diff --git a/plugins/issue-driven-dev/references/config-protocol.md b/plugins/issue-driven-dev/references/config-protocol.md
index 1366acc..6717b96 100644
--- a/plugins/issue-driven-dev/references/config-protocol.md
+++ b/plugins/issue-driven-dev/references/config-protocol.md
@@ -433,14 +433,24 @@ If a group's `when` matches AND a candidate's `when` matches, **groups take prec
 
   "ask_each_time": false,                        // OPTIONAL. If true and candidates/groups exist, always prompt.
 
-  "pr_policy": "ask"                             // OPTIONAL. "always" | "never" | "ask" (default).
+  "pr_policy": "ask",                            // OPTIONAL. "always" | "never" | "ask" (default).
                                                  // Controls idd-implement PR vs direct-commit path.
                                                  // Fork detection always overrides to "always".
                                                  // See references/pr-flow.md for full algorithm.
+
+  "collaborators": [                             // OPTIONAL. Identity registry — resolve a person's
+    {                                            //   alias / email / display-name → GitHub @login WITHOUT guessing.
+      "github_login": "hardy1yang",              // REQUIRED. Canonical handle — the only string GitHub notifies.
+      "display_name": "Hau-Hung Yang",           // REQUIRED. Real name (may be 中文), echoed back on resolve.
+      "role": "collaborator",                    // OPTIONAL. maintainer | collaborator | advisor | external.
+      "aliases": ["hardy", "楊浩弘", "s1093301"],// OPTIONAL. Nicknames / student IDs / romanizations for fuzzy match.
+      "email": "hardy@example.edu"               // OPTIONAL, PII — private/gitignored config ONLY, never committed/public.
+    }
+  ]
 }
 ```
 
-**Backward compatibility**: configs without `candidates` / `groups` / `ask_each_time` / `pr_policy` work exactly as before — they're plain single-target configs. All new fields are additive.
+**Backward compatibility**: configs without `candidates` / `groups` / `ask_each_time` / `pr_policy` / `collaborators` work exactly as before — they're plain single-target configs. All new fields are additive.
 
 ### `pr_policy` field
 
@@ -456,6 +466,20 @@ Controls whether `idd-implement` opens a PR or commits directly.
 
 `idd-all` always enforces `--pr` regardless of `pr_policy`. Full path contract: [pr-flow.md](pr-flow.md).
 
+### `collaborators[]` field
+
+An OPTIONAL identity registry so IDD can resolve a person's alias / email / display-name → their GitHub `@login` **without guessing** — the hard rule set by [tagging-collaborators.md](../rules/tagging-collaborators.md). It is a *resolution accelerator*, **not** an authority: a table hit is still existence-verified via `gh api users/<login>` before any mention is posted, because the table can go stale.
+
+| Field | Req? | Meaning |
+|-------|------|---------|
+| `github_login` | ✅ | Canonical GitHub handle — the only string GitHub actually notifies. Charset `A-Za-z0-9-`. |
+| `display_name` | ✅ | Real name (may be 中文), echoed back to the user on resolve as a sanity check. |
+| `role` | — | `maintainer` \| `collaborator` \| `advisor` \| `external`. Informational only. |
+| `aliases` | — | Nicknames / student IDs / romanizations for fuzzy input matching. Unique across the whole registry. |
+| `email` | — | **PII.** See boundary below. |
+
+**PII boundary (important).** `email` is personally-identifiable and MUST NOT live in a committed / public config. Keep the non-PII fields (`github_login` / `display_name` / `role` / `aliases`) in the normal walked-up config; put `email` only in a **private / gitignored** config layer. `idd-config validate` emits a PII reminder whenever it sees an `email` in a registry entry, so a leak into a tracked config is surfaced early. This mirrors the git-privacy boundary: a person's raw email is third-party PII, not your own derivative content.
+
 ## Resolution algorithm (canonical)
 
