Fable Mode
PYPI · FABLE-ENGINE · SCANNED SEP 21
Mechanical write gates, evidence receipts, and adversarial review for AI coding agents.
Available components
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. How we score → Why this is hard to score →
Supply Chain Security94
- No malware found by supply-chain analysis.Pass
- No known CVEs affecting this package version or its production dependencies.Pass
- 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. View diagnostics → Partial
- No production dependencies, so there is no dependency health to assess. View diagnostics → Pass
Provenance & Transparency32
- Source repository is publicly reachable at the declared URL. View diagnostics → Pass
- Provenance check failed: no build-provenance attestation is published. See how to fix → View diagnostics → Fail
- License check failed: the license (MIT License) isn't a recognized OSI-approved license. See how to fix → Fail
- Actively maintained (last published 2 days ago).Pass
- Disclosure check failed: no security disclosure policy was found in the source repository. See how to fix → Fail
Schema Quality & AI Usability65
- AI-judged instruction clarity (excellent).Pass
- 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. See how to fix → Fail
- Usage-examples check failed: none of the tools include examples. See how to fix → Fail
Stability & Change Management20
- Stability observed for 6 of 30 days with no destabilising changes; credit accrues until the full window elapses.Partial
Tool Coverage100
- 100% of tools have a non-trivial description (not blank, and not just the tool's name).Pass
- 100% of tool parameters carry a description.Pass
- Structured output schemas are declared (57% of tools); any adoption earns full credit.Pass
Tool Safety100
- No prompt-injection markers were found in the server instructions, tool names or descriptions we captured.Pass
- We read all 14 captured tool definition(s), and no name or description among them implies an irreversible operation.Pass
- An AI judge read all 14 captured unit(s) of tool text and found none that tries to manipulate the model reading it.Pass
Capabilities20
- Spec-recency check failed: implements MCP spec 2024-11-05; the latest is 2026-07-28. See how to fix → Fail
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.
pypi · fable-engine
claude mcp add rex-codebase-fable-mode -- uvx fable-engine
{
"mcpServers": {
"rex-codebase-fable-mode": {
"command": "uvx",
"args": [
"fable-engine"
]
}
}
} {
"servers": {
"rex-codebase-fable-mode": {
"command": "uvx",
"args": [
"fable-engine"
]
}
}
} codex mcp add rex-codebase-fable-mode -- uvx fable-engine
{
"$schema": "https://opencode.ai/config.json",
"mcp": {
"rex-codebase-fable-mode": {
"type": "local",
"command": [
"uvx",
"fable-engine"
],
"enabled": true
}
}
} openclaw mcp add rex-codebase-fable-mode --command uvx --arg fable-engine
mcp_servers:
rex-codebase-fable-mode:
command: "uvx"
args: ["fable-engine"] {
"McpServers": {
"rex-codebase-fable-mode": {
"Transport": "stdio",
"Command": "uvx",
"Arguments": [
"fable-engine"
]
}
}
} assistant mcp add rex-codebase-fable-mode -t stdio -c uvx -a fable-engine
{
"mcpServers": {
"rex-codebase-fable-mode": {
"command": "uvx",
"args": [
"fable-engine"
]
}
}
} Every change we have recorded for this component, newest first. Security-relevant changes are always shown. ▲ marks a change for the better, ▼ a change for the worse; unmarked changes are neutral.
- 21 Sept 26 +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.
- 18 Sept 26 +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.
- 17 Sept 26 +15
- Malware scan: unverified → pass ▲ security
- 16 Sept 26 −1
- Install scripts: pass → partial ▼ security
- The scripts that run when this package is installed changed: build_backend → build_backend,cmdclass security
- Schema quality: 227 → 265 ▼ functional
- 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 functional
- 15 Sept 26 48
First indexed and scored.
Diagnostic detail from the automated scan of this channel: what the scanner observed at each step, so you can see exactly where a check passed or failed. It is informational only and never changes the trust score.
Captured 21 Sept 2026 · Analysed pypi/fable-engine@1.3.5
Provenance No attestation
The registry publishes no build provenance for this version, so there is nothing to verify.
| Result | No attestation |
|---|---|
| Ecosystem | pypi |
Background: How many MCP packages publish verified provenance →
Install scripts 2 scripts
| Hook | Tier | Command |
|---|---|---|
| build_backend | allowlisted | setuptools.build_meta |
| cmdclass | unreviewed | <custom, unreviewed> |
Background: Why install scripts are a supply-chain risk →
Dependencies 0 packages
| Packages resolved | 0 |
|---|---|
| Tree resolution | Complete |
Background: SBOMs and build attestations, explained →
The tools this component advertises to a client, with an estimated token cost for each. Expand a tool to see its parameters and schema. The per-tool counts are indicative and are not scored directly; the schema's total context footprint is one signal in Schema Quality & AI Usability. A tool's description is untrusted text the model reads on every call, which is what makes this list a security surface and not just an inventory: how tool poisoning works →
browser_back Go back in browser history ~98
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.
| Name | Type | Req | Description |
|---|---|---|---|
| session_id | string | – | Optional browser tab/session identifier. |
Structured output declared, but exposes no named fields.
