diff --git a/packages/subagent/subagent/src/continuation.ts b/packages/subagent/subagent/src/continuation.ts
index b50903fa70..c5b8782e28 100644
--- a/packages/subagent/subagent/src/continuation.ts
+++ b/packages/subagent/subagent/src/continuation.ts
@@ -294,7 +294,7 @@ function agentMessageSource(sender: Agent): AgentMessageSource {
 function agentMessage(sender: Agent, content: ContentBlock[]) {
   return createUserMessage({
     content: [
-      { type: 'text' as const, text: `Agent ${sender.id} sent a message:` },
+      { type: 'text' as const, text: `Agent ${sender.id} sent a message: ` },
       ...content,
     ],
     source: agentMessageSource(sender),
diff --git a/packages/subagent/subagent/tests/continuation.spec.ts b/packages/subagent/subagent/tests/continuation.spec.ts
index 3acaf20af1..68072264e3 100644
--- a/packages/subagent/subagent/tests/continuation.spec.ts
+++ b/packages/subagent/subagent/tests/continuation.spec.ts
@@ -1954,7 +1954,7 @@ describe('continuable adjacent-Agent delivery', () => {
       senderSessionId: started.childId,
     })
     expect(delivered?.content).toEqual([
-      { type: 'text', text: `Agent ${started.childId} sent a message:` },
+      { type: 'text', text: `Agent ${started.childId} sent a message: ` },
       { type: 'text', text: 'an explicit message' },
     ])
 
diff --git a/packages/subagent/tool-subagent-control/tests/tool-subagent-control.spec.ts b/packages/subagent/tool-subagent-control/tests/tool-subagent-control.spec.ts
index 021d64b6f6..da7fe7a51b 100644
--- a/packages/subagent/tool-subagent-control/tests/tool-subagent-control.spec.ts
+++ b/packages/subagent/tool-subagent-control/tests/tool-subagent-control.spec.ts
@@ -221,7 +221,7 @@ describe('dsh-tool-subagent-control', () => {
       senderSessionId: started.childId,
     })
     expect(delivered[0]?.message.content).toEqual([
-      { type: 'text', text: `Agent ${started.childId} sent a message:` },
+      { type: 'text', text: `Agent ${started.childId} sent a message: ` },
       { type: 'text', text: 'CHILD_FINDING' },
     ])
 
@@ -257,7 +257,7 @@ describe('dsh-tool-subagent-control', () => {
       senderSessionId: parent.id,
     })
     expect(followUp?.type === 'user/message' && followUp.data.content).toEqual([
-      { type: 'text', text: `Agent ${parent.id} sent a message:` },
+      { type: 'text', text: `Agent ${parent.id} sent a message: ` },
       { type: 'text', text: 'and then?' },
     ])
   })
@@ -288,7 +288,7 @@ describe('dsh-tool-subagent-control', () => {
       : [])
     expect(prompts).toEqual([
       'long work',
-      `Agent ${parent.id} sent a message:`,
+      `Agent ${parent.id} sent a message: `,
       'also consider Y',
     ])
   })
@@ -411,9 +411,9 @@ describe('dsh-tool-subagent-control interrupt_agent', () => {
       : [])
     expect(prompts).toEqual([
       'long work',
-      `Agent ${parent.id} sent a message:`,
+      `Agent ${parent.id} sent a message: `,
       'parked follow-up',
-      `Agent ${parent.id} sent a message:`,
+      `Agent ${parent.id} sent a message: `,
       'wake up',
     ])
   })
diff --git a/snapshots/sdk/subagent-continuable/session.1.v2.jsonl b/snapshots/sdk/subagent-continuable/session.1.v2.jsonl
index 21b2ccbcc9..4212579c43 100644
--- a/snapshots/sdk/subagent-continuable/session.1.v2.jsonl
+++ b/snapshots/sdk/subagent-continuable/session.1.v2.jsonl
@@ -6,8 +6,8 @@
