diff --git a/src/loop/bridge.ts b/src/loop/bridge.ts
index b0bc3d5..93caac7 100644
--- a/src/loop/bridge.ts
+++ b/src/loop/bridge.ts
@@ -96,6 +96,14 @@ const isRecord = (value: unknown): value is Record<string, unknown> =>
 const asString = (value: unknown): string | undefined =>
   typeof value === "string" && value.trim() ? value : undefined;
 
+const normalizeLowerString = (value: unknown): string | undefined => {
+  if (typeof value !== "string") {
+    return undefined;
+  }
+  const normalized = value.trim().toLowerCase();
+  return normalized || undefined;
+};
+
 const normalizeAgent = (value: unknown): Agent | undefined => {
   if (value === "claude" || value === "codex") {
     return value;
@@ -435,7 +443,7 @@ const claudeChannelInstructions = (): string =>
     `Messages from the Codex agent arrive as <channel source="${BRIDGE_SERVER}" chat_id="..." user="${CLAUDE_CHANNEL_USER}" ...>.`,
     'When you are replying to an inbound channel message, use the "reply" tool and pass back the same chat_id.',
     "Never answer the human when the inbound message came from Codex. Send the response back through the bridge tools instead.",
-    'Use the "send_to_agent" tool for proactive messages to Codex that are not direct replies to a channel message.',
+    'Use the "send_to_agent" tool with target: "codex" for proactive messages that are not direct replies to a channel message.',
     'Use "bridge_status" only when direct delivery appears stuck.',
   ].join("\n");
 
@@ -711,13 +719,22 @@ const handleSendToAgentTool = async (
   source: Agent,
   args: Record<string, unknown>
 ): Promise<void> => {
-  const target = normalizeAgent(args.target);
+  const normalizedTarget = normalizeLowerString(args.target);
   const message = asString(args.message);
+  if (!normalizedTarget) {
+    writeError(
+      id,
+      MCP_INVALID_PARAMS,
+      "send_to_agent requires a non-empty target"
+    );
+    return;
+  }
+  const target = normalizeAgent(normalizedTarget);
   if (!target) {
     writeError(
       id,
       MCP_INVALID_PARAMS,
-      "send_to_agent requires target=claude|codex"
+      `Unknown target "${normalizedTarget}" - expected "claude" or "codex"`
     );
     return;
   }
diff --git a/src/loop/paired-loop.ts b/src/loop/paired-loop.ts
index 54405e0..9474174 100644
--- a/src/loop/paired-loop.ts
+++ b/src/loop/paired-loop.ts
@@ -80,6 +80,8 @@ export const pairedLoopInternals = {
 const capitalize = (value: string): string =>
   value.slice(0, 1).toUpperCase() + value.slice(1);
 
+const bridgeTargetLiteral = (agent: Agent): string => `target: "${agent}"`;
+
 const resolvePairedReviewers = (
   review: ReviewMode | undefined,
   agent: Agent
@@ -91,9 +93,10 @@ const pairedResumeHint = (runId: string): void => {
 
 const bridgeGuidance = (agent: Agent): string => {
   const peer = agent === "claude" ? "Codex" : "Claude";
+  const target = agent === "claude" ? "codex" : "claude";
   return [
     "Paired mode:",
-    `You are in a persistent Claude/Codex pair. Use the MCP tool "send_to_agent" when you want ${peer} to act, review, or answer.`,
+    `You are in a persistent Claude/Codex pair. Use the MCP tool "send_to_agent" with ${bridgeTargetLiteral(target)} when you want ${peer} to act, review, or answer.`,
     'Do not ask the human to relay messages between agents or answer the human on the other agent\'s behalf. Use "bridge_status" if you need the current bridge state.',
     'If "bridge_status" shows pending messages addressed to you, call "receive_messages" to read them.',
   ].join("\n");
@@ -109,7 +112,7 @@ const reviewDeliveryGuidance = (reviewer: Agent, opts: Options): string => {
     return "If review is needed, keep the actionable notes in your review body before the final review signal.";
   }
 
