diff --git a/.changeset/agent-core-mcp-hash-and-injection-compaction.md b/.changeset/agent-core-mcp-hash-and-injection-compaction.md
new file mode 100644
index 000000000..85a28f89a
--- /dev/null
+++ b/.changeset/agent-core-mcp-hash-and-injection-compaction.md
@@ -0,0 +1,6 @@
+---
+"@moonshot-ai/kimi-code": patch
+"@moonshot-ai/kimi-code-sdk": patch
+---
+
+Fix two agent-core edge cases: long MCP tool names now always get an 8-char hex hash suffix (a signed-hash bug could emit a 9-char `-xxxxxxxx` suffix), and an injected system reminder that gets folded into a compaction summary is now cleared instead of being pinned to the summary message — so plugin session-start blocks re-inject and plan-mode reminders stay at full strength after compaction.
diff --git a/packages/agent-core/src/agent/injection/injector.ts b/packages/agent-core/src/agent/injection/injector.ts
index 504e412de..404403a23 100644
--- a/packages/agent-core/src/agent/injection/injector.ts
+++ b/packages/agent-core/src/agent/injection/injector.ts
@@ -11,8 +11,14 @@ export abstract class DynamicInjector {
 
   onContextCompacted(compactedCount: number): void {
     if (this.injectedAt !== null) {
+      // applyCompaction replaces the first `compactedCount` messages with a
+      // single summary at index 0, so a surviving injection (old index >=
+      // compactedCount) maps to new index >= 1. An injection that was inside
+      // the compacted prefix — including the last one (injectedAt ===
+      // compactedCount - 1, which yields 0) — was folded into the summary and
+      // must become null rather than pointing at the summary itself.
       const newInjectedAt = this.injectedAt - compactedCount + 1;
-      this.injectedAt = newInjectedAt >= 0 ? newInjectedAt : null;
+      this.injectedAt = newInjectedAt >= 1 ? newInjectedAt : null;
     }
   }
 
diff --git a/packages/agent-core/src/mcp/tool-naming.ts b/packages/agent-core/src/mcp/tool-naming.ts
index 87dca29a5..749eed8bb 100644
--- a/packages/agent-core/src/mcp/tool-naming.ts
+++ b/packages/agent-core/src/mcp/tool-naming.ts
@@ -45,5 +45,9 @@ function stableHash8(input: string): string {
     hash ^= input.codePointAt(i)!;
     hash = Math.trunc(Math.imul(hash, 0x01000193));
   }
-  return hash.toString(16).padStart(8, '0');
+  // `Math.imul` yields a signed 32-bit int, so coerce to unsigned before
+  // hex-encoding — otherwise a negative hash renders as a 9-char `-xxxxxxxx`
+  // suffix (the `-` is not a hex digit), breaking the documented 8-char hash.
+  const unsigned = hash < 0 ? hash + 0x1_0000_0000 : hash;
+  return unsigned.toString(16).padStart(8, '0');
 }
diff --git a/packages/agent-core/test/agent/injection/manager.test.ts b/packages/agent-core/test/agent/injection/manager.test.ts
index a8a91ea93..63f8b6297 100644
--- a/packages/agent-core/test/agent/injection/manager.test.ts
+++ b/packages/agent-core/test/agent/injection/manager.test.ts
@@ -37,6 +37,19 @@ class BoomInjector extends DynamicInjector {
   }
 }
 
+class ProbeInjector extends DynamicInjector {
+  override readonly injectionVariant = 'probe_test';
+  protected override getInjection(): string | undefined {
+    return undefined;
+  }
+  setInjectedAt(value: number | null): void {
+    (this as unknown as { injectedAt: number | null }).injectedAt = value;
+  }
+  getInjectedAt(): number | null {
+    return (this as unknown as { injectedAt: number | null }).injectedAt;
+  }
+}
+
 function installInjectors(manager: InjectionManager, injectors: DynamicInjector[]): void {
   (manager as unknown as { injectors: DynamicInjector[] }).injectors = injectors;
 }
@@ -112,3 +125,28 @@ describe('InjectionManager registration', () => {
     expect(injectors.some((injector) => injector instanceof TodoListReminderInjector)).toBe(true);
   });
 });
+
+describe('DynamicInjector.onContextCompacted index remapping', () => {
+  it('remaps a surviving injection to its post-summary index', () => {
+    const ctx = testAgent();
+    const probe = new ProbeInjector(ctx.agent);
+    probe.setInjectedAt(5); // old index 5
+    probe.onContextCompacted(3); // first 3 messages folded into the summary at index 0
+    expect(probe.getInjectedAt()).toBe(3); // 5 - 3 + 1
+  });
+
+  it('nulls an injection folded into the summary, including the last compacted message', () => {
+    const ctx = testAgent();
+    // Boundary: injectedAt === compactedCount - 1 was previously remapped to 0
+    // (pointing at the summary) instead of null.
+    const last = new ProbeInjector(ctx.agent);
+    last.setInjectedAt(2);
+    last.onContextCompacted(3); // indices 0..2 compacted away
+    expect(last.getInjectedAt()).toBeNull();
+
+    const earlier = new ProbeInjector(ctx.agent);
+    earlier.setInjectedAt(0);
+    earlier.onContextCompacted(3);
+    expect(earlier.getInjectedAt()).toBeNull();
+  });
+});
diff --git a/packages/agent-core/test/mcp/tool-naming.test.ts b/packages/agent-core/test/mcp/tool-naming.test.ts
index 7c26b8855..f0b349536 100644
--- a/packages/agent-core/test/mcp/tool-naming.test.ts
+++ b/packages/agent-core/test/mcp/tool-naming.test.ts
@@ -43,6 +43,11 @@ describe('qualifyMcpToolName', () => {
     const name = qualifyMcpToolName(server, tool);
     expect(name.length).toBeLessThanOrEqual(64);
     expect(name.startsWith('mcp__')).toBe(true);
+    // The suffix must be a deterministic 8-char lowercase-hex hash. `Math.imul`
+    // yields a signed int, so a negative hash must not leak a `-` sign (which
+    // would also make the suffix 9 chars). These inputs hash negative.
+    const suffix = name.slice(name.lastIndexOf('_') + 1);
+    expect(suffix).toMatch(/^[0-9a-f]{8}$/);
     // Same input → same output (stable hash).
     expect(qualifyMcpToolName(server, tool)).toBe(name);
   });