diff --git a/package-lock.json b/package-lock.json
index 89d984a0..4999c54a 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -2898,6 +2898,16 @@
 				"undici-types": "~6.21.0"
 			}
 		},
+		"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": "0.12.0",
 			"resolved": "https://registry.npmjs.org/@types/retry/-/retry-0.12.0.tgz",
@@ -5756,6 +5766,32 @@
 			"integrity": "sha512-3ouUOpQhtgrbOa17J7+uxOTpITYWaGP7/AhoR3+A+/1e9skrzelGi/dXzEYyvbxubEF6Wn2ypscTKiKJFFn1ag==",
 			"license": "MIT"
 		},
+		"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/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/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/proxy-from-env": {
 			"version": "1.1.0",
 			"resolved": "https://registry.npmjs.org/proxy-from-env/-/proxy-from-env-1.1.0.tgz",
@@ -7176,6 +7212,7 @@
 				"glob": "^11.0.3",
 				"jiti": "^2.6.1",
 				"marked": "^15.0.12",
+				"proper-lockfile": "^4.1.2",
 				"sharp": "^0.34.2"
 			},
 			"bin": {
@@ -7184,6 +7221,7 @@
 			"devDependencies": {
 				"@types/diff": "^7.0.2",
 				"@types/node": "^24.3.0",
+				"@types/proper-lockfile": "^4.1.4",
 				"typescript": "^5.7.3",
 				"vitest": "^3.2.4"
 			},
diff --git a/packages/coding-agent/package.json b/packages/coding-agent/package.json
index 51f1da6f..2dfdeceb 100644
--- a/packages/coding-agent/package.json
+++ b/packages/coding-agent/package.json
@@ -49,11 +49,13 @@
 		"glob": "^11.0.3",
 		"jiti": "^2.6.1",
 		"marked": "^15.0.12",
+		"proper-lockfile": "^4.1.2",
 		"sharp": "^0.34.2"
 	},
 	"devDependencies": {
 		"@types/diff": "^7.0.2",
 		"@types/node": "^24.3.0",
+		"@types/proper-lockfile": "^4.1.4",
 		"typescript": "^5.7.3",
 		"vitest": "^3.2.4"
 	},
diff --git a/packages/coding-agent/src/core/auth-storage.ts b/packages/coding-agent/src/core/auth-storage.ts
index e76b04c6..a9f88c9e 100644
--- a/packages/coding-agent/src/core/auth-storage.ts
+++ b/packages/coding-agent/src/core/auth-storage.ts
@@ -1,6 +1,9 @@
 /**
  * Credential storage for API keys and OAuth tokens.
  * Handles loading, saving, and refreshing credentials from auth.json.
+ *
+ * Uses file locking to prevent race conditions when multiple pi instances
+ * try to refresh tokens simultaneously.
  */
 
 import {
@@ -16,6 +19,7 @@ import {
 } from "@mariozechner/pi-ai";
 import { chmodSync, existsSync, mkdirSync, readFileSync, writeFileSync } from "fs";
 import { dirname } from "path";
+import lockfile from "proper-lockfile";
 
 export type ApiKeyCredential = {
 	type: "api_key";
@@ -198,12 +202,93 @@ export class AuthStorage {
 		this.remove(provider);
 	}
 
