# Fable Mode (pypi · fable-engine)

Mechanical write gates, evidence receipts, and adversarial review for AI coding agents.

- Trust score: 64/100 (medium)
- Registry status: active
- Liveness: live
- Owner verified: no
- Last scored: 2026-09-21

## Components

- pypi · `fable-engine`: 64/100 (this document), [markdown](https://verifymcp.io/servers/rex-codebase-fable-mode/fable-engine.md), [page](https://verifymcp.io/servers/rex-codebase-fable-mode/fable-engine)

## Channel facts

- Registry: `pypi`
- Package: `fable-engine`
- Version: `1.3.5`
- Transport: `stdio`

## Trust breakdown

How this component scores in each security and reliability category. Every signal is checked automatically from public evidence about the published package, including repeated runs of it in an isolated sandbox, and we only credit what we can confirm. Scores are 0–100 per category. Scoring method: https://verifymcp.io/docs/scoring (what has changed: https://verifymcp.io/docs/scoring/changelog)

Scored 2026-09-21.

- **Supply Chain Security**: 94/100
  - No malware found by supply-chain analysis.
  - No known CVEs affecting this package version or its production dependencies.
  - Runs a script at install time (build_backend,cmdclass) that we could not recognise. It may be perfectly ordinary, but we do not read the published tarball, so we cannot say what it does.
  - No production dependencies, so there is no dependency health to assess.
- **Provenance & Transparency**: 32/100
  - Source repository is publicly reachable at the declared URL.
  - Provenance check failed: no build-provenance attestation is published.
  - License check failed: the license (MIT License) isn't a recognized OSI-approved license.
  - Actively maintained (last published 2 days ago).
  - Disclosure check failed: no security disclosure policy was found in the source repository.
- **Schema Quality & AI Usability**: 65/100
  - AI-judged instruction clarity (excellent).
  - Context-footprint check failed: tool/resource definitions use about 3713 tokens (~265/item across 14 items; 14 tools + 0 resources), over budget; trim descriptions and params.
  - Usage-examples check failed: none of the tools include examples.
- **Stability & Change Management**: 20/100
  - Stability observed for 6 of 30 days with no destabilising changes; credit accrues until the full window elapses.
- **Tool Coverage**: 100/100
  - 100% of tools have a non-trivial description (not blank, and not just the tool's name).
  - 100% of tool parameters carry a description.
  - Structured output schemas are declared (57% of tools); any adoption earns full credit.
- **Tool Safety**: 100/100
  - No prompt-injection markers were found in the server instructions, tool names or descriptions we captured.
  - We read all 14 captured tool definition(s), and no name or description among them implies an irreversible operation.
  - An AI judge read all 14 captured unit(s) of tool text and found none that tries to manipulate the model reading it.
- **Capabilities**: 20/100
  - Spec-recency check failed: implements MCP spec 2024-11-05; the latest is 2026-07-28.

## Install

### How do I install the Fable Mode MCP server?

Fable Mode runs locally as a PyPI package, launched with uvx fable-engine. Ready-made configuration for Claude, Cursor, VS Code, Codex and 5 more is on this page, copied from each client's own documentation.

### Claude

```bash
claude mcp add rex-codebase-fable-mode -- uvx fable-engine
```

### Cursor

```json
{
  "mcpServers": {
    "rex-codebase-fable-mode": {
      "command": "uvx",
      "args": [
        "fable-engine"
      ]
    }
  }
}
```

### VS Code

```json
{
  "servers": {
    "rex-codebase-fable-mode": {
      "command": "uvx",
      "args": [
        "fable-engine"
      ]
    }
  }
}
```

### Codex

```bash
codex mcp add rex-codebase-fable-mode -- uvx fable-engine
```

### opencode

```json
{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "rex-codebase-fable-mode": {
      "type": "local",
      "command": [
        "uvx",
        "fable-engine"
      ],
      "enabled": true
    }
  }
}
```

