diff --git a/CHANGELOG.md b/CHANGELOG.md
index a3c80b965ce..f13c4e4bbdb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -43,6 +43,7 @@ Docs: https://docs.openclaw.ai
 
 - Gateway/hooks: route non-delivered hook completion and error summaries to the target agent's main session instead of the default agent session, preserving multi-agent hook isolation. Fixes #24693; carries forward #68667. Thanks @abersonFAC and @bluesky6868.
 - Discord: own the Carbon interaction listener and hand off Discord slash/component handling asynchronously, so compaction or long session locks no longer trip `InteractionEventListener` listener timeouts. Fixes #73204. Thanks @slideshow-dingo.
+- Compaction/diagnostics: keep unknown compaction failure classifications stable while logging sanitized detail for unclassified provider errors such as missing Ollama provider adapters. Thanks @gzsiang.
 - Gateway/startup: keep value-option foreground starts on the gateway fast path and skip proxy bootstrap unless proxy env is configured, reducing normal gateway startup RSS and avoiding full CLI graph loading. Thanks @vincentkoc.
 - Heartbeat/models: show heartbeat model bleed guidance on context-overflow resets when the last runtime model matches configured `heartbeat.model`, so smaller local heartbeat models point users to `isolatedSession` or `lightContext` instead of only compaction-buffer tuning. Fixes #67314. Thanks @Knightmare6890.
 - Subagents/models: persist `sessions_spawn.model` and configured subagent models as child-session model overrides before the first turn, so spawned subagents actually run on the requested provider/model instead of reverting to the target agent default. Fixes #73180. Thanks @danielzinhu99.
diff --git a/src/agents/pi-embedded-runner/compact-reasons.test.ts b/src/agents/pi-embedded-runner/compact-reasons.test.ts
index 85f8101cb58..6018b371a0d 100644
--- a/src/agents/pi-embedded-runner/compact-reasons.test.ts
+++ b/src/agents/pi-embedded-runner/compact-reasons.test.ts
@@ -1,5 +1,9 @@
 import { describe, expect, it } from "vitest";
-import { classifyCompactionReason, resolveCompactionFailureReason } from "./compact-reasons.js";
+import {
+  classifyCompactionReason,
+  formatUnknownCompactionReasonDetail,
+  resolveCompactionFailureReason,
+} from "./compact-reasons.js";
 
 describe("resolveCompactionFailureReason", () => {
   it("replaces generic compaction cancellation with the safeguard reason", () => {
@@ -37,4 +41,30 @@ describe("classifyCompactionReason", () => {
       ),
     ).toBe("guard_blocked");
   });
+
+  it("keeps unclassified provider errors in the stable unknown bucket", () => {
+    expect(classifyCompactionReason("No API provider registered for api: ollama")).toBe("unknown");
+  });
+});
+
+describe("formatUnknownCompactionReasonDetail", () => {
+  it("formats unknown reasons as single-token diagnostic detail", () => {
+    expect(formatUnknownCompactionReasonDetail("No API provider registered for api: ollama")).toBe(
+      "No_API_provider_registered_for_api:_ollama",
+    );
+  });
+
+  it("strips terminal escapes and log separators from unknown reasons", () => {
+    expect(
+      formatUnknownCompactionReasonDetail("\u001b[31mNo API\u001b[0m provider = ollama\nnext"),
+    ).toBe("No_API_provider_ollama_next");
+  });
+
+  it("omits empty unknown reason detail", () => {
+    expect(formatUnknownCompactionReasonDetail(" \n\t ")).toBeUndefined();
+  });
+
+  it("limits unknown reason detail length", () => {
+    expect(formatUnknownCompactionReasonDetail("x".repeat(120))).toHaveLength(100);
+  });
 });
diff --git a/src/agents/pi-embedded-runner/compact-reasons.ts b/src/agents/pi-embedded-runner/compact-reasons.ts
index 642889cbc4d..5e98181435e 100644
--- a/src/agents/pi-embedded-runner/compact-reasons.ts
+++ b/src/agents/pi-embedded-runner/compact-reasons.ts
@@ -1,4 +1,7 @@
 import { normalizeLowercaseStringOrEmpty } from "../../shared/string-coerce.js";
+import { sanitizeForLog } from "../../terminal/ansi.js";
+
+const MAX_COMPACTION_REASON_DETAIL_CHARS = 100;
 
 function isGenericCompactionCancelledReason(reason: string): boolean {
   const normalized = normalizeLowercaseStringOrEmpty(reason);
@@ -59,3 +62,15 @@ export function classifyCompactionReason(reason?: string): string {
   }
   return "unknown";
 }
+
+export function formatUnknownCompactionReasonDetail(reason?: string): string | undefined {
+  const sanitized = sanitizeForLog((reason ?? "").replace(/\s+/g, " "))
+    .trim()
+    .replace(/[^A-Za-z0-9._:@/+~-]+/g, "_")
+    .replace(/_+/g, "_")
+    .replace(/^_+|_+$/g, "");
+  if (!sanitized) {
+    return undefined;
+  }
+  return sanitized.slice(0, MAX_COMPACTION_REASON_DETAIL_CHARS);
+}
diff --git a/src/agents/pi-embedded-runner/compact.ts b/src/agents/pi-embedded-runner/compact.ts
index 5d666f68d02..b173d4501bc 100644
--- a/src/agents/pi-embedded-runner/compact.ts
+++ b/src/agents/pi-embedded-runner/compact.ts
@@ -97,7 +97,11 @@ import {
   resolveSkillsPromptForRun,
 } from "../skills.js";
 import { resolveSystemPromptOverride } from "../system-prompt-override.js";
-import { classifyCompactionReason, resolveCompactionFailureReason } from "./compact-reasons.js";
+import {
+  classifyCompactionReason,
+  formatUnknownCompactionReasonDetail,
+  resolveCompactionFailureReason,
+} from "./compact-reasons.js";
 import type { CompactEmbeddedPiSessionParams, CompactionMessageMetrics } from "./compact.types.js";
 import { dedupeDuplicateUserMessagesForCompaction } from "./compaction-duplicate-user-messages.js";
 import {
@@ -351,10 +355,14 @@ export async function compactEmbeddedPiSessionDirect(
   let thinkLevel: ThinkLevel = params.thinkLevel ?? "off";
   const attemptedThinking = new Set<ThinkLevel>();
   const fail = (reason: string): EmbeddedPiCompactResult => {
+    const failureReason = classifyCompactionReason(reason);
+    const detail =
+      failureReason === "unknown" ? formatUnknownCompactionReasonDetail(reason) : undefined;
+    const detailSuffix = detail ? ` detail=${detail}` : "";
     log.warn(
       `[compaction-diag] end runId=${runId} sessionKey=${params.sessionKey ?? params.sessionId} ` +
         `diagId=${diagId} trigger=${trigger} provider=${provider}/${modelId} ` +
-        `attempt=${attempt} maxAttempts=${maxAttempts} outcome=failed reason=${classifyCompactionReason(reason)} ` +
+        `attempt=${attempt} maxAttempts=${maxAttempts} outcome=failed reason=${failureReason}${detailSuffix} ` +
         `durationMs=${Date.now() - startedAt}`,
     );
     return {