🐾 fix(mantle): survive Mantle's 20k output ceiling on tool-heavy turns

Two new Mantle shims, auto-installed alongside the existing pair:

- Opt out of the fine-grained-tool-streaming beta. Under it, an
  output-token cutoff mid-tool_use ends the stream without
  content_block_stop; fast-agent raises "Streaming completed but tool
  call never finished" and burns its retry ladder against the same
  wall (the observed ~700s Alan revise_workspace_file failures on
  Taurus).

- Clamp default maxTokens to Mantle's observed 20 000-token server
  ceiling, so the model stops gracefully (proper block close +
  stop_reason=max_tokens) instead of being cut off by the gateway.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/bedrock.md

@@ -218,7 +218,7 @@ anthropic:
 
 That's the whole configuration. Pallas auto-detects the
 `bedrock-mantle` hostname in `anthropic.base_url` at startup and installs
-two compatibility shims so fast-agent's default request shape matches
+four compatibility shims so fast-agent's default request shape matches
 what Mantle expects (see `pallas/mantle_shims.py`):
 
 1. **Wire-name prefix** — re-adds the `anthropic.` prefix that fast-agent's
@@ -233,6 +233,25 @@ what Mantle expects (see `pallas/mantle_shims.py`):
    Input should be a valid dictionary or object"`, which would otherwise
    break the MCP tool-use loop on the second turn.
 
+3. **Fine-grained tool streaming opt-out** — stops fast-agent sending the
+   `fine-grained-tool-streaming-2025-05-14` beta. Under that beta an
+   output-token cutoff mid-`tool_use` block ends the stream without
+   `content_block_stop`, which fast-agent surfaces as
+   `Streaming completed but tool call never finished` and then retries
+   into the same wall (~700 s agent failures on large tool bodies).
+   Without the beta a cutoff closes blocks properly and lands in
+   fast-agent's graceful `stop_reason=max_tokens` handling.
+
+4. **`max_tokens` clamp** — Mantle enforces a server-side output ceiling
+   of 20 000 tokens per response regardless of the requested
+   `max_tokens` (fast-agent asks for the model's full 128 000). The shim
+   clamps default `maxTokens` to `MANTLE_MAX_OUTPUT_TOKENS` (20 000) so
+   the model stops gracefully at the limit instead of the gateway
+   cutting the stream. A single agent turn — thinking, prose, and tool
+   input combined — cannot exceed this on Mantle; agents that must emit
+   more output in one turn need to split the work (e.g. patch-style
+   edits instead of full-document rewrites).
+
 The Anthropic SDK appends `/v1/messages` to `base_url` automatically.
 
 **Feature support.** Mantle accepts the same Messages API request shape