### OpenClaw

```bash
openclaw mcp add rex-codebase-fable-mode --command uvx --arg fable-engine
```

### Hermes

```yaml
mcp_servers:
  rex-codebase-fable-mode:
    command: "uvx"
    args: ["fable-engine"]
```

### Netclaw

```json
{
  "McpServers": {
    "rex-codebase-fable-mode": {
      "Transport": "stdio",
      "Command": "uvx",
      "Arguments": [
        "fable-engine"
      ]
    }
  }
}
```

### Vellum

```bash
assistant mcp add rex-codebase-fable-mode -t stdio -c uvx -a fable-engine
```

### Other

```json
{
  "mcpServers": {
    "rex-codebase-fable-mode": {
      "command": "uvx",
      "args": [
        "fable-engine"
      ]
    }
  }
}
```

## Changelog

Every change recorded for this component, newest first. Days that predate change tracking, or that we cannot explain, say so: "we were watching and nothing happened" and "we were not watching" are different claims.

### 2026-09-21 (score 64, +1)

No change was recorded against any check on this day. Stability & Change Management went from 17 to 20. That category is still filling its 30-day observation window: 5 days of observed history at the previous scan, 6 at this one. The score rises as the window fills, whether or not the server changes.

### 2026-09-18 (score 63, +1)

No change was recorded against any check on this day. Stability & Change Management went from 7 to 10. That category is still filling its 30-day observation window: 2 days of observed history at the previous scan, 3 at this one. The score rises as the window fills, whether or not the server changes.

### 2026-09-17 (score 62, +15)

- [security improvement] Malware scan: unverified → pass

### 2026-09-16 (score 47, −1)

- [security regression] Install scripts: pass → partial
- [security] The scripts that run when this package is installed changed: build_backend → build_backend,cmdclass
- [functional regression] Schema quality: 227 → 265
- [functional improvement] Stability: unverified → 0.03
- [functional] First check of Tool coverage: 57
- [functional] Package version: 1.3.2 → 1.3.5
- [functional] Package version: 1.3.2 → 1.3.3

### 2026-09-15 (score 48)

First indexed and scored.

## MCP tools (14)

### `fable_session` (~2612 tokens)

Fable session, evidence, and review workflow

Runs one Fable operation selected by `action`. Use it for Fable sessions, gates, evidence/proofs, review rubrics, checkpoints, compression, design audits, research scrapers, and explicitly experimental System 3 operations; use browser_* tools for web-page navigation and interaction. Start stateful workflows with `create_session` and a `session_name`; later session actions reuse that name. Only fields documented for the selected action are read, and missing or invalid inputs return an `Error:` message without raising an MCP transport error. Status/list/telemetry/get/view/check operations are read-only. Create, log, record, set, advance, checkpoint, restore, track, evaluate, register, compress/accumulate, evolve, generate, and scraper operations can persist Fable state or artifacts. `unlock_execution` remains blocked until its time and rationale gates pass; `apply_auto_update` can replace installed Fable files. The result is action-specific human-readable text in `result`; inspect that text for success or `Error:` before continuing.

Input parameters:

