diff --git a/cli/commands/build.ts b/cli/commands/build.ts
index bc3f48e1..2d552aea 100644
--- a/cli/commands/build.ts
+++ b/cli/commands/build.ts
@@ -3,6 +3,7 @@ import { execa } from "execa";
 import { exists } from "fs-extra";
 import { mkdir, readFile, writeFile } from "node:fs/promises";
 import { join } from "node:path";
+import { lock, unlockSync } from "proper-lockfile";
 import { cliError } from "../error.js";
 import { isCandidCompatible } from "../helpers/is-candid-compatible.js";
 import { resolveCanisterConfigs } from "../helpers/resolve-canisters.js";
@@ -70,115 +71,149 @@ export async function build(
     motokoPath = resolveConfigPath(motokoPath);
     const wasmPath = join(outputDir, `${canisterName}.wasm`);
     const mostPath = join(outputDir, `${canisterName}.most`);
-    let args = [
-      "-c",
-      "--idl",
-      "--stable-types",
-      "-o",
-      wasmPath,
-      motokoPath,
-      ...(await sourcesArgs()).flat(),
-      ...getGlobalMocArgs(config),
-    ];
-    args.push(
-      ...collectExtraArgs(config, canister, canisterName, options.extraArgs),
-    );
-
-    const isPublicCandid = true; // always true for now to reduce corner cases
-    const candidVisibility = isPublicCandid ? "icp:public" : "icp:private";
-    if (isPublicCandid) {
-      args.push("--public-metadata", "candid:service");
-      args.push("--public-metadata", "candid:args");
+
+    // per-canister lock to prevent parallel builds of the same canister from clobbering output files
+    const lockTarget = join(outputDir, `.${canisterName}.buildlock`);
+    await writeFile(lockTarget, "", { flag: "a" });
+
+    let release: (() => Promise<void>) | undefined;
+    try {
+      release = await lock(lockTarget, {
+        stale: 300_000,
+        retries: { retries: 60, minTimeout: 500, maxTimeout: 5_000 },
+      });
+    } catch {
+      cliError(
+        `Failed to acquire build lock for canister ${canisterName} — another build may be stuck`,
+      );
     }
+
+    // proper-lockfile registers its own signal-exit handler, but it doesn't reliably
+    // fire on process.exit(). This manual handler covers that gap. Double-unlock is
+    // harmless (the second call throws and is caught).
+    const exitCleanup = () => {
+      try {
+        unlockSync(lockTarget);
+      } catch {}
+    };
+    process.on("exit", exitCleanup);
+
     try {
-      if (options.verbose) {
-        console.log(chalk.gray(mocPath, JSON.stringify(args)));
+      let args = [
+        "-c",
+        "--idl",
+        "--stable-types",
+        "-o",
+        wasmPath,
+        motokoPath,
+        ...(await sourcesArgs()).flat(),
+        ...getGlobalMocArgs(config),
+      ];
+      args.push(
+        ...collectExtraArgs(config, canister, canisterName, options.extraArgs),
+      );
+
+      const isPublicCandid = true; // always true for now to reduce corner cases
+      const candidVisibility = isPublicCandid ? "icp:public" : "icp:private";
+      if (isPublicCandid) {
+        args.push("--public-metadata", "candid:service");
+        args.push("--public-metadata", "candid:args");
       }
-      const result = await execa(mocPath, args, {
-        stdio: options.verbose ? "inherit" : "pipe",
-        reject: false,
-      });
+      try {
+        if (options.verbose) {
+          console.log(chalk.gray(mocPath, JSON.stringify(args)));
+        }
+        const result = await execa(mocPath, args, {
+          stdio: options.verbose ? "inherit" : "pipe",
+          reject: false,
+        });
 
-      if (result.exitCode !== 0) {
-        if (!options.verbose) {
-          if (result.stderr) {
-            console.error(chalk.red(result.stderr));
-          }
-          if (result.stdout?.trim()) {
-            console.error(chalk.yellow("Build output:"));
-            console.error(result.stdout);
+        if (result.exitCode !== 0) {
+          if (!options.verbose) {
+            if (result.stderr) {
+              console.error(chalk.red(result.stderr));
+            }
+            if (result.stdout?.trim()) {
+              console.error(chalk.yellow("Build output:"));
+              console.error(result.stdout);
+            }
           }
+          cliError(
+            `Build failed for canister ${canisterName} (exit code: ${result.exitCode})`,
+          );
         }
-        cliError(
-          `Build failed for canister ${canisterName} (exit code: ${result.exitCode})`,
-        );
-      }
-
-      if (options.verbose && result.stdout && result.stdout.trim()) {
-        console.log(result.stdout);
-      }
 
-      options.verbose &&
-        console.log(chalk.gray(`Stable types written to ${mostPath}`));
+        if (options.verbose && result.stdout && result.stdout.trim()) {
+          console.log(result.stdout);
+        }
 
-      const generatedDidPath = join(outputDir, `${canisterName}.did`);
-      const resolvedCandidPath = canister.candid
-        ? resolveConfigPath(canister.candid)
-        : null;
+        options.verbose &&
+          console.log(chalk.gray(`Stable types written to ${mostPath}`));
 
