diff --git a/src/slack/monitor/context.test.ts b/src/slack/monitor/context.test.ts
index c52c5e919..d63b1c71a 100644
--- a/src/slack/monitor/context.test.ts
+++ b/src/slack/monitor/context.test.ts
@@ -60,3 +60,61 @@ describe("resolveSlackSystemEventSessionKey", () => {
     );
   });
 });
+
+describe("isChannelAllowed with groupPolicy and channelsConfig", () => {
+  it("allows unlisted channels when groupPolicy is open even with channelsConfig entries", () => {
+    // Bug fix: when groupPolicy="open" and channels has some entries,
+    // unlisted channels should still be allowed (not blocked)
+    const ctx = createSlackMonitorContext({
+      ...baseParams(),
+      groupPolicy: "open",
+      channelsConfig: {
+        C_LISTED: { requireMention: true },
+      },
+    });
+    // Listed channel should be allowed
+    expect(ctx.isChannelAllowed({ channelId: "C_LISTED", channelType: "channel" })).toBe(true);
+    // Unlisted channel should ALSO be allowed when policy is "open"
+    expect(ctx.isChannelAllowed({ channelId: "C_UNLISTED", channelType: "channel" })).toBe(true);
+  });
+
+  it("blocks unlisted channels when groupPolicy is allowlist", () => {
+    const ctx = createSlackMonitorContext({
+      ...baseParams(),
+      groupPolicy: "allowlist",
+      channelsConfig: {
+        C_LISTED: { requireMention: true },
+      },
+    });
+    // Listed channel should be allowed
+    expect(ctx.isChannelAllowed({ channelId: "C_LISTED", channelType: "channel" })).toBe(true);
+    // Unlisted channel should be blocked when policy is "allowlist"
+    expect(ctx.isChannelAllowed({ channelId: "C_UNLISTED", channelType: "channel" })).toBe(false);
+  });
+
+  it("blocks explicitly denied channels even when groupPolicy is open", () => {
+    const ctx = createSlackMonitorContext({
+      ...baseParams(),
+      groupPolicy: "open",
+      channelsConfig: {
+        C_ALLOWED: { allow: true },
+        C_DENIED: { allow: false },
+      },
+    });
+    // Explicitly allowed channel
+    expect(ctx.isChannelAllowed({ channelId: "C_ALLOWED", channelType: "channel" })).toBe(true);
+    // Explicitly denied channel should be blocked even with open policy
+    expect(ctx.isChannelAllowed({ channelId: "C_DENIED", channelType: "channel" })).toBe(false);
+    // Unlisted channel should be allowed with open policy
+    expect(ctx.isChannelAllowed({ channelId: "C_UNLISTED", channelType: "channel" })).toBe(true);
+  });
+
+  it("allows all channels when groupPolicy is open and channelsConfig is empty", () => {
+    const ctx = createSlackMonitorContext({
+      ...baseParams(),
+      groupPolicy: "open",
+      channelsConfig: undefined,
+    });
+    expect(ctx.isChannelAllowed({ channelId: "C_ANY", channelType: "channel" })).toBe(true);
+  });
+});
diff --git a/src/slack/monitor/context.ts b/src/slack/monitor/context.ts
index bd2425103..83acfbae6 100644
--- a/src/slack/monitor/context.ts
+++ b/src/slack/monitor/context.ts
@@ -327,7 +327,11 @@ export function createSlackMonitorContext(params: {
         );
         return false;
       }
-      if (!channelAllowed) {
+      // When groupPolicy is "open", only block channels that are EXPLICITLY denied
+      // (i.e., have a matching config entry with allow:false). Channels not in the
+      // config (matchSource undefined) should be allowed under open policy.
+      const hasExplicitConfig = Boolean(channelConfig?.matchSource);
+      if (!channelAllowed && (params.groupPolicy !== "open" || hasExplicitConfig)) {
         logVerbose(`slack: drop channel ${p.channelId} (${channelMatchMeta})`);
         return false;
       }