-  return `If review is needed, send the actionable notes to ${capitalize(opts.agent)} with "send_to_agent" before returning your final review signal.`;
+  return `If review is needed, send the actionable notes to ${capitalize(opts.agent)} with "send_to_agent" using ${bridgeTargetLiteral(opts.agent)} before returning your final review signal.`;
 };
 
 const reviewToolGuidance = (reviewer: Agent, opts: Options): string =>
diff --git a/src/loop/tmux.ts b/src/loop/tmux.ts
index b4dcc45..e088722 100644
--- a/src/loop/tmux.ts
+++ b/src/loop/tmux.ts
@@ -135,6 +135,8 @@ const capitalize = (value: string): string =>
 const peerAgent = (agent: Agent): Agent =>
   agent === "claude" ? "codex" : "claude";
 
+const bridgeTargetLiteral = (agent: Agent): string => `target: "${agent}"`;
+
 const appendProofPrompt = (parts: string[], proof: string): void => {
   const trimmed = proof.trim();
   if (!trimmed) {
@@ -150,13 +152,13 @@ const pairedBridgeGuidance = (agent: Agent, runId: string): string => {
     return [
       `Your bridge MCP server is "${serverName}". All bridge tool calls must use the mcp__${serverName}__ prefix.`,
       'Reply to inbound Codex channel messages with the MCP tool "reply" and the same chat_id.',
-      'Use "send_to_agent" only for new proactive messages to Codex; do not send Codex-facing responses as a human-facing message.',
+      `Use "send_to_agent" with ${bridgeTargetLiteral("codex")} only for new proactive messages to Codex; do not send Codex-facing responses as a human-facing message.`,
       'Use "bridge_status" or "receive_messages" only if delivery looks stuck.',
     ].join("\n");
   }
 
   return [
-    'Message Claude with "send_to_agent", not a human-facing message.',
+    `Use "send_to_agent" with ${bridgeTargetLiteral("claude")} for Claude-facing messages, not a human-facing message.`,
     'Use "bridge_status" or "receive_messages" only if delivery looks stuck.',
   ].join("\n");
 };
diff --git a/tests/loop/bridge.test.ts b/tests/loop/bridge.test.ts
index 42f7d8c..bd28bd3 100644
--- a/tests/loop/bridge.test.ts
+++ b/tests/loop/bridge.test.ts
@@ -253,6 +253,119 @@ test("bridge MCP send_to_agent queues a direct message through the CLI path", as
   rmSync(root, { recursive: true, force: true });
 });
 
