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Start an interactive session in Plan mode with claude --permission-mode plan. It lets Claude Code read files and run read-only shell commands to explore without editing source files. For a stronger barrier, deny file-editing tools and use an operating-system or container-level read-only mount for the files you need to protect.
Start Claude Code in Plan mode
Run this command in a new interactive session:
claude --permission-mode planPlan mode is Anthropic’s built-in choice for analysis and read-only exploration. It is a practical default when you want Claude to inspect a project and reason about it without editing source files. To avoid relying on a default that can differ by interface or configuration, select the mode explicitly.
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Make Plan mode the default
To start later sessions in Plan mode, set permissions.defaultMode to plan in the applicable Claude Code settings file. A settings configuration can also deny the file-editing tools:
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{
"permissions": {
"defaultMode": "plan",
"deny": ["Edit", "Write"]
}
}Confirm that Edit and Write are the relevant tool names in your installed version and environment before relying on those rules. Deny rules block matching tools and take priority over ask and allow rules.
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Block editing tools for one session
For a CLI session, pass the editing tools as disallowed:
claude --disallowedTools "Edit" "Write"The current CLI reference also documents --tools to restrict the built-in tools available in a session and --settings to load a settings JSON file or inline JSON. If a flag behaves differently on an older installation, check the Anthropic CLI reference for the version you are using.
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Check which permission rules are active
In Claude Code, run /permissions to see and manage active rules, including which settings file each rule came from. This helps identify whether a setting is coming from your user configuration or another applicable settings file.
Know the limit of tool permissions
Claude Code’s permissions control which tools it may use; they do not make the project directory itself read-only. In particular, an approved Bash command can still write to files, including beyond the file-tool working-directory boundary. If writes must be prevented regardless of the command path, place the protected files on an operating-system- or container-level read-only mount.
Anthropic’s Security documentation says, “Manual mode: Claude Code starts with read-only permissions.” That describes Manual mode; the same documentation describes Auto mode as the built-in starting mode for interactive terminal and VS Code sessions. Defaults vary by interface and configuration, so explicitly choose Plan mode or configure the protections you need rather than assuming a session starts read-only. See Anthropic’s Claude Code Security documentation and Configure permissions.
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| Approach | What it does | Best fit |
|---|---|---|
| Plan mode | Supports read-only exploration without editing source files. | Quick analysis sessions where convenience matters. |
| Deny editing tools | Prevents use of matching tools such as Edit and Write; deny takes priority over ask and allow. |
When Claude should not have its normal file-editing tools available. |
| Read-only filesystem mount | Enforces a filesystem boundary against writes, including attempts through command execution. | When writes must be prevented, not merely disallowed through Claude Code tools. |
These protections can be combined: use Plan mode, deny editing tools, and enforce a read-only mount for files that must not change. For setup details and permission behavior, consult Set up Claude Code and Configure permissions.
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