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IntentGuard

IntentGuard is a Python guardrail layer for MCP tool calls. It runs as a proxy between an agent client and an MCP server, enforcing both static policy checks and optional semantic intent checks before a tool call is forwarded.

What is implemented (MVP)

The current implementation covers all 4 roadmap phases from agent.md:

  1. CLI Interceptor (Phase 1)
    intent_guard/proxy.py + intent_guard/sdk/mcp_proxy.py implement a stdio proxy that intercepts tools/call JSON-RPC requests and can block/allow calls.
  2. Static Engine (Phase 2)
    intent_guard/sdk/engine.py loads YAML policy and enforces:
    • forbidden_tools
    • protected_paths (glob/fnmatch style)
    • max_tokens_per_call
    • custom_policies (tool-specific argument requirements/forbidden arguments)
  3. Semantic Guardrail Providers (Phase 3)
    intent_guard/sdk/providers.py supports:
    • OllamaProvider (POST /api/generate)
    • LiteLLMProvider (litellm.completion) using LLM_MODEL and OPENAI_API_KEY / ANTHROPIC_API_KEY from env Both providers include retries (exponential backoff + jitter) and a circuit breaker.
  4. Pause & Resume Feedback Loop (Phase 4)
    terminal_approval_prompt provides interactive approval for flagged calls (Allow? [y/N]).

Repository layout

intent_guard/
├── __init__.py
├── proxy.py
└── sdk/
    ├── __init__.py
    ├── engine.py
    ├── mcp_proxy.py
    └── providers.py
schema/
└── policy.yaml
tests/
├── conftest.py
└── test_integration_phases.py

Installation

python3 -m venv .venv
.venv/bin/pip install -r requirements.txt

Run tests

.venv/bin/pytest -q

Run live Ollama semantic tests only (requires local Ollama + llama3.1:8b available):

.venv/bin/pytest -q -m runOllamaProvider

If local model responses are slow, increase timeout (seconds):

OLLAMA_TIMEOUT_SECONDS=120 .venv/bin/pytest -q -m runOllamaProvider

The live semantic suite defaults to OLLAMA_RAW=false and bounded generation tuned for llama3.1:8b. You can tune:

OLLAMA_TIMEOUT_SECONDS=60 OLLAMA_NUM_PREDICT=256 OLLAMA_RAW=false \
  .venv/bin/pytest -q -m runOllamaProvider

Integration tests cover all phases:

  • phase 1: interception and logging behavior
  • phase 2: static policy blocking
  • phase 3: semantic provider flow (mocked Ollama HTTP call)
  • phase 4: approval allow/deny behavior

Policy file

Use schema/policy.yaml as a starting point:

static_rules:
  forbidden_tools: ["delete_database", "purge_all"]
  protected_paths: ["/etc/*", ".env", "src/auth/*"]
  max_tokens_per_call: 4000
  rate_limits:
    enabled: 1 # required to turn rate limiting on; 0/false bypasses checks
    default:
      max_calls: 60
      window_seconds: 60
    by_tool:
      write_file:
        max_calls: 10
        window_seconds: 60

custom_policies:
  - tool_name: write_file
    args:
      all_present: ["path", "content"]
      should_not_present: ["sudo"]

semantic_rules:
  provider: ollama # or litellm
  mode: enforce # off | enforce | advisory
  prompt_version: "v2"
  guardrail_model: llama3.1:8b
  critical_intent_threshold: 0.85
  retry_attempts: 2
  retry_base_delay_seconds: 0.25
  retry_max_delay_seconds: 2.0
  retry_jitter_ratio: 0.2
  circuit_breaker_failures: 3
  circuit_breaker_reset_seconds: 30
  provider_fail_mode:
    default: advisory # fail-open
    by_tool:
      delete_database: enforce # fail-closed
  constraints:
    - intent: modify_source_code
      allowed_scope: Actions must only affect UI components or styles.
      forbidden_scope: Should not modify database schemas or auth logic.

Rubric scoring (v2)

Set prompt_version: "v2" to switch from opaque LLM-assigned scores to multi-signal rubric scoring. Instead of asking the LLM for a single confidence number, the engine asks concrete yes/no questions across multiple dimensions and computes the score deterministically from the answers.

semantic_rules:
  prompt_version: "v2"
  critical_intent_threshold: 0.85
  scoring:
    dimensions:
      tool_task_alignment:
        weight: 0.25
      argument_scope_compliance:
        weight: 0.30
      no_forbidden_scope_violation:
        weight: 0.30
      no_side_effect_risk:
        weight: 0.15

Default dimensions (used when scoring is omitted):

Dimension Question Default weight
tool_task_alignment Is this tool appropriate for the stated task? 0.25
argument_scope_compliance Are arguments within the allowed scope? 0.30
no_forbidden_scope_violation Do arguments avoid the forbidden scope? 0.30
no_side_effect_risk Is the call free of destructive/exfil risk? 0.15

Score formula: Σ(weight × pass) / Σ(weight). With 4 equal-pass dimensions the score is 1.0; any single failure drops below the 0.85 threshold.

