diff --git a/README.md b/README.md
index 4c42891..a17b7f1 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,7 @@ curl -fsSL https://raw.githubusercontent.com/laosb/agentc/main/install.sh | sh
 agentc run                          # start default agent (claude) in $PWD
 agentc run -c claude,copilot        # activate multiple configurations
 agentc run "explain this code"      # forward args to the agent entrypoint
+agentc run -e TZ=Europe/Berlin      # set a container environment variable
 agentc sh                           # open a shell in the container
 agentc sh -- ls -la /home/agent     # run a command inside the container
 agentc version                      # print version info
diff --git a/Sources/AgentIsolation/AgentSession.swift b/Sources/AgentIsolation/AgentSession.swift
index e18a3d7..23b2b97 100644
--- a/Sources/AgentIsolation/AgentSession.swift
+++ b/Sources/AgentIsolation/AgentSession.swift
@@ -218,8 +218,8 @@ public final class AgentSession<Runtime: ContainerRuntime>: Sendable {
       break
     }
 
-    // Environment: pass configurations and optional entrypoint override to bootstrap
-    var environment: [String: String] = [:]
+    // Environment: start with user values, excluding reserved bootstrap controls.
+    var environment = config.environment.filter { !$0.key.hasPrefix("AGENTC_") }
     environment["AGENTC_CONFIGURATIONS"] = config.configurations.joined(separator: ",")
     if config.verbose {
       environment["AGENTC_VERBOSE"] = "1"
diff --git a/Sources/AgentIsolation/IsolationConfig.swift b/Sources/AgentIsolation/IsolationConfig.swift
index f1862fa..2fda813 100644
--- a/Sources/AgentIsolation/IsolationConfig.swift
+++ b/Sources/AgentIsolation/IsolationConfig.swift
@@ -44,6 +44,10 @@ public struct IsolationConfig: Sendable {
   /// Arguments forwarded to the container entrypoint.
   public var arguments: [String]
 
+  /// User-defined environment variables passed to the container.
+  /// Internal `AGENTC_*` variables take precedence when the session starts.
+  public var environment: [String: String]
+
   /// Whether to allocate a pseudo-TTY. Typically true when stdin is a terminal.
   public var allocateTTY: Bool
 
@@ -80,6 +84,7 @@ public struct IsolationConfig: Sendable {
     configurations: [String] = ["claude"],
     bootstrapMode: BootstrapMode = .imageDefault,
     arguments: [String] = [],
+    environment: [String: String] = [:],
     allocateTTY: Bool = false,
     cpuCount: Int = 1,
     memoryLimitMiB: Int = 1536,
@@ -95,6 +100,7 @@ public struct IsolationConfig: Sendable {
     self.configurations = configurations
     self.bootstrapMode = bootstrapMode
     self.arguments = arguments
+    self.environment = environment
     self.allocateTTY = allocateTTY
     self.cpuCount = cpuCount
     self.memoryLimitMiB = memoryLimitMiB
diff --git a/Sources/AgentIsolation/ProjectSettings.swift b/Sources/AgentIsolation/ProjectSettings.swift
index 08cf021..ce1dbc4 100644
--- a/Sources/AgentIsolation/ProjectSettings.swift
+++ b/Sources/AgentIsolation/ProjectSettings.swift
@@ -22,6 +22,7 @@ public struct ProjectSettings: Codable, Sendable, Equatable {
     public var excludes: [String]?
     public var configurations: [String]?
     public var additionalMounts: [String]?
+    public var environment: [String: String]?
     public var defaultArguments: [String]?
     public var additionalArguments: [String]?
     public var cpus: Int?
@@ -35,6 +36,7 @@ public struct ProjectSettings: Codable, Sendable, Equatable {
       excludes: [String]? = nil,
       configurations: [String]? = nil,
       additionalMounts: [String]? = nil,
+      environment: [String: String]? = nil,
       defaultArguments: [String]? = nil,
       additionalArguments: [String]? = nil,
       cpus: Int? = nil,
@@ -47,6 +49,7 @@ public struct ProjectSettings: Codable, Sendable, Equatable {
       self.excludes = excludes
       self.configurations = configurations
       self.additionalMounts = additionalMounts
+      self.environment = environment
       self.defaultArguments = defaultArguments
       self.additionalArguments = additionalArguments
       self.cpus = cpus
diff --git a/Sources/agentc/Commands/InitCommand.swift b/Sources/agentc/Commands/InitCommand.swift
index 64d6704..0c78343 100644
--- a/Sources/agentc/Commands/InitCommand.swift
+++ b/Sources/agentc/Commands/InitCommand.swift
@@ -124,6 +124,11 @@ struct InitCommand: AsyncParsableCommand {
         excludes: excludes,
         configurations: configurations,
         additionalMounts: options.additionalMount.isEmpty ? nil : options.additionalMount,
+        environment: options.env.isEmpty
+          ? nil
+          : Dictionary(
+            options.env.map { ($0.key, $0.value) },
+            uniquingKeysWith: { _, latest in latest }),
         cpus: options.cpuCount ?? 1,
         memoryMiB: options.memoryLimitMiB ?? 1536,
         bootstrap: options.bootstrapScript,
diff --git a/Sources/agentc/SessionRunner.swift b/Sources/agentc/SessionRunner.swift
index ebf088f..76b2fbb 100644
--- a/Sources/agentc/SessionRunner.swift
+++ b/Sources/agentc/SessionRunner.swift
@@ -66,6 +66,7 @@ enum SessionRunner {
       configurations: configNames,
       bootstrapMode: bootstrapMode,
       arguments: arguments,
+      environment: options.resolveEnvironment(projectSettings: projectSettings),
       allocateTTY: allocateTTY,
       cpuCount: options.resolveCpuCount(projectSettings: projectSettings),
       memoryLimitMiB: options.resolveMemoryLimitMiB(projectSettings: projectSettings),
diff --git a/Sources/agentc/SharedOptions.swift b/Sources/agentc/SharedOptions.swift
index 4c9e5e7..779d291 100644
--- a/Sources/agentc/SharedOptions.swift
+++ b/Sources/agentc/SharedOptions.swift
@@ -7,6 +7,26 @@ import ArgumentParser
   import Foundation
 #endif
 
