diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index bd2740f15..4a0ede1a8 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -10,6 +10,7 @@
 
 - Fixed reversible secret placeholders sharing a case-folded hash base across ASCII case variants, which let a prompt-injected model synthesize a never-provider-visible sibling secret's keyed token by swapping the case hint (`#…:L#` → `#…:U#`) in a tool-call argument. Placeholder bases are now keyed on the exact secret value, so each casing variant gets an independent base and a synthesized sibling token deobfuscates to nothing on live provider/tool-call paths ([#2465](https://github.com/can1357/oh-my-pi/issues/2465)).
 - Fixed an auto-collected environment secret that is also declared as a plain `mode: "replace"` entry with the same content still forcing creation of the persisted `secret-placeholder.key`. Replace mappings run before obfuscate mappings, so the value is one-way replaced and the obfuscate entry never emits a reversible placeholder; the key-need check now ignores such replace-shadowed obfuscate entries, so an effectively replace-only secret set no longer requires (or writes) the key file and no longer fails startup when the agent config dir is unwritable ([#2465](https://github.com/can1357/oh-my-pi/issues/2465)).
+- Fixed a default `mode: "replace"` regex that matches every non-whitespace candidate (e.g. `\S{n}`) shipping the raw secret unchanged when its deterministic replacement collided with the secret value, since every alphanumeric/punctuation candidate still matched. The redaction search now falls back to a same-length whitespace run (space/tab), which `\S`-style patterns never re-match, so the secret is redacted to a stable nonmatching value instead of leaking to the provider ([#2465](https://github.com/can1357/oh-my-pi/issues/2465)).
 
 ## [16.2.3] - 2026-06-28
 
diff --git a/packages/coding-agent/src/secrets/obfuscator.ts b/packages/coding-agent/src/secrets/obfuscator.ts
index d6ebb29b6..cbe43baf2 100644
--- a/packages/coding-agent/src/secrets/obfuscator.ts
+++ b/packages/coding-agent/src/secrets/obfuscator.ts
@@ -26,6 +26,14 @@ export type JsonRecord = { [key: string]: JsonValue | undefined };
 
 const REPLACEMENT_CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
 const NONMATCHING_REPLACEMENT_CHARS = `${REPLACEMENT_CHARS}!#$%&()*+,-./:;<=>?@[]^_{|}~`;
+// Last-resort redaction bytes for a default replace regex that matches every
+// non-whitespace candidate (e.g. `\S{n}`): a same-length run of a single
+// whitespace byte is still a stable nonmatching redaction, so the secret is
+// replaced rather than shipped raw. Only `space`/`tab` are used — never a line
+// terminator — so a `.`-style match-everything regex (which matches space and
+// tab but not `\n`) still exhausts to the sentinel instead of redacting to a
+// newline run.
+const WHITESPACE_REPLACEMENT_CHARS = " \t";
 
 /** Generate a deterministic same-length replacement string from a secret value. */
 function generateDeterministicReplacement(secret: string): string {
@@ -72,9 +80,12 @@ function ensureDistinctReplacement(replacement: string, secret: string): string
  * over a stable ASCII alphabet: alphanumerics first (usually enough), then
  * punctuation fallback bytes when the regex covers every alphanumeric candidate.
  * This keeps the common case readable while still finding a nonmatching
- * same-length redaction for patterns such as `[A-Za-z0-9]{2}`. The sweep is
- * bounded so a match-everything regex (`.`/`[\s\S]`) terminates, returning
- * undefined to let the caller keep the sentinel as the only available fixed point.
+ * same-length redaction for patterns such as `[A-Za-z0-9]{2}`. When the regex
+ * covers every non-whitespace candidate (e.g. `\S{n}`), a same-length run of a
+ * single whitespace byte (space/tab) is tried as a last resort. The sweep is
+ * bounded so a match-everything regex (`.`/`[\s\S]`, which also matches space and
+ * tab) terminates, returning undefined to let the caller keep the sentinel as the
+ * only available fixed point.
  */
 function findNonMatchingReplacement(value: string, regex: RegExp): string | undefined {
 	const len = value.length;
@@ -95,7 +106,7 @@ function findNonMatchingReplacement(value: string, regex: RegExp): string | unde
 			regex.lastIndex = 0;
 			if (!regex.test(candidate)) return candidate;
 		}
-		return undefined;
+		return findWhitespaceFallbackReplacement(value, regex);
 	}
 	// Longer collisions stay bounded. First exhaust every single-position
 	// substitution against the deterministic baseline (`AAAA…`, then `!AAA…`,
@@ -119,6 +130,24 @@ function findNonMatchingReplacement(value: string, regex: RegExp): string | unde
 		regex.lastIndex = 0;
 		if (!regex.test(candidate)) return candidate;
 	}
+	return findWhitespaceFallbackReplacement(value, regex);
+}
+
+/**
+ * Last-resort fallback for a default replace regex that matches every
+ * non-whitespace candidate: a same-length run of one whitespace byte (space, then
+ * tab) is a stable nonmatching redaction. `space` already covers the common
+ * `\S`-style case; `tab` extends it to space-only exclusions. A match-everything
+ * regex (`.`/`[\s\S]`) also matches both, so this still returns undefined there,
+ * keeping the caller's sentinel as the sole fixed point.
+ */
+function findWhitespaceFallbackReplacement(value: string, regex: RegExp): string | undefined {
+	for (const ch of WHITESPACE_REPLACEMENT_CHARS) {
+		const candidate = ch.repeat(value.length);
+		if (candidate === value) continue;
+		regex.lastIndex = 0;
+		if (!regex.test(candidate)) return candidate;
+	}
 	return undefined;
 }
 
diff --git a/packages/coding-agent/test/secrets-obfuscator.test.ts b/packages/coding-agent/test/secrets-obfuscator.test.ts
index 80b916d62..981d3d29f 100644
--- a/packages/coding-agent/test/secrets-obfuscator.test.ts
+++ b/packages/coding-agent/test/secrets-obfuscator.test.ts
@@ -842,6 +842,21 @@ describe("SecretObfuscator friendlyName placeholders", () => {
 		expect(obf.obfuscate(out)).toBe(out);
 	});
 
+	it("redacts a default replace regex that matches every non-whitespace candidate", () => {
+		// `\S{5}` matches every non-whitespace value, so the alphanumeric and
+		// punctuation candidates are all exhausted. A same-length whitespace run is a
+		// stable nonmatching redaction, so the colliding sentinel value is replaced
+		// rather than shipped raw to the provider.
+		const obf = new SecretObfuscator([{ type: "regex", mode: "replace", content: "\\S{5}" }], "Q".repeat(43));
+
+		const out = obf.obfuscate("ZZLB6");
+
+		expect(out).not.toBe("ZZLB6");
+		expect(out).toHaveLength(5);
+		expect(/\S{5}/.test(out)).toBe(false);
+		expect(obf.obfuscate(out)).toBe(out);
+	});
+
 	it("keeps the sentinel only when no same-length value avoids the regex", () => {
 		// A match-everything regex has no nonmatching same-length redaction, so the
 		// search exhausts and the sentinel is kept as the sole fixed point. Such a