diff --git a/CHANGELOG.md b/CHANGELOG.md
index e237baed0..02e252410 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,6 +31,8 @@ Docs: https://docs.clawd.bot
 - Messaging: mirror outbound sends into target session keys (threads + dmScope) and create session entries on send. (#1520)
 - Sessions: normalize session key casing to lowercase for consistent routing.
 - BlueBubbles: normalize group session keys for outbound mirroring. (#1520)
+- Slack: match auto-thread mirroring channel ids case-insensitively. (#1520)
+- Gateway: lowercase provided session keys when mirroring outbound sends. (#1520)
 - Skills: gate bird Homebrew install to macOS. (#1569) Thanks @bradleypriest.
 - Slack: honor open groupPolicy for unlisted channels in message + slash gating. (#1563) Thanks @itsjaydesu.
 - Agents: show tool error fallback when the last assistant turn only invoked tools (prevents silent stops).
diff --git a/docs/refactor/outbound-session-mirroring.md b/docs/refactor/outbound-session-mirroring.md
index e9b2b8bcc..cbcf5711c 100644
--- a/docs/refactor/outbound-session-mirroring.md
+++ b/docs/refactor/outbound-session-mirroring.md
@@ -40,6 +40,8 @@ Outbound sends were mirrored into the *current* agent session (tool session key)
   - Mattermost targets now strip `@` for DM session key routing.
   - Zalo Personal uses DM peer kind for 1:1 targets (group only when `group:` is present).
   - BlueBubbles group targets strip `chat_*` prefixes to match inbound session keys.
+  - Slack auto-thread mirroring matches channel ids case-insensitively.
+  - Gateway send lowercases provided session keys before mirroring.
 
 ## Decisions
 - **Gateway send session derivation**: if `sessionKey` is provided, use it. If omitted, derive a sessionKey from target + default agent and mirror there.
diff --git a/src/gateway/server-methods/send.test.ts b/src/gateway/server-methods/send.test.ts
index abd961965..8c50da881 100644
--- a/src/gateway/server-methods/send.test.ts
+++ b/src/gateway/server-methods/send.test.ts
@@ -137,6 +137,34 @@ describe("gateway send mirroring", () => {
     );
   });
 
+  it("lowercases provided session keys for mirroring", async () => {
+    mocks.deliverOutboundPayloads.mockResolvedValue([{ messageId: "m-lower", channel: "slack" }]);
+
+    const respond = vi.fn();
+    await sendHandlers.send({
+      params: {
+        to: "channel:C1",
+        message: "hi",
+        channel: "slack",
+        idempotencyKey: "idem-lower",
+        sessionKey: "agent:main:slack:channel:C123",
+      },
+      respond,
+      context: makeContext(),
+      req: { type: "req", id: "1", method: "send" },
+      client: null,
+      isWebchatConnect: () => false,
+    });
+
+    expect(mocks.deliverOutboundPayloads).toHaveBeenCalledWith(
+      expect.objectContaining({
+        mirror: expect.objectContaining({
+          sessionKey: "agent:main:slack:channel:c123",
+        }),
+      }),
+    );
+  });
+
   it("derives a target session key when none is provided", async () => {
     mocks.deliverOutboundPayloads.mockResolvedValue([{ messageId: "m3", channel: "slack" }]);
 
diff --git a/src/gateway/server-methods/send.ts b/src/gateway/server-methods/send.ts
index e4d292924..1d3adbb6f 100644
--- a/src/gateway/server-methods/send.ts
+++ b/src/gateway/server-methods/send.ts
@@ -145,7 +145,7 @@ export const sendHandlers: GatewayRequestHandlers = {
         );
         const providedSessionKey =
           typeof request.sessionKey === "string" && request.sessionKey.trim()
-            ? request.sessionKey.trim()
+            ? request.sessionKey.trim().toLowerCase()
             : undefined;
         const derivedAgentId = resolveSessionAgentId({ config: cfg });
         // If callers omit sessionKey, derive a target session key from the outbound route.
diff --git a/src/infra/outbound/message-action-runner.threading.test.ts b/src/infra/outbound/message-action-runner.threading.test.ts
index c8d8c5ba2..462a33cd3 100644
--- a/src/infra/outbound/message-action-runner.threading.test.ts
+++ b/src/infra/outbound/message-action-runner.threading.test.ts
@@ -89,4 +89,30 @@ describe("runMessageAction Slack threading", () => {
     const call = mocks.executeSendAction.mock.calls[0]?.[0];
     expect(call?.ctx?.mirror?.sessionKey).toBe("agent:main:slack:channel:c123:thread:111.222");
   });
+
+  it("matches auto-threading when channel ids differ in case", async () => {
+    mocks.executeSendAction.mockResolvedValue({
+      handledBy: "plugin",
+      payload: {},
+    });
+
+    await runMessageAction({
+      cfg: slackConfig,
+      action: "send",
+      params: {
+        channel: "slack",
+        target: "channel:c123",
+        message: "hi",
+      },
+      toolContext: {
+        currentChannelId: "C123",
+        currentThreadTs: "333.444",
+        replyToMode: "all",
+      },
+      agentId: "main",
+    });
+
+    const call = mocks.executeSendAction.mock.calls[0]?.[0];
+    expect(call?.ctx?.mirror?.sessionKey).toBe("agent:main:slack:channel:c123:thread:333.444");
+  });
 });
diff --git a/src/infra/outbound/message-action-runner.ts b/src/infra/outbound/message-action-runner.ts
index 2a57db17c..50ddce227 100644
--- a/src/infra/outbound/message-action-runner.ts
+++ b/src/infra/outbound/message-action-runner.ts
@@ -217,7 +217,7 @@ function resolveSlackAutoThreadId(params: {
   if (context.replyToMode !== "all" && context.replyToMode !== "first") return undefined;
   const parsedTarget = parseSlackTarget(params.to, { defaultKind: "channel" });
   if (!parsedTarget || parsedTarget.kind !== "channel") return undefined;
-  if (parsedTarget.id !== context.currentChannelId) return undefined;
+  if (parsedTarget.id.toLowerCase() !== context.currentChannelId.toLowerCase()) return undefined;
   if (context.replyToMode === "first" && context.hasRepliedRef?.value) return undefined;
   return context.currentThreadTs;
 }