Decisions include dimension_scores with per-dimension passed and evidence for full auditability.

CLI usage

INTENT_GUARD_TASK="Only update frontend styles" \
python -m intent_guard.proxy \
  --policy schema/policy.yaml \
  --target "npx @modelcontextprotocol/server-filesystem /path/to/repo" \
  --model llama3.1:8b \
  --approval-webhook "https://approval.internal/intent-guard" \
  --approval-timeout 10 \
  --approval-default-action deny

Flags

  • --policy: YAML policy path
  • --target: target MCP server command
  • --model: optional Ollama model name for semantic checks
  • --task: optional task context (or set INTENT_GUARD_TASK)
  • --ask-approval: prompt user before allowing flagged calls
  • --approval-webhook: call this webhook for non-interactive approval decisions
  • --approval-timeout: timeout (seconds) for webhook approvals
  • --approval-default-action: allow or deny when webhook approval times out/fails

Native hook integration

IntentGuard can run as the policy engine behind native hooks in Claude Code, Copilot, and Cursor.

Evaluate command

Use the unified command:

intent-guard evaluate --policy schema/policy.yaml

Input:

  • Reads a hook payload JSON object from stdin
  • Supports generic keys like tool_name, arguments, task_context
  • Also supports nested payloads (params.name, params.arguments) and common aliases (tool_input, args, prompt)

Output:

  • Prints a GuardDecision JSON object to stdout
  • Exit code 0 for allow, 1 for block, 2 for invalid input

Hook config templates

Template files are shipped under hooks/:

  • hooks/claude-code/settings.json
  • hooks/copilot/hooks.json
  • hooks/cursor/hooks.json

Each template invokes:

cat | intent-guard evaluate --policy schema/policy.yaml

This lets platform-native hooks call IntentGuard directly instead of wrapping only MCP servers.

Encoded payload detection

Static checks can decode and normalize argument payloads before matching:

  • URL decoding
  • Unicode normalization (NFKC)
  • Base64 decoding (when valid)

Enable or disable via:

static_rules:
  decode_arguments: true

When enabled, injection, sensitive-data, and protected-path checks run against decoded variants to catch obfuscated bypasses.

Response-side inspection

IntentGuard can inspect MCP server responses before forwarding them to the client.

Configure response_rules in policy:

response_rules:
  action: block # block | warn | redact
  detect_base64: true
  patterns:
    - name: "GitHub Token"
      pattern: "gh[ps]_[A-Za-z0-9_]{36,}"

Behavior:

  • block: return JSON-RPC error and suppress original response
  • warn: forward response and log warning decision
  • redact: redact matched text and forward sanitized response

Tool description change detection (rug-pull protection)

IntentGuard can snapshot MCP tools/list metadata and detect changes over time.

Configure:

tool_change_rules:
  enabled: true
  action: warn # warn | block

Behavior:

  • On first tools/list, stores snapshot in .intent-guard/tool-snapshots/<server-hash>.json
  • On subsequent tools/list, compares name, description, and inputSchema
  • warn: log warning and continue
  • block: block response when drift is detected

Semantic mode and provider failure behavior

semantic_rules.mode controls normal semantic enforcement:

  • off: semantic check disabled
  • enforce: semantic failures block tool calls
  • advisory: semantic failures are logged as warnings but calls are allowed

semantic_rules.provider_fail_mode controls behavior when semantic provider is unavailable:

  • supports default and per-tool by_tool override
  • values use the same mode set: off|enforce|advisory

Behavior matrix for tool criticality tiers (example mapping):

Tool tier provider_fail_mode Outcome on provider outage
Critical tools enforce Fail-closed (block + approval required)
Standard tools advisory Fail-open with warning decision
Low-risk tools off Fail-open without warning severity

Define tiers by assigning tools in provider_fail_mode.by_tool.

semantic_rules.prompt_version is copied into every semantic decision and log entry as semantic_prompt_version so prompt changes are auditable.

Semantic decision caching

To reduce repeated provider calls for identical semantic evaluations:

semantic_rules:
  decision_cache:
    enabled: true
    max_size: 256
    ttl_seconds: 300

Cache key uses (tool_name, arguments, task_context). Static checks always run; only semantic verdicts are cached.

LiteLLM provider

To use the API provider, set in .env (or process env):

LLM_MODEL=claude-3-5-sonnet-20241022
ANTHROPIC_API_KEY=...
# or OPENAI_API_KEY=...