- `a_matrix` (array|string): Observation likelihood matrix A [O x S] for Active Inference.
- `action` (string, required): The Fable session action to perform.
- `actions` (array|string): List of control action identifiers for Active Inference POMDP.
- `affected_invariants` (array|string): List or string of invariant names affected by this file change.
- `antithesis_title` (string): Title of the antithesis / adversarial critique.
- `archetype` (string): Haute aesthetic archetype identifier for design tokens or scaffolding.
- `archetype_override` (string): Optional override for Haute aesthetic archetype.
- `architectural_refinement` (string): Concrete architectural evolution, optimization, or algorithm change derived from the critique.
- `artifact_path` (string): Absolute filesystem path to blueprint artifact, proof, or benchmark script documenting the refinement.
- `auto_log_epistemic` (boolean): Whether to automatically record retrieved research findings as a [HYPOTHESIS] candidate item in Fable Session's epistemic ledger for subsequent cross-verification.
- `axioms` (array|string): List of reference axiom names or strings for proof oracle.
- `b_matrices` (object|string): State transition matrices B [Action -> S x S] for Active Inference.
- `base_step` (number): Base geodesic step distance for hyperbolic tree embedding (default 1.0).
- `broken_scenarios` (array|string): List or JSON string of broken attack scenarios found during red-team review.
- `c_preferences` (array|string): Prior preference distribution C over observations for Active Inference.
- `cas_ref` (string): CAS reference URI (cas://<sha256_hex>) or 64-char hash.
- `change_type` (string): Classification of file change.
- `claim` (string): Fact, hypothesis statement, or unknown parameter to track in epistemic ledger.
- `code` (string): Source code content (HTML, JSX, CSS, or TSX) to audit or validate against anti-slop gates.
- `code_snippet` (string): Alternative alias for source-code string input. Dynamic target execution from source-code strings is disabled and requires a separate sandboxed executor.
- `content` (string): Raw text content or payload to store/compress into Content-Addressed Storage (CAS).
- `context` (object|string): Hypothesis typing context dictionary {name: type} for proof oracle.
- `contradiction_density` (number): Contradiction density index [0.0, 1.0] for cognitive gear shifting.
- `contradictions` (array|string): List of parameter trade-offs / contradictions to resolve with TRIZ principles.
- `criteria` (array|string|object): List of criteria pointers or JSON string of rubric items for goal score evaluation.
- `critique_or_bottleneck` (string): Critical flaw, vulnerability, edge case, memory/latency bottleneck, or assumption scrutinized.
- `crossover_rate` (number): Genetic crossover rate probability (default 0.80).
- `curvature` (number): Sectional curvature c > 0 of Poincaré manifold (default 1.0).
- `custom_hypotheses` (array|string): List or JSON string of custom adversarial hypotheses / attack vectors.
- `d_prior` (array|string): Prior initial state belief distribution D for Active Inference.
- `dials_override` (object): Optional discrete dials override: variance (1-10), motion (1-10), density (1-10).
- `diff_summary` (string): Concise summary of file changes, diff, or slated edits.
- `dimension` (integer): Dimension of Poincaré ball manifold (default 2).
- `domain` (string): Domain boundary for the invariant or cortical plasticity consolidation.
- `edges` (array|string): List of directed edge dictionaries for Causal DAG construction.
- `end_line` (integer): Ending line number (1-indexed inclusive) for windowed line slice viewing.
- `epistemic_uncertainty` (number): Epistemic uncertainty index [0.0, 1.0] for arbitration.
- `evaluations` (array|object|string): Alias for item_evaluations: list or dict of criterion evaluation updates.
- `evaluator_cmd` (string): Alias for evaluator_command: shell command or tool invocation for candidate evaluator.
- `evaluator_command` (string): Shell command or tool invocation for candidate evaluator.
- `evidence` (string): Source file, command output, line number, or URL supporting the claim.
- `failure_count` (integer): Historical failure count for tri-level cognitive gear arbitration.
- `failure_modes` (array|string): List of adversarial failure modes identified in critique.
- `file_path` (string): Target file path for file change tracking or proof verification.
- `focus_area` (string): Specific subsystem, component, algorithm, or interface being critically re-evaluated.
- `force_flush` (boolean): Force flush buffered micro-payloads immediately into a composite frame.
- `formal_statement` (string): Mathematical or formal contract specification that must never be violated.
- `formula` (string): CTL / modal formula string to verify against Kripke structure (e.g. 'AG(safe)', 'EF(goal)').
- `gamma` (number): Policy precision inverse temperature gamma for Active Inference softmax (default 16.0).
- `generations` (integer): Number of evolutionary generations to run (default 3).
- `generator_cmd` (string): Alias for generator_command: shell command or tool invocation for candidate generator.
- `generator_command` (string): Shell command or tool invocation for candidate generator.
- `include_line_numbers` (boolean): Whether to format line slice with line numbers.
- `initial_world` (string): Initial world ID for Kripke model checking.
- `interventions` (object|string): Dictionary of Pearl's do-operator interventions: {node_id: value}.
