diff --git a/packages/agent/CHANGELOG.md b/packages/agent/CHANGELOG.md
index 76af5a22..7b89b46e 100644
--- a/packages/agent/CHANGELOG.md
+++ b/packages/agent/CHANGELOG.md
@@ -10,6 +10,7 @@
 
 ### Fixed
 
+- Fixed oversized harness shell execution timeouts to fail with a clear validation error instead of being clamped to an immediate timeout ([#6181](https://github.com/earendil-works/pi/issues/6181)).
 - Fixed `Agent.prepareNextTurn` to keep receiving the run abort signal instead of the next-turn context.
 
 ## [0.80.2] - 2026-06-23
diff --git a/packages/agent/src/harness/env/nodejs.ts b/packages/agent/src/harness/env/nodejs.ts
index 3d929c82..4cf3331e 100644
--- a/packages/agent/src/harness/env/nodejs.ts
+++ b/packages/agent/src/harness/env/nodejs.ts
@@ -28,6 +28,23 @@ import {
 	toError,
 } from "../types.ts";
 
+const MAX_TIMEOUT_MS = 2_147_483_647;
+const MAX_TIMEOUT_SECONDS = MAX_TIMEOUT_MS / 1000;
+
+function resolveTimeoutMs(timeout: number | undefined): Result<number | undefined, ExecutionError> {
+	if (timeout === undefined) return ok(undefined);
+	if (!Number.isFinite(timeout)) {
+		return err(new ExecutionError("timeout", "Invalid timeout: must be a finite number of seconds"));
+	}
+	if (timeout <= 0) return ok(undefined);
+
+	const timeoutMs = timeout * 1000;
+	if (timeoutMs > MAX_TIMEOUT_MS) {
+		return err(new ExecutionError("timeout", `Invalid timeout: maximum is ${MAX_TIMEOUT_SECONDS} seconds`));
+	}
+	return ok(timeoutMs);
+}
+
 function resolvePath(cwd: string, path: string): string {
 	return isAbsolute(path) ? path : resolve(cwd, path);
 }
@@ -258,6 +275,9 @@ export class NodeExecutionEnv implements ExecutionEnv {
 		},
 	): Promise<Result<{ stdout: string; stderr: string; exitCode: number }, ExecutionError>> {
 		if (options?.abortSignal?.aborted) return err(new ExecutionError("aborted", "aborted"));
+		const timeoutMsResult = resolveTimeoutMs(options?.timeout);
+		if (!timeoutMsResult.ok) return err(timeoutMsResult.error);
+		const timeoutMs = timeoutMsResult.value;
 
 		const cwd = options?.cwd ? resolvePath(this.cwd, options.cwd) : this.cwd;
 		const shellConfig = await getShellConfig(this.shellPath);
@@ -310,13 +330,13 @@ export class NodeExecutionEnv implements ExecutionEnv {
 			}
 
 			timeoutId =
-				typeof options?.timeout === "number"
+				timeoutMs !== undefined
 					? setTimeout(() => {
 							timedOut = true;
 							if (child?.pid) {
 								killProcessTree(child.pid);
 							}
-						}, options.timeout * 1000)
+						}, timeoutMs)
 					: undefined;
 
 			if (options?.abortSignal) {
diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index c5e0ca1d..0ee4422a 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed oversized bash tool timeouts to fail with a clear validation error instead of being clamped to an immediate timeout ([#6181](https://github.com/earendil-works/pi/issues/6181)).
+
 ## [0.80.3] - 2026-06-30
 
 ### New Features
diff --git a/packages/coding-agent/src/core/tools/bash.ts b/packages/coding-agent/src/core/tools/bash.ts
index da6934e7..d5e080cc 100644
--- a/packages/coding-agent/src/core/tools/bash.ts
+++ b/packages/coding-agent/src/core/tools/bash.ts
@@ -21,6 +21,23 @@ import { getTextOutput, invalidArgText, str } from "./render-utils.ts";
 import { wrapToolDefinition } from "./tool-definition-wrapper.ts";
 import { DEFAULT_MAX_BYTES, DEFAULT_MAX_LINES, formatSize, type TruncationResult } from "./truncate.ts";
 
+const MAX_TIMEOUT_MS = 2_147_483_647;
+const MAX_TIMEOUT_SECONDS = MAX_TIMEOUT_MS / 1000;
+
+function resolveTimeoutMs(timeout: number | undefined): number | undefined {
+	if (timeout === undefined) return undefined;
+	if (!Number.isFinite(timeout)) {
+		throw new Error("Invalid timeout: must be a finite number of seconds");
+	}
+	if (timeout <= 0) return undefined;
+
+	const timeoutMs = timeout * 1000;
+	if (timeoutMs > MAX_TIMEOUT_MS) {
+		throw new Error(`Invalid timeout: maximum is ${MAX_TIMEOUT_SECONDS} seconds`);
+	}
+	return timeoutMs;
+}
+
 const bashSchema = Type.Object({
 	command: Type.String({ description: "Bash command to execute" }),
 	timeout: Type.Optional(Type.Number({ description: "Timeout in seconds (optional, no default timeout)" })),
@@ -66,15 +83,16 @@ export interface BashOperations {
 export function createLocalBashOperations(options?: { shellPath?: string }): BashOperations {
 	return {
 		exec: async (command, cwd, { onData, signal, timeout, env }) => {
+			const timeoutMs = resolveTimeoutMs(timeout);
+			if (signal?.aborted) {
+				throw new Error("aborted");
+			}
 			const shellConfig = getShellConfig(options?.shellPath);
 			try {
 				await fsAccess(cwd, constants.F_OK);
 			} catch {
 				throw new Error(`Working directory does not exist: ${cwd}\nCannot execute bash commands.`);
 			}
-			if (signal?.aborted) {
-				throw new Error("aborted");
-			}
 
 			const commandFromStdin = shellConfig.commandTransport === "stdin";
 			const child = spawn(shellConfig.shell, commandFromStdin ? shellConfig.args : [...shellConfig.args, command], {
@@ -97,11 +115,11 @@ export function createLocalBashOperations(options?: { shellPath?: string }): Bas
 
 			try {
 				// Set timeout if provided.
-				if (timeout !== undefined && timeout > 0) {
+				if (timeoutMs !== undefined) {
 					timeoutHandle = setTimeout(() => {
 						timedOut = true;
 						if (child.pid) killProcessTree(child.pid);
-					}, timeout * 1000);
+					}, timeoutMs);
 				}
 				// Stream stdout and stderr.
 				child.stdout?.on("data", onData);