 {"type":"agent/inbox/spliced","data":{"target":"next-turn","start":0,"inserted":[{"content":[{"type":"text","text":"Reply with exactly the word CHILD_OK and nothing else."},{"type":"text","text":"Your parent agent id is \"{{session:1}}\". Before you finish, send your result to that agent with send_message({ agent_id: \"{{session:1}}\", message: \"<self-contained result>\" }). The parent shares your workspace but does not automatically receive your transcript, tool output, or reasoning. Send earlier messages as well when a finding changes what the parent should do next; sending a message does not end your turn."}],"source":{"kind":"user"},"role":"user","id":"{{message:14}}"}]}}
 {"type":"turn/start","data":{"turn":1}}
 {"type":"agent/inbox/spliced","data":{"target":"next-turn","start":0,"removedCount":1,"inserted":[]}}
-{"type":"agent/inbox/spliced","data":{"target":"next-step","start":0,"inserted":[{"content":[{"type":"text","text":"Agent {{session:1}} sent a message:"},{"type":"text","text":"Now reply with exactly SECOND_OK."}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:1}}"},"role":"user","id":"{{message:15}}"}]}}
-{"type":"agent/inbox/spliced","data":{"target":"next-step","start":1,"inserted":[{"content":[{"type":"text","text":"Agent {{session:1}} sent a message:"},{"type":"text","text":"Now reply with exactly THIRD_OK."}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:1}}"},"role":"user","id":"{{message:16}}"}]}}
+{"type":"agent/inbox/spliced","data":{"target":"next-step","start":0,"inserted":[{"content":[{"type":"text","text":"Agent {{session:1}} sent a message: "},{"type":"text","text":"Now reply with exactly SECOND_OK."}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:1}}"},"role":"user","id":"{{message:15}}"}]}}
+{"type":"agent/inbox/spliced","data":{"target":"next-step","start":1,"inserted":[{"content":[{"type":"text","text":"Agent {{session:1}} sent a message: "},{"type":"text","text":"Now reply with exactly THIRD_OK."}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:1}}"},"role":"user","id":"{{message:16}}"}]}}
 {"type":"step/start","data":{"turn":1,"step":1}}
 {"type":"user/message","data":{"content":[{"type":"text","text":"Reply with exactly the word CHILD_OK and nothing else."},{"type":"text","text":"Your parent agent id is \"{{session:1}}\". Before you finish, send your result to that agent with send_message({ agent_id: \"{{session:1}}\", message: \"<self-contained result>\" }). The parent shares your workspace but does not automatically receive your transcript, tool output, or reasoning. Send earlier messages as well when a finding changes what the parent should do next; sending a message does not end your turn."}],"source":{"kind":"user"},"role":"user","id":"{{message:14}}"},"surfaceOp":"append"}
 {"type":"user/message","data":{"content":[{"type":"text","text":"Current runtime context. This snapshot supersedes earlier runtime-context snapshots.\n\nCurrent DSH file policy: danger-full-access. The DSH file sandbox does not restrict file modifications by available operations.\n\nApproval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`).\n\nYou are a delegated subagent: your permission scope was fixed when you were started and cannot be widened from inside this session — operations that require approval are rejected automatically. When the task needs access beyond that scope, do not retry the denied operation; state the limitation in your reply so the delegating agent can handle it."}],"source":{"kind":"plugin","plugin":"@deepseek-ai/dsh-system-prompt","form":"snapshot","sections":[{"name":"sandbox:policy","text":"Current DSH file policy: danger-full-access. The DSH file sandbox does not restrict file modifications by available operations."},{"name":"approval:policy","text":"Approval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`)."},{"name":"subagent:delegation","text":"You are a delegated subagent: your permission scope was fixed when you were started and cannot be widened from inside this session — operations that require approval are rejected automatically. When the task needs access beyond that scope, do not retry the denied operation; state the limitation in your reply so the delegating agent can handle it."}]},"role":"user","id":"{{message:17}}"},"surfaceOp":"append"}