Then set semantic_rules.provider: litellm (or just set LLM_MODEL and omit explicit provider).

CI break-glass options

  • INTENT_GUARD_BREAK_GLASS_TOKEN: if set, flagged calls are auto-approved with override metadata.
  • INTENT_GUARD_BREAK_GLASS_SIGNED_TOKEN + INTENT_GUARD_BREAK_GLASS_SIGNING_KEY: optional HMAC-signed break-glass token for CI. Token format is <base64url(json payload)>.<base64url(signature)> where signature is HMAC-SHA256(payload_part, signing_key) and payload contains future exp (unix timestamp), for example {"exp": 4102444800}.
  • INTENT_GUARD_APPROVAL_AUTH_TOKEN: bearer token added to webhook approval requests.

SDK usage (Python)

from intent_guard import IntentGuardSDK

guard = IntentGuardSDK(
    policy_path="schema/policy.yaml",
    local_model="llama3.1:8b",
    task_context="Only modify UI components"
)

decision = guard.evaluate("write_file", {"path": "src/auth/config.py"})
print(decision.allowed, decision.reason)

GuardDecision contract (stable)

GuardDecision now includes machine-readable metadata for enforcement and analytics:

  • decision_id (UUID)
  • code
  • severity
  • policy_name
  • policy_version
  • rule_id
  • timestamp (UTC ISO-8601)
  • override (who/why/ttl, when manually approved)
  • semantic_prompt_version (when semantic checks are applied)

Backward compatibility:

  • Existing fields allowed, reason, requires_approval, semantic_score are unchanged.
  • New fields are always present with safe defaults, so existing consumers can ignore them.

Semantic eval harness

IntentGuard ships a lightweight semantic eval harness used in tests to measure model behavior on known-safe and known-unsafe tool calls.

  • Dataset fixtures: tests/fixtures/semantic_eval_dataset.json
  • Replay verdicts: tests/fixtures/semantic_eval_verdicts.json
  • Metrics computed: precision, recall, accuracy

This enables reproducible regression checks for semantic policy quality.

Versioning/migration strategy:

  • Keep parsing logic tolerant of unknown fields.
  • Use policy_version + code + rule_id for downstream contract evolution and dashboards.
  • Prefer adding new fields over changing/removing existing field semantics.

Usage examples with popular tools

1) Claude Code (MCP server proxy)

Configure the MCP server command to run through IntentGuard:

{
  "mcpServers": {
    "filesystem": {
      "command": "python",
      "args": [
        "-m",
        "intent_guard.proxy",
        "--policy",
        "schema/policy.yaml",
        "--target",
        "npx @modelcontextprotocol/server-filesystem /path/to/repo",
        "--ask-approval"
      ],
      "env": {
        "INTENT_GUARD_TASK": "Refactor UI only; do not touch auth or database"
      }
    }
  }
}

2) Codex (MCP command wrapping)

For Codex setups that support MCP server command configuration, point the server command to IntentGuard first, then to your real MCP server as --target:

python -m intent_guard.proxy \
  --policy schema/policy.yaml \
  --target "npx @modelcontextprotocol/server-filesystem /path/to/repo" \
  --ask-approval

Use that command as the configured MCP server entry in your Codex environment.

3) LangSmith / LangChain workflows

Use IntentGuard before each tool execution and keep normal LangSmith tracing:

from langsmith import traceable
from intent_guard import IntentGuardSDK

guard = IntentGuardSDK(
    policy_path="schema/policy.yaml",
    task_context="Only update docs and UI text"
)

@traceable(name="guarded_tool_call")
def guarded_call(tool_name: str, args: dict, tool_callable):
    decision = guard.evaluate(tool_name, args)
    if not decision.allowed:
        raise PermissionError(f"IntentGuard blocked: {decision.reason}")
    return tool_callable(**args)

This keeps execution decisions visible in traces while enforcing IntentGuard policy at runtime.

Build and publish (pip / Artifactory)

Build source and wheel distributions:

python3 -m venv .venv
.venv/bin/pip install -U pip build twine
.venv/bin/python -m build

Publish to your Artifactory PyPI repository:

export TWINE_USERNAME="<artifactory-username>"
export TWINE_PASSWORD="<artifactory-password-or-token>"
.venv/bin/python -m twine upload \
  --repository-url "https://<artifactory-host>/artifactory/api/pypi/<pypi-repo>/local" \
  dist/*

Integration testing and Docker

Current integration tests are in-process (tests/test_integration_phases.py) and do not require a database or cache service. If a future change adds external DB/cache dependencies, run those services in Docker for tests (same pattern as temp-noob/rule-engine) so test setup remains reproducible.

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