record<string, Record<string, unknown>> = {};
+  for (const [field, propertySchema] of Object.entries(properties)) {
+    if (isPlainObject(propertySchema)) {
+      result[field] = propertySchema;
+    }
+  }
+  return result;
+}
+
+function readExpectedType(schema: Record<string, unknown>): string | null {
+  const type = schema.type;
+  if (typeof type !== "string") return null;
+  return SUPPORTED_PROPERTY_TYPES.has(type) ? type : null;
+}
+
+function matchesType(value: unknown, expectedType: string): boolean {
+  switch (expectedType) {
+    case "array":
+      return Array.isArray(value);
+    case "boolean":
+      return typeof value === "boolean";
+    case "integer":
+      return Number.isInteger(value);
+    case "number":
+      return typeof value === "number" && Number.isFinite(value);
+    case "object":
+      return isPlainObject(value);
+    case "string":
+      return typeof value === "string";
+    default:
+      return true;
+  }
+}
+
+function formatExpectedType(expectedType: string): string {
+  return expectedType === "integer" ? "an integer" : `a ${expectedType}`;
+}
+
+function isPlainObject(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
 }
diff --git a/tests/harness-behavior.test.ts b/tests/harness-behavior.test.ts
index f1858c3..8ceab82 100644
--- a/tests/harness-behavior.test.ts
+++ b/tests/harness-behavior.test.ts
@@ -13,6 +13,7 @@ import type { ChatParams, ModelProvider } from "../src/models/provider.js";
 import { EventStore } from "../src/storage/event-store.js";
 import { SessionStore } from "../src/storage/session-store.js";
 import { ToolRegistry } from "../src/tools/registry.js";
+import { runCommandTool } from "../src/tools/run-command.js";
 import type { ModelResponse, SessionState } from "../src/types/agent.js";
 import type { Message } from "../src/types/messages.js";
 import type { ToolDefinition } from "../src/types/tools.js";
@@ -614,3 +615,78 @@ test("unknown tool calls are logged and returned as error tool results", async (
     rmSync(cwd, { recursive: true, force: true });
   }
 });
+
+test("malformed run_command input returns a tool error without policy evaluation", async () => {
+  const cwd = createTempDir();
+  const session = createSession(cwd);
+  const registry = new ToolRegistry();
+  registry.register(runCommandTool);
+  const responses: ModelResponse[] = [
+    {
+      stopReason: "tool_use",
+      content: [
+        {
+          type: "tool_use",
+          id: "tool-1",
+          name: "run_command",
+          input: {},
+        },
+      ],
+    },
+    {
+      stopReason: "end_turn",
+      content: [{ type: "text", text: "I need to include a command." }],
+    },
+  ];
+  const provider: ModelProvider = {
+    async chat() {
+      const response = responses.shift();
+      assert.ok(response);
+      return response;
+    },
+  };
+  const { loop, eventStore, sessionStore } = createHarness(cwd, provider, {
+    registry,
+    policyEngine: PolicyEngine.withDefaults(),
+  });
+
+  try {
+    await silenceConsole(() => loop.run(session, "Run a command"));
+
+    const saved = await sessionStore.load(session.id);
+    assert.ok(saved);
+    assert.deepEqual(saved.messages.at(-2), {
+      role: "user",
+      content: [
+        {
+          type: "tool_result",
+          toolUseId: "tool-1",
+          content:
+            'Invalid input for tool "run_command": missing required field "command".',
+          isError: true,
+          metadata: {
+            validationErrors: ['missing required field "command".'],
+          },
+        },
+      ],
+    });
+
+    const events = await eventStore.getEvents(session.id);
+    assert.deepEqual(
+      events.map((event) => event.type),
+      ["user_message", "assistant_response", "tool_call", "tool_result", "assistant_response"]
+    );
+    assert.deepEqual(events[3].data, {
+      toolCallId: "tool-1",
+      tool: "run_command",
+      content:
+        'Invalid input for tool "run_command": missing required field "command".',
+      isError: true,
+      metadata: {
+        validationErrors: ['missing required field "command".'],
+      },
+    });
+  } finally {
+    rmSync(cwd, { recursive: true, force: true });
+  }
+});
diff --git a/tests/tool-registry.test.ts b/tests/tool-registry.test.ts
index 1d3ce86..ad300b2 100644
--- a/tests/tool-registry.test.ts
+++ b/tests/tool-registry.test.ts
@@ -47,3 +47,33 @@ test("ToolRegistry.toSchemas returns deterministic name-sorted schemas", () => {
     parameters: alpha.inputSchema,
   });
 });
+
+test("ToolRegistry.execute returns validation errors for malformed tool input", async () => {
+  const registry = new ToolRegistry();
+  registry.register(createTool("alpha"));
+
+  const result = await registry.execute("alpha", {});
+
+  assert.deepEqual(result, {
+    content: 'Invalid input for tool "alpha": missing required field "value".',
+    isError: true,
+    metadata: {
+      validationErrors: ['missing required field "value".'],
+    },
+  });
+});
+
+test("ToolRegistry.execute validates primitive property types", async () => {
+  const registry = new ToolRegistry();
+  registry.register(createTool("alpha"));
+
+  const result = await registry.execute("alpha", { value: 42 });
+
+  assert.deepEqual(result, {
+    content: 'Invalid input for tool "alpha": field "value" must be a string.',
+    isError: true,
+    metadata: {
+      validationErrors: ['field "value" must be a string.'],
+    },
+  });
+});