+	/**
+	 * Refresh OAuth token with file locking to prevent race conditions.
+	 * Multiple pi instances may try to refresh simultaneously when tokens expire.
+	 * This ensures only one instance refreshes while others wait and use the result.
+	 */
+	private async refreshOAuthTokenWithLock(
+		provider: OAuthProvider,
+	): Promise<{ apiKey: string; newCredentials: OAuthCredentials } | null> {
+		// Ensure auth file exists for locking
+		if (!existsSync(this.authPath)) {
+			const dir = dirname(this.authPath);
+			if (!existsSync(dir)) {
+				mkdirSync(dir, { recursive: true, mode: 0o700 });
+			}
+			writeFileSync(this.authPath, "{}", "utf-8");
+			chmodSync(this.authPath, 0o600);
+		}
+
+		let release: (() => Promise<void>) | undefined;
+
+		try {
+			// Acquire exclusive lock with retry and timeout
+			// Use generous retry window to handle slow token endpoints
+			release = await lockfile.lock(this.authPath, {
+				retries: {
+					retries: 10,
+					factor: 2,
+					minTimeout: 100,
+					maxTimeout: 10000,
+					randomize: true,
+				},
+				stale: 30000, // Consider lock stale after 30 seconds
+			});
+
+			// Re-read file after acquiring lock - another instance may have refreshed
+			this.reload();
+
+			const cred = this.data[provider];
+			if (cred?.type !== "oauth") {
+				return null;
+			}
+
+			// Check if token is still expired after re-reading
+			// (another instance may have already refreshed it)
+			if (Date.now() < cred.expires) {
+				// Token is now valid - another instance refreshed it
+				const needsProjectId = provider === "google-gemini-cli" || provider === "google-antigravity";
+				const apiKey = needsProjectId
+					? JSON.stringify({ token: cred.access, projectId: cred.projectId })
+					: cred.access;
+				return { apiKey, newCredentials: cred };
+			}
+
+			// Token still expired, we need to refresh
+			const oauthCreds: Record<string, OAuthCredentials> = {};
+			for (const [key, value] of Object.entries(this.data)) {
+				if (value.type === "oauth") {
+					oauthCreds[key] = value;
+				}
+			}
+
+			const result = await getOAuthApiKey(provider, oauthCreds);
+			if (result) {
+				this.data[provider] = { type: "oauth", ...result.newCredentials };
+				this.save();
+				return result;
+			}
+
+			return null;
+		} finally {
+			// Always release the lock
+			if (release) {
+				try {
+					await release();
+				} catch {
+					// Ignore unlock errors (lock may have been compromised)
+				}
+			}
+		}
+	}
+
 	/**
 	 * Get API key for a provider.
 	 * Priority:
 	 * 1. Runtime override (CLI --api-key)
 	 * 2. API key from auth.json
-	 * 3. OAuth token from auth.json (auto-refreshed)
+	 * 3. OAuth token from auth.json (auto-refreshed with locking)
 	 * 4. Environment variable
 	 * 5. Fallback resolver (models.json custom providers)
 	 */
@@ -221,23 +306,44 @@ export class AuthStorage {
 		}
 
 		if (cred?.type === "oauth") {
-			// Filter to only oauth credentials for getOAuthApiKey
-			const oauthCreds: Record<string, OAuthCredentials> = {};
-			for (const [key, value] of Object.entries(this.data)) {
-				if (value.type === "oauth") {
-					oauthCreds[key] = value;
-				}
-			}
+			// Check if token needs refresh
+			const needsRefresh = Date.now() >= cred.expires;
 
-			try {
-				const result = await getOAuthApiKey(provider as OAuthProvider, oauthCreds);
-				if (result) {
-					this.data[provider] = { type: "oauth", ...result.newCredentials };
-					this.save();
-					return result.apiKey;
+			if (needsRefresh) {
+				// Use locked refresh to prevent race conditions
+				try {
+					const result = await this.refreshOAuthTokenWithLock(provider as OAuthProvider);
+					if (result) {
+						return result.apiKey;
+					}
+				} catch (err) {
+					// Refresh failed - re-read file to check if another instance succeeded
+					this.reload();
+					const updatedCred = this.data[provider];
+
+					if (updatedCred?.type === "oauth" && Date.now() < updatedCred.expires) {
+						// Another instance refreshed successfully, use those credentials
+						const needsProjectId =
+							provider === "google-gemini-cli" || provider === "google-antigravity";
+						return needsProjectId
+							? JSON.stringify({ token: updatedCred.access, projectId: updatedCred.projectId })
+							: updatedCred.access;
+					}
+
+					// Refresh truly failed - DO NOT remove credentials
+					// User can retry or re-authenticate manually
+					const errorMessage = err instanceof Error ? err.message : String(err);
+					throw new Error(
+						`OAuth token refresh failed for ${provider}: ${errorMessage}. ` +
+							`Please try again or re-authenticate with /login.`,
+					);
 				}
-			} catch {
-				this.remove(provider);
+			} else {
+				// Token not expired, use current access token
+				const needsProjectId = provider === "google-gemini-cli" || provider === "google-antigravity";
+				return needsProjectId
+					? JSON.stringify({ token: cred.access, projectId: cred.projectId })
+					: cred.access;
 			}
 		}