- `invariant_name` (string): Identifier or title of the formal invariant (e.g., 'INV-01: Zero-Deadlock Ring Buffer').
- `item_evaluations` (array|object|string): Item evaluations mapping, list, or JSON string with pointer_id, satisfied, score, evidence_receipt_id.
- `label` (string): Optional label or description for compressed CAS node.
- `max_debate_rounds` (integer): Maximum number of dialectical debate rounds for synthesis (default 4).
- `max_iterations` (integer): Maximum closed-loop iterations before halting (default 10).
- `metadata` (object): Optional metadata dictionary attached to accumulated micro-payload.
- `mockups` (array|string): List of concept dictionaries for visual mockup recording.
- `model_name` (string): Name for the causal DAG model in System 3 simulation.
- `mutation_rate` (number): Genetic mutation rate probability (default 0.15).
- `name` (string): Name identifier for automation pipeline spec.
- `next_phase` (string): Target phase to advance the session into.
- `node_labels` (object|string): Optional mapping of node IDs to readable labels.
- `nodes` (array|string): List of node dictionaries for Causal DAG construction.
- `objective` (string): High-level goal or problem statement for the reasoning session.
- `objective_weights` (object|string): Optional dictionary of weights across the 10 Pareto dimensions.
- `observation` (string): Current sensory observation for Active Inference belief updating.
- `observations` (array|string): List of observation identifiers for Active Inference POMDP.
- `payload` (string): Micro-payload text for adaptive batch accumulator or CAS storage.
- `phase_summary` (string): Concise summary of findings or deliverables completed in the previous phase.
- `pipeline_name` (string): Alternative alias for automation pipeline name.
- `pipeline_type` (string): Type of autonomous pipeline (default 'closed_loop').
- `policies` (array|string): List of candidate action sequence policies for Expected Free Energy evaluation.
- `population_size` (integer): Population size for evolutionary gene pool (default 12).
- `prior_report` (object|string): Prior red-team breakage report dictionary or JSON string to verify remediation against.
- `proof_or_rationale` (string): Proof sketch, rationale, or inductive argument establishing the invariant.
- `proof_type` (string): Deterministic proof type.
- `rationale` (string): Justification for unlocking code execution after satisfying cognitive gates.
- `refinement_type` (string): Type of rethink-refine cycle (e.g. 'archetype_exploration', 'triz_resolution', 'adversarial_falsification', 'benchmark_probe', 'failure_mode_analysis').
- `remediated_code` (string): Remediated source-code string input. Dynamic target execution from source-code strings is disabled and requires a separate sandboxed executor.
- `root_id` (string): Root node ID for hyperbolic tree embedding or initial world.
- `rubric_id` (string): Identifier of the goal rubric.
- `seed_paradigms` (array|string): Optional list of initial paradigm definitions to seed the gene pool.
- `selected_concept` (string): Identifier of the selected visual concept archetype.
- `session_name` (string): Unique identifier / name for the Fable session.
- `start_line` (integer): Starting line number (1-indexed inclusive) for windowed line slice viewing.
- `states` (array|string): List of hidden state identifiers for Active Inference POMDP.
- `subagent_prompt` (string): The delegation prompt or contract text for the subagent to validate.
- `tag` (string): Epistemic classification tag for truth calibration.
- `target` (string): Target URL, ID, subreddit, repo, paper ID, handle, or search query string for research scraping actions.
- `target_code` (string): Source-code string input. Dynamic target execution from source-code strings is disabled and requires a separate sandboxed executor.
- `target_metric` (string): Target KPI node ID for sensitivity and structural brittleness analysis.
- `target_name` (string): Identifier or name of the target module, class, or function for red-team review.
- `target_residual_threshold` (number): Target residual contradiction score threshold for convergence (default 0.15).
- `target_resource` (string): Target resource (file path, receipt ID) for proof verification.
- `target_score` (number): Target composite goal score threshold (default 0.95, min 0.0, max 1.0).
- `target_threshold` (number): Target threshold score for pipeline iteration termination (default 0.95).
- `task_complexity` (number): Task complexity index [0.0, 1.0] for tri-level cognitive arbitration.
- `task_id` (string): Task identifier for cortical plasticity consolidation or session lineage.
- `task_objective` (string): Task objective or target outcome for goal scoring rubric.
- `terminal_probe_results` (string): Live empirical probe output, benchmark figures, latency numbers, or profiling stats supporting the refinement.
- `thesis_description` (string): Core architecture description and assumptions for the thesis candidate.
- `thesis_title` (string): Title of the thesis paradigm for System 3 dialectical synthesis.
- `time_budget_minutes` (number): Immutable outer authority budget in minutes. set_timer can only change the internal pacing timer.
- `transitions` (array|object|string): List of transitions or adjacency dictionary for Kripke structure.
- `tree` (object|array|string): Tree hierarchy adjacency list or nested dict for hyperbolic embedding.
- `user_prompt` (string): Alias for prompt: high-level design prompt describing desired web experience.
- `worlds` (array|string): List of world/state definitions for Kripke structure.

