One turn between the
drop
and the 400
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Agent loop
Vertex (Gemini 3.5)
Turn 1
1
turn-1 request
2
streams tool call
glob
extra_content.google.thought_signature: "Cv…=="
3
stores
id
·
name
·
arguments
thought_signature
dropped
4
executes
glob
Turn 2
5
turn-2 request — replays assistant tool call
thought_signature
+ tool result
6
400 INVALID_ARGUMENT
missing
thought_signature
Step through
the run, or hover any step. The signature Vertex sends in step 2 has to come back in step 5 — it doesn’t.
The thought signature must round-trip. It was lost on the way in — one full turn before the failure surfaced.