diff --git a/gateway/session.py b/gateway/session.py
index f7dc5263d..d0bf0cfe4 100644
--- a/gateway/session.py
+++ b/gateway/session.py
@@ -183,6 +183,16 @@ class SessionContext:
         }
 
 
+_PII_SAFE_PLATFORMS = frozenset({
+    Platform.WHATSAPP,
+    Platform.SIGNAL,
+    Platform.TELEGRAM,
+})
+"""Platforms where user IDs can be safely redacted (no in-message mention system
+that requires raw IDs).  Discord is excluded because mentions use ``<@user_id>``
+and the LLM needs the real ID to tag users."""
+
+
 def build_session_context_prompt(
     context: SessionContext,
     *,
@@ -196,10 +206,14 @@ def build_session_context_prompt(
     - What platforms are connected
     - Where it can deliver scheduled task outputs
 
-    When *redact_pii* is True, phone numbers are stripped and user/chat IDs
+    When *redact_pii* is True **and** the source platform is in
+    ``_PII_SAFE_PLATFORMS``, phone numbers are stripped and user/chat IDs
     are replaced with deterministic hashes before being sent to the LLM.
+    Platforms like Discord are excluded because mentions need real IDs.
     Routing still uses the original values (they stay in SessionSource).
     """
+    # Only apply redaction on platforms where IDs aren't needed for mentions
+    redact_pii = redact_pii and context.source.platform in _PII_SAFE_PLATFORMS
     lines = [
         "## Current Session Context",
         "",
diff --git a/tests/gateway/test_pii_redaction.py b/tests/gateway/test_pii_redaction.py
index ca2570723..1982f5e88 100644
--- a/tests/gateway/test_pii_redaction.py
+++ b/tests/gateway/test_pii_redaction.py
@@ -130,3 +130,27 @@ class TestBuildSessionContextPromptRedaction:
         p1 = build_session_context_prompt(ctx1, redact_pii=True)
         p2 = build_session_context_prompt(ctx2, redact_pii=True)
         assert p1 != p2
+
+    def test_discord_ids_not_redacted_even_with_flag(self):
+        """Discord needs real IDs for <@user_id> mentions."""
+        ctx = _make_context(user_id="123456789", platform=Platform.DISCORD)
+        prompt = build_session_context_prompt(ctx, redact_pii=True)
+        assert "123456789" in prompt
+
+    def test_whatsapp_ids_redacted(self):
+        ctx = _make_context(user_id="+15551234567", platform=Platform.WHATSAPP)
+        prompt = build_session_context_prompt(ctx, redact_pii=True)
+        assert "+15551234567" not in prompt
+        assert "user_" in prompt
+
+    def test_signal_ids_redacted(self):
+        ctx = _make_context(user_id="+15551234567", platform=Platform.SIGNAL)
+        prompt = build_session_context_prompt(ctx, redact_pii=True)
+        assert "+15551234567" not in prompt
+        assert "user_" in prompt
+
+    def test_slack_ids_not_redacted(self):
+        """Slack may need IDs for mentions too."""
+        ctx = _make_context(user_id="U12345ABC", platform=Platform.SLACK)
+        prompt = build_session_context_prompt(ctx, redact_pii=True)
+        assert "U12345ABC" in prompt
diff --git a/website/docs/user-guide/configuration.md b/website/docs/user-guide/configuration.md
index 8eb28e850..1df9c491f 100644
--- a/website/docs/user-guide/configuration.md
+++ b/website/docs/user-guide/configuration.md
@@ -839,7 +839,7 @@ privacy:
   redact_pii: false  # Strip PII from LLM context (gateway only)
 ```
 
-When `redact_pii` is `true`, the gateway redacts personally identifiable information from the system prompt before sending it to the LLM:
+When `redact_pii` is `true`, the gateway redacts personally identifiable information from the system prompt before sending it to the LLM on supported platforms:
 
 | Field | Treatment |
 |-------|-----------|
@@ -849,6 +849,8 @@ When `redact_pii` is `true`, the gateway redacts personally identifiable informa
 | Home channel IDs | Numeric portion hashed |
 | User names / usernames | **Not affected** (user-chosen, publicly visible) |
 
+**Platform support:** Redaction applies to WhatsApp, Signal, and Telegram. Discord and Slack are excluded because their mention systems (`<@user_id>`) require the real ID in the LLM context.
+
 Hashes are deterministic — the same user always maps to the same hash, so the model can still distinguish between users in group chats. Routing and delivery use the original values internally.
 
 ## Speech-to-Text (STT)