How do I install the Cedar skill?

Run npx skillmds@latest add tomevault-io/cedar in your terminal (requires Node.js), paste this page's agent-chat prompt into Claude, Cursor, or any MCP-connected agent, or download the SKILL.md file and copy it into your agent's skills directory.

Is Cedar safe to use?

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This skill is tagged as working with Claude Code, Claude.ai, OpenAI Codex. SKILL.md is an open format, so most agents that read a skills directory can load it too.

Is Cedar free to use?

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tomevault-io (@tomevault-io) published this skill. Their other Agent Skills are listed on their SkillMD profile.

Cedar by tomevault-io — Agent Skill

Name: Cedar
Author: tomevault-io

Cedar

Authors, validates, and tests Cedar authorization policies and schemas for Sondera AI agent governance. Use when writing .cedar or .cedarschema files, working with agent guardrails, YARA signatures, sensitivity labels, or information flow control policies. Use when this capability is needed.

tomevault-io 43370e2 2 files · 4.5 KB Updated Sep 17, 2026

File contents

Cedar Policy Authoring for Sondera

Write Cedar policies that govern AI coding agent behavior — shell commands, file operations, web fetches, and prompts — using YARA signature detection, policy compliance checks, and sensitivity labels.

Cedar language reference: See CEDAR_REFERENCE.md for full syntax, schema authoring, data types, operators, templates, authorization patterns, and best practices.

MCP Tools

The cedar MCP server (configured in .mcp.json) provides four tools:

Tool	Purpose
`validate_policy`	Validate a single Cedar policy — returns effect, id, annotations, formatted text
`validate_policy_set`	Validate multiple Cedar policies at once — returns count, policy summaries, formatted text
`validate_schema`	Validate a Cedar schema (JSON or Cedar format) — returns entity types, actions, principals, resources
`format_policy`	Format Cedar policy text to canonical form

Workflow

Follow this checklist when authoring Cedar policies:

- [ ] Step 1: Read and validate schema
- [ ] Step 2: Write policy
- [ ] Step 3: Validate policy
- [ ] Step 4: Format policy

Step 1: Read and validate schema

Read policies/base.cedarschema to understand the available entity types, actions, and context fields
Call validate_schema with the schema contents (use schema_format: "cedar" for .cedarschema files) to confirm it is valid and to see the list of entity types, actions, principals, and resources

Step 2: Write policy

Use the Sondera context features below and the syntax in CEDAR_REFERENCE.md.

Step 3: Validate policy

For a single policy: call validate_policy with the policy text to catch syntax errors
For multiple policies: call validate_policy_set with all policies to validate them together

Both tools return the parsed representation including effect, id, and annotations.

Step 4: Format policy

Call format_policy for consistent style before saving.

Key Pitfalls

like is an infix operator, not a method: context.path like "*.py" (correct) — NOT context.path.like("*.py")
* wildcard matches broadly: context.command like "*rm*" matches rm but also format, firmware
Guard optional attributes: always check has before access — resource has "location" && resource.location == "US"
Explicit deny wins: a single matching forbid overrides all permit policies
Default deny: if no policy matches, access is denied
Scope over conditions: use principal/action/resource constraints in policy scope, not in when clauses

Quick Reference

POLICY:   @id("name") permit|forbid (principal, action, resource) when {...} unless {...};
SCOPE:    == (exact) | in (hierarchy) | is (type check) | in [...] (set)
STRING:   like "pattern*"   (* = wildcard)
SET:      .contains(x)  .containsAll(s)  .containsAny(s)  .isEmpty()
ATTR:     entity.attr  entity["attr"]  entity has "attr"
LOGIC:    &&  ||  !  if...then...else
COMPARE:  ==  !=  <  <=  >  >=
ARITH:    +  -  *  (Long only, no division)
ENTITY:   Type::"id"  Namespace::Type::"id"

Source: sondera-ai/sondera-harness-python — distributed by TomeVault.

---name: cedardescription: Authors, validates, and tests Cedar authorization policies and schemas for Sondera AI agent governance. Use when writing .cedar or .cedarschema files, working with agent guardrails, YARA signatures, sensitivity labels, or information flow control policies. Use when this capability is needed.---# Cedar Policy Authoring for SonderaWrite Cedar policies that govern AI coding agent behavior — shell commands, file operations, web fetches, and prompts —using YARA signature detection, policy compliance checks, and sensitivity labels.**Cedar language reference**: See [CEDAR_REFERENCE.md](CEDAR_REFERENCE.md) for full syntax, schema authoring, datatypes, operators, templates, authorization patterns, and best practices.## MCP ToolsThe `cedar` MCP server (configured in `.mcp.json`) provides four tools:| Tool | Purpose ||------------------------|---------------------------------------------------------------|| `validate_policy` | Validate a single Cedar policy — returns effect, id, annotations, formatted text || `validate_policy_set` | Validate multiple Cedar policies at once — returns count, policy summaries, formatted text || `validate_schema` | Validate a Cedar schema (JSON or Cedar format) — returns entity types, actions, principals, resources || `format_policy` | Format Cedar policy text to canonical form |## WorkflowFollow this checklist when authoring Cedar policies:```- [ ] Step 1: Read and validate schema- [ ] Step 2: Write policy- [ ] Step 3: Validate policy- [ ] Step 4: Format policy```**Step 1: Read and validate schema**1. Read `policies/base.cedarschema` to understand the available entity types, actions, and context fields2. Call `validate_schema` with the schema contents (use `schema_format: "cedar"` for `.cedarschema` files) to confirm it is valid and to see the list of entity types, actions, principals, and resources**Step 2: Write policy**Use the Sondera context features below and the syntax in [CEDAR_REFERENCE.md](CEDAR_REFERENCE.md).**Step 3: Validate policy**- For a single policy: call `validate_policy` with the policy text to catch syntax errors- For multiple policies: call `validate_policy_set` with all policies to validate them togetherBoth tools return the parsed representation including effect, id, and annotations.**Step 4: Format policy**Call `format_policy` for consistent style before saving.---## Key Pitfalls- **`like` is an infix operator**, not a method: `context.path like "*.py"` (correct) — NOT `context.path.like("*.py")`- **`*` wildcard matches broadly**: `context.command like "*rm*"` matches `rm` but also `format`, `firmware`- **Guard optional attributes**: always check `has` before access — `resource has "location" && resource.location == "US"`- **Explicit deny wins**: a single matching `forbid` overrides all `permit` policies- **Default deny**: if no policy matches, access is denied- **Scope over conditions**: use principal/action/resource constraints in policy scope, not in `when` clauses---## Quick Reference```POLICY: @id("name") permit|forbid (principal, action, resource) when {...} unless {...};SCOPE: == (exact) | in (hierarchy) | is (type check) | in [...] (set)STRING: like "pattern*" (* = wildcard)SET: .contains(x) .containsAll(s) .containsAny(s) .isEmpty()ATTR: entity.attr entity["attr"] entity has "attr"LOGIC: && || ! if...then...elseCOMPARE: == != < <= > >=ARITH: + - * (Long only, no division)ENTITY: Type::"id" Namespace::Type::"id"```---> Source: [sondera-ai/sondera-harness-python](https://github.com/sondera-ai/sondera-harness-python) — distributed by [TomeVault](https://tomevault.io).

Frequently asked questions