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+# Session Summary – Modrinth Mod Install Integration
+
+**Date:** 2026-02-23
+**Type:** Foundational Architecture + Implementation
+**Status:** Core flow complete, polish pending
+
+---
+
+## Objective
+
+Implement automated mod installation from Modrinth into Minecraft game containers via:
+
+```
+Portal → API → Agent → Filesystem
+```
+
+With security controls, validation, and production-grade guardrails.
+
+---
+
+## What Was Completed
+
+### 1. Modrinth Resolver (API Side)
+
+Implemented `resolveCompatibleVersion()`:
+
+- Fetches project versions from Modrinth API
+- Filters by loader (`neoforge`, `fabric`, `quilt`, etc.) and game version (e.g. `1.21.4`)
+- Excludes alpha builds
+- Sorts by publish date (newest first)
+- Selects primary file and extracts `filename`, `downloadUrl`, `sha512` (preferred), `sha1` (fallback), `size`
+
+Hardened:
+- `.jar` only enforcement
+- Host allowlist (`cdn.modrinth.com`)
+- Required hash presence validation
+- Proper `400` mapping for "not found" and "no compatible version"
+
+### 2. Engine Metadata Bug Fix
+
+**Before (broken):**
+```
+engineType = "minecraft"
+engineVersion = "neoforge-1.21.4"
+```
+
+**After (correct):**
+```
+engineType = "neoforge"
+engineVersion = "1.21.4"
+```
+
+Loader filtering now works correctly. The version string no longer includes the loader prefix.
+
+### 3. API → Agent Payload Contract Fix
+
+**API was sending:**
+```json
+{
+  "fileName": "...",
+  "downloadUrl": "...",
+  "size": 12345,
+  "sha512": "..."
+}
+```
+
+**Agent expected:**
+```json
+{
+  "source": "modrinth",
+  "download_url": "...",
+  "filename": "...",
+  "sha512": "..."
+}
+```
+
+Mismatch caused `invalid mod_id` errors. API payload corrected to match agent contract.
+
+### 4. Agent Host Allowlist Expanded
+
+Added `cdn.modrinth.com` alongside existing `artifacts.zerolaghub.com`.
+
+Redirect policy still enforced:
+- HTTPS only
+- Must resolve to allowed host
+- Max 3 redirects
+
+### 5. Agent Filename Validation Updated
+
+Modrinth filenames contain `+` (e.g. `sodium-neoforge-0.6.13+mc1.21.4.jar`).
+
+Updated allowed character set from `[a-zA-Z0-9._-]` to `[a-zA-Z0-9._+-]`.
+
+Still blocks: `/`, `\`, whitespace, path traversal, control characters.
+
+### 6. End-to-End Install Verified
+
+Mod successfully:
+- Downloaded from Modrinth CDN
+- SHA verified
+- Written to `/tmp/zlh-agent/mods`
+- Moved to `/opt/zlh/minecraft/neoforge/world/mods/`
+- Ownership set to `minecraft`, permissions `0644`
+
+Second install correctly returned `mod already exists` — duplicate protection confirmed working.
+
+---
+
+## Security Controls Now In Place
+
+**Agent level:**
+- HTTPS-only downloads
+- Host allowlist enforcement
+- Redirect limit (max 3)
+- 200MB hard cap
+- SHA256 verification
+- Filename sanitization
+- Duplicate prevention
+- Ownership enforcement (`minecraft:minecraft`)
+- Temp file cleanup on failure
+- Controlled mod directory write path
+- Cache invalidation
+
+**API level:**
+- Auth + ownership enforcement
+- VMID validation
+- Timeout protection with `AbortController`
+- Resolver filtering (loader + version + stability)
+- Correct payload contract to agent
+
+---
+
+## Current System State
+
+| Component | Status |
+|-----------|--------|
+| Modrinth resolver | ✅ Stable |
+| API route | ✅ Working |
+| Agent integration | ✅ Working |
+| Install flow | ✅ Confirmed |
+| Duplicate protection | ✅ Working |
+| Security model | ✅ Intact |
+
+---
+
+## Remaining Issues
+
+### Response Corruption (Needs Verification)
+
+One early `curl` output appeared corrupted — part of the curl command echoed in the response. Needs clean reproduction to determine root cause:
+
+```bash
+curl -sS -D headers.txt -o body.txt ...
+```
+
+Could be: terminal wrap artifact, API returning raw non-JSON, agent returning malformed error, or logging leaking into response body.
+
+**Reproduce before wiring portal.**
+
+### API Error Mapping Refinement
+
+Currently all agent non-2xx responses return `502`. Should improve:
+
+- `"mod already exists"` → `409 Conflict`
+
+Small API enhancement, low effort.
+
+---
+
+## Next Steps
+
+1. **Reproduce and resolve** response corruption issue with clean curl test
+2. **Tighten API error mapping** (`409` for duplicate, etc.)
+3. **Wire install endpoint to portal UI** — display `Installing`, `Already installed`, `Failed (reason)`
+4. **Begin dev-server linking system** (see `docs/architecture/dev-to-game-artifact-pipeline.md`)
+
+---
+
+## Future Work (Not Blocked On)
+
+- Automatic server restart after install
+- Update detection and mod version upgrade logic
+- Rollback integration (snapshot + shadow — see `docs/architecture/mod-deployment-safety.md`)
+- Mod state tracking in DB
+- Curated artifact coordinator
+
+---
+
+## Strategic Note
+
+This session was not routine bug fixing. The work done today:
+
+- Aligned contract layers across API and agent
+- Corrected the loader/version metadata architecture
+- Established a secure mod ingestion path
+- Created the foundation for the full mod management system
+
+The architecture decisions locked here (host allowlist, filename validation, channel separation, payload contract) will apply to every future mod install path including dev promotion, curated updates, and self-healing rollback.
+
+**This was a foundational milestone.**