-      if (resolvedCandidPath) {
-        try {
-          const compatible = await isCandidCompatible(
-            generatedDidPath,
-            resolvedCandidPath,
-          );
+        const generatedDidPath = join(outputDir, `${canisterName}.did`);
+        const resolvedCandidPath = canister.candid
+          ? resolveConfigPath(canister.candid)
+          : null;
 
-          if (!compatible) {
-            cliError(
-              `Candid compatibility check failed for canister ${canisterName}`,
+        if (resolvedCandidPath) {
+          try {
+            const compatible = await isCandidCompatible(
+              generatedDidPath,
+              resolvedCandidPath,
             );
-          }
 
-          if (options.verbose) {
-            console.log(
-              chalk.gray(
-                `Candid compatibility check passed for canister ${canisterName}`,
-              ),
+            if (!compatible) {
+              cliError(
+                `Candid compatibility check failed for canister ${canisterName}`,
+              );
+            }
+
+            if (options.verbose) {
+              console.log(
+                chalk.gray(
+                  `Candid compatibility check passed for canister ${canisterName}`,
+                ),
+              );
+            }
+          } catch (err: any) {
+            cliError(
+              `Error during Candid compatibility check for canister ${canisterName}${err?.message ? `\n${err.message}` : ""}`,
             );
           }
-        } catch (err: any) {
-          cliError(
-            `Error during Candid compatibility check for canister ${canisterName}${err?.message ? `\n${err.message}` : ""}`,
-          );
         }
-      }
 
-      options.verbose &&
-        console.log(chalk.gray(`Adding metadata to ${wasmPath}`));
-      const candidPath = resolvedCandidPath ?? generatedDidPath;
-      const candidText = await readFile(candidPath, "utf-8");
-      const customSections: CustomSection[] = [
-        { name: `${candidVisibility} candid:service`, data: candidText },
-      ];
-      if (canister.initArg) {
-        customSections.push({
-          name: `${candidVisibility} candid:args`,
-          data: canister.initArg,
-        });
-      }
-      const wasmBytes = await readFile(wasmPath);
-      const newWasm = getWasmBindings().add_custom_sections(
-        wasmBytes,
-        customSections,
-      );
-      await writeFile(wasmPath, newWasm);
-    } catch (err: any) {
-      if (err.message?.includes("Build failed for canister")) {
-        throw err;
+        options.verbose &&
+          console.log(chalk.gray(`Adding metadata to ${wasmPath}`));
+        const candidPath = resolvedCandidPath ?? generatedDidPath;
+        const candidText = await readFile(candidPath, "utf-8");
+        const customSections: CustomSection[] = [
+          { name: `${candidVisibility} candid:service`, data: candidText },
+        ];
+        if (canister.initArg) {
+          customSections.push({
+            name: `${candidVisibility} candid:args`,
+            data: canister.initArg,
+          });
+        }
+        const wasmBytes = await readFile(wasmPath);
+        const newWasm = getWasmBindings().add_custom_sections(
+          wasmBytes,
+          customSections,
+        );
+        await writeFile(wasmPath, newWasm);
+      } catch (err: any) {
+        if (err.message?.includes("Build failed for canister")) {
+          throw err;
+        }
+        cliError(
+          `Error while compiling canister ${canisterName}${err?.message ? `\n${err.message}` : ""}`,
+        );
       }
-      cliError(
-        `Error while compiling canister ${canisterName}${err?.message ? `\n${err.message}` : ""}`,
-      );
+    } finally {
+      process.removeListener("exit", exitCleanup);
+      try {
+        await release?.();
+      } catch {}
     }
   }
 