 ```
diff --git a/plugins/issue-driven-dev/rules/tagging-collaborators.md b/plugins/issue-driven-dev/rules/tagging-collaborators.md
index f68e34b..bdf788f 100644
--- a/plugins/issue-driven-dev/rules/tagging-collaborators.md
+++ b/plugins/issue-driven-dev/rules/tagging-collaborators.md
@@ -55,6 +55,19 @@ git log --pretty=format:'%an <%ae>' | sort -u > /tmp/idd-commit-authors.txt
 - Handles inferred from git config / email domains
 - Handles from `~/.gitconfig` / `~/.ssh/config` / `gh auth status`
 
+### Step 2.5: Consult the config registry first (acceleration, not authority)
+
+If the walked-up IDD config has a `collaborators[]` array (schema in [references/config-protocol.md](../references/config-protocol.md)), use it as the **first** resolution attempt — it carries the human's own curated alias/name → `@login` mapping, so it resolves `Hardy` / `楊浩弘` / a student ID that the raw API list can't. Match the input, in priority order:
+
+1. `github_login` exact (case-insensitive)
+2. `aliases[]` exact (case-insensitive)
+3. `email` exact — **only if** the input literally is an email
+4. `display_name` substring
+
+On a **hit**, resolve to that entry's `github_login` — **but the table is an accelerator, never an authority.** It can go stale (a collaborator removed, a login renamed after the config was written), so a hit MUST still be existence-verified via `gh api users/<login>` before it counts as resolved and before `--mention-attested` is passed. A hit that fails existence-verification falls through to Step 3 (treat as no config match).
+
+On a **miss** (no `collaborators[]`, or no entry matches), fall through to Step 3 and fuzzy-match against the API-fetched list from Step 2 as before. The registry never replaces Step 2's fetch — it only front-runs the resolution when it can.
+
 ### Step 3: Resolve user input → @login
 
