Plan Mode Mechanics: Enter vs Tab, Syntax Highlighting and Inline Editing

plan-modesteer-modeenter-vs-tabsyntax-highlightingtuiinteraction-mechanics

Plan Mode Mechanics: Enter vs Tab, Syntax Highlighting and Inline Editing

Codex CLI’s terminal interface has evolved well beyond a simple prompt-and-response loop. Between plan mode, steer mode, and a rich set of TUI enhancements, the CLI now offers a genuinely interactive development experience. This article unpacks the specific mechanics — the keystrokes, the mode transitions, and the visual tooling — that make it all work.

Collaboration Modes: Plan, Pair and Execute

Since v0.96.0, Codex CLI ships with three collaboration modes enabled by default1. Each mode represents a different level of agent autonomy:

  • Plan — design only. The model reads files and analyses the codebase, then proposes an implementation plan without writing anything. File mutations are suppressed at the prompt level2.
  • Pair — interactive implementation. The agent reads and writes files, but every change requires your approval before it lands.
  • Execute — autonomous execution. The agent works through the task independently, applying changes as it goes.

You cycle through them with Shift+Tab, or jump directly with the /plan slash command1. The current mode is displayed in the TUI footer and via /status.

The Prompt-Level Constraint

An important subtlety: plan mode is enforced at the prompt level, not via a runtime sandbox2. The system prompt instructs the model not to write files, but there is no physical block on mutations. For genuinely safety-critical work, combine plan mode with a git stash or an isolated branch as a belt-and-braces measure.

flowchart LR
    A["Plan Mode\n(read-only design)"] -->|Shift+Tab| B["Pair Mode\n(interactive edits)"]
    B -->|Shift+Tab| C["Execute Mode\n(autonomous)"]
    C -->|Shift+Tab| A
    A -->|"/plan"| A
    B -->|Approve plan| C

Enter vs Tab: The Steer Mode Mechanics

Steer mode, stable since v0.98.03, is arguably the CLI’s most distinctive interaction pattern. Rather than waiting for the agent to finish a turn before providing feedback, you can redirect it mid-execution. Two keys control the timing:

Key Behaviour Use Case
Enter Sends instructions immediately — the agent sees them during the current turn Urgent corrections: “Use bcrypt instead of argon2”
Tab Queues instructions for the next turn — the agent finishes its current work first Follow-up tasks: “After this, add the migration script”

This distinction matters. Enter interrupts; Tab appends. Choosing wrong doesn’t break anything, but it changes the agent’s context window at the point of injection.

When to Use Each

flowchart TD
    S["Agent is running"] --> Q{"Is this urgent?"}
    Q -->|"Yes — wrong approach,\nbug, or priority change"| E["Press Enter\n(immediate injection)"]
    Q -->|"No — additional work\nfor after this turn"| T["Press Tab\n(queue for next turn)"]
    E --> R1["Agent sees instruction\nduring current turn"]
    T --> R2["Agent processes instruction\nafter current turn completes"]

Enter examples:

  • The agent is refactoring a module but chose the wrong design pattern — redirect immediately
  • You spot a bug in the approach before it propagates further
  • Requirements changed mid-task and the current direction is now invalid

Tab examples:

  • You want tests written after the current implementation finishes
  • You’d like the agent to update documentation once it’s done with the code
  • You want to queue a scope addition without disrupting the current flow

Syntax Highlighting and the /theme System

Version 0.105.0 introduced syntax highlighting for fenced code blocks and file diffs in the TUI4. The implementation uses syntect paired with two-face, providing approximately 250 language grammars and 32 bundled themes5 — the same engine that powers tools like bat and delta.

Configuring Themes

The /theme command opens an interactive picker with live preview5. Your selection persists to ~/.codex/config.toml under the tui.theme key:

[tui]
theme = "Monokai Extended"

For custom themes, drop standard .tmTheme files into ~/.codex/themes/ and they appear in the picker alongside the bundled options4.

