diff --git a/packages/coding-agent/src/tools/write.ts b/packages/coding-agent/src/tools/write.ts
index 94f76e4f8..f1bb1bcd3 100644
--- a/packages/coding-agent/src/tools/write.ts
+++ b/packages/coding-agent/src/tools/write.ts
@@ -402,8 +402,9 @@ export class WriteTool implements AgentTool<typeof writeSchema, WriteToolDetails
 			// Decode the device JSON payload and evaluate the mounted tool's own
 			// approval (which may be argument-dependent, e.g. ast_edit is read-tier
 			// for internal-URL paths, debug is read-tier for inspection actions).
-			// Malformed JSON, non-object payloads, and missing content stay exec so
-			// the gate fails closed — the dispatch itself rejects them too.
+			// Malformed JSON, non-object payloads, missing content, and approval
+			// functions that reject schema-invalid objects stay exec so the gate
+			// fails closed — the dispatch itself rejects invalid arguments too.
 			const rawContent = (args as Partial<WriteParams>).content;
 			if (typeof rawContent !== "string") return "exec";
 			let parsed: unknown;
@@ -413,7 +414,11 @@ export class WriteTool implements AgentTool<typeof writeSchema, WriteToolDetails
 				return "exec";
 			}
 			if (!isRecord(parsed)) return "exec";
-			return resolveToolTier(inst, parsed);
+			try {
+				return resolveToolTier(inst, parsed);
+			} catch {
+				return "exec";
+			}
 		}
 		// Remote SSH writes open an outbound connection and run a remote shell —
 		// gate them like the exec-tier `ssh` tool, ahead of the handler-write
diff --git a/packages/coding-agent/test/write-xdev-dispatch.test.ts b/packages/coding-agent/test/write-xdev-dispatch.test.ts
index 969fa3621..bd168e33f 100644
--- a/packages/coding-agent/test/write-xdev-dispatch.test.ts
+++ b/packages/coding-agent/test/write-xdev-dispatch.test.ts
@@ -115,11 +115,13 @@ describe("read and write route xd:// device URLs", () => {
 			expect(tier("xd://debug", JSON.stringify({ action: "sessions" }))).toBe("read");
 			expect(tier("xd://debug", JSON.stringify({ action: "launch", program: "./app" }))).toBe("exec");
 
-			// Fail closed: malformed JSON, non-object payloads, missing content,
-			// and unknown devices all stay exec so the gate never under-prompts.
+			// Fail closed: malformed JSON, non-object or schema-invalid payloads,
+			// missing content, and unknown devices all stay exec so the gate never
+			// under-prompts.
 			expect(tier("xd://ast_edit", "{ not json")).toBe("exec");
 			expect(tier("xd://ast_edit", "[1,2,3]")).toBe("exec");
 			expect(tier("xd://ast_edit", '"a string"')).toBe("exec");
+			expect(tier("xd://ast_edit", JSON.stringify({ paths: [null] }))).toBe("exec");
 			expect(approval({ path: "xd://ast_edit" })).toBe("exec");
 			expect(tier("xd://no_such_device", "{}")).toBe("exec");
 		} finally {