Output parameters:

- `result` (string): Action-specific result text. Failures begin with `Error:`.

### `browser_open` (~82 tokens)

Opens a URL in the stealth agent browser using persistent local logins/profile (<=20 MB RAM ceiling). Supports local dev servers across all languages.

Input parameters:

- `session_id` (string): Optional browser tab/session identifier.
- `timeout` (number): Timeout in seconds for page load (maximum 30 seconds).
- `url` (string, required): Target URL or localhost address to open.

### `browser_navigate` (~149 tokens)

Navigate current browser session

Loads `url` into an existing browser session, replacing its current document and adding a history entry. Use this instead of browser_open when continuing in a known session; if `session_id` is omitted, the active session is reused or a default session is created. It waits up to `timeout` seconds and returns the resulting URL, title, viewport, scroll position, and indexed elements; network, timeout, size, and parse failures return an error object without changing committed page state.

Input parameters:

- `session_id` (string): Optional browser tab/session identifier.
- `timeout` (number): Timeout in seconds for page load (maximum 30 seconds).
- `url` (string, required): Target URL to navigate to.

### `browser_click` (~59 tokens)

Navigates to the href of a link element by its stable element ID. Elements without an href are unsupported.

Input parameters:

- `element_id` (string, required): Stable element ID of an href-bearing link.
- `session_id` (string): Optional browser tab/session identifier.

### `browser_type` (~124 tokens)

Replace text field value

Replaces the full value of a text-like input or textarea identified by `element_id`; it does not submit the form. Use browser_press for single-key edits or activation. An omitted `session_id` targets the active/default session. Returns status `typed`, the element ID, and the supplied text; missing or non-editable elements return an error object and are not changed.

Input parameters:

- `element_id` (string, required): Stable element ID to type into.
- `session_id` (string): Optional browser tab/session identifier.
- `text` (string, required): Text content to enter into element.

### `browser_scroll` (~42 tokens)

Scrolls the page vertically by delta pixels.

Input parameters:

- `delta_y` (integer, required): Vertical pixel scroll offset.
- `session_id` (string): Optional browser tab/session identifier.

### `browser_snapshot_layers` (~138 tokens)

Capture layered page snapshots

Captures up to `max_layers` PNGs from the top of the current document, one 1280x800 viewport per layer, without changing the session scroll position. Use this for multi-viewport page coverage; use browser_screenshot for only the current viewport. An omitted `session_id` targets the active/default session. Returns layer count, offsets, dimensions, and base64 PNG data; `max_layers` defaults to 3 and is capped at 10.

Input parameters:

- `max_layers` (integer): Maximum number of screen-sized viewport layers to capture (default 3).
- `session_id` (string): Optional browser tab/session identifier.

### `browser_screenshot` (~85 tokens)

Capture current viewport

Captures one 1280x800 PNG of the current viewport without navigation or scroll changes. Use this for the visible region; use browser_snapshot_layers for multi-viewport coverage. An omitted `session_id` targets the active/default session. Returns one layer with its offset, dimensions, and base64 PNG data.

