diff --git a/.changeset/fix-approx-count-serialize.md b/.changeset/fix-approx-count-serialize.md
new file mode 100644
index 0000000..b2c0f0d
--- /dev/null
+++ b/.changeset/fix-approx-count-serialize.md
@@ -0,0 +1,5 @@
+---
+"flint": patch
+---
+
+Harden `approxCount` (and the `count` fallback) against non-serializable tool-call arguments. `JSON.stringify` could throw on circular references or `BigInt` values, or return `undefined` for a bare `undefined`, crashing a pure token estimate. Such arguments now contribute 0 tokens instead of throwing.
diff --git a/.changeset/fix-memory-append-type.md b/.changeset/fix-memory-append-type.md
new file mode 100644
index 0000000..68d7610
--- /dev/null
+++ b/.changeset/fix-memory-append-type.md
@@ -0,0 +1,5 @@
+---
+"flint": patch
+---
+
+Fix `ConversationMemory.append` type signature: it was declared as returning `void` but the implementation is `async` and performs summarization. The interface now correctly returns `Promise<void>` so callers don't silently drop the promise.
diff --git a/.changeset/fix-redact-private-keys.md b/.changeset/fix-redact-private-keys.md
new file mode 100644
index 0000000..5697ca7
--- /dev/null
+++ b/.changeset/fix-redact-private-keys.md
@@ -0,0 +1,5 @@
+---
+"flint": patch
+---
+
+Broaden the private-key pattern in the `secretPatterns` redaction preset. The previous regex only matched key types made of uppercase letters and spaces, so it missed the most common modern formats — generic PKCS#8 `-----BEGIN PRIVATE KEY-----` (no type word), `ENCRYPTED PRIVATE KEY`, and types containing digits or hyphens. These are now redacted.
diff --git a/packages/flint/src/memory.ts b/packages/flint/src/memory.ts
index d10be24..72e64fe 100644
--- a/packages/flint/src/memory.ts
+++ b/packages/flint/src/memory.ts
@@ -58,7 +58,7 @@ export type ConversationMemoryOpts = {
 };
 
 export type ConversationMemory = {
-  append(m: Message): void;
+  append(m: Message): Promise<void>;
   messages(): Message[];
   summary(): string | undefined;
   clear(): void;
diff --git a/packages/flint/src/primitives/approx-count.ts b/packages/flint/src/primitives/approx-count.ts
index 5bb61f1..98ebb67 100644
--- a/packages/flint/src/primitives/approx-count.ts
+++ b/packages/flint/src/primitives/approx-count.ts
@@ -29,7 +29,16 @@ export function approxCount(messages: Message[]): number {
     if (msg.role === 'assistant' && msg.toolCalls) {
       for (const tc of msg.toolCalls) {
         total += ROLE_OVERHEAD;
-        total += textTokens(JSON.stringify(tc.arguments));
+        // tc.arguments is `unknown` — JSON.stringify can throw (circular refs,
+        // BigInt) or return undefined (a bare undefined value). Either case must
+        // not crash a pure token estimate, so fall back to 0 for the arguments.
+        let serialized: string | undefined;
+        try {
+          serialized = JSON.stringify(tc.arguments);
+        } catch {
+          serialized = undefined;
+        }
+        total += serialized === undefined ? 0 : textTokens(serialized);
       }
     }
   }
diff --git a/packages/flint/src/safety/redact.ts b/packages/flint/src/safety/redact.ts
index c057368..f81c722 100644
--- a/packages/flint/src/safety/redact.ts
+++ b/packages/flint/src/safety/redact.ts
@@ -39,7 +39,7 @@ export const secretPatterns: RegExp[] = [
   /gho_[a-zA-Z0-9]{36}/g,
   /xox[baprs]-[a-zA-Z0-9-]{10,}/g,
   /sk_(?:live|test)_[a-zA-Z0-9]{24,}/g,
-  /-----BEGIN [A-Z ]+ PRIVATE KEY-----[\s\S]+?-----END [A-Z ]+ PRIVATE KEY-----/g,
+  /-----BEGIN (?:[A-Z0-9 -]+ )?PRIVATE KEY-----[\s\S]+?-----END (?:[A-Z0-9 -]+ )?PRIVATE KEY-----/g,
   /\b\d{3}-\d{2}-\d{4}\b/g,
   /\b(?:\d{4}[\s-]?){3}\d{4}\b/g,
 ];
diff --git a/packages/flint/test/count.test.ts b/packages/flint/test/count.test.ts
index 03964fc..6c69a04 100644
--- a/packages/flint/test/count.test.ts
+++ b/packages/flint/test/count.test.ts
@@ -55,6 +55,25 @@ describe('approxCount', () => {
     const more: Message[] = [...base, { role: 'assistant', content: 'hi back' }];
     expect(approxCount(more)).toBeGreaterThanOrEqual(approxCount(base));
   });
+
+  it('does not throw on non-serializable tool arguments', () => {
+    const circular: Record<string, unknown> = {};
+    circular.self = circular;
+    const msgs: Message[] = [
+      {
+        role: 'assistant',
+        content: '',
+        toolCalls: [
+          { id: 'c1', name: 'circular', arguments: circular },
+          { id: 'c2', name: 'bigint', arguments: { n: 1n } },
+          { id: 'c3', name: 'undef', arguments: undefined },
+        ],
+      },
+    ];
+    // role 4 + 3 tool-call overheads (4 each); unserializable args contribute 0.
+    expect(() => approxCount(msgs)).not.toThrow();
+    expect(approxCount(msgs)).toBe(4 + 4 * 3);
+  });
 });
 
 describe('count', () => {
diff --git a/packages/flint/test/safety/redact.test.ts b/packages/flint/test/safety/redact.test.ts
index 426c7d1..5f8b2d5 100644
--- a/packages/flint/test/safety/redact.test.ts
+++ b/packages/flint/test/safety/redact.test.ts
@@ -91,6 +91,18 @@ describe('secretPatterns preset', () => {
     ['Stripe live key', 'sk_live_abcdefghijklmnopqrstuvwx'],
     ['SSN', 'SSN: 123-45-6789'],
     ['Credit card', 'card 4111-1111-1111-1111'],
+    [
+      'RSA private key',
+      '-----BEGIN RSA PRIVATE KEY-----\nMIIBOgIBAAJBAKj\n-----END RSA PRIVATE KEY-----',
+    ],
+    [
+      'generic PKCS#8 private key',
+      '-----BEGIN PRIVATE KEY-----\nMIGHAgEAMBMGBy\n-----END PRIVATE KEY-----',
+    ],
+    [
+      'encrypted private key',
+      '-----BEGIN ENCRYPTED PRIVATE KEY-----\nMIIFLTBXBgkq\n-----END ENCRYPTED PRIVATE KEY-----',
+    ],
   ];
 
   for (const [label, text] of cases) {