fix: strip Anthropic params from 3P resume paths (#479)

* fix: strip Anthropic-specific params from 3P provider paths

Three silent failure modes affecting all third-party provider users:

1. Thinking blocks serialized as <thinking> text corrupt multi-turn
   context — strip them instead of converting to raw text tags.

2. Unknown models fall through to 200k context window default, so
   auto-compact never triggers — use conservative 8k for unknown
   3P models with a warning log.

3. Session resume with thinking blocks causes 400 or context corruption
   on 3P providers — strip thinking/redacted_thinking content blocks
   from deserialized messages when resuming against a non-Anthropic
   provider.

Addresses findings 2, 3, and 5 from #248.

* test: align resume stripping expectation with orphan-thinking filter

* test: isolate provider env in conversation recovery tests

* test: move provider-sensitive resume coverage behind module mocks

* test: trim extra blank lines in conversation recovery test

Keep the focused provider-resume test diff clean so the regression branch stays easy to review.

Co-Authored-By: Claude Opus 4.6 <noreply@openclaude.dev>

---------

Co-authored-by: Claude Opus 4.6 <noreply@openclaude.dev>

src/utils/conversationRecovery.ts

@@ -24,6 +24,7 @@ import {
   type FileHistorySnapshot,
 } from './fileHistory.js'
 import { logError } from './log.js'
+import { getAPIProvider } from './model/providers.js'
 import {
   createAssistantMessage,
   createUserMessage,
@@ -177,6 +178,25 @@ export type DeserializeResult = {
   turnInterruptionState: TurnInterruptionState
 }
 
+/**
+ * Remove thinking/redacted_thinking content blocks from assistant messages.
+ * Messages that become empty after stripping are removed entirely.
+ */
+function stripThinkingBlocks(messages: NormalizedMessage[]): NormalizedMessage[] {
+  return messages.reduce<NormalizedMessage[]>((acc, msg) => {
+    if (msg.type !== 'assistant' || !Array.isArray(msg.message?.content)) {
+      acc.push(msg)
+      return acc
+    }
+    const filtered = msg.message.content.filter(
+      (block: { type?: string }) => block.type !== 'thinking' && block.type !== 'redacted_thinking',
+    )
+    if (filtered.length === 0) return acc
+    acc.push({ ...msg, message: { ...msg.message, content: filtered } })
+    return acc
+  }, [])
+}
+
 /**
  * Deserializes messages from a log file into the format expected by the REPL.
  * Filters unresolved tool uses, orphaned thinking messages, and appends a
@@ -227,10 +247,19 @@ export function deserializeMessagesWithInterruptDetection(
       filteredToolUses,
     ) as NormalizedMessage[]
 
+    // Strip thinking/redacted_thinking content blocks from assistant messages
+    // when resuming against a 3P provider. These Anthropic-specific blocks cause
+    // 400 errors or context corruption on OpenAI-compatible providers (issue #248 finding 5).
+    const provider = getAPIProvider()
+    const isThirdPartyProvider = provider !== 'firstParty' && provider !== 'bedrock' && provider !== 'vertex' && provider !== 'foundry'
+    const thinkingStripped = isThirdPartyProvider
+      ? stripThinkingBlocks(filteredThinking)
+      : filteredThinking
+
     // Filter out assistant messages with only whitespace text content.
     // This can happen when model outputs "\n\n" before thinking, user cancels mid-stream.
     const filteredMessages = filterWhitespaceOnlyAssistantMessages(
-      filteredThinking,
+      thinkingStripped,
     ) as NormalizedMessage[]
 
     const internalState = detectTurnInterruption(filteredMessages)