macp-runtime 0.7.2

MACP reference runtime: a coordination kernel and gRPC server enforcing session boundaries, message validation, append-only history, modes, and governance policy.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180

# Governance Policy

This page covers the runtime's implementation of the governance policy framework: how to register policies via gRPC, what rule schemas look like in practice, how the evaluation engine works internally, and how errors are surfaced. For the protocol-level policy specification -- identifiers, lifecycle, determinism guarantees, and the full rule schema definitions -- see the [protocol policy documentation](https://www.multiagentcoordinationprotocol.io/docs/policy).

## Managing policies

Policies are managed through five gRPC RPCs. Any authenticated sender can perform these operations.

| RPC | Purpose |
|-----|---------|
| `RegisterPolicy` | Add a new policy to the registry |
| `UnregisterPolicy` | Remove a policy (does not affect sessions already using it) |
| `GetPolicy` | Retrieve a policy by its identifier |
| `ListPolicies` | List all policies, optionally filtered by target mode |
| `WatchPolicies` | Stream notifications when the registry changes |

The built-in `policy.default` is always present and cannot be registered or removed.

## Registering a policy

Here is a complete example of registering a Decision Mode policy that requires majority voting with a confidence threshold:

```json
{
  "policy_id": "policy.fraud-review.majority-vote",
  "mode": "macp.mode.decision.v1",
  "description": "Require majority vote with 0.7 confidence threshold",
  "schema_version": 1,
  "rules": {
    "voting": {
      "algorithm": "majority",
      "threshold": 0.5,
      "quorum": { "type": "percentage", "value": 60 }
    },
    "evaluation": {
      "required_before_voting": true,
      "minimum_confidence": 0.7
    },
    "objection_handling": {
      "critical_severity_vetoes": true,
      "veto_threshold": 1
    },
    "commitment": {
      "authority": "initiator_only"
    }
  }
}
```

At registration, the runtime validates the rules against the target mode's schema. It enforces structural constraints: a `weighted` voting algorithm requires a non-empty `weights` map, `supermajority` requires a threshold above 0.5, and `designated_role` commitment authority requires a non-empty `designated_roles` list. The `schema_version` must be `1`. Rules that fail to deserialize into the target mode's Rust struct are rejected with `INVALID_POLICY_DEFINITION`.

## Rule examples by mode

### Decision Mode

```json
{
  "voting": {
    "algorithm": "supermajority",
    "threshold": 0.67,
    "quorum": { "type": "count", "value": 3 },
    "weights": {}
  },
  "evaluation": {
    "required_before_voting": true,
    "minimum_confidence": 0.7
  },
  "objection_handling": {
    "critical_severity_vetoes": true,
    "veto_threshold": 1
  },
  "commitment": {
    "authority": "initiator_only",
    "designated_roles": [],
    "require_vote_quorum": true
  }
}
```

Supported voting algorithms: `none`, `majority`, `supermajority`, `unanimous`, `weighted`, `plurality`.

### Proposal Mode

```json
{
  "acceptance": { "criterion": "all_parties" },
  "counter_proposal": { "max_rounds": 5 },
  "rejection": { "terminal_on_any_reject": false },
  "commitment": { "authority": "initiator_only" }
}
```

Acceptance criteria: `all_parties`, `counterparty`, `initiator`.

### Task Mode

```json
{
  "assignment": { "allow_reassignment_on_reject": true },
  "completion": { "require_output": true },
  "commitment": { "authority": "initiator_only" }
}
```

### Handoff Mode

```json
{
  "acceptance": { "implicit_accept_timeout_ms": 30000 },
  "commitment": { "authority": "initiator_only" }
}
```

### Quorum Mode

```json
{
  "threshold": { "threshold_type": "percentage", "value": 66 },
  "abstention": { "counts_toward_quorum": false, "interpretation": "neutral" },
  "commitment": { "authority": "initiator_only" }
}
```

Threshold types: `n_of_m`, `percentage`, `count`. Abstention interpretations: `neutral`, `implicit_reject`, `ignored`.

## How evaluation works

Each standard mode has a dedicated evaluator in `crates/macp-policy/src/evaluator.rs`. Evaluation runs when a `Commitment` envelope arrives, after the mode's own validation has passed. It is a pure function of three inputs: the resolved policy rules, the accumulated accepted message history, and the session's declared participants. No wall-clock time, external calls, or out-of-session state are involved.

| Evaluator | What it checks |
|-----------|---------------|
| `evaluate_decision_commitment` | Qualifying evaluations meet the confidence threshold, critical objection count stays below veto threshold, vote quorum is met, voting algorithm threshold is satisfied. REVIEW-type evaluations are excluded from confidence checks. |
| `evaluate_proposal_commitment` | Counter-proposal count is within `max_rounds` |
| `evaluate_task_commitment` | Output is present if `require_output` is set |
| `evaluate_handoff_commitment` | Always allows (implicit timeout is handled by the mode) |
| `evaluate_quorum_commitment` | Effective voter count (adjusted for abstention rules) satisfies the threshold |

## Commitment authority

The `commitment.authority` rule determines who can send the terminal commitment. This is enforced in `crates/macp-modes/src/mode/util.rs` and applies across all modes:

| Value | Who can commit |
|-------|---------------|
| `initiator_only` (default) | The session initiator |
| `any_participant` | Any declared participant or the initiator |
| `designated_role` | Only agents listed in the `designated_roles` array |

## Error handling

| Error code | When it occurs | gRPC status |
|-----------|----------------|-------------|
| `UNKNOWN_POLICY_VERSION` | The `policy_version` in SessionStart is not found in the registry | InvalidArgument |
| `POLICY_DENIED` | A commitment is rejected because governance rules are not satisfied | PermissionDenied |
| `INVALID_POLICY_DEFINITION` | A policy fails schema validation at registration time | InvalidArgument |

When a commitment is denied, the error includes structured reasons explaining which rules were not met:

```json
{
  "reasons": [
    "vote quorum not met: 1 voters of 3 participants (quorum: 60 percentage)",
    "no qualifying evaluation meets minimum confidence threshold: 0.70"
  ]
}
```

## Default policy

The default policy (`policy.default`) is always registered with mode `"*"` and no governance constraints:

```json
{
  "voting": { "algorithm": "none", "quorum": { "type": "count", "value": 0 } },
  "objection_handling": { "critical_severity_vetoes": false, "veto_threshold": 1 },
  "evaluation": { "required_before_voting": false, "minimum_confidence": 0.0 },
  "commitment": { "authority": "initiator_only", "designated_roles": [], "require_vote_quorum": false }
}
```

Sessions with an empty `policy_version` automatically resolve to this default. It allows commitment whenever the mode's own built-in rules are satisfied.