No examples provided.
browser_click ~59
Navigates to the href of a link element by its stable element ID. Elements without an href are unsupported.
| Name | Type | Req | Description |
|---|---|---|---|
| element_id | string | yes | Stable element ID of an href-bearing link. |
| session_id | string | – | Optional browser tab/session identifier. |
No output schema declared.
No examples provided.
browser_close Close browser session ~88
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.
| Name | Type | Req | Description |
|---|---|---|---|
| session_id | string | – | Optional browser tab/session identifier. |
Structured output declared, but exposes no named fields.
No examples provided.
browser_forward Go forward in browser history ~95
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.
| Name | Type | Req | Description |
|---|---|---|---|
| session_id | string | – | Optional browser tab/session identifier. |
Structured output declared, but exposes no named fields.
No examples provided.
browser_navigate Navigate current browser session ~149
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.
| Name | Type | Req | Description |
|---|---|---|---|
| session_id | string | – | Optional browser tab/session identifier. |
| timeout | number | – | Timeout in seconds for page load (maximum 30 seconds). |
| url | string | yes | Target URL to navigate to. |
Structured output declared, but exposes no named fields.
No examples provided.
browser_open ~82
Opens a URL in the stealth agent browser using persistent local logins/profile (<=20 MB RAM ceiling). Supports local dev servers across all languages.
| Name | Type | Req | Description |
|---|---|---|---|
| session_id | string | – | Optional browser tab/session identifier. |
| timeout | number | – | Timeout in seconds for page load (maximum 30 seconds). |
| url | string | yes | Target URL or localhost address to open. |
No output schema declared.
No examples provided.
browser_press ~70
Edits supported text controls, moves focus with Tab, or activates links with Enter.
| Name | Type | Req | Description |
|---|---|---|---|
| element_id | string | – | Optional element ID target. |
| key | string | yes | Keyboard key to press (e.g. 'Enter', 'Tab'). |
| session_id | string | – | Optional browser tab/session identifier. |
No output schema declared.
No examples provided.
browser_reload ~31
Reloads the current page in the browser session.
| Name | Type | Req | Description |
|---|---|---|---|
| session_id | string | – | Optional browser tab/session identifier. |
No output schema declared.
No examples provided.
browser_screenshot Capture current viewport ~85
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.
| Name | Type | Req | Description |
|---|---|---|---|
| session_id | string | – | Optional browser tab/session identifier. |
Structured output declared, but exposes no named fields.
No examples provided.
browser_scroll ~42
Scrolls the page vertically by delta pixels.
| Name | Type | Req | Description |
|---|---|---|---|
| delta_y | integer | yes | Vertical pixel scroll offset. |
| session_id | string | – | Optional browser tab/session identifier. |
No output schema declared.
No examples provided.
browser_snapshot_layers Capture layered page snapshots ~138
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.
| Name | Type | Req | Description |
|---|---|---|---|
| max_layers | integer | – | Maximum number of screen-sized viewport layers to capture (default 3). |
| session_id | string | – | Optional browser tab/session identifier. |
Structured output declared, but exposes no named fields.
No examples provided.
browser_type Replace text field value ~124
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.
| Name | Type | Req | Description |
|---|---|---|---|
| element_id | string | yes | Stable element ID to type into. |
| session_id | string | – | Optional browser tab/session identifier. |
| text | string | yes | Text content to enter into element. |
Structured output declared, but exposes no named fields.
No examples provided.
browser_wait ~40
Waits for a specified duration in seconds.
| Name | Type | Req | Description |
|---|---|---|---|
| seconds | number | yes | Seconds to wait. |
| session_id | string | – | Optional browser tab/session identifier. |
No output schema declared.
No examples provided.
fable_session Fable session, evidence, and review workflow ~2,612
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.
| Name | Type | Req | Description |
|---|---|---|---|
| a_matrix | array|string | – | Observation likelihood matrix A [O x S] for Active Inference. |
| action | string | yes | 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. |
| Name | Type | Req | Description |
|---|---|---|---|
| result | string | yes | Action-specific result text. Failures begin with `Error:`. |
No examples provided.
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.