diff --git a/cli/package-lock.json b/cli/package-lock.json
index 3f62f1dc..458fa84f 100644
--- a/cli/package-lock.json
+++ b/cli/package-lock.json
@@ -45,6 +45,7 @@
         "prettier-plugin-motoko": "0.13.0",
         "promisify-child-process": "4.1.2",
         "prompts": "2.4.2",
+        "proper-lockfile": "4.1.2",
         "semver": "7.7.1",
         "stream-to-promise": "3.0.0",
         "string-width": "7.2.0",
@@ -66,6 +67,7 @@
         "@types/ncp": "2.0.8",
         "@types/node": "24.0.3",
         "@types/prompts": "2.4.9",
+        "@types/proper-lockfile": "4.1.4",
         "@types/semver": "7.5.8",
         "@types/stream-to-promise": "2.2.4",
         "@types/tar": "6.1.13",
@@ -3966,6 +3968,23 @@
         "kleur": "^3.0.3"
       }
     },
+    "node_modules/@types/proper-lockfile": {
+      "version": "4.1.4",
+      "resolved": "https://registry.npmjs.org/@types/proper-lockfile/-/proper-lockfile-4.1.4.tgz",
+      "integrity": "sha512-uo2ABllncSqg9F1D4nugVl9v93RmjxF6LJzQLMLDdPaXCUIDPeOJ21Gbqi43xNKzBi/WQ0Q0dICqufzQbMjipQ==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/retry": "*"
+      }
+    },
+    "node_modules/@types/retry": {
+      "version": "0.12.5",
+      "resolved": "https://registry.npmjs.org/@types/retry/-/retry-0.12.5.tgz",
+      "integrity": "sha512-3xSjTp3v03X/lSQLkczaN9UIEwJMoMCA1+Nb5HfbJEQWogdeQIyVtTvxPXDQjZ5zws8rFQfVfRdz03ARihPJgw==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/semver": {
       "version": "7.5.8",
       "resolved": "https://registry.npmjs.org/@types/semver/-/semver-7.5.8.tgz",
@@ -12442,6 +12461,23 @@
         "node": ">= 6"
       }
     },
+    "node_modules/proper-lockfile": {
+      "version": "4.1.2",
+      "resolved": "https://registry.npmjs.org/proper-lockfile/-/proper-lockfile-4.1.2.tgz",
+      "integrity": "sha512-TjNPblN4BwAWMXU8s9AEz4JmQxnD1NNL7bNOY/AKUzyamc379FWASUhc/K1pL2noVb+XmZKLL68cjzLsiOAMaA==",
+      "license": "MIT",
+      "dependencies": {
+        "graceful-fs": "^4.2.4",
+        "retry": "^0.12.0",
+        "signal-exit": "^3.0.2"
+      }
+    },
+    "node_modules/proper-lockfile/node_modules/signal-exit": {
+      "version": "3.0.7",
+      "resolved": "https://registry.npmjs.org/signal-exit/-/signal-exit-3.0.7.tgz",
+      "integrity": "sha512-wnD2ZE+l+SPC/uoS0vXeE9L1+0wuaMqKlfz9AMUo38JsyLSBWSFcHR1Rri62LZc12vLr1gb3jl7iwQhgwpAbGQ==",
+      "license": "ISC"
+    },
     "node_modules/punycode": {
       "version": "2.3.1",
       "resolved": "https://registry.npmjs.org/punycode/-/punycode-2.3.1.tgz",
@@ -12772,6 +12808,15 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/retry": {
+      "version": "0.12.0",
+      "resolved": "https://registry.npmjs.org/retry/-/retry-0.12.0.tgz",
+      "integrity": "sha512-9LkiTwjUh6rT555DtE9rTX+BKByPfrMzEAtnlEtdEwr3Nkffwiihqe2bWADg+OQRjt9gl6ICdmB/ZFDCGAtSow==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 4"
+      }
+    },
     "node_modules/reusify": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/reusify/-/reusify-1.1.0.tgz",
diff --git a/cli/package.json b/cli/package.json
index ab75c462..1813d422 100644
--- a/cli/package.json
+++ b/cli/package.json
@@ -86,6 +86,7 @@
     "prettier-plugin-motoko": "0.13.0",
     "promisify-child-process": "4.1.2",
     "prompts": "2.4.2",
+    "proper-lockfile": "4.1.2",
     "semver": "7.7.1",
     "stream-to-promise": "3.0.0",
     "string-width": "7.2.0",
@@ -102,6 +103,7 @@
     "@types/ncp": "2.0.8",
     "@types/node": "24.0.3",
     "@types/prompts": "2.4.9",
+    "@types/proper-lockfile": "4.1.4",
     "@types/semver": "7.5.8",
     "@types/stream-to-promise": "2.2.4",
     "@types/tar": "6.1.13",
diff --git a/cli/tests/build.test.ts b/cli/tests/build.test.ts
index 777c44fd..f6bf25d3 100644
--- a/cli/tests/build.test.ts
+++ b/cli/tests/build.test.ts
@@ -100,6 +100,23 @@ describe("build", () => {
     }
   });
 
+  test("parallel builds of the same canister both succeed", async () => {
+    const cwd = path.join(import.meta.dirname, "build/success");
+    try {
+      const [a, b] = await Promise.all([
+        cli(["build", "foo"], { cwd }),
+        cli(["build", "foo"], { cwd }),
+      ]);
+      expect(a.exitCode).toBe(0);
+      expect(b.exitCode).toBe(0);
+      expect(existsSync(path.join(cwd, ".mops/.build/foo.wasm"))).toBe(true);
+      expect(existsSync(path.join(cwd, ".mops/.build/foo.did"))).toBe(true);
+      expect(existsSync(path.join(cwd, ".mops/.build/foo.most"))).toBe(true);
+    } finally {
+      cleanFixture(cwd);
+    }
+  });
+
   // Regression: bin/mops.js must route through environments/nodejs/cli.js
   // so that setWasmBindings() is called before any command runs.
   // The dev entry point (npm run mops) uses tsx and always worked;
diff --git a/cli/tests/build/success/.gitignore b/cli/tests/build/success/.gitignore
index 38505c9e..1e04dc29 100644
--- a/cli/tests/build/success/.gitignore
+++ b/cli/tests/build/success/.gitignore
@@ -1,2 +1,3 @@
 .mops/
 cli-output-test/
+*.most