Performance Guardrails

The highlighting engine caps processing at 512 KB or 10,000 lines per block5, preventing pathological delays on massive diffs. If a block exceeds these limits, it falls back to plain-text rendering. The total binary size increase from adding syntect was roughly 3 MB — about 1% of the Codex CLI binary5.

Known Limitations

The diff view has a documented issue where background colours defined in .tmTheme scope rules for markup.inserted, markup.deleted, and markup.changed are ignored6. The diff view renders with hardcoded red/green backgrounds regardless of theme settings. This is tracked upstream and may be resolved in a future release.

TUI Commands Reference

Beyond the mode-switching and highlighting features, the TUI includes a set of utility commands that have accumulated across recent releases:

Command Description Notes
/plan Switch to plan mode (optionally with an inline prompt) Accepts /plan "redesign the API"
/copy Copy the latest completed output to clipboard Unavailable before first output7
/clear Clear the terminal and start a fresh chat Resets conversation history
/compact Summarise conversation to free tokens Preserves critical context7
/theme Open the syntax highlighting theme picker Live preview, persistent config4
/title Set the terminal window title v0.117.0+3
/status Display session config, mode, and token usage Shows remaining context capacity7
/model Switch model and reasoning effort mid-session No restart required
/permissions Adjust approval policy mid-session Toggle between Auto and Read Only

Keyboard Shortcuts

Shortcut Action
Shift+Tab Cycle Plan → Pair → Execute
Ctrl+L Clear screen (preserves conversation)
Ctrl+G Open prompt in external editor
Ctrl+C Cancel current operation (twice to quit)
Esc + Esc Edit previous message (when composer is empty)
@ Fuzzy file search — attach files to context
! Execute shell command inline
↑/↓ Navigate draft history

Note the distinction between /clear and Ctrl+L: the slash command resets both the terminal display and the conversation, whilst the keyboard shortcut only clears the visual output7.

Plan Mode in Practice: A Decision Framework

Choosing the right collaboration mode depends on task risk and complexity. Here’s a practical framework:

flowchart TD
    T["New task"] --> R{"High risk?\n(production, security,\ndata migration)"}
    R -->|Yes| P["Start in Plan mode"]
    R -->|No| C{"Complex?\n(multi-file, architectural)"}
    C -->|Yes| P
    C -->|No| D{"Familiar pattern?"}
    D -->|Yes| X["Execute mode"]
    D -->|No| PR["Pair mode"]
    P --> REV["Review plan"]
    REV --> IMPL["Switch to Pair/Execute\nfor implementation"]

For high-risk or architecturally complex work, start in plan mode. Review the proposed approach, iterate on it with follow-up prompts, then switch to pair or execute mode once the plan is solid. For routine, well-understood tasks — running a familiar refactoring pattern, adding a standard endpoint — execute mode gets out of the way.

The combination of plan-first design and mid-execution steering means you can maintain tight control over the agent’s direction without sacrificing the speed of autonomous execution. The key insight: plan mode sets the direction; steer mode keeps it on course.

Citations

  1. Codex Plan Mode: Stop Code Drift with Plan→Execute (2026) — SmartScope ↩ ↩2

  2. Introducing Plan Mode: A Game Changer for Codex CLI Users — Oreate AI Blog ↩ ↩2

  3. Codex CLI: The Definitive Technical Reference — Blake Crosley ↩ ↩2

  4. [Features — Codex CLI OpenAI Developers](https://developers.openai.com/codex/cli/features)

    ↩ ↩2 ↩3

  5. feat(tui): syntax highlighting via syntect with theme picker — GitHub PR #11447 ↩ ↩2 ↩3 ↩4

  6. Codex CLI themes are genuinely unusable — GitHub Issue #12912 ↩

  7. Slash commands in Codex CLI — OpenAI Developers ↩ ↩2 ↩3 ↩4