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Output Modes

The output field on an agent step tells Gump what the agent will produce and how to handle it.

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diff (default)

The agent writes or modifies code in the worktree. Gump captures the git patch after the agent finishes. 
- name: impl
  agent: claude-sonnet
  output: diff
This is the default — if you omit output, Gump assumes diff. The result is a structured Diff Contract containing the base commit, head commit, patch, and list of changed files.

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plan

The agent produces a JSON array of items. Gump parses it, validates the schema, and stores it in the state bag. A foreach step can then iterate over the items. 
- name: decompose
  agent: claude-opus
  output: plan
  prompt: |
    Decompose {spec} into independent items.
  gate: [schema]
The agent writes the plan to .gump/out/plan.json. Each item has a name, a description, and optionally a files array (the blast radius). 
[
  {
    "name": "add-healthz",
    "description": "Add the /healthz endpoint",
    "files": ["internal/server/healthz.go", "internal/server/healthz_test.go"]
  }
]
Implicit guard: no_write: true — the agent is prevented from modifying code files during planning.

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artifact

The agent produces free-form text — an analysis, a decision, a specification. Gump stores the text in the state bag. No code changes expected. 
- name: arbiter
  agent: claude-opus
  output: artifact
  prompt: |
    Analyze both reviews and produce actionable instructions.
The agent writes to .gump/out/artifact.txt. Other steps can reference the artifact via {steps.arbiter.output}. Implicit guard: no_write: true.

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review

The agent produces a JSON verdict: pass or fail, with a comment. 
- name: code-review
  agent: claude-opus
  output: review
  session: fresh
  prompt: |
    Review the implementation for correctness.
    Diff: {steps.impl.output}
The agent writes to .gump/out/review.json: 
{
  "pass": false,
  "comment": "Missing error handling on the database connection timeout."
}
If pass is false, it triggers the step’s on_failure handler (same as a gate failure). Implicit guard: no_write: true.

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Implicit guards

The plan, artifact, and review modes automatically enable no_write: true — the agent is killed if it writes files outside .gump/out/. This prevents a planning or review agent from accidentally modifying code. You can override this with guard: { no_write: false } if needed.