@@ -18,8 +18,8 @@
 {"type":"step/end","data":{"turn":1,"step":1}}
 {"type":"agent/inbox/spliced","data":{"target":"next-step","start":0,"removedCount":2,"inserted":[]}}
 {"type":"step/start","data":{"turn":1,"step":2}}
-{"type":"user/message","data":{"content":[{"type":"text","text":"Agent {{session:1}} sent a message:"},{"type":"text","text":"Now reply with exactly SECOND_OK."}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:1}}"},"role":"user","id":"{{message:15}}"},"surfaceOp":"append"}
-{"type":"user/message","data":{"content":[{"type":"text","text":"Agent {{session:1}} sent a message:"},{"type":"text","text":"Now reply with exactly THIRD_OK."}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:1}}"},"role":"user","id":"{{message:16}}"},"surfaceOp":"append"}
+{"type":"user/message","data":{"content":[{"type":"text","text":"Agent {{session:1}} sent a message: "},{"type":"text","text":"Now reply with exactly SECOND_OK."}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:1}}"},"role":"user","id":"{{message:15}}"},"surfaceOp":"append"}
+{"type":"user/message","data":{"content":[{"type":"text","text":"Agent {{session:1}} sent a message: "},{"type":"text","text":"Now reply with exactly THIRD_OK."}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:1}}"},"role":"user","id":"{{message:16}}"},"surfaceOp":"append"}
 {"type":"assistant/message","data":{"turn":1,"step":2,"message":{"role":"assistant","content":[{"type":"text","text":"SECOND_OK"}],"source":{"kind":"model","provider":"deepseek-official","model":"deepseek-v4-flash"},"id":"{{message:19}}"},"usage":{"inputTokens":10,"outputTokens":5},"stream":[{"type":"chunk","time":1788269696707,"chunk":{"type":"block-start","index":0,"blockType":"text"}},{"type":"text-chunks","time0":1788269696707,"index":0,"dt":[],"texts":["SECOND_OK"]},{"type":"chunk","time":1788269696707,"chunk":{"type":"block-end","index":0,"block":{"type":"text","text":"SECOND_OK"}}},{"type":"chunk","time":1788269696707,"chunk":{"type":"usage","usage":{"inputTokens":10,"outputTokens":5}}},{"type":"chunk","time":1788269696707,"chunk":{"type":"finish","reason":{"kind":"stop"}}}]},"surfaceOp":"append"}
 {"type":"step/end","data":{"turn":1,"step":2}}
 {"type":"turn/end","data":{"turn":1,"reason":{"kind":"completed"}}}
diff --git a/snapshots/sdk/subagent-send-message/session.v2.jsonl b/snapshots/sdk/subagent-send-message/session.v2.jsonl
index 4d9a232e60..d80daddee2 100644
--- a/snapshots/sdk/subagent-send-message/session.v2.jsonl
+++ b/snapshots/sdk/subagent-send-message/session.v2.jsonl
@@ -19,13 +19,13 @@
 {"type":"assistant/message","data":{"turn":1,"step":2,"message":{"role":"assistant","content":[{"type":"text","text":"STARTED"}],"source":{"kind":"model","provider":"deepseek-official","model":"deepseek-v4-flash"},"id":"{{message:5}}"},"usage":{"inputTokens":10,"outputTokens":5},"stream":[{"type":"chunk","time":1788269697354,"chunk":{"type":"block-start","index":0,"blockType":"text"}},{"type":"text-chunks","time0":1788269697354,"index":0,"dt":[],"texts":["STARTED"]},{"type":"chunk","time":1788269697354,"chunk":{"type":"block-end","index":0,"block":{"type":"text","text":"STARTED"}}},{"type":"chunk","time":1788269697354,"chunk":{"type":"usage","usage":{"inputTokens":10,"outputTokens":5}}},{"type":"chunk","time":1788269697354,"chunk":{"type":"finish","reason":{"kind":"stop"}}}]},"surfaceOp":"append"}