+struct EnvironmentVariableOption: ExpressibleByArgument, Sendable, Equatable {
+  let key: String
+  let value: String
+
+  init?(argument: String) {
+    guard let separator = argument.firstIndex(of: "="), separator != argument.startIndex else {
+      return nil
+    }
+
+    let key = String(argument[..<separator])
+    let value = String(argument[argument.index(after: separator)...])
+    guard !key.contains("\0"), !value.contains("\0") else {
+      return nil
+    }
+
+    self.key = key
+    self.value = value
+  }
+}
+
 struct SharedOptions: ParsableArguments {
   @Option(name: .shortAndLong, help: "Container runtime.")
   var runtime: RuntimeChoice?
@@ -55,6 +75,14 @@ struct SharedOptions: ParsableArguments {
   )
   var additionalMount: [String] = []
 
+  @Option(
+    name: .shortAndLong,
+    help: ArgumentHelp(
+      "Set a container environment variable. Repeat to set multiple variables.",
+      valueName: "KEY=VALUE")
+  )
+  var env: [EnvironmentVariableOption] = []
+
   @Option(
     name: [.short, .customLong("configurations")],
     help: "Comma-separated agent configuration names."
@@ -226,6 +254,16 @@ extension SharedOptions {
     return result
   }
 
+  /// Resolve container environment variables.
+  /// Project settings provide defaults; CLI values override matching keys.
+  func resolveEnvironment(projectSettings: ProjectSettings? = nil) -> [String: String] {
+    var result = projectSettings?.agent?.environment ?? [:]
+    for variable in env {
+      result[variable.key] = variable.value
+    }
+    return result
+  }
+
   /// Resolve entrypoint arguments.
   ///
   /// - `defaultArguments`: used when no CLI rest arguments are given; CLI overrides.
diff --git a/Tests/AgentIsolationDockerRuntimeTests/DockerRuntimeTests.swift b/Tests/AgentIsolationDockerRuntimeTests/DockerRuntimeTests.swift
index 8c814f1..0565046 100644
--- a/Tests/AgentIsolationDockerRuntimeTests/DockerRuntimeTests.swift
+++ b/Tests/AgentIsolationDockerRuntimeTests/DockerRuntimeTests.swift
@@ -494,6 +494,36 @@
 
