feat: add integrity-reactions feature flag to inject endorsement/disapproval reactions into MCPG allow-only policy

[lpcox]

Summary

Add a new feature flag integrity-reactions that, when enabled, injects endorsement-reactions and disapproval-reactions fields into the MCPG allow-only integrity policy. This enables maintainer reaction-based integrity promotion and demotion as described in github/gh-aw-mcpg#3618.

Requires MCPG v0.2.18+ — the feature flag must be version-gated so the compiler only emits these fields when the resolved MCPG version is >= v0.2.18.

Background

MCPG v0.2.18 adds support for reaction-based integrity evaluation (gh-aw-mcpg#3618):

• Endorsement reactions (e.g., 👍, ❤️) from maintainers promote content integrity to approved
• Disapproval reactions (e.g., 👎, 😕) from maintainers demote content integrity (default: none)
• Disapproval overrides endorsement (safe default)

The compiler needs to support injecting these fields into the allow-only policy JSON that is passed to MCPG.

Proposed Frontmatter Syntax

features:
  integrity-reactions: true

tools:
  github:
    min-integrity: approved
    endorsement-reactions: ["THUMBS_UP", "HEART"]
    disapproval-reactions: ["THUMBS_DOWN", "CONFUSED"]
    disapproval-integrity: none           # optional, default: none
    endorser-min-integrity: approved      # optional, default: approved

Implementation Plan

1. Add feature flag constant

In pkg/constants/feature_constants.go:

// IntegrityReactionsFeatureFlag enables reaction-based integrity
// promotion/demotion in the MCPG allow-only policy.
// Requires MCPG >= v0.2.18.
IntegrityReactionsFeatureFlag FeatureFlag = "integrity-reactions"

2. Add MCPG minimum version constant

In pkg/constants/version_constants.go:

// MCPGIntegrityReactionsMinVersion is the minimum MCPG version that supports
// endorsement-reactions and disapproval-reactions in the allow-only policy.
const MCPGIntegrityReactionsMinVersion Version = "v0.2.18"

3. Parse new fields from tools.github config

In pkg/workflow/tools_parser.go, parse the new fields from tools.github:

• endorsement-reactions — []string (valid values: THUMBS_UP, THUMBS_DOWN, HEART, HOORAY, CONFUSED, ROCKET, EYES, LAUGH)
• disapproval-reactions — []string (same valid values)
• disapproval-integrity — string (valid values: none, unapproved, approved, merged)
• endorser-min-integrity — string (valid values: approved, unapproved, merged)

4. Inject into allow-only policy (both code paths)

When the feature flag is enabled AND MCPG version >= v0.2.18, inject the reaction fields into the allow-only policy object. This needs to happen in two places:

a. MCP Gateway path (pkg/workflow/mcp_github_config.go — getGitHubGuardPolicies()):

Add after the existing approval-labels line (~line 281):

if isFeatureEnabled(constants.IntegrityReactionsFeatureFlag, data) && mcpgSupportsIntegrityReactions(gatewayConfig) {
    if endorsement, ok := toolConfig["endorsement-reactions"]; ok {
        policy["endorsement-reactions"] = endorsement
    }
    if disapproval, ok := toolConfig["disapproval-reactions"]; ok {
        policy["disapproval-reactions"] = disapproval
    }
    if disapprovalIntegrity, ok := toolConfig["disapproval-integrity"]; ok {
        policy["disapproval-integrity"] = disapprovalIntegrity
    }
    if endorserMinIntegrity, ok := toolConfig["endorser-min-integrity"]; ok {
        policy["endorser-min-integrity"] = endorserMinIntegrity
    }
}

b. DIFC proxy path (pkg/workflow/compiler_difc_proxy.go — buildDIFCProxyPolicy()):

Same injection logic for the proxy policy object (~line 190).

5. Add validation

In pkg/workflow/tools_validation.go:

• Validate that reaction values are valid GitHub ReactionContent enum values
• Validate that endorsement-reactions and disapproval-reactions require min-integrity to be set
• Validate that disapproval-integrity and endorser-min-integrity use valid integrity levels
• Emit a validation error if integrity-reactions feature flag is enabled but MCPG version < v0.2.18

6. Add version gate helper

func mcpgSupportsIntegrityReactions(gatewayConfig *MCPGatewayRuntimeConfig) bool {
    version := gatewayConfig.Version
    if version == "" {
        version = string(constants.DefaultMCPGatewayVersion)
    }
    return semver.Compare(version, string(constants.MCPGIntegrityReactionsMinVersion)) >= 0
}

7. Update schema

Add the new fields to the frontmatter JSON schema in pkg/parser/schemas/ so they pass validation. Rebuild with make build after schema changes.

Files to Modify

File	Change
pkg/constants/feature_constants.go	Add IntegrityReactionsFeatureFlag
pkg/constants/version_constants.go	Add MCPGIntegrityReactionsMinVersion = "v0.2.18"
pkg/workflow/tools_parser.go	Parse new reaction fields from tools.github
pkg/workflow/tools_validation.go	Validate reaction values and version gate
pkg/workflow/mcp_github_config.go	Inject reactions into gateway allow-only policy
pkg/workflow/compiler_difc_proxy.go	Inject reactions into proxy allow-only policy
pkg/parser/schemas/	Add new fields to frontmatter schema
Tests for each modified file	Unit tests for parsing, validation, and policy generation

Acceptance Criteria

• features: integrity-reactions: true + valid config → reactions appear in allow-only policy JSON
• Feature flag disabled (default) → no reaction fields in policy (backward compatible)
• MCPG version < v0.2.18 + feature flag enabled → validation error
• Invalid reaction values → validation error with list of valid values
• Reaction fields without min-integrity → validation error
• Both gateway and DIFC proxy paths emit consistent policy JSON
• make recompile produces no changes for existing workflows (feature flag defaults to off)

References

• MCPG implementation: github/gh-aw-mcpg#3618
• Existing feature flag pattern: CliProxyFeatureFlag + AWFCliProxyMinVersion in pkg/constants/
• Existing allow-only policy generation: getGitHubGuardPolicies() in mcp_github_config.go