Input parameters:

- `session_id` (string): Optional browser tab/session identifier.

### `browser_close` (~88 tokens)

Close browser session

Closes and permanently discards one in-memory browser session, including its current document and history; persistent profile cookies remain saved. Use it when that tab is no longer needed. `session_id` targets a specific session; if omitted, the active session is closed. Returns `closed`, or `not_found` without changing another session.

Input parameters:

- `session_id` (string): Optional browser tab/session identifier.

### `browser_back` (~98 tokens)

Go back in browser history

Loads the previous history entry in the active or named browser session without adding a new history entry. Use it for history traversal rather than browser_navigate. If history is already at its first entry, it is a no-op that returns current page status. A navigation failure returns an error and preserves the history index; omitting `session_id` reuses or creates the active/default session.

Input parameters:

- `session_id` (string): Optional browser tab/session identifier.

### `browser_forward` (~95 tokens)

Go forward in browser history

Loads the next history entry in the active or named browser session without adding a new history entry. Use it after browser_back rather than browser_navigate. If no forward entry exists, it is a no-op that returns current page status. A navigation failure returns an error and preserves the history index; omitting `session_id` reuses or creates the active/default session.

Input parameters:

- `session_id` (string): Optional browser tab/session identifier.

### `browser_wait` (~40 tokens)

Waits for a specified duration in seconds.

Input parameters:

- `seconds` (number, required): Seconds to wait.
- `session_id` (string): Optional browser tab/session identifier.

### `browser_press` (~70 tokens)

Edits supported text controls, moves focus with Tab, or activates links with Enter.

Input parameters:

- `element_id` (string): Optional element ID target.
- `key` (string, required): Keyboard key to press (e.g. 'Enter', 'Tab').
- `session_id` (string): Optional browser tab/session identifier.

### `browser_reload` (~31 tokens)

Reloads the current page in the browser session.

Input parameters:

- `session_id` (string): Optional browser tab/session identifier.

## Diagnostics

Captured diagnostic sections: Provenance, Install scripts, Dependencies. The full working is on the page: https://verifymcp.io/servers/rex-codebase-fable-mode/fable-engine#diagnostics

## Score history

- 2026-09-21: 64
- 2026-09-20: 63
- 2026-09-19: 63
- 2026-09-18: 63
- 2026-09-17: 62
- 2026-09-16: 47
- 2026-09-15: 48

## Common questions

### What is the Fable Mode MCP server?

Fable Mode is an MCP server listed in the public MCP registry as io.github.REX-codebase/fable-mode. Mechanical write gates, evidence receipts, and adversarial review for AI coding agents. This page covers its PyPI package (fable-engine).

### Is the Fable Mode MCP server safe to use?

Fable Mode scores 64 out of 100 on VerifyMCP. We found no known CVEs affecting it as of 21 September 2026. That is a record of what we were able to check automatically, not an endorsement. The category breakdown on this page shows every signal behind the number, including the ones we could not confirm.

### What tools does the Fable Mode MCP server expose?

Fable Mode exposes 14 tools: fable_session, browser_open, browser_navigate, browser_click, browser_type, and 9 more. Their descriptions and schemas cost roughly 3,713 tokens of context every time the server is loaded.

### Is the Fable Mode MCP server still maintained?

Fable Mode is still listed as active in the MCP registry. We last reached this channel on 21 September 2026. Those dates come from our own scans of the registry and the channel itself, not from anything the publisher announced.

## Links

- PyPI project: https://pypi.org/project/fable-engine/
- Socket report: https://socket.dev/pypi/package/fable-engine
- Repository: https://github.com/REX-codebase/fable-mode
- Changelog RSS feed: https://verifymcp.io/servers/rex-codebase-fable-mode/fable-engine.xml
- Changelog JSON feed: https://verifymcp.io/servers/rex-codebase-fable-mode/fable-engine.json
- HTML version of this page: https://verifymcp.io/servers/rex-codebase-fable-mode/fable-engine