       try await runtime.removeContainer(container)
     }
+
+    @Test("runContainer passes environment variables")
+    func runContainerEnvironment() async throws {
+      let runtime = makeRuntime()
+      defer { Task { try? await runtime.shutdown() } }
+      try await runtime.prepare()
+
+      _ = try await runtime.pullImage(ref: "alpine:latest")
+
+      let stdout = MockWriter()
+      let containerConfig = ContainerConfiguration(
+        entrypoint: ["/bin/sh", "-c", "printf '%s|%s' \"$TZ\" \"$LC_ALL\""],
+        environment: [
+          "TZ": "America/Los_Angeles",
+          "LC_ALL": "en_US.UTF-8",
+        ],
+        io: .custom(stdin: EmptyReaderStream(), stdout: stdout, stderr: MockWriter())
+      )
+
+      let container = try await runtime.runContainer(
+        imageRef: "alpine:latest",
+        configuration: containerConfig
+      )
+
+      let exitCode = try await container.wait(timeoutInSeconds: 30)
+      try await runtime.removeContainer(container)
+
+      #expect(exitCode == 0)
+      #expect(stdout.string == "America/Los_Angeles|en_US.UTF-8")
+    }
   }
 
   // MARK: - Custom IO Integration Tests
diff --git a/Tests/AgentIsolationTests/AgentSessionTests.swift b/Tests/AgentIsolationTests/AgentSessionTests.swift
index ea3a2f7..19b56b9 100644
--- a/Tests/AgentIsolationTests/AgentSessionTests.swift
+++ b/Tests/AgentIsolationTests/AgentSessionTests.swift
@@ -521,6 +521,44 @@ struct ConfigurationTests {
     #expect(env["AGENTC_CONFIGURATIONS"] == "claude,swift")
   }
 