 {"type":"step/end","data":{"turn":1,"step":2}}
 {"type":"turn/end","data":{"turn":1,"reason":{"kind":"completed"}}}
-{"type":"agent/inbox/spliced","data":{"target":"next-step","start":0,"inserted":[{"content":[{"type":"text","text":"Agent {{session:2}} sent a message:"},{"type":"text","text":"CHILD_MESSAGE_OK"}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:2}}"},"role":"user","id":"{{message:6}}"}]}}
+{"type":"agent/inbox/spliced","data":{"target":"next-step","start":0,"inserted":[{"content":[{"type":"text","text":"Agent {{session:2}} sent a message: "},{"type":"text","text":"CHILD_MESSAGE_OK"}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:2}}"},"role":"user","id":"{{message:6}}"}]}}
 {"type":"agent/inbox/spliced","data":{"target":"next-turn","start":0,"inserted":[{"content":[{"type":"text","text":"Background subagent {{session:2}} finished and will do no further work unless you send it more."},{"type":"text","text":"Its closing message:"},{"type":"text","text":"Message sent."}],"source":{"kind":"subagent-settled","form":"notice","summary":"Background subagent {{session:2}} finished and will do no further work unless you send it more.","senderSessionId":"{{session:2}}"},"role":"user","id":"{{message:7}}"}]}}
 {"type":"turn/start","data":{"turn":2}}
 {"type":"agent/inbox/spliced","data":{"target":"next-step","start":0,"removedCount":1,"inserted":[]}}
 {"type":"agent/inbox/spliced","data":{"target":"next-turn","start":0,"removedCount":1,"inserted":[]}}
 {"type":"step/start","data":{"turn":2,"step":1}}
-{"type":"user/message","data":{"content":[{"type":"text","text":"Agent {{session:2}} sent a message:"},{"type":"text","text":"CHILD_MESSAGE_OK"}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:2}}"},"role":"user","id":"{{message:6}}"},"surfaceOp":"append"}
+{"type":"user/message","data":{"content":[{"type":"text","text":"Agent {{session:2}} sent a message: "},{"type":"text","text":"CHILD_MESSAGE_OK"}],"source":{"kind":"agent-message","form":"relay","senderSessionId":"{{session:2}}"},"role":"user","id":"{{message:6}}"},"surfaceOp":"append"}
 {"type":"user/message","data":{"content":[{"type":"text","text":"Background subagent {{session:2}} finished and will do no further work unless you send it more."},{"type":"text","text":"Its closing message:"},{"type":"text","text":"Message sent."}],"source":{"kind":"subagent-settled","form":"notice","summary":"Background subagent {{session:2}} finished and will do no further work unless you send it more.","senderSessionId":"{{session:2}}"},"role":"user","id":"{{message:7}}"},"surfaceOp":"append"}
 {"type":"assistant/message","data":{"turn":2,"step":1,"message":{"role":"assistant","content":[{"type":"text","text":"SUBAGENT_SETTLED_NOTED"}],"source":{"kind":"model","provider":"deepseek-official","model":"deepseek-v4-flash"},"id":"{{message:8}}"},"usage":{"inputTokens":10,"outputTokens":5},"stream":[{"type":"chunk","time":1788269697428,"chunk":{"type":"block-start","index":0,"blockType":"text"}},{"type":"text-chunks","time0":1788269697428,"index":0,"dt":[],"texts":["SUBAGENT_SETTLED_NOTED"]},{"type":"chunk","time":1788269697428,"chunk":{"type":"block-end","index":0,"block":{"type":"text","text":"SUBAGENT_SETTLED_NOTED"}}},{"type":"chunk","time":1788269697428,"chunk":{"type":"usage","usage":{"inputTokens":10,"outputTokens":5}}},{"type":"chunk","time":1788269697428,"chunk":{"type":"finish","reason":{"kind":"stop"}}}]},"surfaceOp":"append"}
 {"type":"step/end","data":{"turn":2,"step":1}}