perf(agent-core): drop dynamic timestamp from system prompt to restore prefix-cache hits

.changeset/perf-system-prompt-cache.md

@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Replace the per-session timestamp in the default system prompt with a static time-lookup reminder. The timestamp changed on every new session, breaking DeepSeek's byte-prefix cache from that point onward — including the ~16.8k-token tools definition — so every new session's first LLM call missed the cache. First-turn cache-miss input drops from ~19.5k tokens to ~88 tokens (99.6% cache hit measured on the built artifact). The static `## Date and Time` section keeps the instruction to fetch the real current time from the environment (e.g. via the `date` command) when it matters, without the cache-breaking value.

packages/agent-core-v2/src/app/agentProfileCatalog/system.md

@@ -82,7 +82,7 @@ The operating environment is not in a sandbox. Any actions you do will immediate
 
 ## Date and Time
 
-The current date and time in ISO format is `${now}`. This was captured when the session started and does not update as the session continues, so in a long or resumed session it may be hours or days stale. Treat it only as a rough reference; whenever the real current time matters (web-result freshness, age or expiry checks, anything time-sensitive), get it fresh from the environment — for example by running `date` if you have a shell tool — instead of trusting this value.
+The current time is not embedded in this prompt. Whenever the real current time matters — web-result freshness, age or expiry checks, anything time-sensitive — get it fresh from the environment, for example by running `date` if you have a shell tool.
 
 ## Working Directory
 

packages/agent-core-v2/test/app/agentProfileCatalog/profile-shared.test.ts

@@ -197,6 +197,21 @@ describe('renderSystemPrompt', () => {
     );
   });
 
+  it('keeps a static time reminder without a per-session timestamp', () => {
+    const nowA = '2026-08-02T10:00:00.000Z';
+    const nowB = '2026-08-03T10:00:00.000Z';
+    const promptA = renderSystemPrompt('', { now: nowA }, { skillActive: true });
+    const promptB = renderSystemPrompt('', { now: nowB }, { skillActive: true });
+
+    // Byte-identical across sessions — the property the prefix cache depends on.
+    expect(promptA).toBe(promptB);
+    // The dynamic value must not leak into the rendered prompt...
+    expect(promptA).not.toContain(nowA);
+    // ...but the static time-lookup guidance stays.
+    expect(promptA).toContain('## Date and Time');
+    expect(promptA).toContain('get it fresh from the environment');
+  });
+
   it('renders the builtin template with no leftover placeholders', () => {
     // Every placeholder in the builtin template must be bound in the variable
     // table — an unbound one would stay verbatim in the output.

packages/agent-core/src/profile/default/system.md

@@ -86,7 +86,7 @@ The operating environment is not in a sandbox. Any actions you do will immediate
 
 ## Date and Time
 
-The current date and time in ISO format is `{{ KIMI_NOW }}`. This was captured when the session started and does not update as the session continues, so in a long or resumed session it may be hours or days stale. Treat it only as a rough reference; whenever the real current time matters (web-result freshness, age or expiry checks, anything time-sensitive), get it fresh from the environment — for example by running `date` if you have a shell tool — instead of trusting this value.
+The current time is not embedded in this prompt. Whenever the real current time matters — web-result freshness, age or expiry checks, anything time-sensitive — get it fresh from the environment, for example by running `date` if you have a shell tool.
 
 ## Working Directory
 

packages/agent-core/test/profile/default-agent-profiles.test.ts

@@ -31,6 +31,14 @@ describe('default agent profiles', () => {
     expect(prompt).toContain('/workspace');
   });
 
+  it('keeps a static time reminder without the dynamic timestamp', () => {
+    const prompt = DEFAULT_AGENT_PROFILES['agent']?.systemPrompt(promptContext) ?? '';
+
+    expect(prompt).not.toContain(promptContext.now);
+    expect(prompt).toContain('## Date and Time');
+    expect(prompt).toContain('get it fresh from the environment');
+  });
+
   it('keeps static instructions before dynamic prompt context', () => {
     const prompt = DEFAULT_AGENT_PROFILES['agent']?.systemPrompt(promptContext) ?? '';