+  @Test("Passes custom environment variables and reserves AGENTC names")
+  func passesCustomEnvironment() async throws {
+    let runtime = MockRuntime(config: .init(storagePath: "/tmp"))
+    let base = URL(fileURLWithPath: "/tmp/agentc-test-env-\(UUID().uuidString)")
+    let profileDir = base.appendingPathComponent("home")
+    let configsDir = try makeConfigsDir(configs: [:])
+    defer {
+      try? FileManager.default.removeItem(at: base)
+      try? FileManager.default.removeItem(at: configsDir)
+    }
+
+    let config = IsolationConfig(
+      image: "test:latest",
+      profileHomeDir: profileDir,
+      workspace: URL(fileURLWithPath: "/tmp"),
+      configurationsDir: configsDir,
+      configurations: ["claude"],
+      arguments: ["echo"],
+      environment: [
+        "TZ": "America/Los_Angeles",
+        "LC_ALL": "en_US.UTF-8",
+        "EMPTY": "",
+        "AGENTC_CONFIGURATIONS": "overridden",
+        "AGENTC_ENTRYPOINT_OVERRIDE": "1",
+      ]
+    )
+    let session = AgentSession(config: config, runtime: runtime)
+    try await session.start()
+    _ = try await session.wait()
+
+    let environment = runtime.lastContainerConfiguration!.environment
+    #expect(environment["TZ"] == "America/Los_Angeles")
+    #expect(environment["LC_ALL"] == "en_US.UTF-8")
+    #expect(environment["EMPTY"] == "")
+    #expect(environment["AGENTC_CONFIGURATIONS"] == "claude")
+    #expect(environment["AGENTC_ENTRYPOINT_OVERRIDE"] == nil)
+  }
+
   @Test("Creates additional mounts from single configuration")
   func additionalMountsSingle() async throws {
     let runtime = MockRuntime(config: .init(storagePath: "/tmp"))
diff --git a/Tests/AgentIsolationTests/ProjectSettingsTests.swift b/Tests/AgentIsolationTests/ProjectSettingsTests.swift
index 314d682..075cc6b 100644
--- a/Tests/AgentIsolationTests/ProjectSettingsTests.swift
+++ b/Tests/AgentIsolationTests/ProjectSettingsTests.swift
@@ -17,6 +17,10 @@ struct ProjectSettingsDecodingTests {
           "excludes": [".git", "node_modules"],
           "configurations": ["claude", "copilot"],
           "additionalMounts": ["/data/models"],
+          "environment": {
+            "TZ": "America/Los_Angeles",
+            "LC_ALL": "en_US.UTF-8"
+          },
           "defaultArguments": ["--model", "opus"],
           "additionalArguments": ["--verbose"],
           "cpus": 4,
@@ -35,6 +39,12 @@ struct ProjectSettingsDecodingTests {
     #expect(agent.excludes == [".git", "node_modules"])
     #expect(agent.configurations == ["claude", "copilot"])
     #expect(agent.additionalMounts == ["/data/models"])
+    #expect(
+      agent.environment
+        == [
+          "TZ": "America/Los_Angeles",
+          "LC_ALL": "en_US.UTF-8",
+        ])
     #expect(agent.defaultArguments == ["--model", "opus"])
     #expect(agent.additionalArguments == ["--verbose"])
     #expect(agent.cpus == 4)
@@ -71,6 +81,7 @@ struct ProjectSettingsDecodingTests {
     #expect(agent.excludes == nil)
     #expect(agent.configurations == nil)
     #expect(agent.additionalMounts == nil)
+    #expect(agent.environment == nil)
     #expect(agent.defaultArguments == nil)
     #expect(agent.additionalArguments == nil)
     #expect(agent.memoryMiB == nil)
diff --git a/Tests/AgentcIntegrationTests/InitCommandIntegrationTests.swift b/Tests/AgentcIntegrationTests/InitCommandIntegrationTests.swift
index f6acada..6b6069b 100644
--- a/Tests/AgentcIntegrationTests/InitCommandIntegrationTests.swift
+++ b/Tests/AgentcIntegrationTests/InitCommandIntegrationTests.swift
@@ -53,6 +53,9 @@ struct InitCommandIntegrationTests {
         "--image", "custom:latest",
         "--configurations", "claude,copilot",
         "--exclude", "node_modules,.git",
+        "--env", "TZ=America/Los_Angeles",
+        "-e", "LC_ALL=en_US.UTF-8",
+        "-e", "EMPTY=",
       ]
     )
     #expect(result.exitCode == 0)
@@ -66,6 +69,30 @@ struct InitCommandIntegrationTests {
     #expect(agent["memoryMiB"] as? Int == 4096)
     #expect(agent["configurations"] as? [String] == ["claude", "copilot"])
     #expect(agent["excludes"] as? [String] == ["node_modules", ".git"])
+    let environment = agent["environment"] as? [String: String]
+    #expect(environment?["TZ"] == "America/Los_Angeles")
+    #expect(environment?["LC_ALL"] == "en_US.UTF-8")
+    #expect(environment?["EMPTY"] == "")
+  }
+
+  @Test("agentc init rejects malformed environment options")
+  func initRejectsMalformedEnvironment() async throws {
+    let base = URL(
+      fileURLWithPath: "/tmp/__TEST_agentc_init_env.\(UUID().uuidString.prefix(6))")
+    try FileManager.default.createDirectory(at: base, withIntermediateDirectories: true)
+    defer { try? FileManager.default.removeItem(at: base) }
+
+    let result = await runAgentc(
+      args: [
+        "init",
+        base.path,
+        "--skip-container-init",
+        "--env", "MISSING_SEPARATOR",
+      ]
+    )
+
+    #expect(result.exitCode != 0)
+    #expect(!FileManager.default.fileExists(atPath: base.appendingPathComponent(".agentc").path))
   }
 
   @Test("agentc init with --profile writes profile to settings")
diff --git a/Tests/AgentcIntegrationTests/ProjectSettingsIntegrationTests.swift b/Tests/AgentcIntegrationTests/ProjectSettingsIntegrationTests.swift
index 27ca23a..90f5e8b 100644
--- a/Tests/AgentcIntegrationTests/ProjectSettingsIntegrationTests.swift
+++ b/Tests/AgentcIntegrationTests/ProjectSettingsIntegrationTests.swift
@@ -114,6 +114,40 @@ struct ProjectSettingsIntegrationTests {
     #expect(result.stdout.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty)
   }
 
+  @Test("--agentc-folder applies agent.environment setting")
+  func agentcFolderAppliesEnvironment() async throws {
+    let base = URL(fileURLWithPath: "/tmp/__TEST_agentc_ps_env.\(UUID().uuidString.prefix(6))")
+    defer { try? FileManager.default.removeItem(at: base) }
+
+    let settingsDir = base.appendingPathComponent("settings")
+    try writeProjectSettings(
+      """
+      {
+        "agent": {
+          "environment": {
+            "TZ": "America/Los_Angeles",
+            "LC_ALL": "en_US.UTF-8"
+          }
+        }
+      }
+      """,
+      at: settingsDir)
+
+    let result = await runAgentc(
+      args: [
+        "sh",
+        "--profile", sharedProfile,
+        "--configurations-dir", sharedConfigurationsDir,
+        "--no-update-image",
+        "--agentc-folder", settingsDir.appendingPathComponent(".agentc").path,
+        "--", "printf '%s|%s' \"$TZ\" \"$LC_ALL\"",
+      ]
+    )
+
+    #expect(result.exitCode == 0)
+    #expect(result.stdout == "America/Los_Angeles|en_US.UTF-8")
+  }
+
   // MARK: - CLI Override
 