 Apply fuzzy matching against the real list:
diff --git a/plugins/issue-driven-dev/scripts/tests/collaborators-schema/test.sh b/plugins/issue-driven-dev/scripts/tests/collaborators-schema/test.sh
new file mode 100644
index 0000000..2336dad
--- /dev/null
+++ b/plugins/issue-driven-dev/scripts/tests/collaborators-schema/test.sh
@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+# test.sh — schema-consistency drift guard for the collaborators[] identity
+# registry (PsychQuant/issue-driven-development#86).
+#
+# #86 adds an OPTIONAL `collaborators[]` config array so IDD can resolve a
+# person's alias / email / display-name → their GitHub @login WITHOUT guessing
+# (feeds rules/tagging-collaborators.md Step 2-3 as a resolution accelerator).
+# The schema is documented across THREE files that MUST agree:
+#   - references/config-protocol.md   (schema source of truth + PII boundary)
+#   - rules/tagging-collaborators.md  (consumer: table-lookup then verify)
+#   - skills/idd-config/SKILL.md       (validate: schema checks)
+#
+# This is a C_shared_module_coord change, so the real failure mode is DRIFT:
+# rename a field in one file, forget the other two, and the tagging protocol
+# resolves against a stale schema. There is deliberately no behavioral test —
+# the resolution itself is LLM-executed prose (consistent with the whole rules/
+# corpus), so there is no resolver binary to unit-test. What IS mechanically
+# checkable, and what breaks in practice, is that all three files still describe
+# the same distinctive field set + the PII boundary. The needles below are
+# 0-occurrence in all three files BEFORE #86 (verified), so each is a genuine
+# drift signal, not a token that happens to pre-exist for another reason
+# (`role`, e.g., pre-exists via groups[].repos[].role — deliberately not tested).
+set -u
+
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+. "$HERE/../../lib/assert-helpers.sh"
+ROOT="$(cd "$HERE/../../.." && pwd)"     # → plugins/issue-driven-dev
+
+PROTOCOL="$ROOT/references/config-protocol.md"
+TAGGING="$ROOT/rules/tagging-collaborators.md"
+IDDCONFIG="$ROOT/skills/idd-config/SKILL.md"
+
+echo "collaborators-schema (3-file drift guard, #86)"
+
+# ── config-protocol.md — the schema source of truth ──
+assert_output_grep "protocol declares the collaborators[] field"   "collaborators[" "$PROTOCOL"
+assert_output_grep "protocol: github_login (required @-handle)"    "github_login"   "$PROTOCOL"
+assert_output_grep "protocol: display_name (required real name)"   "display_name"   "$PROTOCOL"
+assert_output_grep "protocol: aliases (optional fuzzy-match keys)" "aliases"        "$PROTOCOL"
+assert_output_grep "protocol: PII boundary documented"             "PII"            "$PROTOCOL"
+
+# ── tagging-collaborators.md — consumes the registry as a lookup accelerator ──
+assert_output_grep "tagging references the collaborators[] registry" "collaborators[" "$TAGGING"
+assert_output_grep "tagging resolves to github_login"                "github_login"   "$TAGGING"
+# regression lock: a table HIT must STILL existence-verify (table can be stale).
+assert_output_grep "tagging keeps the gh api users/ existence-verify" "users/"        "$TAGGING"
+
+# ── skills/idd-config/SKILL.md — validate schema-checks the registry ──
+assert_output_grep "idd-config validate covers collaborators[]"    "collaborators[" "$IDDCONFIG"
+assert_output_grep "idd-config validate checks github_login format" "github_login"  "$IDDCONFIG"
+assert_output_grep "idd-config validate checks aliases uniqueness"  "aliases"       "$IDDCONFIG"
+# the PII-reminder contract lives in idd-config too — keep it anchored so it
+# can't be dropped from validate while config-protocol.md still advertises it.
+assert_output_grep "idd-config validate carries the PII reminder"   "PII"           "$IDDCONFIG"
+
+print_summary
diff --git a/plugins/issue-driven-dev/skills/idd-config/SKILL.md b/plugins/issue-driven-dev/skills/idd-config/SKILL.md
index 312a388..d8a2d3c 100644
--- a/plugins/issue-driven-dev/skills/idd-config/SKILL.md
+++ b/plugins/issue-driven-dev/skills/idd-config/SKILL.md
@@ -261,6 +261,7 @@ Local-only（`TARGET=""`）：不寫 `github_repo`，提示 GitHub-backed idd-*
 ```
 TaskCreate(name="validate_load", description="讀 IDD config（.claude/.idd/local.json 優先,legacy 次之,#195），JSON parse")
 TaskCreate(name="validate_schema", description="檢查 required fields + 各 candidates/groups 結構")
+TaskCreate(name="validate_collaborators", description="若有 collaborators[]：github_login 必填+格式、display_name 必填、role enum、aliases 全域唯一；email 出現→PII warning（#86）")
 TaskCreate(name="validate_repo_exists", description="對 github_repo / candidates[].github_repo / groups[].repos[].github_repo 跑 gh repo view 驗證實際存在")
 TaskCreate(name="validate_predicate_form", description="when 區塊 path_contains / title_matches 等 key 是 known set")
 TaskCreate(name="validate_report", description="輸出 PASS / list of issues")
@@ -272,6 +273,8 @@ TaskCreate(name="validate_report", description="輸出 PASS / list of issues")
 - `github_repo` 形式：`owner/repo`（regex `^[\w\-\.]+/[\w\-\.]+$`）
 - `candidates[].github_repo` 同上
 - `groups[]` 必須有**剛好一個** `role: "primary"`
+- `collaborators[]`（若存在，schema 見 [config-protocol.md](../../references/config-protocol.md)「`collaborators[]` field」）：每個 entry `github_login` **必填**且符合 GitHub login charset `^[A-Za-z0-9-]+$`（明顯錯字如含空白/`@`/中文 → error）；`display_name` **必填**非空；`role`（若有）∈ `{maintainer, collaborator, advisor, external}`（其餘 → warning）；**`aliases` 全域唯一** —— 跨所有 entry 的 `aliases` + `github_login` 攤平後不得重複（否則 fuzzy match 會 ambiguous，這是 error）
+- `collaborators[].email` 若出現 → **PII 提醒**（warning，非 error）：email 是可識別個資，不該進 committed/public config，只放 private/gitignored config layer（見 config-protocol.md「PII boundary」）
 - `when.path_contains` / `path_matches` / `title_matches` / `label_in` / `git_remote_matches` / `git_branch_matches` / `all` / `any` / `not` — 不認識的 key → warning（不 fail）
 - `gh repo view` 對每個 repo 跑（warning 而非 error，因為 private repo 無權限會失敗但 config 本身可能合法）