+test("bridge MCP send_to_agent normalizes target case and whitespace", async () => {
+  const bridge = await loadBridge();
+  const root = makeTempDir();
+  const runDir = join(root, "run");
+  mkdirSync(runDir, { recursive: true });
+
+  const result = await runBridgeProcess(
+    runDir,
+    "codex",
+    [
+      encodeFrame({
+        id: 1,
+        jsonrpc: "2.0",
+        method: "tools/call",
+        params: {
+          arguments: {
+            message: "ship it",
+            target: "  CLAUDE  ",
+          },
+          name: "send_to_agent",
+        },
+      }),
+      "\n",
+    ].join("")
+  );
+
+  expect(result.code).toBe(0);
+  expect(result.stderr).toBe("");
+  expect(result.stdout).toContain("queued");
+  expect(bridge.readPendingBridgeMessages(runDir)).toEqual([
+    expect.objectContaining({
+      message: "ship it",
+      source: "codex",
+      target: "claude",
+    }),
+  ]);
+  rmSync(root, { recursive: true, force: true });
+});
+
+test("bridge MCP send_to_agent rejects an empty target after trimming", async () => {
+  const root = makeTempDir();
+  const runDir = join(root, "run");
+  mkdirSync(runDir, { recursive: true });
+
+  const result = await runBridgeProcess(
+    runDir,
+    "codex",
+    [
+      encodeFrame({
+        id: 1,
+        jsonrpc: "2.0",
+        method: "tools/call",
+        params: {
+          arguments: {
+            message: "ship it",
+            target: "   ",
+          },
+          name: "send_to_agent",
+        },
+      }),
+      "\n",
+    ].join("")
+  );
+
+  expect(result.code).toBe(0);
+  expect(JSON.parse(result.stdout)).toMatchObject({
+    error: {
+      code: -32_602,
+      message: "send_to_agent requires a non-empty target",
+    },
+    id: 1,
+    jsonrpc: "2.0",
+  });
+  rmSync(root, { recursive: true, force: true });
+});
+
+test("bridge MCP send_to_agent rejects an unknown normalized target", async () => {
+  const root = makeTempDir();
+  const runDir = join(root, "run");
+  mkdirSync(runDir, { recursive: true });
+
+  const result = await runBridgeProcess(
+    runDir,
+    "codex",
+    [
+      encodeFrame({
+        id: 1,
+        jsonrpc: "2.0",
+        method: "tools/call",
+        params: {
+          arguments: {
+            message: "ship it",
+            target: "  FOO  ",
+          },
+          name: "send_to_agent",
+        },
+      }),
+      "\n",
+    ].join("")
+  );
+
+  expect(result.code).toBe(0);
+  expect(JSON.parse(result.stdout)).toMatchObject({
+    error: {
+      code: -32_602,
+      message: 'Unknown target "foo" - expected "claude" or "codex"',
+    },
+    id: 1,
+    jsonrpc: "2.0",
+  });
+  rmSync(root, { recursive: true, force: true });
+});
+
 test("bridge MCP handles standard empty-list and ping requests through the CLI path", async () => {
   const root = makeTempDir();
   const runDir = join(root, "run");
diff --git a/tests/loop/paired-loop.test.ts b/tests/loop/paired-loop.test.ts
index 8e0a7c4..3fc6d64 100644
--- a/tests/loop/paired-loop.test.ts
+++ b/tests/loop/paired-loop.test.ts
@@ -865,7 +865,7 @@ test("runPairedLoop preserves claudex reviewers in paired mode", async () => {
       "concrete file paths, commands, and code locations that must change"
     );
     expect(reviewPrompts[1]?.prompt).toContain(
-      'send the actionable notes to Claude with "send_to_agent"'
+      'send the actionable notes to Claude with "send_to_agent" using target: "claude"'
     );
   });
 });
diff --git a/tests/loop/tmux.test.ts b/tests/loop/tmux.test.ts
index ac61a5b..afa42ed 100644
--- a/tests/loop/tmux.test.ts
+++ b/tests/loop/tmux.test.ts
@@ -770,7 +770,7 @@ test("tmux prompts keep the paired review workflow explicit", () => {
   );
   expect(primaryPrompt).toContain("Wait briefly if it arrives");
   expect(primaryPrompt).toContain(
-    'Message Claude with "send_to_agent", not a human-facing message'
+    'Use "send_to_agent" with target: "claude" for Claude-facing messages'
   );
   expect(primaryPrompt).toContain("worktree isolation");
   expect(peerPrompt).toContain("You are the reviewer/support agent.");
@@ -778,7 +778,7 @@ test("tmux prompts keep the paired review workflow explicit", () => {
   expect(peerPrompt).toContain("Wait for Codex to send you a targeted request");
   expect(peerPrompt).toContain('"reply"');
   expect(peerPrompt).toContain(
-    'Use "send_to_agent" only for new proactive messages to Codex; do not send Codex-facing responses as a human-facing message.'
+    'Use "send_to_agent" with target: "codex" only for new proactive messages to Codex; do not send Codex-facing responses as a human-facing message.'
   );
   expect(primaryPrompt).not.toContain("mcp__loop-bridge-1__ prefix");
   expect(peerPrompt).toContain("mcp__loop-bridge-1__ prefix");
@@ -797,7 +797,7 @@ test("interactive tmux prompts tell both agents to wait for the human", () => {
   expect(primaryPrompt).toContain("No task has been assigned yet.");
   expect(primaryPrompt).toContain("Wait for the first human task");
   expect(primaryPrompt).toContain(
-    'Message Claude with "send_to_agent", not a human-facing message'
+    'Use "send_to_agent" with target: "claude" for Claude-facing messages'
   );
   expect(primaryPrompt).toContain("worktree isolation");
   expect(peerPrompt).toContain("No task has been assigned yet.");
@@ -805,7 +805,7 @@ test("interactive tmux prompts tell both agents to wait for the human", () => {
   expect(peerPrompt).toContain("human clearly assigns you separate work");
   expect(peerPrompt).toContain('"reply"');
   expect(peerPrompt).toContain(
-    'Use "send_to_agent" only for new proactive messages to Codex; do not send Codex-facing responses as a human-facing message.'
+    'Use "send_to_agent" with target: "codex" only for new proactive messages to Codex; do not send Codex-facing responses as a human-facing message.'
   );
   expect(peerPrompt).toContain(
     "If you are answering Codex, use the bridge tools instead of a human-facing reply."