   @Test("CLI --cpus overrides project settings agent.cpus")
@@ -145,6 +179,41 @@ struct ProjectSettingsIntegrationTests {
     #expect(reported == "3")
   }
 
+  @Test("CLI --env overrides matching project environment variables")
+  func cliOverridesProjectEnvironment() async throws {
+    let base = URL(fileURLWithPath: "/tmp/__TEST_agentc_ps_envov.\(UUID().uuidString.prefix(6))")
+    defer { try? FileManager.default.removeItem(at: base) }
+
+    let settingsDir = base.appendingPathComponent("settings")
+    try writeProjectSettings(
+      """
+      {
+        "agent": {
+          "environment": {
+            "TZ": "Asia/Shanghai",
+            "LANG": "en_US.UTF-8"
+          }
+        }
+      }
+      """,
+      at: settingsDir)
+
+    let result = await runAgentc(
+      args: [
+        "sh",
+        "--profile", sharedProfile,
+        "--configurations-dir", sharedConfigurationsDir,
+        "--no-update-image",
+        "--agentc-folder", settingsDir.appendingPathComponent(".agentc").path,
+        "--env", "TZ=Europe/Berlin",
+        "--", "printf '%s|%s' \"$TZ\" \"$LANG\"",
+      ]
+    )
+
+    #expect(result.exitCode == 0)
+    #expect(result.stdout == "Europe/Berlin|en_US.UTF-8")
+  }
+
   // MARK: - Merge Behavior
 
   @Test("CLI --exclude and project excludes are both applied")
diff --git a/docs/project-settings.md b/docs/project-settings.md
index a4d8a05..a6b097f 100644
--- a/docs/project-settings.md
+++ b/docs/project-settings.md
@@ -44,6 +44,9 @@ All fields are optional. Only the values you specify take effect.
     "excludes": ["<string>", ...],
     "configurations": ["<string>", ...],
     "additionalMounts": ["<string>", ...],
+    "environment": {
+      "<name>": "<value>"
+    },
     "defaultArguments": ["<string>", ...],
     "additionalArguments": ["<string>", ...],
     "cpus": "<int>",
@@ -63,6 +66,7 @@ All fields are optional. Only the values you specify take effect.
 | `agent.excludes` | `--exclude` | Workspace sub-folders to mask with empty overlays. |
 | `agent.configurations` | `--configurations`, `-c` | Agent configuration names to activate. |
 | `agent.additionalMounts` | `--additional-mount` | Additional host directories to mount. |
+| `agent.environment` | `--env`, `-e` | Environment variables passed to the container. |
 | `agent.defaultArguments` | positional args after `--` | Default arguments passed to the entrypoint. |
 | `agent.additionalArguments` | *(none)* | Arguments always appended to entrypoint args. |
 | `agent.cpus` | `--cpus` | Number of CPUs to allocate. |
@@ -82,6 +86,7 @@ When both CLI flags and project settings specify a value, the behavior depends o
 
 - `excludes` — CLI and project excludes are all applied.
 - `additionalMounts` — CLI and project mounts are all mounted.
+- `environment` — Variables are merged by name; CLI values override matching project values.
 
 **Arguments have special handling:**
 
@@ -124,6 +129,29 @@ For fields with override behavior, the full priority chain is:
 }
 ```
 
+### Customize Locale and Time Zone
+
+```json
+{
+  "agent": {
+    "environment": {
+      "TZ": "America/Los_Angeles",
+      "LANG": "en_US.UTF-8",
+      "LC_ALL": "en_US.UTF-8"
+    }
+  }
+}
+```
+
+Use repeatable CLI options for one-off overrides:
+
+```sh
+agentc run --env TZ=America/Los_Angeles --env LC_ALL=en_US.UTF-8
+```
+
+Project settings provide defaults. A CLI value with the same name takes precedence.
+Variables used internally by agentc (`AGENTC_*`) are reserved and cannot be overridden.
+
 ### Use Image Entrypoint (No Bootstrap)
 
 ```json